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Deepu Dharmarajan - Posted 3 years ago
CH1 | THE PURPOSE OF SIGNALLING
SignallingSIGNALLING BOOK | CHAPTER 1 CONTENTS 1.Introduction 2.The Problems to be Solved 3.Basic Requirements 4.Lineside Signals 5.The Absolute Block System 6.Interlocking of Points and Signals 7.Single Lines 8.Further Developments 1. INTRODUCTION In general, the railway traveller assumes that his journey will be safe. This high standard of safety which is taken for granted is the result of a long history of development. As human errors and deficiencies in safety systems become evident, often as a result of an accident, improvements are made which are then incorporated into new generations of equipment. This is certainly true of railway signalling. It also appears to be a continuing process. We have not yet reached the situation where absolute safety can be assured. It is useful to start by looking back at some of the early history of signalling development. In the early days of railways, trains were few and speeds were low. The risk of a serious collision between two trains was minimal. Better track and more powerful locomotives allowed trains to run faster (requiring greater stopping distances). Railway traffic increased, requiring more and larger trains. The risks thus became greater and some form of control over train movements became necessary. The need for railway signalling had been identified. 2. THE PROBLEMS TO BE SOLVED 2.1 Collision with a Preceding Train When one train follows another on to the same section of line, there is a risk that, if the first train travels more slowly or stops, the second train will run into the rear of the first. Initially, trains were separated using a system of "time interval" working, only permitting a train to leave a station when a prescribed time had elapsed after the departure of the previous train. Although this reduced the risk of collisions, a minimum safe distance between trains could not be guaranteed. However, in the absence of any proper communication between stations, it was the best that could be achieved at that time. 2.2 Conflicting Movements at Junctions Where railway lines cross or converge, there is the risk of two trains arriving simultaneously and both attempting to enter the same portion of track. Some method of regulating the passage of trains over junctions was therefore needed. This should ensure that one train is stopped, if necessary, to give precedence to the other. 2.3 Ensuring that the Correct Route is Set Where facing points are provided to allow a train to take alternative routes, the points must be held in the required position before the train is allowed to proceed and must not be moved until the train has completely passed over the points. Depending on the method of point operation, it may also be necessary to set trailing (i.e. converging) points to avoid damage to them. 2.4 Control of Single Lines Where traffic in both directions must use the same single line, trains must not be allowed to enter the single line from both ends at the same time. Although this could in theory be controlled by working to a strict timetable, problems could still arise if trains were delayed or cancelled. 3. BASIC REQUIREMENTS We therefore have the basic requirements of any railway signalling system. The method of implementation has changed over the years but the purpose remains the same:- To provide a means of communicating instructions to the driver (signals) to enable him to control his train safely according to the track and traffic conditions ahead. To maintain a safe distance between following trains on the same line so that a train cannot collide with a preceding train which has stopped or is running more slowly. To provide interlocking between points and the signals allowing trains to move over them so that conflicting movements are prevented and points are held in the required position until the train has passed over them. To prevent opposing train movements on single lines. All the above requirements place restrictions on train movements, but it is vital that the signalling system will allow trains to run at the frequency demanded by the timetable to meet commercial requirements. This must be done without reduction of safety below an acceptable level. Signalling involves not only the provision of equipment but the adoption of a consistent set of operating rules and communication procedures which can be understood and implemented by all staff responsible for railway operation. 4. LINESIDE SIGNALS It will probably be evident that the decisions regarding the movement of two or more trains over any portion of the railway can only be made by a person on the ground who has sufficient knowledge of the current traffic situation. His decision must be passed on to the driver of each train passing through his area of control. In the early days railways employed policemen whose duties would include the display of hand signals to approaching trains. As the policemen also had many other duties, it soon became impractical for them to be correctly positioned at all times. Fixed signals of various designs, often boards of different shapes and colours, were provided. The policeman could then set these and attend to his other duties. The simplest signals would only tell a driver whether or not he could proceed. From this evolved a standard layout of signals at most small stations; a "home" signal on the approach side controlling entry to the station and a "starting" signal protecting the section of line to the next station. Between these signals, each train would be under the direct control of the policeman. These signals could give only two indications, STOP or PROCEED. They therefore became collectively known as "stop" signals. As line speeds increased, "distant" signals were introduced which gave advance warning of the state of stop signals ahead. A distant signal could be associated with one or more stop signals and would be positioned to give an adequate braking distance to the first stop signal. It could give a CAUTION indication to indicate the need to stop further ahead or a CLEAR indication, assuring the driver that the stop signal(s) ahead were showing a proceed indication. With the addition of distant signals, trains were no longer restricted to a speed at which they could stop within signal sighting distance. It is important to understand the difference between stop and distant signals. A train must never pass a stop signal at danger. A distant signal at caution can be passed but the driver must control his train ready to stop, if necessary, at a stop signal ahead. The earliest signals were "semaphore" signals (i.e. moveable boards). To enable operation at night, these often had oil lamps added. With the advent of reliable electric lamps, the semaphore signal became unnecessary and a light signal could be used by day and night. Red is universally used as the colour for danger while green is the normal colour for proceed or clear. Initially, red was also used for the caution indication of distant signals but many railway administrations changed this to yellow so that there was no doubt that a red light always meant stop. If necessary, stop and distant signals can be positioned at the same point along the track. Alternatively, certain types of signal can display three or more indications to act as both stop and distant signals. 5. THE ABSOLUTE BLOCK SYSTEM Although time-interval working may seem crude, it is important to remember that nothing better was possible until some means of communication was invented. The development of the electric telegraph made the Block System possible. On many railways, time-interval working on double track lines is still the last resort if all communication between signal boxes is lost. 5.1 Block Sections In the Block Signalling system, the line is divided into sections, called "Block Sections". The Block Section commences at the starting signal (the last stop signal) of one signal box, and ends at the outermost home signal (the first stop signal) of the next box. With Absolute Block working, only one train is allowed in the Block Section at a time. The signalman may control movements within "Station Limits" without reference to adjacent signal boxes. The accompanying diagram shows a block section between two signal boxes on a double track railway. To understand the method of working, we will look at the progress of a train on the up line. Signalbox A controls entry to the block section but it is only signalman B who can see a train leaving the section, whether it is complete (usually checked by observation of the tail lamp) and who thus knows whether or not the section is clear. Signalbox B must therefore control the working of the UP line block section. Similarly, signalbox A controls the DOWN line block section. 5.2 Block Bell The signalmen at each end of a block section must be able to communicate with each other. Although a telephone circuit is a practical means of doing this, a bell is normally used to transmit coded messages. It consists of a push switch ("tapper") at one box, operating a single-stroke bell at the adjacent box (normally over the same pair of wires). The use of a bell enforces the use of a standard set of codes for the various messages required to signal a train through the section and imposes a much greater discipline than a telephone, although a telephone may be provided as well, often using the same circuit as the block bell. 5.3 Block Indicator This provides the signalman at the entrance to the section with a continuous visual indication of the state of the section, to reinforce the bell codes. It is operated by the signalman at the exit of the block section. Early block instruments were "two position" displaying only two indications; line clear and line blocked. Later instruments display at least 3 indications. The most usual are:- Line clear Giving permission to the rear signalman to admit a train to the section. Normal or Line Blocked Refusing permission. The signalman at the entrance to the section must maintain his starting signal at danger. Train on Line There is a Train in the block section. 5.4 Method of Working When signalbox A has an UP train approaching to send to box B, the signalman at A will offer it forward to box B, using the appropriate bell code (so that signalman B knows what type of train it is). If the signalman B is unable to accept the train for any reason, he will ignore A's bell, and leave the UP line block indicator at "Normal". If he is able to accept the train, signalman B will repeat the bell code back to box A, and change the indication to "Line Clear". When signalman A sees his block repeater go to "Line Clear", then he can clear his starting signals to admit the train to the section. When the train actually enters the section, signalman A sends the "Train Entering Section" bell code to box B. Signalman B will acknowledge this by repeating the bell code back to A, and turning the block indicator to "Train on Line". When the train leaves the block section at B, the signalman there checks that it is complete by watching for its tail lamp. He then turns his block indicator to "Normal" again. He also sends the "Train out of Section" bell code to A, which A acknowledges by repeating it back. The system is now back to normal, ready for the next train. On multiple track railways, a pair of block instruments as above is required for each line. 5.5 Extra Safeguards The basic three-position block system, as described, relies on the correct sequence of operations for safety. A signalman could forget that he has a train in section and turn the indicator to "line clear", allowing a second train in. A detailed record (the train register) is kept of the actual times of train arrival and departure, and the times at which the bell signals are exchanged. In most places, additional safeguards have been added to the basic system. An electric lock on the starting signal will prevent it being operated unless the block indicator is at line clear. Track circuit occupation may be used to set the block instruments to ''Train on Line" if the signalman forgets to do so. Electric locking may also be used to ensure that signals are operated for one train movement only and replaced to danger before another movement is permitted to approach. Although it is unusual for absolute block working to be installed on any new signalling installation today, there are many railways on which it is in widespread use. The block system, by ensuring that only one train may occupy a section of line at any time, maintains a safe distance between following trains. 6. INTERLOCKING OF POINTS AND SIGNALS On all early railways, points were moved by hand levers alongside the points. They could therefore be moved independently of the signals controlling the movement of trains. A great improvement in safety (as well as efficiency) was possible by connecting the point switches via rodding to a single central control point (the signal box). Similarly the signals could also be operated by wire from levers in the signal box. With the control of points and signals all in one place the levers could be directly interlocked with each other. This had the following benefits:- Signals controlling conflicting routes could not be operated at the same time. A signal could only be operated if all the points were in the correct position. The points could not be moved while a signal reading over them was cleared. In early signalling installations, all point and signal operation, together with any interlocking, was mechanical. Although it was a great technological advancement to be able to control a station from one place, the effort required to operate the levers restricted control of points to within about 300 metres from the signal box and signals up to about 1500 metres. At large stations, more than one signal box would often be necessary. The possibility still existed for a signalman to set the points, clear the signal, the train to proceed and then for the signalman to replace the signal to normal. This could free the locking on the points before the train had completely passed over them. Signalmen's instructions usually required the complete train to pass over the points before the signal was replaced to danger. 7. SINGLE LINES On most single line railways trains are infrequent. It is not normally necessary for two trains to follow each other closely in the same direction. Single lines were therefore treated in the same way as a normal block section with the important extra condition that trains could not be signalled in both directions at the same time. To enforce this condition and also to reassure the driver that he could safely enter the single line, some form of physical token was used as authority to travel over the single line. On the simplest of systems only one token existed. This caused problems whenever the pattern of service differed from alternate trains in each direction. If the timetable required two trains to travel over the single line in the same direction, the driver of the first train would be shown the token (or train staff as it is commonly known) to assure the driver that no other train was on the single line. His authority to enter the single line would however be a written ticket . The following train would convey the train staff. Although workable, this system would cause problems if trains did not work strictly to the timetable. A further improvement was to provide several tokens, but to hold them locked in instruments at either end of the single line. The instruments would be electrically interlocked with each other to prevent more than one token being withdrawn at a time. The one token could however be withdrawn from either instrument. If the single line block equipment fails, many railways employ a member of the operating personnel as a human token. The "pilotman", as he is usually known, will either travel with the train or instruct the driver to pass through the section. No other person may allow a train on to the single line. Operationally, this is the equivalent of the train staff and ticket system described earlier. 8. FURTHER DEVELOPMENTS The main functions of the signalling system had now been defined, although they were to be continuously improved as the available technology developed. All signalling systems would be required to maintain a safe distance between trains, interlock points and signals and thus prevent conflicting movements, and provide the necessary information so that the speed of all trains can be safely controlled. In recent years, the signal engineer has been asked to provide further facilities within the general scope of the signalling system. These include, train information to the operating staff, train information for passengers, detection of defective vehicles, identification of vehicles and the increasing automation of tasks previously carried out by humans. The technology exists to completely operate a railway without human intervention although the level of automation desirable for a particular railway is for that railway administration to decide. Factors such as cost, maintainability, reliability, staffing policy, passenger security and sometimes political considerations must be taken into account. In many cases the final decision on the type of signalling to be provided is outside the direct control of the signal engineer. However, he should always endeavour to provide the best possible information and propose cost-effective solutions to particular problems so that the best decisions can be made.
Deepu Dharmarajan - Posted 3 years ago
CH2 | BASIC SIGNALLING PRINCIPLES | PART 1
SignallingSIGNALLING BOOK | CHAPTER 2 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Signal Aspects - In Part 1 3. Signalling Principles - In Part 2 4. Drawing Standards - In Part 2 5. Interlocking Principles - In Part 2 6. Train Detection & Track Circuit Block - In Part 2 7. Colour Light Signals - In Part 2 8. Control Panels & Other Methods of Operation - In Part 2 9. Colour Light Signalling Controls - In Part 2 1. INTRODUCTION Whatever type of signalling system is provided on a railway, its basic functions will remain the same. Safety must be ensured by preventing trains colliding with each other and locking points over which the train is to pass. The means of achieving these functions may vary from one railway administration to another but a set of rules must be laid down to define:- The positioning of signals The types of signals The aspects to be displayed by the signals and the instructions to be conveyed by those aspects The controls to be applied to the signals The method of controlling points The method of interlocking points with signals The standardisation of human interfaces Many countries have sytems of signalling based on British railway signalling practice. The basic British system is very simple having only a small number of different signal aspects displayed to the driver. The driver is responsible for knowing the route over which he is to pass. The signal engineer must, in turn, provide sufficient information for the driver to safely control the speed of his train and, where necessary, to inform him which route he is to take. Other signalling systems have developed along a different path. The driver is given specific instructions to travel up to or reduce to an indicated speed. Route indications are optional. This will generally require a more complex set of signal aspects. This section will deal mainly with the principles and practices of the State Rail Authority of New South Wales, with reference to other systems where appropriate. 2. SIGNAL ASPECTS There are three principal types of signal, each serving a different purpose:- Main or Running signals control the normal movement of passenger and freight trains on running lines. The great majority of movements will be controlled by main signals. Subsidiary signals, mounted on the same post or structure as running signals, control movements other than for normal running, such as the shunting or coupling of trains. Independent shunting signals, generally similar to the subsidiary signals above, are provided for shunting movements at positions where there is no need for a running signal. We will examine the aspects displayed by each type of signal and the instructions and/or information conveyed by them. 2.1 Main or Running Signals As all early signals were semaphore signals, displaying a light for night time use, the aspects of colour light signals are usually based on the indications of the semaphore signals which they replaced. As most new signalling installations are likely to employ colour light signals, this section will concentrate on colour light signalling only. SRA (Currentlly TfNSW) employs two methods of signalling on main lines; single light and double light. As the name suggests, double light signals will always display at least two lights to the driver. Double light signalling is generally used in the Sydney metropolitan area. Single light signals normally use only one light to convey instructions to the driver, although a second marker light may be illuminated to aid the driver in locating the signal. Single light signalling is mostly used on lines outside the Sydney metropolitan area. Although there are similarities between the two systems, we will deal with each separately. We will then make a comparison with the corresponding aspects displayed by the British system to enable readers to read signalling plans drawn in British style. 2.1.1 Double Light Signalling This is intended for use in areas where signals are closely spaced. Each stop signal is therefore required to carry a distant signal for the signal ahead. To give the driver a consistent indications, each signal carries two separate signal heads. The upper signal head can be considered as the stop signal. It will always be capable of displaying, at least, stop and proceed aspects. The lower signal head can be considered as the distant for the next signal ahead. An additional green light may be provided below the distant. This is used for a "low speed" indication . FIGURE 1 shows the normal running aspects for double light signalling. Four aspects are used for normal running :- STOP is denoted by two red lights, one above the other. Note that the lower signal head will always display a red if the upper signal is at red, even if the signal ahead is showing a proceed aspect. This is important to avoid misleading or confusing the driver. CAUTION is denoted by green over red. In other words, this signal is at "proceed" (top signal head) but the next signal is at "stop" (bottom signal head acts as distant). The caution indication tells the driver to be prepared to stop at the next signal. MEDIUM is a preliminary warning of the need to stop. It is denoted by green over yellow. Signals in urban areas may be closely spaced. The one signal section between the caution and the stop may provide insufficient braking distance for a train travelling at full line speed. The medium indication tells the driver that the next signal is at caution. This implies that he may have to stop at the second signal ahead. CLEAR allows the train to proceed at maximum speed. A clear indication is two green lights. This will tell the driver that there is no need to reduce speed (other than for fixed speed restrictions) before the next signal. All the above indications require the driver to know where the next signal is, to safely control the speed of his train and be able to stop where required. An additional indication is provided on some signal, A LOW SPEED indication, consisting of a small green light below a normal stop aspect tells the driver to proceed at no more than 27km/h towards the next signal. This is generally more restrictive than the caution. The low speed aspect is used when the track is only clear for a very short distance beyond the next signal. Fig 1: DOUBLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING *Where a low speed indication is provided. Fig 2: DOUBLE LIGHT SIGNALLING - TURNOUT ASPECTS NOTE: A full clear indication is not given for turnouts. Note the difference in the indications given by Multi-light signals at a turnout. The yellow over red indicates "Proceed" at Medium speed through Turnout, next signal at "Stop". The Yellow over Yellow indicates "Proceed at Medium speed through Turnout" the lower Yellow is cautioning the driver to continue at Medium Speed towards the next signal which is indicating either "CAUTION" or "CLEAR" *Where a low speed indication is provided. **In the Sydney and Strathfield resignaled areas this indication represents a 'low speed' with the train stop at stop. In this case the signal in the rear will show a caution indication. Figure 2 shows the indications for double light turnout movements. If there is more than one route from a main signal, the driver must be told whether he is to take the main line or through route or whether he is to take a lower speed diverging turnout. This information is necessary to prevent the driver running through a turnout at too high a speed. The upper signal head is used to display a distinct proceed aspect for a turnout. Instead of the green normally displayed, a yellow light will tell the driver that he is to take the turnout. The proceed aspects for turnouts are:- CAUTION TURNOUT (yellow over red, also described in some operating documents as medium caution) tells the driver to expect the next signal to be at red. MEDIUM TURNOUT (yellow over yellow) tells the driver that the next signal ahead is displaying a proceed aspect. This is the least restrictive aspect for a turnout. There is no equivalent of a clear aspect for trains signalled over a turnout. To give a clearer indication to the driver where several routes are possible from one signal, the main signal aspect may be used instead, in conjunction with a theatre route indicator. The route indicator contains a matrix of small lunar white lights which can be illuminated to display a Jetter or number. Each character will be associated with a distinct route. The route indicator is not illuminated when the signal is at stop. When the signal is required. to clear, the route indicator will illuminate for the appropriate route. LOW SPEED aspects may be used. If a low speed aspect is provided for a turnout route, this is no different in appearance to a low speed for the main line route. This is not a problem as this aspect conveys a specific speed instruction. As the speed is likely to be lower than that of most turnouts, route information is not essential 2.1.2 Single Light Signalling On lines where single light signalling is installed, the spacing of signals may vary widely. Therefore some signals may be combined stop and distant signals (as for double light) but there may also be signals which are stop signals only or distant signals only. The instruction to the driver is therefore generally conveyed by a single light. A second marker light is provided below the main light to aid the driver in locating the signal. The meaning of the signal aspects is equivalent to the double light aspects but the appearance to the driver is different. FIGURE 3 shows the normal running aspects for single light signalling. The appearance of each aspect is as follows:- STOP consists of a red light. The marker light also displays a red, except on some older signals where it is lunar white. CAUTION consists of a single steady yellow light. The marker light is extinguished, except on some older signals where it is lunar white. If the main light should fail the marker light will display a red on stop signals or yellow on distant signals. MEDIUM, where this aspect is necessary, will be a flashing or pulsating yellow light. The marker light will operate as for the caution aspect. CLEAR is a green light. The marker light will operate as for the caution aspect. LOW SPEED aspects may be used in single light signalling where required. An additional small green light is provided below the marker light. The complete low speed aspect will be a main red light over a red marker light with the additional green light illuminated . Fig 3: SINGLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING Fig 4: SINGLE LIGHT SIGNALLING - TURNOUT ASPECTS The indication displayed by a Home signal for a turnout movement through facing points into a Loop Refuge siding or important siding consists of a band of three yellow lights in a subsidiary light unit (inclined towards the direction of the movement). The Red light is displayed in the Main line signal , as shown. The marker light for the main line signal, contained in the subsiduary light unit will be extinguished when the main line or turnout signal indication is displayed. 4.1 ROUTE INDICATIONS MAIN LINE At locations where more than one turnout is provided one signal indication is some times given and in such cases a route indicator working in conjunction with th e signal is provided, this enables drivers to ascertain the route for which th e signal has been cleared. The route indicator will not show any indication when the signal is at stop, but when the points have been set for the turnout movement a yellow light will appear in the signal in conjunction with the route indication showing a letter to denote the line to which the train will travel, e.g. Figure 4 shows the indications for single light turnout movements. For junction signals, two distinct methods are used according to the situation. For a simple turnout into a loop or siding, a separate turnout signal is provided below the main aspect, incorporating the marker light. For a CAUTION TURNOUT aspect, the main aspect remains at red, the marker light is extinguished and the three yellow lights of the turnout signal are illuminated . The row of lights is inclined in the direction of the turnout. For a MEDIUM TURNOUT aspect, the turnout signal will flash. Otherwise the appearance is the same as above. As for double light signalling, a theatre route indicator may be used in conjunction with the main signal aspect, where several routes are possible from one signal. Again the route indicator is not illuminated when the signal is at stop. When the signal is required to clear, the route indicator will illuminate for the appropriate route. The normal construction of signals is to provide a separate lamp unit for each light to be displayed . Some signals, however, are of the "searchlight" type. In this type of signal, the lamp is continuously illuminated and coloured lenses are moved in front of the lamp according to the aspect to be displayed. The lenses are moved by a relay mechanism inside the signal head. The lights visible to the driver are the same for either type of signal. 2.2 Subsidiary Signals Associated with Main Signals As well as normal running movements, signals may be required for some of the following movements:- Entering an occupied section Shunting into a siding Running on to a line used for traffic in the opposite direction. Attaching or detaching vehicles or locomotives. The main types are described below. As for the running signals, only current practice is covered in detail. Although the SRA (TfNSW) practice will allow subsidiary signals to clear immediately the route is set, many railway administrations employ approach control to delay clearance of the subsidiary signal until the train has come at or almost to a stand at the signal. This is usually achieved by timed track circuit occupation. The driver will receive a caution at the previous signal and will be preparing to stop. Subsidiary signals are short range signals which are only visible within a short distance of the signal. 2.2.1 Subsidiary Shunt and Calling-on Signals These authorise a driver to pass a main signal at stop for shunting purposes or to enter an occupied section. The driver must be prepared to stop short of any train or other obstruction on the line ahead. He must therefore control the speed of his train so that he can stop within the distance he can see. The appearance of these signals is a small yellow light below the main aspect. On some older double light signals the letters "CO" in a round lens illuminated in white may be used. 2.2.2 Shunt Ahead Signal This signal is generally found on single and double lines worked under absolute block conditions. It permits the movement of a train past the starting signal for shunting purposes. It does not require a block release from the signal box ahead and the movement will eventually come back behind the starting signal when shunting is complete. As this method of working is generally only found outside the suburban area, a shunt ahead signal will normally be provided on single light signals only. It consists of a small flashing yellow signal below the main running signal and marker light. 2.2.3 Close-up Signal This is similar in appearance and application to the low speed signal. 2.2.4 Dead-end Signal This is for entering short dead end sidings directly from a running line. The only difference between this and a subsidiary shunting or calling-on signal is that the dead end signal is offset from the post on the same side as the siding leads off the main line. Subsidiary signals display no aspect when not in use. The associated main signal will remain at stop when the subsidiary signal is in use. Route indicators may be used in conjunction with subsidiary aspects to give an indication to the driver where multiple routes are available. In the case of a movement on to another running line in the wrong direction (i.e. opposite to normal direction of traffic) a route indication is always provided. 2.4 Shunting Signals Signals may also be required for shunting movements in positions where no main signal is necessary. The most common locations are:- Entrance to and exit from sidings. At crossovers to allow a wrong road movement to regain the right line. In yards and depots where main signals are not required. As they have no associated main signal, they must display a stop as well as a proceed aspect. 2.5 Dwarf and Position-light Signals Two main types are in use, the dwarf signal, with all lights arranged vertically and the position light signal. The diagrams show the various signal profiles. Both types display two red lights for stop. The proceed aspect is normally only a yellow indicating caution. Shunting signals do not always prove track circuits clear and the driver must be ready to stop if there is another train occupying the section ahead. The proceed aspect may be accompanied by a route indication. Shunting signals are normally mounted at ground level, although they may be elevated if required for sighting. 2.3.2 Stop Boards If a wrong road movement is authorised from a shunting or subsidiary signal there must be another signal ahead to limit the wrong road movement. If no such signal was provided, the movement could continue past the protecting signal for the normal direction of traffic and cause a collision. The usual signal is an illuminated notice board carrying the words "SHUNTING LIMIT". It can be considered as a shunting signal permanently at danger. 2.3.3 Facing Shunt Signals A shunting signal is sometimes needed in a position where it is passed in the normal direction by running movements. To avoid confusing the driver by displaying a yellow light for a movement which may well be running under the authority of clear signals, these facing shunt signals are provided with an additional green light (clear aspect) for use only in this situation. 2.3.4 Point Indicators Although not signals in the same way as those just described, point indicators are important in sidings to avoid derailments and damage to equipment. They provide a visible indication of the position of hand operated points. Operation may be either mechanical or electrical. Located alongside the point switches, they display an illuminated arrow in the direction of the line for which the points are set. 2.5 British Signalling Aspects For the benefit of those who may at some time have to read signalling plans drawn to British standards (e.g. for the IRSE examination), this is a brief summary of the aspects in use, their meanings and how they are drawn. 2.5.1 Main Signals As with SRA (Currently TfNSW) practice, three colours are used, red, yellow and green. On the plan, a red light is denoted by a circle with a horizontal line across it. A yellow light has the line at 45° and a green light has a vertical line. The "normaJ" aspect of the signaJ (i.e. with no routes set and all track circuits clear) is shown by a double line in the appropriate light(s). There are four available aspects; STOP is a red light, CAUTION is a single yellow light, PRELIMINARY CAUTION is two yellow lights and CLEAR is green.The stop, caution and clear signals have the same meanings as the corresponding SRA aspects. The preliminary caution is similar to the MEDIUM indication of SRA signaJs. The double yellow aspect is only used in situations where signals must be positioned closer together than braking distance. Many lines use red, single yellow and green only. Marker lights are not used. There is no equivalent of the LOW SPEED signal. An equivalent control (to allow trains to close up provided they are running at very low speed) is provided by delayed clearance of the yellow aspect. The train must be almost stationary at the signal before the aspect will change from red to yellow. This is achieved by applying approach control with timed track circuit occupation. 2.5.2 Junction Signalling Where a signal has more than one route, a distinct route indication must be given for each route, except that the highest speed or straight route need not have a route indication. This may take one of two forms, a junction indicator (a row of five white lights) normally above the main signal and pointing in the direction of the divergence or a multi lamp or fibre optic route indicator displaying one or two characters. There are six available junction indicator positions. Positions 1, 2 and 3 (at 45°, 90" and 135° respectively) indicate diverging routes to the left. Positions 4, 5 and 6 provide equivalent indications to the right. Multi-lamp or fibre optic route indicators are restricted to routes with a speed of 40 mph (64 km/h) or less. Where necessary, clearance of the junction signal is delayed by occupation of the approach track circuit (timed if necessary) to enforce a speed reduction. This is because the driver may receive no warning at previous signals of the route set from the junction signal 2.5.3 Subsidiary Signals The standard subsidiary signal is a position light with two white lights at 45°. The proceed aspect is both lights illuminated. There is no stop aspect - the associated main signal remains at red. Route indicators are provided where necessary but are not obligatory - if they are provided, route indications must be displayed for all routes. The subsidiary signal is used for all shunting and calling on moves. This is a short range signal. An approaching train must be brought to a stand before clearance of the subsidiary aspect. 2.5.4 Shunting Signals The position light shunting signal has two white lights and one red light. The proceed aspect is identical to the subsidiary signal. The stop aspect is one red and one white light, horizontally placed. The white light at the lower right {the "pivot" light) therefore remains continuously lit. A shunting signal with two red lights only is used as a "limit of shunt" indicator. 2.6 Summary Whatever the system of signalling, the signal engineer must have a detailed knowledge of the aspects displayed to the driver and the instructions conveyed. He must then design the layout of the signalling and the associated controls so that the driver can safely obey all signal aspects. This must apply for all types of train likely to use a line. When required to reduce speed or stop, trains must have adequate braking distance under all conditions. The driver of a train must never be given an instruction by a signal that he is unable to comply with. TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 2 | PART 2...........
Deepu Dharmarajan - Posted 3 years ago
CH3 | SIGNALLING A LAYOUT | PART 1
SignallingSIGNALLING BOOK | CHAPTER 3 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Headway - In Part 1 3. Positioning of Running Signals - In Part 2 4. Types of Signal - In Part 2 5. Points and Crossings - In Part 3 6. Track Circuits - In Part 3 7. Identification of Signals, Points & Track Circuits - In Part 3 8. Examples - In Part 3 1. INTRODUCTION One of the first steps in any signalling project is to determine the method of train working. Having decided this, it is then necessary to decide the position and spacing of signals. This section will assume throughout that colour light signalling to track circuit block principles will be provided on all main lines. Although other methods of working may well be more appropriate, particularly for lightly used single lines, these will be covered later in the course. It is useful at an early stage to determine whether 2, 3 or 4 aspect signalling will be required. This will be governed by the required line capacity, which in turn will be determined by the timetable to be operated. Having this information and an approximate signal spacing, we can then proceed to position the signals on a scale plan of the track layout. Their position relative to stations, junctions etc. will be decided largely by operating requirements. The most economical arrangement that meets all operating requirements is the one that should be adopted. In order to produce a safe and economical signalling scheme, the designer must use his knowledge of signalling principles and be provided with all necessary details of the train service pattern required, the track layout, gradient profiles, line speeds and train characteristics. If this information is not immediately available, it must be requested from the appropriate authority. Sometimes operating requirements conflict with each other and with safety standards — the engineer must then use his experience to reach a satisfactory compromise whilst maintaining the safety standard. 2. HEADWAY The headway of a line is the closest spacing between two following trains, so that the second train can safely maintain the same speeds as the first. This usually means that the second train is sufficiently far behind the first that its driver does not see an unduly restrictive signal aspect. Headways can be expressed in terms of distance but more usefully as a time (e.g. 2 1/2 minutes between following non-stop trains). It can also be converted to a line capacity (trains per hour). Care must be taken when using a "trains per hour" figure if the trains are not evenly spaced in the timetable. The signalling must be able to handle the minimum headway, not the average. Headway will depend on a number of factors:- D = Service Braking Distance d = Distance between STOP signals S = Sighting Distance (usually 200 yds/metres or distance travelled in 10 seconds) O = Overlap Length L = Train Length (less than 100 yds/metres for a short suburban train but possibly over 1km for a heavy freight train) V = Line Speed (or actual train speed if lower) a = Braking rate Where any of these factors are not given to you, you should always state your assumptions. In practical situations, it is vital to obtain accurate information regarding the braking performance of trains. It is also vital to standardise your units of distance and time. If you work in imperial, yards and seconds are most useful; in metric, metres and seconds would be most appropriate. Whichever you decide, you must use the same set of units consistently throughout to avoid confusion and error. 2.1. Service Braking Distance This is the distance in which a train can stop without causing undue passenger discomfort. It will depend on the line speed, gradient, and type of train. It is usually significantly greater than the emergency braking distance. Theoretically, the Service Braking Distance can be calculated using the line speed and braking rate This is derived from the 3rd Law of Motion. This calculation will depend upon the braking characteristics of the type(s) of train using the line and must take into account the worst case combination of train speed and braking rate. If this calculation is to be performed frequently, it is useful to show the service braking distances for different combinations of speed and gradient in tabular or graphical form. Gradient should always be taken into account. A falling gradient will increase braking distance, a rising gradient will reduce it. As gradients are rarely uniform between signals, we need to calculate an average gradient using the formula: where G is the average gradient D is the total distance g and d are the individual gradients & distances. For a gradient of 1 in 100, G = 100. If the gradient is expressed as a percentage, G is the reciprocal of the percentage gradient. Falling gradients taken as negative, rising gradients as positive. 2.2. 2 Aspect Signalling 2 aspect signalling will generally be adequate on lines where traffic density is low. The required length of block section is much greater than braking distance. Only two types of signal are used, a stop signal showing stop and clear only and a distant signal showing caution or clear. Each stop signal will have its associated distant signal. As 2 aspect signalling will mainly be found outside the suburban area, the example shows single light signals. The distance (d) between stop signals is variable according to the geography of the line, positions of stations, loops etc. The headway distance can be calculated as: H = D + d + S + O + L giving a headway time: Note that the headway time for the line is that of the longest section and cannot be averaged. To obtain the greatest signal spacing to achieve a specified headway, we transpose the equation to give: d = (V x T) - ( D + S + O + L) 2.3. 3 Aspect Signalling With 2 aspect signalling, as the required headway reduces, each stop signal will become closer to the distant signal ahead. it is therefore more economic to put both signals on the same post. This then becomes 3 aspect signalling. Each signal can display either stop, caution or clear. The distance (d) between signals must never be less than braking distance (D), but to ensure that the driver does not forget that he has passed a distant at caution, (d) should not be excessively greater than the service braking distance. The current SRA recommendation is for signal spacing to be no greater than three times braking distance. BR has adopted a maximum of 50% (i.e. 1.5D) although this is often exceeded at low speeds. The headway distance is given by:- H = 2d + S + O + L So the best possible headway, when the signals are as close as possible (exactly braking distance), is: H = 2D + S + O + L The headway with signals spaced 50% over service braking distance is: H = 3D + S + O + L The headway with signals spaced at three times braking distance is: H = 6D + S + O + L 2.4. 4 Aspect Signalling Where signals are closer together than braking distance, a preliminary caution or medium aspect is needed to give trains sufficient warning of a signal at danger. This medium aspect must not be less than braking distance (D) from the stop aspect, so the distance (d) between successive signals must on average be no less than 0.5D. The headway distance is given by:- H = 3d + S + O + L where d > 0.5 D So the best possible headway with 4 aspect signalling is given by:- H = 1.5 D + S + O + L In practice, the geographical constraints of the track layout will probably prevent regular spacing of signals at 0.5D. If the total length of two consecutive signal sections is less than braking distance, an additional medium aspect will be required at the previous signal. In other words, the first warning of a signal at stop must be greater than braking distance away. If more than two warnings are required, the medium aspect is repeated, not the caution. Signals should however be positioned so that this situation is as far as possible avoided. 2.5. Application of Low Speed Signals and Conditional Caution Aspects In normal use, the addition of a low speed signal provides the driver with a fifth aspect. It is important to realise that this does not have any effect on the headway of through or non-stopping trains running at their normal speed. In this situation, the engineer will arrange the signals so that each driver should, under normal conditions, see only clear aspects. The preceding headway calculations apply regardless of whether low speed signals are provided or not. A low speed signal tells the driver that he has little or no margin for error beyond the next signal and should control the speed of his train accordingly. The benefit of low speed signals is in allowing a second train to close up behind a stationary or slow moving train by reducing the length of the overlap, provided the speed of the second train has been sufficiently reduced. The same effect can be achieved by delaying the clearance of the caution aspect. This is now preferred, provided an overlap of the order of 100 metres can be achieved. The clearance of the signal should be delayed to give a passing speed of approximately 35km/h. Low speed signals should only be used where the reduced overlap is very short (less than 50 metres) and/or there are fouling moves within 100 metres of the stop signal. 2.5.1. Station Stops With an overlap of 500 metres, a train stopped at a station will have at least 500 metres of clear track behind it. A following train will stop at the first signal outside this distance. By the addition of a low speed signal or a conditionally cleared caution, the overlap distance can be reduced and the second train can approach closer to the station. When the first train leaves the station, the second train can enter the platform earlier, thus giving a better headway for stopping trains. A conditionally cleared caution aspect will normally be used unless the overlap is less than 50 metres. 2.5.2. Approaching Junctions Trains awaiting the clearance of another movement across a junction can approach closer to the junction while keeping the overlap clear of other routes across the junction. A low speed aspect will normally be used in this situation. 2.5.3. Recovery from Delays A line which is operating at or near its maximum capacity will be susceptible to disruption from even minor train delays (e.g. extended station stops at busy times). Low speed signals and or conditionally cleared caution aspects can allow trains to keep moving, even if only slowly, to improve recovery from the delay. The total length of a queue of trains will be less and the area over which the delay has an impact will be reduced. 2.6. Summary For 2 aspect signalling, the headway distance is:- H = D + d + [S + O + L] For 3 aspect signalling, the headway distance is:- H = 2D + [S + O + L] (minimum) where signals are spaced at braking distance H = 2d + [S + O+ L] (general case) for an actual signal spacing of d For 4 aspect signalling, the headway distance is:- H = 1.5 D + [S + O + L] (minimum) where signals are spaced at braking distance H = 3d + [S + O + L] (general case) for an actual signal spacing of d Note the factor [S + O + L] is common to all equations. Headway time is then calculated as: 2.7. Determining Signal Type and Spacing Because cost is generally proportional to the number of signals, the arrangement of signalling which needs the smallest number of signals is usually the most economic. It must, however, meet the headway requirements of the operators. For non-stop headways it is likely that the same type of signalling should be provided throughout. Otherwise there will be large variations in the headway. Remember that the headway of the line is limited by the signal section which individually has the greatest headway. This section will briefly describe a technique for determining the optimum signalling for a line. There may need to be localised variations (e.g. a 2 aspect signalled line may need 3 aspect signals in the vicinity of a station or a 3-aspect line may need to change to 4 aspect through a complex junction area). These variations will depend on the requirements for positioning individual signals and can be dealt with after the general rules have been determined. Firstly, determine braking distance, train length and overlap length required. Each must be the worst case. Knowing the required minimum headway, use the H = 2D + S + O + L equation to determine the best possible headway for 3 aspect. Compare the results with the required headway to check whether "best case" 3 aspect signalling is adequate. There should be a margin of 25–30% between the theoretical headway and that required by the timetable to allow for some flexibility to cope with delays. 2.7.1. If the Headway is Worse than Required 3 aspect will not be adequate and 4 aspect must be used. Recalculate for 4 aspect to confirm that this does meet the headway requirement. T = (1.5D + S + O + L) / V If the non-stop headway requires 4 aspect signalling, it is likely that station stops will cause further problems. Signal spacing near stations should be kept to a minimum and low speed signals or conditionally cleared cautions with reduced overlaps may also be required. 2.7.2. If the Headway is Much Better Much better generally means a headway time of 30% or less than that required by the timetable. If this is the case 2 aspect will generally be adequate. Calculate the greatest signal spacing that will achieve the headway with 2 aspect signalling. d=(V x T) - (D + S + O + L) Remember that in this distance d there will be two signals, a stop signal and a distant signal. Then compare this with the maximum permissible signal spacing for 3 aspect. In the absence of any firm rules, a judgement must be made on the amount of excess braking which is acceptable. SRA recommends that signal spacing is no more than three times braking distance while BR signalling principles specify no more than 1.5 times braking distance. If the two calculations produce a similar total number of signals (i.e. d for 2 aspect is approximately twice the value of d for 3 aspect) a 3 aspect system will be the better choice. The cost of the signals will be similar and the operator may as well benefit from the improved headway provided by 3 aspect. 2.7.3. If the Headway is Slightly Better It is probable that 3 aspect is the correct choice. Check that there is sufficient margin between the required and theoretical headway. 2.7.4. Signal Spacing Having evaluated that the chosen arrangement of signalling will provide the required headway, the relevant equation should be transposed to calculate the greatest possible signal spacing that can be allowed with the specified headway: eg. for 3 aspect signalling: V x T = H = 2d + S + O + L therefore 2d = (V x T) - (S + O + L) from which the post to post spacing (d) can be calculated Remember, there may be a constraint on the maximum signal spacing. The value of d should not exceed this. Geographical constraints may also require signals to be closer together than braking distance, in which case the 4th (medium) aspect is used where required. It does not need to be used throughout unless for headway [puposes]. 2.8. Example Information given:- Max. Line Speed...... 90 km/h Gradients .......... Level Train Length.............. 250 metres Headway Required..... 2 1/2 mins. (non-stop) Before we start, we need the Service Braking Distance, either by calculation or from tables/curves (where available). We will assume that D = 625 metres. Note : S assumed to be 200 metres. O assumed to be 500 metres (although overlaps may need to be more accurately calculated if trainstops used) V = 90 km/h = 25m/s First, check 3 aspect signalling:- H = (2D + S + O + L) = (1250 + 200 + 500 + 250) = 2200 metres so T = H/V = 88 seconds. This is much less than the 150 seconds (21/2 mins) specified. We will therefore consider the alternative of 2 aspect signalling. We cannot calculate a theoretical headway for 2 aspect signalling as the signal spacing is not fixed. Instead, we calculate the greatest 2 aspect signal spacing to give us the 150 second headway specified : d = (V x T) - (D + S + O + L) = (25 x 150) - (625 + 200 + 500 + 250) = 3750 - 1575 metres = 2175 metres Hence 2 aspect signalling, with the stop signals no more than 2175 metres apart, would give the 2 1/2 min. headway required. However, each stop signal also requires a distant signal. Two signals are therefore required every 2175 metres. 3 aspect signalling with signals every 1088 metres would require no more signals but would give a better headway of: H = 2d + (S + O = L) = 2175 + (200 + 500 + 250) = 3125 metres So T = H/V = 3125/25 = 125 seconds In fact, the signal spacing could be extended further within the headway requirement of 150 seconds. This would give a better headway with fewer signals than 2 aspect. This demonstrates that 2 aspect is generally worth considering only for very long headways. We could now calculate the maximum possible 3 aspect signal spacing allowed by the headway : V x T = H = 2d + S + O + L therefore 2d = (V x T) - (S + O + L) = (25 x 150) - (200 + 500 + 250) = 3750 - 950 = 2800 metres d = 1400m As this is over twice braking distance, it should be confirmed that this signal spacing is operationally acceptable TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 3 | PART 2...........
Deepu Dharmarajan - Posted 3 years ago
SRA (TfNSW) SIGNALLING & ELECTRICAL SYMBOLS
SignallingCONTENTS The following pages show the signalling and schematic symbols used by the State Rail Authority of New South Wales on their plans and drawings. They are provided for reference use throughout the course. They are reproduced from SRA (Currently TfNSW) training material. TRACK PLAN AND WORKING SKETCH SYMBOLS MECHANICAL SIGNALS POWER WORKED SIGNALS SINGLE LIGHT INDICATION WAYSIDE BUILDING AND STRUCTURES TRACK CIRCUIT DEVICES INTERLOCKING SYMBOLS APPARATUS HOUSINGS RELAYS AND CONTACTS CONTACTS OPERATED BY SIGNALS LOWER QUADRANT SEMAPHORE SIGNALS, INCLUDING BANNER SIGNALS CONTACTS OPERATED BY POINTS LEVER CONTACTS CATCH ROD CONTACTS MISCELLANEOUS APPARATUS
Deepu Dharmarajan - Posted 3 years ago
CH4 | HEADWAYS
SignallingSIGNALLING BOOK | CHAPTER 4 CONTENTS Introduction Theoretical Headways Practical Headways Headway Charts Producing a Headway Chart Station Stops Speed Restrictions & Junctions Gradients Varying Train Speeds Single Lines Terminal Stations 1. INTRODUCTION The prime function of a signalling system ,irrespective of fixed block or moving block is to protect trains so that they run safely, including maintenance of a safe distance between following trains. Design headway can be defined as the theoretical time separation between two Trains travelling in the same direction on the same track. It is calculated from the time the head-end of the leading Train passes a given reference point to the time the head-end of the following Train passes the same reference point.The run profile for both trains shall be the minimum run time that the rollingstock and track conditions permit. For a fixed block signalling once the signals have been positioned, this minimum distance has effectively been fixed. This in tum governs the capacity of the line, or how many trains per hour can use it,whereas for a moving block headway is the minimum distance can be maintained between two moving block (Rollingstock) with a safety margin. Refer figure 1 below Figure 1 Head way of Moving block system It would be ideal to learn the headway with a fixed block signalling.Line capacity is derived from the minimum headway time between trains. Although this can be deduced mathematically from a knowledge of Signalling Principles and Equations of Motion, it is very laborious and time-consuming to go through these calculations for every signal, although it is often useful to do so for a rough assessment and to examine any critical sections. In practice, the headway time is often found graphically, by producing a HEADWAY CHART, or time-distance-speed curve. These are often drawn for lines where headways are important, such as those with an intensive service or major junctions or termini. On the majority of lines fitted with 4 aspect signalling, the headway provided is usually much better than actually required, so a headway chart for the whole line is unnecessary. However, it is still important to have an appreciation of those factors which affect line capacity when signalling a layout. Additionally, headways are often adversely affected if the trains do not actually behave according to the simplified theoretical performance assumed when positioning the signals. This section will also examine some of the problems which arise and suggest possible solutions. Although the problems of optimising headway have been known to the signal engineer for many years, after introduction of computers that most engineers have had readily available computer aided design facilities and problem solving. The solution of headway problems is well suited to the application of computers. The details of signals, gradients, speed restrictions and the other geographical features of the railway can be held on a database. Train performance generally follows fairly simple mathematical equations. Given the correct data, computer simulations of the passage of trains along the line may be performed very quickly, providing the signal engineer with an accurate output of the headway at each signal. However, it is important that the engineer understands how these results are derived. These notes will therefore concentrate on the graphical calculation of headways. 2. THEORETICAL HEADWAYS The headway time of a given line can be calculated theoretically by the equations: From the 3 basic equations of motion: we can derive the relationship: where B is the braking rate, and D the braking distance. Substituting this in the 3 aspect headway formula, for example, gives To understand how the headway varies with train speed, we will use the second of the above expressions. It consists of two parts. Both have units of time (normally seconds). The first part (V/B) represents the time taken to cover two signal sections. As braking distance (which determines signal spacing) increases in proportion to the square of the speed and time taken to cover a given distance is inversely proportional to speed, the resultant time increases directly in proportion to speed. If we could ignore factors such as train length, overlaps and sighting distance, a headway could always be improved to the required value by reducing the speed. This is however offset by the second part of the expression, (S+O+L)/V, which represents the time for the train to traverse a distance equal to the sum of the overlap, the sighting distance and the length of the train. This distance is usually fairly constant (although sighting and overlaps can be reduced to a limited extent at low speed). The time taken therefore reduces as the speed of the train increases. Figure 2 Headway-Line speed Graph As V/B increases, (S+O+L)/V decreases. There is therefore an optimum speed at which the headway is at a minimum. Increasing or decreasing this speed will reduce line capacity. Figure 2 shows this effect. It can be shown mathematically that the best possible headway is defined by the equation:- At low speeds, train length, overlap and sighting distances dominate the value of the headway. At higher speeds, the time to traverse the required number of signal sections will be the most significant. However, in practice, the signal engineer is usually presented with a situation where these factors, particularly the line speed and the braking performance of the trains, have all been previously decided. This theoretical treatment should nevertheless assist in the understanding of specific situations. 3. PRACTICAL HEADWAYS The equations and formulae we have dealt with so far are for the very simple case of two identical trains running at the same constant speed as each other, with signals placed at ideal positions for headway purposes. In practice these are unrealistic assumptions, and the headway of a line is affected by:- station stops speed restrictions signal positioning constraints different types/speeds of trains trains travelling at less than line To calculate the headway mathematically, taking all these factors into account would be very time consuming. It would be necessary to calculate the headway time individually for each signal to find the "worst case". For the level of accuracy required, the use of graphical techniques (the headway chart) will usually produce a solution more quickly. It is also possible to perform the same calculations with the aid of a computer. To ensure a thorough understanding, this section will concentrate on graphical solutions. Some of the diagrams refer to British signal aspects. The Australian equivalents are:- 4. HEADWAY CHARTS The headway chart, or time-distance-speed curve, is a plot of the train's position against time, from which headways can be measured directly. Where the headway times achieved do not meet the Operators' requirements, then the positions of signals may be adjusted by eye, or extra intermediate signals added, provided that braking distances are not infringed. Ideally, signals should be spaced to give equal headways, even where the "worst case" is better than that required, to avoid creating a "bottleneck". The chart is drawn with distance along the horizontal (x) axis, and time along the vertical (y) axis. It is once again important to use consistent units throughout, time in seconds and distance in meters. A train travelling at constant speed is therefore represented by a straight line, the higher the speed the closer the line becomes to the horizontal, while a stationary train is represented as a vertical line. A plot of a train running at constant speed, stopping at a station, and then accelerating away again to reach the same speed as before would look like in figure 3 & 4 Figure 3 Time- Distance Graph Figure 4 Time -Distance Graph 5. PRODUCING A HEADWAY CHART 5.1 The horizontal and vertical scales are drawn, with the position of signals shown on the horizontal scale. Ends of overlaps, stations and any significant speed restrictions are also drawn. 5.2 The path of a train is then plotted on the chart. This may be done using point-to-point times (where known), or by assuming that the train will travel at the maximum permitted or attainable Suitable time should be allowed for a station stop. If the timetable shows this information, it should be used, otherwise 30 seconds is probably a reasonable assumption. It is very important to obtain accurate information. Remember that at the busiest times of day, when the shortest headway will be needed , trains often take longer to load and unload when much larger numbers of passengers are travelling. Accelerations and decelerations are curved plots, calculated from S=at²/2, often drawn on a template or stencil, while constant speed is a straight line calculated from S =vt.The time/distance curve is constructed by joining the acceleration/deceleration curves together with the constant speed lines, ensuring that the transition from one curve/line to another occurs at points of equal speed. 5.3 Having drawn the curve, you should check that the line speed and other speed restrictions have not been 5.4 For headway purposes, it is necessary to show both the front and rear of the train, so an identical curve is then drawn for the rear of the train, displaced horizontally a scale train length in rear of the first 5.5 The headway distance for a 3 aspect signal is from its sighting point to when the rear of the previous train clears the overlap beyond the appropriate signal in advance. Rather than calculate how long it would take for a train to cover this distance, we can read the headway time directly from the chart. From the sighting point of each signal project a horizontal line forward to the end of the overlap beyond the headway signal, and then measure the running time vertically from this line to the REAR of the train. It is usual on a headway chart to show the sighting allowance as a time (10 seconds) rather than a distance, although it is a simple matter to measure off a distance if preferred for a particular situation. 5.6 This process is repeated for each signal, and the headway time for each aspect noted on the horizontal line from that signal. Figure 5 Headway Plotted Curve 5.7 The headway of the line is then given by the worst-case signal For non-stop services the headway on clear aspects is always quoted, but if lower speed stopping services can in practice run at their normal speed on double yellow (medium) aspects, then that headway may often be quoted. In addition, there is generally no objection to stopping trains entering a platform on a caution aspect (i.e. platform starter at red/stop), but the platform "starting" signal should permit a train to make an unrestricted departure after the station stop ,ideally showing green/clear before the train is due to leave. Separate curves are often plotted for stopping and non-stopping services. 5.8 When quoting headway times from a chart, you should make allowance for errors in plotting the curves and intersections, for the reaction time of signalmen, and for the fact that in practice drivers do not make a uniform brake application but reduce speed in stages. An allowance of typically 25% is often added to the derived values to take these and other factors into 6. STATION STOPS Considering the headway chart for stopping trains, we see that:- 6.1 Providing a platform "starting" signal allows signals in rear of the station to clear as quickly as possible when a train · 6.2 The headways of signals in rear of the station include the braking period, the stopping time, and at least a portion of the acceleration Any signal which includes any of these factors in its headway should be located as close to the station as possible to keep the headway figure down. 6.3 Similarly, any signals in advance should be located as close to the station as possible, if they control the headways of signals approaching the station. In severe cases, if the stopping headway is most critical, it may be necessary to impose speed restrictions on the approach to a station in order to close up the signals sufficiently. Where conditional caution or low speed aspects are provided (with reduced overlaps), the clearance of the signal should be measured from the point at which the rear of the train clears the reduced overlap. It must be remembered that a separate curve may need to be drawn to take account of the speed reduction due to the approach control. 6.4 It is very often the case that 4 aspect signalling is necessary to provide adequate headway for station stops even though 3 aspect would have been satisfactory for non-stopping 7. SPEED RESTRICTIONS & JUNCTIONS S peed restrictions at junctions and on plain line have a similar but less marked effect than station stops. When a train slows down for a speed restriction, any following train will still be travelling at full line speed and so will tend to catch up with the train ahead. If the speed restriction is severe, this can have a serious affect on headways. As with station stops, it is desirable to have the signals either side of the restriction spaced closer together. Signals can be closer together after the restriction because the maximum attainable speed is less, but on the approach side braking for full line speed must be maintained. Temporary speed restrictions can have a serious effect on headways and line capacity, particularly as the speeds imposed are often low. Unfortunately, the signal engineer has little control over temporary speed restrictions. In severe cases, the imposition of a temporary speed restriction can make a timetable based on normal line speeds unworkable. This is another good reason why the headway provided should always be better than that required by the operators. 8. GRADIENTS Signals will generally need to be spaced further apart on falling gradients because of the greater braking distanc.es. This will increase the headway and thus reduce line capacity. On rising gradients, the braking distances (and therefore signal spacing) will be reduced, which may improve the headway, but train speeds may also be reduced by the gradient, making line speed unattainable. The effect on headways may be particularly noticeable if the speeds of some trains are more reduced than others (e.g. heavy freight trains on rising gradients). 9. VARYING TRAIN SPEEDS So far we have only considered the headways between similar trains. However, if a line is used by both fast and slow, or stopping and non-stop services, this can have a marked effect on line capacity. Fast trains will "catch up" slower trains ahead, while a slow train starting out closely behind a faster train will follow a progressively larger distance behind as it travels down the line. This effectively makes some of the line capacity unusable. Either trains must be timetabled so that the "catching up" does not occur or faster trains will have their speed reduced and journey time extended by a slower train ahead. There is thus a compromise between line capacity and the attainable speed of the faster trains. Consider the case of two passenger trains running at 90 km/h over a 30 km section of line, with an intervening 60 km/h freight train. Figure 6 Time -Distance Graph The freight train will take 10 mins. longer than the faster passenger trains, so if the headway of the line is 2 mins, then the "fast to slow" headway will be 12 mins. Mixing dissimilar services in this way can lead to a very low line capacity. The situation can be improved by either running similar trains together in groups ("flights"), or by providing loops or sections with additional running lines to allow slower trains to be overtaken. The best line capacities are always obtained on those lines where all trains perform identically. 10. SINGLE LINES 10.1. Calculation of maximum capacity The maximum number of trains that can use a single line is set by the number of crossing(passing) loops: Figure 7 Single Line with Passing Loops T he capacity of a single line is set by the spacing between loops. If the loops are not equidistant, then trains will be delayed awaiting entry to the longest single-line section, which will determine the minimum headway for the whole line. Figure 8 Passing loop case In the example in Figure 8 , once train A has cleared the section, train B has to travel a distance (d + D + S + L) before train C can follow A, which sets the minimum crossing time between trains in alternate directions. The headway between following trains on an alternating service will be twice this time. 10.2 Additional Factors If trains have to stop at the crossing loops, this can increase the crossing times due to the delay in accelerating and decelerating. For trains to run through a loop without significantly reducing speed, we must ensure: The Loop is long enough Running times between loops are as near as possible the same for each Signals are suitably positioned with free overlaps. The method of signalling does not require trains to stop (eg to exchange tokens). Unfortunately, it is often the case that loops and stations occur together, so trains are required to stop anyway. 10.3 Single Line Section on a Double Line Railway Where a stretch of single line is necessary in an otherwise double line, this can seriously affect the capacity of the line. However, this effect can be minimised by providing extra signals at line headway throughout the single line section, and "flighting" groups of trains through in each direction. For example, if the crossing time is 15 mins, then 4 trains per hour can be run on .an alternating service; however, if it is possible to run trains in flights with a 5 minute headway between trains, then 6 trains per hour can be achieved with 2 trains per flight, or 8 trains per hour with 4 trains per flight. 10.4 Effect of Gradients Many single lines occur in rural and/or mountainous areas where steep gradients prevail. Gradients can affect both line capacity and the speed of trains. Let us examine a line with ideally spaced (equidistant) passing loops operating at or near full capacity, with a train passing one in the opposite direction every time it reaches a passing loop. The line is on a severe gradient and it takes 10 minutes for a train to clear a single line section downhill but 15 minutes for a train going uphill. Although the downhill train may reach the next passing loop in a shorter time it cannot proceed further as there is a train in the single line section coming the other way. The effective capacity of the line is therefore only 2 trains per hour. In addition, the effective speed of the downhill train is no better than that of the uphill train. It simply spends more time waiting at passing loops. The only solution to this problem, where it is necessary for the capacity of the line and the speed of the trains and can be financially justified, is to provide additional lengths of double track over the most critical sections. These will generally be those with long continuous gradients in one direction. 11. TERMINAL STATIONS The capacity of lines entering and leaving a terminal station should ideally be twice that of the main lines served by the station. This is because some arriving movements will completely block the station approach to departing movements and vice-versa (Refer Figure 9). In practice, it may not be possible to provide this capacity. Careful planning of the timetable to maximise the number of parallel arriving and departing movements may give some improvement. However, if a terminal station is operating near to its maximum capacity, late running of a small number of trains can quickly disrupt the entire service. Figure 9 11.1 Signals Approaching the Station At most busy stations the signalman will only be able to set an incoming route shortly before the arrival of the train. To keep trains moving, the driver should see the most favorable aspects possible on the approach to the station. The final signal will of course always be a single yellow. If possible, speed restrictions which reflect the actual attainable speeds should be applied on the approach to the station to permit the closest possible signal spacing. Remember that all trains will be braking in order to stop .at the terminus. On the layout shown in figure 9 , assuming 3 aspect signals approaching the terminus the headway from the signal in rear of signal 2 will be determined by the time taken for the rear of the train to clear the relevant points (103 for the move shown if the following train is destined for platforms 1,2 or 3, 100 for platform 4). However, this time may vary significantly for each platform. The worst case should be used. 11.2 Signals Leaving the Station Platform starting signals are invariably provided. For braking purposes, the first signal after leaving the station can be as close as necessary to the platform starting signals as all trains will be starting from a stand. For operating convenience, the next signal should be at least a train length beyond the platform ends (so that trains will always completely clear the platform on departure). If possible there should be standing room for the longest · train clear of all points and crossings. Another platform starting signal cannot be cleared until the rear of a departing train has cleared the overlap of this signal. Headway requirements may have to override the desire to provide standing room. Platform starting signals are usually 3 aspect (making the first TWO signal sections 3 aspect). This increases the likelihood of a train departing with an unrestricted green (clear) signal and clearing the station as quickly as possible. A more restrictive aspect might encourage the driver to make a slower departure and is not in any case necessary for braking. The spacing of the first few signals leaving the station should be as close as possible. Speed restrictions based upon the attainable speed of the trains rather than the design speed of the track will enable the signals to be more closely spaced.
Deepu Dharmarajan - Posted 3 years ago
CH6 | ASPECT SEQUENCES
SignallingSIGNALLING BOOK | CHAPTER 6 CONTENTS Introduction Plain Line Sequence Selection of Aspects Diverging Junctions Converging Junctions Transition between Aspect Sequences Isolated 4-aspect Signals INTRODUCTION Control tables contain details of the aspects displayed by each signal and the conditions under which they are displayed. To simplify testing, it is common to show the conditions for which each signal displays each of its aspects on a single document. As the final testing of signal aspects requires several persons to observe the aspects of a succession of signals the aspect sequence chart or table is of great assistance to the person coordinating the testing of aspect sequences. The aspects of shunting and subsidiary signals are not shown as they convey no information regarding the aspect of the signal ahead or the distance for which the line is clear. Similarly BR practice has Call On Route ( To send another train to share platform ,already occupied with another train),Proceed On sight Class (When Track equipment status is unknown ,but route reserved in interlocking and point are in desired position ) are not covered. The text of these notes will refer to both British and NSW- New South Wales of Australia (Former State Railway Authority /Railcorp) aspects. New South Wales of Australia has two operators Transport for New South Wales (here by will be referred as TfNSW ) and ARTC (Australian Rail Track Corporation ) exclusively for freight operation. PLAIN LINE SEQUENCE For any signal showing a stop or danger aspect, there will be one or more aspects preceding it, warning of the need to stop. The aspect sequence information should already have been shown on the control tables. It can, if necessary be derived independently according to the following simple rules. Measure back full-service braking distance from each signal at stop. The next signal outside this distance (still measuring back) will display the first caution to be seen by the driver. If there are no other signals between, this will be a caution (British single yellow) aspect. If there is one signal between, this will show caution and the signal outside braking distance will show preliminary caution/medium. British practice does not permit repeated cautionary aspects of any type but, for NSW signalling, if there are two or more signals between the first signal outside braking distance and the signal displaying stop, the medium aspect will be repeated as necessary, and a caution aspect will precede the stop. Move forward to the next signal and repeat the process. When the aspect sequence is complete, a full set of aspects should be written down for each signal. To establish the conditions for a particular aspect to be displayed (e.g., when testing) the line from that aspect should be followed forward to its conclusion to show the aspects of signals in advance. Where low speed and/or conditional caution (warning) aspects are required, the aspect sequence should distinguish between the two by reference to the different overlaps and/or the clearance conditions required. Refer Below diagrams for normal two, three and four aspect sequences. Examples of conditional caution, low speed and repeated medium aspects are also shown. Figure 1: Normal Two Aspect Sequence -New South Wales, Australia Figure 2: Normal Two Aspect Sequence -British Figure 3: Normal 3 Aspect Sequence (Single light)-New South Wales -Australia Figure 4: Normal 3 Aspect -British Figure 5: Normal 4 Aspect Sequence (Double Light) -New South Wales -Australia Figure 6: Normal 4 Aspect Sequence -British Figure 7: Conditional Caution and Low Speed Aspects -New South Wales -Australia Figure 8 Repeated Medium Aspects (Closely Spaced Signals) SELECTION OF ASPECT It will be helpful to know how an Engineer decide 2 aspect or 3 aspect or 4 Aspects are suitable for the line in a brief. However this has been detailed in previous chapter headways. Please refer article HEADWAYS in RailFactor.First caution aspect for a stop signal is installed in advance at braking distance from the stop aspect. This is a trade off between Safety & Service. Safety is the minimum required separation between two signals which must be minimum at breaking distance (S) and must not be greater than 1.5S for a 2 aspect and 3Aspect signal . DIVERGING JUNCTIONS Depending on the type of signalling indication of a route diverging from the main line must be given at the junction signal and may also need to be given at the previous signal or signals. A separate line of the aspect sequence will be required from the signal at which the aspect sequence for the diverging (turnout) route differs from the plain line sequence. British practice requires approach control where a significant speed reduction is required. For Example, a junction turn out where approach locking starts when cleared, train is brought to a signal on low speed displaying red aspect and clears junction indicator as train approach the signal). This should be noted on the aspect sequence chart. NSW, Australia does not use approach control for junction signalling so the aspect sequence is correspondingly simplified. If a junction signal is showing a proceed aspect for the turnout. the previous signal will display medium. It is recommended that the braking distances are checked to ensure that the required speed reduction can be achieved before the turnout. If not, two options are available; the medium aspect could be repeated or the aspect sequence leading up to the junction signal should be the same as that for the signal at stop. It should be noted that normal NSW, Australia practice normally requires the use of the medium aspect for junction signalling even where it is not used for the through route. The following diagrams show the aspect sequences for a junction signal on lines signalled with three and four aspect signals for the main line. The aspect displayed by the junction signal should refer where necessary to any turnout route or junction indicator displayed as this will need to be checked when testing the aspect sequence. Figure 9: Junction Signalling (4 Aspect -Double Light) -NSW Australia Figure 10: Junction Signalling (4 Aspect) British Figure 11: Junction Signalling (3 Aspect -Single Light) -NSW Australia CONVERGING JUNCTIONS Because all trains at converging junctions must make the same speed reduction, and the driver is expected to know this, no special provisions are necessary for converging junctions. The sequences will combine at the first signal beyond the junction. Figure 12: NSW-Australia 3Aspect Sequence at Converging Junction (Single Light ) TRANSITION BETWEEN ASPECT SEQUENCES The transition between different aspect sequences sometimes causes difficulties, for example when running from a 3-aspect line to a 4-aspect line. The engineer must decide exactly where the transition occurs. This should of course have been considered at the time the signalling plan was produced to be able to decide the possible aspects displayed by each signal. The simplest rule to ensure getting the aspect sequence correct is to start from the stop aspect and work back. There should be no caution aspects further back than the first signal at or beyond braking distance. Where a 3-aspect line leads on to a 4-aspect line at a junction, the only reason for carrying the 4-aspect sequence back on to the 3-aspect line is if the signal spacing is inadequate for 3 aspect over the junction or the first section past (taking into account any speed restriction over the junction). Only use a medium aspect if it is necessary to obtain adequate braking (or to warn if a turnout ahead) not simply because the signal can display a medium. With NSW, Australia practice, the aspect sequences through diverging junctions will be similar, regardless of whether plain line aspect sequences are 3 or 4 aspects. In British practice, the aspect sequence should be maintained through the junction (together with any route or junction indicators) according to the aspects used on the diverging route. A junction signal reading on to a 3-aspect line will usually only display yellow (caution) or green (clear) for the turnout. Figure 13: NSW-Australia 3 Aspect to 4 Aspect Transition - Figure 14: NSW-Australia 4 Aspect to 3 Aspect Transition ISOLATED 4 ASPECT SIGNALS At certain locations (approaching stations for example) it may be necessary to position two signals closer than braking distance in what is otherwise a 3-aspect sequence. The remaining signals are all at least braking distance apart. In the example shown on Figure 15, there is insufficient braking distance from 105 to 107. Note that 103, the signal before the section shorter than braking distance is the 4-aspect signal. Note also that when 107 changes from stop to caution, 105 and 103 will both change to a clear aspect together. Figure 15: NSW-Australia Isolated 4 Aspect Signal (Single Light)
Deepu Dharmarajan - Posted 3 years ago
CH8 | LEVEL CROSSINGS
SignallingCONTENTS 1. INTRODUCTION 2. LEVEL CROSSING MODERNISATION 3. AVAILABLE TTYPES OF LEVEL CROSSING 4. MANUALLY CONTROLLED CROSSINGS 5. AUTOMATIC CROSSINGS 6. OPEN CROSSINGS 7. OTHER VARIATIONS 8. WESTERN AUSTRALIAN LEVEL CROSSINGS 9. SINGAPORE LEVEL CROSSING FOR FIRE VEHICLE/MAINTAINER ACCESS 10. LEVELCROSSING PREDICTORS 1. INTRODUCTION One of the early problems encountered by railway engineers was that of crossing existing roads. The operators and the engineers are very fortunate if all crossings can be achieved by the construction of bridges. The level crossing was however a cheap and effective means of dealing with the problem: With the increases in speed and volume of both road and rail traffic, level crossings may cause greater operational problems. However, geographical and cost factors may require many level crossings to be retained. Level crossings have the following disadvantages:- a) They often require additional staff to operate. b) They can reduce line capacity and increase the risk of delays to rail traffic. c) They are an additional safety risk. d) They may be unpopular with road users. When railways were first built. the type of level crossing protection provided varied according to the terrain, the type of train service and the density of population. Political considerations were also significant. In countries like the United Kingdom where centres of population were already established and most land was privately owned, there was generally an obligation on the railway companies to fence off the railway. In Australia, no such obligation exists. Many lines in sparsely populated areas will not be fenced. Early level crossings in the UK therefore consisted of gates which could be placed across the road or the railway to protect one from the other. Many early level crossings in Australia were totally unprotected. There was not the need or the available finance to provide anything more. Looking at the UK example, therefore, most level crossings required an operator or attendant to operate gates across the full width of the roads.Nowa days , most gates have been replaced by lifting barriers. This section will deal with the basic requirements of level crossing protection and ways in which level crossings can be made more economic and efficient in operation. Because most countries have extensive regulations to deal with the control of road traffic, details of crossing layout and construction and the operation of specific types of equipment are not covered in these notes. The general principles of operation of the main types of modem level crossing from the railway operating viewpoint will be dealt with. 2. LEVEL CROSSING MODERNISATION On most railways, the signal engineer is responsible for providing any level crossing protection other than the provision of basic warning signs. There will normally be pressures on the signal engineer to improve the level of protection and/or reduce operating costs. Any equipment provided must, of course, be safe and reliable. Operation of level crossings can be very expensive. In recent years British Rail bas been engaged in an extensive programme of level crossing modernisation. The main factors to justify such a programme are given below. 2.1 Staff Savings This is probably the main reason for modernisation. Where local conditions require the road to be closed across its whole width, some form of human supervision is essential to check that the crossing is completely clear before permitting trains to pass. Instead of providing a local attendant, closed circuit television will permit a signalman or crossing attendant to supervise one or more remote level crossings, often in addition to one adjacent to the crossing/signal box. Alternatively, it may be possible to automate the operation of the level crossing. Some form of local or remote monitoring for correct operation is still required. The level of monitoring for correct operation will depend on local circumstances. In the UK, levels of road and rail traffic require continuous monitoring. Any failures could have a serious impact on the safety and flow of traffic. The Australian approach is to perform a daily inspection or test. In remote areas, the person performing this test may not necessarily be a railway employee. The availability and cost of available persons could lead to some form of remote monitoring being considered in the future. 2.2 Improvements in Line Capacity Manually controlled crossings, whether gates or barriers, are interlocked with the signals. If the driver of an approaching train is not to see a restrictive aspect, the crossing must be closed and the signals cleared some time before the arrival of the train. It may not be possible to open the road to traffic between closely following trains. This may cause severe delays to road traffic. Conversely, leaving the road open for sufficient time to clear a backlog of road traffic may delay an approaching train. Assuming that the options to close the road or to build a bridge have been discounted, the only solution is to reduce the road closure time. This can be done by removing the interlocking with signals and operating crossings automatically. The crossing is then only closed for a short period before the arrival of each train until it bas completely cleared the crossing. To ensure safety, road traffic must not be obstructed on the exit side of the crossing. 2.3 Improvements in Safety The use of barriers is inherently much safer than gates, partiatlarly if used in conjunction with road signals. Opinions differ on the effect of automatic crossings on safety. The reduction in road closure time obviously reduces traffic congestion and gives road users less cause to disobey the road signals (regular users will know that the road will only be closed for a short period). As there are never barriers on the exit side of the crossing, road vehicles and pedestrians cannot get trapped on the crossing. However, the removal of interlocking with signals may also remove the opportunity to stop a train in sufficient time if the crossing becomes obstructed. In all cases road users must be disciplined to obey the signs and signals on the approach to the crossing. Pedestrians may be very diffiatlt to control where there are no barriers or half barriers. The problems will often vary according to the culture of the country. In the UK, level crossing automation has often been perceived by the public as a reduction in protection because the local attendant' is no longer visibly in charge of all traffic. In addition road users do not appear to pay the same regard to road traffic signals as railway personnel do to their signals. In countries having a large number of unprotected open crossings, any form of protection is seen as an improvement. In the UK a large quantity of statistical information has now been built up which appears to indicate that automatic half barrier crossings are in fact very safe (as compared with other types) regardless of the volume of road traffic. Automatic open crossings, to achieve a similar level of safety, must be restricted to situations with lower road traffic density and/or speed. These findings may not always be applicable to other countries. As an example, one of the problems of open and automatic crossings in the United States is that of trying to beat the train to the crossing, regardless of any road signals which may be displayed. This is probably because a long, slow moving train may block the crossing for several minutes (a train 2km long running at 15km/h would block a crossing for over 8 minutes!). The provision of barriers may also vary. Australian practice is to provide half barriers where the road crosses two or more tracks, as a physical reminder to the road user when two trains approach the crossing at the same time. On a single track railway, where this problem does not arise, barriers are not normally provided. In most cases the public perception of level crossings will influence the amount of government regulation. As a minimum, there are usually certain standard road traffic signs which need to be erected. In the UK, government regulation extends to the determination of the type and layout of each level crossing on an individual basis. Any alterations to operation or appearance also have to be approved. 3. AVAILABLE TYPES OF LEVEL CROSSING Modem level crossings can be broadly divided into the following categories:- a) Manually worked, normally with full or half barriers according to local requirements and/or legislation. With local attendant Remotely supervised (closed circuit television - CCTV) User worked Operated by train crew b) Automatic (half barriers or open - no barriers). Remotely monitored (from adjacent signal box) continuously Locally monitored (by driver) with the passage of each train No continuous monitoring but regularly tested and inspected. In the UK, this type of crossing would not be permitted. The period before a failure would become apparent is considered unacceptable. c) Open - no road or rail signals - suitable warning notices only. While some types may be used regardless of traffic density or speed, others have slight or severe practical restrictions on their use. The following descriptions are based on UK practice. 4. MANUALLY CONTROLLED CROSSINGS The most common type is the Manually Controlled Barrier (MCB) which may be either locally controlled or remotely supervised using CCTV. The crossing is directly interlocked with all signal routes over the crossing. The signalling layout for a typical MCB installation is shown on Figure 1 The main features of the MCB crossing are as follows:- Barriers across the full width of the road.2 or 4 barriers may be provided dependent on the width of the road. Lowering of the barriers will be preceded by operation of road traffic signals. On lines with overhead electrification, the barrier arms will normally be earthed. An audible warning will be provided for pedestrians from the start of the operating sequence until the barriers are fully lowered. Figure 1 MANUALLY CONTROLLED BARRIER (MCB) LEVEL CROSSING Although not desirable, overlaps may extend over the crossing without requiring the barriers lowered provided the signal is at least 50m (25m if a platform starter) from the edge of the crossing. Routes may be set while the barriers are raised. Signals will not clear until the barriers are fully lowered and the crossing is clear. The signalman/attendant must operate a special "crossing clear" button for this purpose. The signalman/attendant will have an indication of road signals operating and barriers lowered on his control panel (or equivalent). Signals will clear for one movement only. Under appropriate conditions, facility may be provided to lower the barriers automatically. In most cases, an automatic raise facility is provided which operates as soon as the train bas cleared the crossing and the signal approach locking is released (provided no other routes have been set). Crossings supervised by CCTV are provided with a Local Control Unit (LCU) which permits local operation in the event of CCTV failure or maintenance or for other engineering work. When the LCU is in use the signals are maintained at danger. In general, a signal passed at danger will immediately operate the road traffic signals. Safety of MCB crossings is ensured by: Interlocking with signals. Detecting the barriers down and the road signals operating. Provision of a separate "crossing clear" button. Maintaining the barriers down until the approach locking on all protecting signals is released and the crossing is clear of trains. The MCB is generally the most expensive type of crossing to provide. In the UK there were no restrictions on its use. 5. AUTOMATIC CROSSINGS Automatic crossings will generally have no barriers or half barriers. This is to ensure that vehicles and pedestrians do not become trapped on the crossing. They will always be provided with road traffic signals. In general, the operating sequence will be timed so that at least 27 seconds (UK practice, determined by government regulation) elapses from the start until the arrival of the fastest train. This timing is calculated from the operating sequence of the particular type of road traffic signals in use, the lowering time of the barriers (if any) and a suitable margin of time before the train reaches the crossing. It could therefore vary for other types of road signal and/or barrier equipment. The crossing will reopen to road traffic provided:- a) The train is clear of the crossing. b) The crossing can remain fully open to road traffic for at least 10 seconds after the passage of the train. Therefore, if another train is approaching the crossing within this period, the crossing will remain closed to road traffic until both trains have passed. Automatic crossings may be monitored by an adjacent signal box (remotely monitored) of by the driver (locally monitored). At locally monitored crossings a flashing white light indicates to the driver that the road signals are operating. Provision is generally made for local control, to cover periods of maintenance, failures or track maintenance in the vicinity of the controlling track circuits. Local control may also be necessary in the event of planned or unplanned single line working on a double track railway. It may, however be cost effective to equip crossings for bi-directional working on all lines. The provision of the additional circuitry could well be more economic than the cost of providing crossing attendants for single line working. 5.1 Automatic Half Barrier Level Crossing (AHB) The automatic half barrier crossing is the earliest and most widespread automatic crossing in the UK. Each barrier is pivoted on the left hand side (for left hand road traffic) and covers slightly less than half the width of the road. It is monitored from an adjacent signal box. A dedicated telephone circuit and indications for barriers and power supply are provided. Operation can be initiated by track circuits, treadles (or a combination of both). The running on or "strike-in" end of the track circuit may be provided with a welded stainless steel strip on the rail surface to protect against bad contact due to rust The simplest arrangement for the AHB crossing is on a single line. On most single lines there is no possibility of a second train striking in before the crossing has been open for 10 seconds. Therefore, the operation of the crossing is initiated by a track circuit approaching the crossing from either side becoming occupied. The crossing will remain closed to road traffic until the train has cleared the crossing. A treadle may be provided at the crossing to safeguard against false operation of the track circuit by proving that the front of the train has reached the crossing. The controls are more complicated in the case of a double line. In the example on Figure 2, the crossing operation is initiated by timed occupation of the approach track circuit. Occupation of the other approach track circuit while the crossing is closed to the road will maintain the crossing closed until both trains have passed. The example also shows the provision of emergency replacement on the automatic signal approaching the crossing. There must be a minimum of 50 metres and a maximum of 10 minutes between a stop signal and the crossing. The signal must either be a controlled signal or an automatic signal with an emergency replacement facility. In the opposite direction, a station is located next to the crossing. Special arrangements are necessary if the road is not to be closed for an excessive length of time by a stopping train. The signalman may select between an operating sequence for a stopping or a non stopping train. For a non-stopping train the platform starting signal clears immediately and the normal sequence of operation will apply. For a stopping train, the signal will be maintained at danger for a suitable time (to allow for the station stop). Crossing operation will commence before the signal clears. The signal will clear so as to permit the minimum road closure time before arrival of a train starting from the platform. There is no restriction on the volume of road or rail traffic. The speed of rail traffic must be below 100 mph (160km/h). To limit the time for a road vehicle to cross, a maximum of 2 running lines and 2 other lines are permitted. If any of these conditions cannot be fulfilled or the crossing and approaching road layouts are unsuitable, the MCB type of crossing must be used. Figure 2 AUTOMATIC HALF BARRIER (AHB) LEVEL CROSSING If signals are located within the "strike-in" distance of the level crossing, the controls can become very complicated. The crossing operation must not commence unless a route has been set (or an automatic signal can show a proceed aspect) over the crossing. The clearance of such a signal may need to be delayed to ensure adequate crossing closure time. If a train passes a signal at danger, crossing operation should commence immediately. If a signal is replaced to danger·after showing a proceed aspect and the train is successfully brought to a stand at the signal, the crossing may be opened after the approach locking bas been released. 5.2 Automatic Open Crossing Remotely Monitored (AOCR) This type of crossing is effectively an AHB without barriers. Due to the absence of barriers, its use is restricted to situations where the road traffic is very light. The crossing is also restricted to a maximum of 2 lines and a line speed of 75 mph (120km/b). Additional safeguards due to the absence of barriers are:- a) An illuminated "Another Train Coming" sign which operates as the first train reaches the crossing when two trains are to pass over the crossing before it reopens to road traffic. This is to ensure that road users do not assume it is safe to proceed (or fail to check the main road signals) after the passage of the first train. b) In conjunction with the operation of the "Another Train Coming" sign, the audible warning will change in pitch. 5.3 Automatic Open Crossing Locally Monitored (AOCL) Instead of providing indication and telephone circuits to a remote monitoring point, many crossings can more economically and effectively be monitored by the driver as be approaches the crossing. One vital provision is that be must be able to stop the train short of the crossing in the event of any failure of the crossing equipment or obstruction of the crossing. A flashing white light facing in each direction of rail traffic is provided at the crossing which operates when the road signals are operating correctly. The speed of approaching trains must be restricted so that the driver can stop short of the crossing if the white light fails to operate (or if the crossing is obstructed). Warning boards are provided on the approach to the crossing. An overall maximum speed limit of 55 mph (88km/b) or lower is applied to ensure adequate sighting. If there is a station on the approach side of the level crossing where trains normally stop then ALL trains must stop to ensure correct operation of the crossing. This is normally enforced by a stop board although a signal could be employed instead. It is thought that the provision of a Main signal and a flashing white light signal in the same place could cause confusion. Some crossings therefore exist where the proceed aspect of the signal performs the function of the white light. All trains will initially be brought to a stand by the signal at danger. There is some restriction on the volume of road traffic for which an AOCL is suitable. This is not as severe as for an AOCR. Figure 3 AUTOMATIC OPEN CROSSING ,LOCALLY MONITORED (AOCL) If the train does not reach the crossing within a reasonable time the crossing will reset to open the road. This is quite safe because the driver's white light will already have been extinguished and the driver will therefore be prepared to stop (if a train is actually on the track circuit it will obviously be travelling very slowly). This is a useful safeguard against track circuit failure causing serious road traffic delays. 5.4 Automatic Barrier Crossing Locally Monitored (ABC-L) This is a new addition to the available types of level crossing. It has the operational advantages of the AOCL but is also provided with half barriers. It is therefore suitable for situations with heavier road traffic. Operation is the same as for the AHB. The flashing white light operates when the road signals are operating and the barriers have commenced to lower. Proving the barriers down would effectively reduce the train speed over the crossing (due to the longer operating time and the effective upper limit on sighting) or increase crossing closure time. Automatic reset facilities are provided similar to the AOCL 6. OPEN CROSSINGS On some lines it may be acceptable for all trains to severely reduce speed at a level crossing. If both road and rail traffic are low, the provision of an Open Crossing (without road signals) may be adequate. Suitable road signs are provided on the road approaches and a warning board at braking distance on the rail approaches. A speed restriction of 10 mph (16 km/h) applies to all trains. Road traffic is instructed to give priority to rail traffic. Train drivers must ensure the crossing is clear of obstruction before proceeding. This type of crossing is suitable for single lines only. There are no signals therefore no warning can be given of the approach of a second train. 7. OTHER VARIATIONS On crossings where either the road or rail traffic is very infrequent, other alternatives may be used. If the railway crosses a private road with generally a small number of regular users and protection is considered necessary due to the frequency of rail traffic or the approach view of rail traffic, the crossing may be a barrier or gate crossing operated by the user. Telephone communication and/or warning signals to indicate an approaching train would normally be provided. The gates or barriers would normally be left closed across the road. If it is acceptable for the trains to stop, the crossing may be operated by the train crew. At least one other person in addition to the driver is desirable - the train will have to stop, set down the crossing operator, who then closes the crossing to the road, proceed over the crossing and stop again to pick up the crossing operator after he has reopened the crossing. This method of working will generally not be acceptable for a regular passenger service. 8. WESTERN AUSTRALIAN LEVEL CROSSINGS TransPerth network is mainly electrified hence predictors arenot type approved and level crossing is controlled with Track circuits .There are controlled level crossing for Road Traffic and separate pedestrian crossing Roads are equipped with half boom barriers ,warning flashig light and audible alarm and pedestrian crossings are equipped with electronically controlled swing gate . There are active level crossing with out half boom as well but protected with audible alarm and visual warning lights.All the requirements are in compliance with the Australian Stanadard AS 1742 8.1 Level Crossing Protected with Flashing Light Signals In its quiescent state if no train is detected approaching or passing over the level crossing,flashing light warning signals will be extinguished and audible warnings will be silent. If a train is detected as approaching the level crossing within the approach area the flashing light warning signals will commence and continue to flash alternately and the audible warning will commence and continue to operate. When the rear of the train passes clear of the road area of the level crossing, the flashing light warning signals will become extinguished and the audible warning will be silenced. 8.2 Level Crossings Controlled by Flashing Light Signals, Half-Boom Barriers and Audible Warning In its quiescent state where no train is approaching or passing over the level crossing, all flashing light warning signals will be extinguished, the half-boom barriers will be in the fully raised position and audible warnings will be silent. If a train is detected as approaching the level crossing within the approach area, then the flashing light warning signals will automatically commence and continue to flash alternately and the audible warnings will commence and continue to operate. After a predetermined period (normally a minimum of 6 seconds) the half-boom barriers will commence descent.After a predetermined period (normally 10-12 seconds) the half-boom barriers will reach the fully horizontal position and all of the audible warnings will be silenced unless there is a designated pedestrian crossing. After the minimum design warning period, the front of the approaching train will reach the level crossing. The minimum warning time for all new boom barrier installations will be 25 seconds. When the rear of the approaching train passes clear of the level crossing then both the halfboom barriers willl commence to rise and any audible warning will be silenced. When both half-boom barriers reach the fully vertical position, the flashing light warning signals will become extinguished. In multiple track level crossings, if a second train is approaching the level crossing on another track, as the rear of the first train passes clear of the level crossing, and if there is insufficient time for the half-boom barriers to rise and remain in the fully raised position for the predetermined minimum road opening time (normally 15 seconds) then they remain lowered until the rear of the second train has also passed clear of the level crossing. 8.3 Pedestrian and Cycleway Level Crossings Controlled by Lights and Audible Warnings Only If no train is detected as approaching or passing over the pedestrian level crossing then the warning lights will be extinguished and audible warning devices will be silent. If a train is detected as approaching or passing over the pedestrian level crossing then the warning lights will display and flash red warning lights and audible warning devices will commence and continue to sound. When the rear of the train passes clear of the pedestrian level crossing then the warning lights will become extinguished and the audible warning devices will be silenced. 8.4 Pedestrian Level Crossings Controlled by Lights and AutoLocking Gates If no train is detected as approaching or passing over the pedestrian level crossing then the warning lights will be extinguished, the gates will be fully open and the audible warning devices shall be silent. If a train is detected as approaching or passing over the pedestrian level crossing, then the warning lights will display and flash red lights and the audible warning devices will commence and continue to sound. a. After a predetermined period the gates commence to close. b. After a predetermined period the gates will be fully closed. One or all of the audible warning devices may be reduced in level. c. After the predetermined minimum period the front of the approaching train will reach the level crossing. When the rear of the approaching train passes clear of the level crossing then the gates shall commence to open, the warning lights will become extinguished and the audible warning devices will be silenced. If a second train is approaching the level crossing as the rear of the first train passes clear of the level crossing and there is insufficient time for the gates to open and remain in the fully open position for a predetermined period before commencing to close for the second train then they remain closed until the rear of the second train has also passed clear of the level crossing. 9. SINGAPORE LEVEL CROSSING FOR FIRE VEHICLE/MAINTAINER ACCESS Singapore SMRT operate moving block Grade of Automation 4(GoA4) with a designed headway of 88 seconds maninly on elevated track or tunnled Track with a bit of at grade track .It is practically impossible to maintain a normally open level crossing .They do have test track with design speed 80kmph and unmanned depot operation with operational speed of 18km/hr.Recently built Thomson East Coast Line Depot has an at Grade depot (Mandai Depot) with a test track where access is given through normally closed level crossing .There are four types of such crossing used for safe passage of fire engine ,maintainer and drivers on emergency. 9.1 Type 1 Low speed levelcrossing with gates ,normally closed to road traffic ( Fire Engine & Train Delivery Road ) These are slow speed levelcrossing with gates ,normally closed to road traffic.This type of crossing is used for those level crossings that are occasionally used by road traffic and in depot only. They are suitable for Train Consists travelling up to 18 kph.The gates are electrically detected as closed and locked by double pole SIL 4 detection switches.When gates are detected not closed and locked, the signalling system will safely stop Train Consists which are routed to the level crossing and When gates are detected not closed and locked, the signalling system will safely stop Train Consists which are routed to the level crossing.Train Consists are not allowed to stop on the level crossing. When the gates are detected as closed and locked after the gates are detected not closed and locked, the signalling system prompt the operator at the Depot Control Centre to confirm that train operation at the level crossing can be resumed and operator can remotely request train operation at the level crossing to resume. When the resume train operation request is received, the signalling system will safely check that the gates are detected closed and locked before allowing train operation at the level crossing to resume.Indications are provided for the depot controller as below (a) Gates not closed and locked indication (b) Gates closed and locked indication (c) Prompt to confirm train operation to resume 9.2 Type 2 Slow speed Level Crossing with gates, normally closed to human traffic This type of crossing is used for those level crossings that are occasionally used by human traffic and in depot only. They are suitable for Train Consists travelling up to 18 kph.It is equipped with three-position spring loaded local switches at each side of the level crossing. The three positions of the switches are (a) Request To Use Crossing (b) Normal position (c) Cancel Request to Use Crossing When the switch is set to Request to Use Crossing to cross, the signalling system will safely stop Train Consists which are routed to the level crossing. Train Consists are not allowed stop on the level crossing. When trains have been stopped from approaching the level crossing a safe to proceed lamp at each gate will be lit.t has facilities at the Depot Control Centre to allow the operator to remotely request train operation at the level crossing to resume. When the switch is set to Cancel Request to Use Crossing ,system will prompt the Depot Control Operator to resume train operations and the safe to proceed lamps are e extinguished. Indications at the DCC are : (a) Request to Use Crossing (b) Cancel Request to use Crossing (c) Prompt to confirm train operation to resume 9.3 Type 3 Level Crossing with no gates, normally open to road traffic This type of level crossing shall be used where road traffic across the level crossing is moderately frequent. Its use are restricted to cases where rail traffic across the level crossing is restricted to a maximum speed of 18 km/h. Track circuits as required are utilised for the operation ,along with warning light and audible alarms at either side of the level crossing .Operating Principles are as below (a) When a signalled route is set across the road level crossing, and the berth track circuit to the signal is occupied, the warning lights will flash red along with audible alarm. (b) The railway signal of the route that has been set across the level crossing will not clear until the flashing road crossing signals have been proved illuminated for a pre-determined time. (c) The failure of one lamp of each road crossing signal shall still allow the relevant railway signal(s) that read over the level crossing to clear and failure is alarmed to the Depot Control Centre (d) The failure of both lamps of one road crossing signal will prevent clearance of the relevant railway signals that read overthe crossing. This failure is alarmed to the Depot Contrl Centre. (e) If a route is set across the level crossing, when the berth track circuit to the signal is clear, the level crossing warning lights and audible alarm will not be initiated and the signal will remain at red. When the berth track circuit to the signal becomes occupied, warning lights and audible alarms are initiated as described in (a). (f) In case the Train Consist passes a red railway signal before travelling over the road crossing, the road level crossing warning lights and audible warning will be initiated when any track circuit between the signal and level crossing are occupied. (Unless Train Consist is routed away from level crossing). There are road warning light indication provided to the Depot Controller 9.4 Type 4 Level Crossing with gates, normally closed to road traffic integrated with Fire Alarm Signal This type of crossing is used for those level crossings that are occasionally used by road traffic and in depot only. They are suitable for Train Consists travelling up to 90 kph, e.g. level crossing of test track in the depot track in depot.This gates are equipped with electrically released locks ,which can be electrically detected in closed and locked position with double pole detction switches .Locks are controlled by signalling system "Gates Locked" (RED) and "Gates released" (Green) lamps are provided on each side of the gate .Depot Controller can remotely release the switch to unlock the gate at same time each side of level crossing has three position spring loaded locake switches and the positions are (a) Gates release: to request the gates to be unlocked (b) Normal position (c) Gates lock: to request the gates to be locked Appropriate Fire Alarm signal is received by the interlocking which command to release the lock automatically 10. LEVELCROSSING PREDICTORS These are the relatively new trends in level crossing .Signal engineers releaized that if the train driver dont maintain the allowed speed limit and its possible train can reach the level crossing island much later that required also driver is suppose not to excced his alllwed speed limit for the train to reach the island earlier . Thease are potential threats with the track circuit based controlls.Engineers thought of detecting the speed of the train when it strikes the warning point and activate the crossing accordingly to avoid such threats .Not forgetting the fact that driver cannotexceed the speed after his train strikes the point of level crossing activation Hence a btetter equipped predictors come into existence .It used Narrow band shunts ,wide band shunts to make it accurate .GCP of Siemens(Former Westinghouse) and XP4 of Alstom (Former GE) are well know level crossing predictors .We will discuss level crossing tedictors in a separate chapter with logic ,circuits and settings.
Deepu Dharmarajan - Posted 3 years ago
CH9 | TRACK CIRCUITS
SignallingTRACK CIRCUITS CONTENTS Introduction Principles of Operation Practical Considerations Insulated Block Joints Track Circuit Types D.C. Track Circuits AC. Track Circuits Jointless Track Circuits Impulsing Track Circuits Rail Circuits and Overlay Track Circuits Shunt Assisters Alternatives to Track Circuits INTRODUCTION The track circuit is an important component of modern signalling systems. Its main purpose is to continuously detect the presence and position of traffic within the signalled area. Track circuits are electrical circuits formed in part by the rails of the track. The detection of vehicles is possible because their wheels and axles reduce the rail to rail resistance significantly below the value relating to a clear track condition. Eventhough modern communication based train control system make use of track installed transponders otherwise know as balises or beacon (French) as a primary means to identify the position,over laid secondary train systems are implemented (Can be axle counter as well) to recover the train ,if communication is lost and primary train detection is impossible. The first track circuits were used only as indication devices, to remind the signalman of the presence of a train, for example standing at the home signal or on a section of track which might be obscured from his view. Later they were used to lock facing points, as a replacement for fouling bars. In this role, the track circuit directly controlled the operation of the point lever through an electric lock. Track circuits are now used to control the signalling directly in legacy fixed block signalling system,by preventing a signal being cleared if the track ahead of the signal is occupied by a train, by holding the route after a train has passed the signal, and to lock a set of points if a train is standing on them. It is important to remember that a track circuit is designed primarily to detect the absence of a train and not its presence. There are however many signalling controls which require track circuits to be occupied (e.g. release of overlap after a train has come to a stand) and the track circuit must therefore be reliable enough to be used for this purpose also. A rail circuit is the reverse of a track-circuit, and is used to prove the presence (not absence) of a train at a particular place. Rail-circuits are not as commonly used as track circuits. To understand the basic principles of track circuit operation, the easiest type of track circuit to study is the simple, battery-fed D.C. track circuit. Many other types have been developed to cover particular situations such as electrified areas, jointless track etc. These will be discussed later. Most of the principles of the simple D.C. track apply equally to other types, although the configuration of the equipment may differ. PRINCIPLES OF OPERATION A track circuit employs the rails as a transmission line. The electrical signal, in this case a d.c. voltage, is applied to one end. At the other end a relay is connected across the rails as a receiver of the voltage applied at the opposite end. The physical limits of the track circuit must be defined by insulated rail joints to prevent interference between adjacent track circuits or isolate the track circuit from sections of line where track circuits are not required. When there is no train on the track circuit, the relay will be energised via the full length of each rail. When a train stands on the track circuit the wheels and axles create a short circuit between the rails. The current takes the lower resistance path via the train and the relay becomes de-energised. All track circuits depend for their operation on the wheels of a train forming a low resistance path between the rails. Current from a battery (or rectified power source) at one end of the section, energises a relay at the other end via an adjustable resistor and the rails. Each rail forms one leg of the circuit. Although the rail joints are metallically joined by fishplates, they are electrically unreliable and it is therefore necessary for the rails in each leg to be bonded together to form a good path for conduction of the electrical current. The end rails of the track circuit are insulated from the adjoining rails by means of insulated rail joints. The track circuit is an independent circuit with its own power supply at one end, and its own relay at the other end. Normally, when the track is clear of trains, current flows through the rails and through the coil of the tractive armature electromagnetic relay, thereby energising the relay which closes its front contacts. When a train occupies the track circuit, the two rails of the track circuit are short-circuited by the axles of the train. The relay is shunted by an electrical path of extremely low resistance, provided by the wheels and axles of the train, the result being that virtually no current flows through the relay coil because all but an insignificant amount is diverted through the axles. The relay becomes de-energised and the armature drops to close the back contacts. Thus the condition of the track relay determines whether the track circuit is clear or occupied. The most important feature of the track circuit is the fact that it fails to safety, i. e. is "fail-safe". The moment a rail or a wire breaks, or a metallic obstruction straddles the track, or the power supply fails, the relay will become de-energised indicating the track occupied condition Figure 1 Track Circuit Clear (relay Energized) -Simple DC Track Circuit Figure 2 Track Circuit Occupied (relay De-Energized) -Simple DC Track Circuit The relay is therefore energised to prove that there is no train on the track, and de-energised when there is a train on the track. The relay will also de-energise if: a) A rail connection becomes detached b) The power supply fails c) A rail to rail bond is broken or becomes detached d) A rail breaks The circuit is therefore designed to be "fail-safe", as under most fault conditions it indicates that the track is occupied. This may not be totally true in practice, as will be seen later. 3. PRACTICAL CONSIDERATION Figure 3 Practical Track Circuits A practical track circuit differs significantly from the theoretical example shownin Figure 1 & Figure 2 The two major difficulties are:- a) The rails are not perfectly insulated from each other. Current "leaks" from one rail to the other, through the rail fixings, the sleepers and the ballast. This rail-to-rail resistance is known as "ballast resistance". It can vary widely 300 metres of standard gauge (1435mm) track may have a ballast resistance as high as 50 ohms for well ballasted, dry track, or as low as 0.5 ohm with waterlogged or dirty ballast. b) The presence of the wheels on the rails may not provide a perfect short circuit. The axles may have a small resistance, but the main problem is the contact between wheel and rail. This may be due to rust or other causes such as fallen leaves ,or ice on track during winter . Although the rails also have a resistance, this is normally insignificant compared to the above two problems. Rails will also have an inductance. This will be significant when dealing with some types of a.c. track circuit. 3.1 Ballast Resistance This is a factor over which the signal engineer usually has very little control. It will vary according to the type of ballast, its condition, the type of sleepers, the method of fixing of rails to sleepers, track drainage and, most importantly, the weather conditions. These will all vary in time during the life of the track circuit. Some factors will change slowly, such as the condition of the ballast. Others, like the amount of moisture in the ground, can vary rapidly. The design of the track circuit equipment must be able to compensate for the long term changes by some means of manual or automatic adjustment. The short term changes due to weather must be contained within the normal operation range of the track circuit. If not, failures (either right side or wrong side) will result. The inclusion of a series resistance at the feed end is an essential feature of most track circuits to avoid excessive power consumption when the track is occupied. It also permits components of a lower current rating to be used. This will also make the track circuit more difficult to set up for reliable operation. As ballast resistance decreases, the current drawn will increase, the voltage drop across the feed resistor will increase and the relay voltage will be correspondingly reduced. Too large a reduction in ballast resistance could cause a right side failure of the track circuit. Under the most adverse conditions, the minimum ballast resistance should not be less than 4 ohms per 1000 ft. Short track circuits will of course have a higher ballast resistance. The value of ballast resistance may be calculated from the measurements taken for rail resistance: - Vf = Volts across rails at feed end. Vr = Volts across rails relay end. It = Current to track. Ir = Current to relay. Mean voltage across rails = (Vf + Vr) / 2 Current in ballast = It - Ir Ballast Resistance = (Mean voltage across rails) / (Current in ballast) For a typical DC track circuit calculation, the following figures for a track circuit will be used: - EMF of battery 1.2 volts. Resistance of relay 9 ohms Pick-up value of relay 0.36 volts Drop-away value of relay 0.27 volts Minimum ballast resistance 2 ohms Working voltage 25 per cent above pick-up 0.45 volts Current to relay = Voltage across relay/Resistance of Relay = 0.45/9 = 0.05 ampere. Current to ballast = Voltage across rails/Ballast resistance = 0.45/ 2 = 0.225 amp. Current to track = 0.05 + 0.225= 0.275 ampere. Voltage across feed resistance = EMF of battery - EMF across rails = 1.2 - 0.45 = 0.75 volts Value of feed resistance = Voltage across feed resistance/Current to track = 0.75 / 0.275 = 2.73 ohms The value of the resistance which, when placed across the rails of the track circuit with a ballast resistance of 2 ohms, would reduce the voltage across the relay to the drop-away value, i. e. 0.27 volts, can be calculated as follows Voltage across feed resistance = 1.2V - 0.27V = 0.93 volts. Current to track = 0.93V/2.73= 0.341 ampere. Current to relay = 0.27V/9 = 0.03 ampere. Current to ballast = 0.27V/2 = 0.135 ampere. Current to relay and ballast = 0.03 + 0.13 5 = 0.165 ampere. Current to shunt resistance = 0.341 - 0.165 = 0.176 amp. Shunt resistance = Voltage across track/Current through shunt= 0.27/ 0.176 = 1.53 ohms. The above figures are given as an example to show how the various values can be obtained. 3.2 Rail Resistance This is the total resistance of rails, fishplates and bonds. Its value remains appreciably constant. The rails of a track circuit form the conductors of a simple circuit in that current is supplied at the feed end and less current appears at the relay end. The loss in current is due to leakage and the loss in voltage to rail and cable resistance. The fall in current and voltage is not uniform along the length of the track circuit, but falls faster near the supply end than the relay end. In most cases, however, the rail resistance is small compared with the ballast resistance and it may be assumed that the voltage and current fall off uniformly along the length of the track circuit. The value of the rail resistance in a track circuit may be obtained from the following measurements Vf = Rail voltage at the feed end of track circuit. Vr = Rail voltage at the relay end of track circuit. It = Current to circuit. Ir = Current to relay. From Ohms law it is known that: Rail resistance = (Voltage drop in rails) / (mean current in rails) 3.3 Train Shunt Resistance This is the joint resistance of all the parallel paths from rail to rail of a train or vehicle. The train shunt of a vehicle is greater than that of a train and that of a pair of wheels greater than that of a vehicle. It should be noted that the ordinary resistance of a pair of wheels and axle is not the same as the train shunt of a pair of wheels resting on the track, due to films of oil, scale and rust frequently found between the wheels and rail. 3.4 Train Shunt The train shunt resistance varies with the voltage between the rails, in that as the voltage increases the train shunt resistance decreases. The lowest allowable drop shunt is 0.5 ohm irrespective of the track voltage. The shunt and pick-up of a track relay can be taken with the aid of a variable resistance with 0.1 ohm tappings. When taking the pick-up value the resistance should be set at a low value and gradually increased until the relay armature just picks up. This should be repeated two or three times until consistent results are obtained. When taking the train shunt value the resistance should be set at a high value so that the relay remains in the energised position, and then the resistance is decreased until the armature of the relay just falls away and breaks its energised, i.e. front, contacts. The reading of the adjustable resistance is the drop train shunt of the track circuit. This should also be repeated two or three times to obtain the true value of the drop train shunt. 3.5 Relay Characteristics There are two different values for train shunt. The drop shunt is the largest value of train resistance that will cause the track relay to release. The pick-up shunt is the smallest value of train shunt which will allow the relay to operate. Due to the method of operation of all relays, the drop shunt will always be the lower of the two values. As the energisation of the relay closes the air gap in the relay's magnetic circuit, the reluctance of the magnetic circuit reduces. The relay therefore requires less current to hold it in the energised position than to pick it up initially. Conversely, it will require a significant reduction in this current to release again. Whenever track circuits are tested the drop shunt should always be observed and recorded. Track relays are designed specially to minimise the difference between the pick-up and hold-up currents. Nevertheless, a hold-up current of only 0.5 to 0.7 times the pick-up current is typical. Another problem is the time of release of the track relay. With both relay and rail being inductive, a loop will be formed by the rails, the train and the relay. The back-emf caused by the reduction in current as the train shunts the track will tend to cause a circulating current in this loop which will delay the release of the relay. Short, fast moving trains could momentarily be undetected as they pass from one track circuit to the next. This has serious implications for the release of route locking and the holding of points. If the type of track relay used is not inherently slow to pick up, a repeater relay which is slow to pick must be used in all vital signalling controls. 3.6 Maximum Length The combination of the above factors will generally limit the maximum length of any type of track circuit. The longer the track-circuit, the lower will be its ballast resistance, and so the attainable voltage at the relay will be lower, as more current leaks between the rails. Eventually, a length is reached at which it is no longer possible to operate the track relay. This maximum length will be lower for lower values of ballast resistance. The maximum length must therefore be based on worst case ballast conditions. A track circuit adjusted for minimum ballast resistance (e.g. during wet weather) will have a relay voltage higher than the minimum pick-up voltage when ballast conditions improve. Adjustment must be such that the track relay does not remain energised when a train of maximum value of drop shunt occupies the track circuit. This requirement also effectively imposes a maximum length on the track circuit. Maximum length will depend on the type of track circuit, the components used and the conditions under which it operates. Most track circuits have maximum lengths in the 600 to 2000 metres range. 3.7 Minimum Length For short track circuits, adjustment for reliable operation does not usually present any problem. But if a track circuit is too short, a vehicle with a long wheel-base may stand completely over the track circuit without it being detected. The minimum length of a track circuit is set by the vehicle with the)opgest wheel-base. For bogie vehicles, this distance is measured between the inner axles of each bogie. In most cases, this minimum distance is more likely to be determined by bogie vehicles than by two axle vehicles. 3.8 Staggered Block Joints Ideally, insulated joints are installed in pairs opposite each other. In practice, it is frequently difficult to ensure that they are exactly opposite, such as within pointwork. Joints may be provided to separate adjacent track circuits or to allow a polarity change within a track circuit. Where joints are not exactly opposite, they are said to be staggered. There is a short length of track where both rails are of the same polarity. In this area, an axle will not be detected. If the stagger is too great, a bogie or even a complete vehicle may remain undetected. The maximum stagger is determined by the vehicle with the shortest wheelbase. Although it is still possible to "lose" one axle, the . other axle will be detected, Care must be taken to correctly position joints for clearance purposes to take account of this problem. Staggered joints must not be used to define critical clearance points. If two sets of staggered joints are too close together, both axles of a short, four wheel vehicle could be undetected at the same time. A set of joints which is not staggered but defines the end of the track circuit could also fail to detect a vehicle if too close to staggered joints. Therefore, unless joints are exactly opposite, they must always be separated by the length of the longest vehicle. Figure 4 Track Circuit Minimum Length Figure 5 Staggered Joints -Maximum Stagger Figure 6 Joints in Close Proximity -Loss of Train Detection 3.9 Equipment Details To ensure effective operation equipment types and values must be carefully chosen. Whether a.c. or d.c. power supplies are normally of low voltage. This is desirable as the presence of a train presents a short circuit to the feed set or transmitter. Similarly, there is usually a series impedance in the transmitter/feed set to limit output current, avoid damaging circuit components and prevent excessive power consumption. Where power supplies are derived from an ac mains supply, each track feed will normally have its own supply to prevent track circuit signals feeding through between adjacent track circuits via the power supply. Where a feed resistor is provided it is often adjustable to deal with different track circuit characteristics. When a track circuit is set up, account must be taken of the weather & ground conditions at the time. An incorrectly adjusted track circuit may either fail to detect all trains under conditions of high ballast resistance (dry weather) or fail to energise the relay with no train present if the ballast resistance is extremely low (wet weather). Many audio frequency track circuits are designed to be adjusted by varying the gain of the receiver. This makes setting up easier and also permits track circuits to be centre fed where required. Frequencies for a.c. track circuits are normally within the audio frequency range. Higher frequencies are increasingly attenuated by the inductive impedance of the rail. This would make the maximum length impractically low. Mains frequency may be used in non-electrified or d.c. traction areas (with suitable precautions). Due to the low power used, relays are normally of low coil resistance (typically 4 to 20 ohms). For the same reason, relays may only be provided with two front contacts. Electrical continuity of both rails is essential. Any rails not welded or bonded for traction return must be bonded by the signal engineer. Bolted fishplates do not in practice provide a reliable electrical connection. For rails which carry track circuit currents only, an adequate connection can be made by using two galvanised steel bonds pinned at each end.Welded or brazed connections may also be used. Where electric traction is used, the return traction current will be many times greater than the track circuit currents. The bonds must be much heavier and will often be provided by the electrification engineer. Connections from the rail to lineside equipment should be firmly secured to the rails and employ flexible cable to counteract the effects of vibration. INSULATED BLOCK JOINTS The insulated rail joint is installed in the track at the extremities of a track circuit and also within a track circuit at points and crossings in order to achieve reversal of polarity. When 40-ft. rails were used, the tensile stresses were negligible and to provide an insulating medium the steel fishplate was planed down by 1/4 inch and a specially formed sheet of bone fibre was inserted between the fishplate and the fishing surfaces of the rail. Fibre ferrules and washers were used to insulate the bolts and provided the joint was correctly packed in the ballast bed to prevent excessive flexing of the joint and crushing of the fibre insulation, the result was satisfactory. To insulate the gap between the rails, a fibre sheet shaped to the contour of the rail was inserted between the rail ends. This latter insulation, which is known as an end-post or "T" piece, was amply strong to resist the compressive load when the 40-ft. rails expanded. This form of insulated rail joint gave reasonably good service even when 120-ft. rails came into use. However, the crushing of the fibre fishplate resulting in electrical failure of the insulated joint could not be observed and therefore the failure could not be anticipated. This fact necessitated the use of a joint where signs of impending mechanical failure could be seen long before replacement became necessary. For this joint, the steel fishplate was discarded and replaced by one made of an insulating material, the most common one being resin impregnated, compressed laminated wood. Such joints will take a tensile load of 2000 psi but the bending strength is limited to a smaller loading. The fishplate will be very quickly smashed unless the joint is well supported. From a track point of view a rigidly supported joint is not favoured since track resilience is the ultimate aim. Modern railway practice is to provide a running top without any joints whatsoever as distinct from the use of 40 ft. or 120 ft. rail lengths joined together by means of steel fishplates. Rails of 45 ft. are depot-welded to 360-ft. lengths, transported to site and then thermit-welded into 1440-ft. lengths in the track. Thus the civil engineer has a track which is smooth, resilient and silent, requiring comparatively little maintenance and one which increases the life of rolling stock. The signal engineer, however, is obliged to request that the long-welded rail be cut at selected points in order that insulated rail joints may be installed to suit the track circuiting arrangements. Insulated rail joints, for which a design to resist the stresses set up in long rails has yet to be perfected, are inserted in the track. In order to protect the insulated joint against the high tensile forces, it is normal practice to install a splice joint next to the insulated joint. The splice joint allows the long rail to expand and contract, but apart from its prohibitive cost, it breaks the pattern and, together with the insulated joint, tends to nullify the advantages of long-welded rails. A possible solution to the problem lies in the application of an insulated joint which reverts to the use of steel fishplates which, after all, give maximum bending strength. The joint is known as a "frozen" joint or "glued" joint because over and above the use of high-tensile steel bolts, the entire assembly is rigidly held together by means of an adhesive glue, 1/16 inch thick, which acts both as a strengthening bond and as insulation. The entire assembly must be made to close tolerances under workshop supervision. Two short lengths of rail are used, the completed joint then being taken to site and thermit-welded into the track. Except for jointless track circuits on plain line, insulated block joints are used to define the limits of a track-circuit. It is important that the failure of an insulated joint does not cause a wrong side failure. Joints may be provided in one rail only or in both rails. This will usually be decided by requirements for electric traction return. With joints in both rails, if a single block joint fails, both track circuits should continue to work satisfactorily. If both block joints of a pair fail, there is a danger of a relay being falsely energised from the adjacent track-feed. To avoid this problem, the polarities of the rails on either side of every insulated joint must opposite. If both block joints of a pair fail, the two feeds will short-circuit each other, and both relays will de-energise. In the vicinity of points and crossings,it may be necessary for sections of one rail to be common to two track circuits. Failure of any common blockjoint should cause both track circuits to show occupied. In electrified territory, joints may be in one rail only. Maintenance of opposite polarities at every blockjoint is essential as only one joint separates the track circuits. Figure 7 Track Circuit polarities -Double Rail (Joints in Both rails) Figure 8 Single Rail (Joints in One Rail Only -Other Rail Continuous) TRACK-CIRCUIT TYPES Many different types of track circuit are available, designed to be used in particular applications such as electrified lines, continuous welded rail, etc. A list of the main types of track circuit is given below. As track circuit development continues, further types will no doubt become available. It is not necessarily an exhaustive list. 5.1 D.C. Track Circuits D.C. track circuits have the advantage that they can be simply fed from a battery, either as the main or a standby supply. Installation, setting up and subsequent maintenance is straightforward. They have the disadvantage that they are unsuitable where d.c. traction systems are in use. Battery - fed (primary cell or trickle charged) Non AC - immune AC - immune Coded "Swept" 5.2 Fixed Frequency A.C. Track Circuits All track circuits in this category work from a mains power supply at the same frequency. Mains frequency (e.g. 50Hz) vane relay (single-rail or double-rail) Traction immune, vane relay (operates at a different frequency to the traction supply for immunity from a.c. and d.c. traction currents) A.C./D.C (a.c. track circuit employing a d.c. relay) various types 5.3 Multiple Frequency A.C. Track Circuits Track circuits in this category are normally designed and installed so that adjacent track circuits operate at different frequencies. In most cases they may be used without insulated blockjoints. Reed Aster Hitachi UM71 ML TI-21 Siemens FS2500/FS3000 Alstom CVCM 5.4 Impulse Track Circuits These track circuits employ short pulses at a relatively high voltage to overcome contact resistance at the rail while keeping power consumption at a low level. Jeumont Lucas Although in some cases proprietary or manufacturer's names have been shown above, similar equipment may be available from other manufacturers. 5.5 Functions Other Than Train Detection As the rails are a continuous transmission line from one end of the track circuit to the other, it is possible to use the track circuit for more than simple train detection. To economise on lineside circuits, additional controls can be added into the feed end of the track circuit, typically in automatic sections. For example, the track feed can include proof that the signal ahead of it bas returned to normal after the passage of the previous train and/or proof that the signal is alight. The disadvantage of this type of circuit is that it complicates fault finding and may give the signalman misleading track circuit indications. A more recent application is the coded track circuit. The track circuit current is interrupted at different intervals or modulated at different frequencies to convey information to the train, usually for automatic train control (ATC). The current passing from one rail, through the wheels and returning to the other rail is detected by one or two receivers on the front of the train. Different signals may correspond to different commands to the on-board ATC equipment. To operate correctly, the track feed must always be at the end of the track at which the train leaves. Otherwise the train will shunt its own track circuit signal and the receiver on the train will see no signal· current in the rails. This is not a problem where trains run in one direction only. On bi-directional lines, this means that the track feed and relay circuits must be capable of being switched from one end to the other or two sets of track circuit equipment must be provided for each bi-directional track. Coded track circuits also create complications in train detection at the relay or receiver end. The simplest approach is to build in some form of delay longer than the code pulses to maintain the relay energised. The relay/receiver could be set up to recognise only the available set of codes, any invalid code causing the track circuit to show occupied. The third option is to compare the signals at the feed and relay ends. Figure 9 Coded Track Circuits Operating Principle D.C. TRACK CIRCUITS In its simplest form the d.c. track circuit is as described in the earlier sections. Power was originally derived from one·or more primary cells which would be replaced· as necessary during routine maintenance. Where a lineside signalling power supply is available (normally a.c.) this will be used to operate the track circuit, either via a transformer & rectifier or by trickle charging a battery of secondary cells. In more recent years, solar power has become a viable power source in areas where the climate is suitable. The choice of feed arrangements will depend upon the reliability of the main power supply. If there are standby arrangements for the supply itself, a battery is not normally used. In ac. electrified areas, d.c. track circuits may be used but with special ac. immune feed sets and relays. Immunity to the a.c. traction currents must ensure that the feed circuit cannot be damaged by traction currents and the relay will not pick or be damaged by a.c. A combination of fusing, series chokes and a.c. immune relays will normally provide acceptable protection. Purpose built a.c. immune feed sets are available. Obviously, d.c. track circuits cannot be employed where d.c. traction is in use as there is no satisfactory means of preventing traction currents from passing through the relay. A.C. TRACK CIRCUITS A.C track circuits, normally at mains frequency, can be used in areas where the presence of high levels of d.c. would prevent the use of d.c. track circuits. This is most commonly due to the use of d.c. traction, but may also be due to the presence of certain chemicals in the ground, causing a voltage to appear across the rails due to electrolytic effects. It is also possible to use a.c. track circuits in non-electrified areas. In place of a d.c. feed, an a.c. source is used. A two element a.c. vane relay is usually used in places of the d.c. relay. AC. vane relays cannot be energised by a d.c. voltage. The vane relay is operated by two windings, both of which must be energised simultaneously. One winding is fed locally, the other through the rails. The two relay feeds are then approximately 90" out of phase. If either feed is absent, or the phase angle between them is wrong, the relay will not energise. To isolate the track circuit equipment from the effects of very large traction currents, it is normal to connect the track circuit equipment to the rails via either a transformer or a series capacitor. If a capacitor is used in the feed end, this can be variable to perform the function of the feed resistor in a d.c. track circuit. Fuse protection is also desirable. One point to be borne in mind when designing locations for a.c. track circuits is that the supplies for the two windings must come from the same source. Signalling power supplies are single phase. This is normally derived from a 3-phase supply and adjacent power feeders may not be fed off the same phase. As the relay is phase sensitive, failure to observe this rule could lead to incorrect operation and/or adjustment of the track circuit. If d.c. traction is used, voltages are usually low and currents are therefore extremely high. To provide an acceptable low resistance return current path to the substation, both rails may be needed for traction return. Two basic configurations of A.C. track circuit are required, Double Rail and Single Rail. This refers to the traction current, not the track circuit current. Phase must be alternated between adjacent tracks for the same reason as polarity is staggered with D.C. track circuits (i.e. failure of blockjoints, connecting two supplies in antiphase will produce an effective voltage which is insufficient to energise either of the relays). 7.1 Double Rail Track circuits With low voltage d.c. traction systems, currents of 3,000A or more are commonplace. If current is to pass along both rails, special arrangements are made at track circuit joints to pass d.c. but not a.c. The arrangements include the use of an "impedance bond", which presents a low impedance to d.c., but a high impedance to a.c. by virtue of its two coils being wound in opposite directions. Often the impedance bond is "resonated". The impedance bond is provided with a secondary winding of a larger number of turns than the primary windings. This is connected in circuit with a capacitor, giving a band-stop filter effect at the track circuit frequency to further increase the a.c. impedance. The impedance bond is effectively a centre tapped inductor. The d.c. traction currents in each half of the winding set up opposing fluxes in the magnetic core which cancel each other out. To the a.c. track circuit current, the impedance bond presents a rail to rail impedance which is high compared with the ballast resistance. Failure of a blockjoint or the disconnection of a traction return bond between rails may unbalance the d.c. currents in the impedance bond sufficiently to saturate the magnetic core of the impedance bond and cause the track circuit to fail. In addition to the extremities of a track circuit, impedance bonds will also be found:- a) Where it is necessary to bond the traction return between adjacent tracks (to provide a lower resistance path). Depending on the power supply arrangements, this is usually necessary at approximately 0.9 km (BR third rail 750v) to 1.5 km (TfNSW 1500v overhead) intervals. b) Immediately adjacent to substations for connection to the traction supply. 7.2 Single Rail Track Circuits If traction currents can be kept low enough it may not be necessary to use double rail track circuits throughout an electrified line. Through points and crossings the use of double rail track circuits may not always be possible or desirable. Single Rail track circuits will then be used. The length of single rail track circuits through pointwork should be kept as low as possible where D.C. traction is in use as the impedance of the traction return path will be increased. Figure 10 A.C Double Rail Track Circuits (Typical TfNSW , Former SRA) Figure 11 BR(British) Capacitor Fed Figure 12 Typical Single rail Track Circuits Figure 13 Double Rail Track circuits with Additional Impedance Bond 7.3 Dual Electrified Lines In a few cases, more than one electrification system is in use on the same line (e.g. 750v d.c. third rail and 25kv a.c. overhead) or lines on different systems run in close proximity. If mains frequency a.c. track circuits are used, they will not be immune to the a.c. traction current. Similarly, d.c track circuits cannot be used either. It is however possible to operate at a supply frequency different to the traction supply (and any harmonics), Immunity will then be provided against both traction systems. For a 50Hz traction supply, frequencies of 83.3Hz and 125 Hz have been used. A separate signalling supply at the required frequency can be provided and distributed along the lineside. Alternatively, the supply for each track feed can be produced at each location by an inverter. Filters must be incorporated in both feed and relay ends of the track circuit. Impedance bonds on double rail track circuits must be resonated at the track circuit frequency (not 50Hz). This will require a different (usually lower) value for the resonating c3pacitor. The impedance bond will then appear as a much lower impedance at 50Hz. This arrangement is particularly useful where the traction system is to be converted from d.c. to a.c. where much of the existing track circuit equipment can be retained and track circuit conversion can be undertaken in advance of the traction changeover. The use of such track circuits is now falling in popularity as a number of dual trnction immune audio frequency track circuits are available. 7.4 A.C/D.C. Track Circuits The simple d.c. track circuit has an inherent slow-to-release characteristic, because the track relay has a high inductance, and a low resistance. The d.c. track relay is however much simpler and cheaper to manufacture than the a.c. vane relay. The high inductance delays the release of the relay. This is an unwanted characteristic for a track circuit, because it delays detection of fast moving vehicles and could leave a short train or vehicle undetected as it passes from one track circuit to the next. This obviously has serious implications on the safe operation of route locking. If a relay with higher resistance, and lower inductance is used, the relay releases quicker. However, a relay with higher resistance requires a higher operating voltage. An equivalent effect can be obtained by feeding a d.c. track relay through a step up transformer rectifier arrangement. One example of this is the B.R. "Quick Release" track circuit. The voltage on the rails is ac. The resistance in series with the track relay increases the value of resistance in the shunted circuit, and reduces the release time. The fact that the relay end has a high resistance also makes it possible to locate the relay some distance from the end of the track circuit (useful in restricted access situations such as tunnels). This type of track circuit is not traction immune. It is also not frequency selective, and will operate at frequencies other than 50Hz, which causes problems when used adjacent to audio-frequency jointless track circuits. JOINTLESS TRACK CIRCUITS Insulated block joints are an additional cost in modem welded rail. Although the strength and reliability of insulated joints has generally reached a high level, most permanent way engineers prefer to avoid insulated joints wherever possible. Jointless track circuits have been developed to operate without insulated block joints. Various arrangements of electrical components are used to terminate the track circuits, and define their limits, electrically. All jointless track circuits employ an a.c. signal. A transmitter at a specified frequency is connected to one end of the track circuit. At the opposite end, a receiver is connected which will only respond to the signal produced by its own transmitter. The receiver will normally be used to operate a relay which interfaces with the signalling system in the normal manner. The insulated joint normally takes the form of a filter which will prevent signals of the track circuit's operating frequency from penetrating more than a short distance beyond the transmitter or receiver and ensure that the track circuit is not operated by signals from adjacent track circuits. Adjacent track circuits must therefore operate at different frequencies. The frequencies used must also be immune from mains interference from the signalling power supply and, where necessary, the traction power supply. Early jointless track circuits were generally not traction immune. Several types of jointless track circuit are now available, many of which are immune to both a.c. and d.c. traction. The GEC "Reed" track circuit was also developed to be used as both jointless and conventionally jointed track circuits, immune to both a.c. and d.c. traction. However, problems were encountered with the operation of the track circuit in its jointless form in certain situations and it is at present only used on BR as a conventional jointed track, often in areas with both ac. and d.c. traction together. It will however be described in this section. Due to the absence of insulated joints, the limits of jointless track circuits cannot be as precisely defined as for conventional track circuits. This generally presents no problem on plain line but, as the extremities of track circuits through pointwork need to be precisely defined to ensure safe clearances, block joints must be used. Track circuits which define precise clearances will thereftill need insulated joints although the same jointless track circuits transmitters and receivers may often be used. Joints are also required at the extremities of jointless sections where they adjoin conventional track circuits. To permit the filters to operate correctly, each type of track circuit will often have a minimum length. This may be of the order of 50 metres. Shorter track circuits may therefore be unsuitable for use with jointless track circuit equipment. To permit adequate flexibility in the positioning of track circuits to meet all requirements, at least four distinct frequencies must be used, two on each line of a double line railway. This prevents interference between track circuits on adjacent lines. An early type of jointless track circuit, the Aster 1 watt, employed six frequencies, three for each line. On railways with more than two parallel lines, each set of frequencies could be used on alternate lines. Aster track circuits were not traction immune. A further development was the Aster U type. This employed only four track frequencies (two on each line) and was also not traction immune. To permit alterations (addition or removal of a track circuit) with only two frequencies per line, the track circuit may be centre fed if required. It is still a simple example of a jointless track circuit to study and will be described first. 8.1 Aster Track Circuits The connection of the transmitter and the receiver to the rails is by secondary coils of the track transformer. This defines the approximate position of the boundary between the two track circuits. The tuning units operate in conjunction with the inductance of the rails to form filters. In the diagram below, L1-4 are the inductances of the rails between the tuning units and the track transformer. At frequency F1, C2 and L6 are series resonant and are therefore an effective short circuit to F1. The remaining components present a parallel resonant circuit of high impedance to the F1 signal. F1 will thus reduce in amplitude progressing from Tuning Unit 1 to Tuning Unit 2. A similar situation arises for F2 with L5 and C1 acting as a series resonant short circuit. Four frequencies are used. 1.7KHz and 2.3KHz would alternate on one line and 2.0KHz and 2.6KHz would alternate on the other line. Track circuit length may be from 50m to 1000m long. By using a centre feeding Transmitter the track circuit may be extended up to 2000m either as two individual track circuits or a single long track circuit (the TPR circuit would of course include both receivers in this case). Figure 14 Aster U Type Track Circuit -Tuned Area Between Adjoining Tracks 8.2 ABB /ML Style TI-21 Track Circuits The TI21 track circuit is similar in operation to the Aster track circuit already described, but has been designed to provide traction immunity. This track circuit requires a tuned length of 20m with the tuning unit F1. acting as a short circuit to F2 and Tuning Unit F2 short circuiting F1. Transmitter and Receiver connections are made via the respective tuning units. The security of the system is enhanced and traction immunity provided by operating each track circuit at two frequencies 34 Hz apart and modulating between them at a rate of 4.8 Hz. The receiver must detect both frequencies and the correct rate of modulation to energise the follower relay. It is considered that any signal produced by the electric traction supply and its harmonics would be unable to simulate the track circuit signal for long enough to energise the relay. For traction return purposes, parallel tracks are normally bonded together at regular intervals. It is therefore not possible to re-use the same frequencies on third and fourth parallel tracks. Eight frequencies are available, four of which are preferred and thus used on two track lines. The remaining four are used on third and fourth lines when required. A minimum length of 200m is required (300m if centre fed) and a maximum of 1100m is permitted (1000m per end if centre fed). Length can be reduced to 50m in a special low power mode of operation but the adjoining track circuits either side must then be connected with the transmitter nearest to the low power track to prevent the signal feeding through and falsely energising the receiver of the next track of the same frequency. On d.c. electrified lines, double rail traction return is usually required and impedance bonds will be required at feeder points and wherever adjacent tracks are bonded together. They will not be required at the extremities of the tracks. Special resonating units are provided to resonate the bonds to the track circuit frequencies. Figure 15 TI21 Transmitter & Receiver Connections 8.3 GEC Type RT Reed Track Circuits The GEC Type RT Jointless Track Circuit operates by detecting the current in the track circuit loop, as opposed to the voltage at the end of the track circuit like the jointless track circuits already described. It is always centre fed in the jointless mode, although the last track before block joints may be end fed. The Type RT Track circuit may also be used as a jointed track circuit, in which case it may be centre or end fed. Each track circuit operates at a frequency defined by a mechanical reed filter in the transmitter. The filter is very stable in frequency and has a bandwidth of less than 1 Hz. This allows track frequencies to be only 3 Hz apart, using 366, 369, 372, 375, 378, 381 and 408 Hz. On each line a four frequency cycle should be maintained. The transmitter is connected directly to the rails. The receiver only responds to its own frequency due also to a reed filter at its input. When used in its jointless configuration, each track circuit is centre fed. The receiver is energised by a_n aerial positioned between the rails. The aerial comprises 36 cores of a 37 core cable wired to give a 36 turn inductive loop. The 37th core is connected to a series resonant shunt which terminates the track circuit. At any frequency other than the track circuit frequency, insufficient current will flow in this loop to energise the receiver. As the limits of the track circuit may not extend as far as the rail connection to the loop, the ends of adjacent tracks must overlap as shown. The inefficiency of the shunt has led to problems in practice on B.R. where track circuit lengths are often short and/or more than two parallel tracks occur on electrified lines. The result is that signals from nearby track circuits of the same frequency feed through sufficiently to be detected by other receivers. This type of track circuit is only used on B.R. in its conventional jointed form, where it provides dual (a.c. & d.c.) traction immunity. Receiver connection is then directly to the rails. Tracks may be either centre or end fed. Figure 16 GEC Reed Track Circuit Arrangement for Jointless Operation 8.4 Siemens FS2500 Track Circuit This is another recent addition to the range of jointless track circuits available. The location equipment can be mounted on a standard BR930 relay rack. The track circuit is traction immune and its circuitry includes the use of microprocessors to process the track circuit signal digitally. It employs a similar signal to the 1121 track circuit, a modulated ac. signal where the level of modulation and the modulation frequency must be correct to indicate a clear track circuit. An interface has been designed to connect direct to a Solid State Interlocking (SSI) trackside module without the use of a track relay. The receiver has a built in "slow to operate" feature. It employs four sets of frequencies, two on each line. It may be centre or end fed. In addition, the receiver may be positioned remote from the end of the track circuit, useful in tunnel situations. Different termination arrangements are used in each situation. For a series of end fed track circuits, each transmitter and receiver is connected to the rails via a tuning unit. The tuning unit performs three functions:- a) A high impedance to its own frequency b) A low impedance to the frequency of the adjacent track circuit. This prevents transmission of the signal through to the next track circuit of the same frequency and shunting of the track by a train outside its nominal limits. c) A transformer for transmission of the signal from transmitter to rails and from rails to receiver. Adjustment of the signal level can be performed at both transmitter and receiver. It is usual for the transmitter to be set up on installation according to the length of the track circuit. Finer adjustment according to individual track characteristics is at the receiver. At the end of a jointless section, an end tuning unit, which is slightly different to the normal tuning units for track circuit separation, is connected across the rails. The same tuning unit is used at any centre feed to provide a high impedance connection between transmitter and rails. It can also be used in conjunction with an impedance bond where connection between tracks, to traction feeders or to conventional jointed track circuits. Air cored inductors (often referred to as SI units) are used where it is necessary to make earth connections to the track without direct connection to the rails. These are installed in the centre of the tuned area between tuning units. Intermediate receivers can be installed within a track circuit to indicate the occupation of part of a track circuit, for example approaching level crossings. Maximum track circuit length is 1.3km end fed (although TfNSW adopts a maximum of 900m). Minimum track circuit length is 50 metres. 8.4.1 Siemens FS3000 Track Circuit FS3000 is the latest addition to the family .This Audio frequency track circuit has tranceiver unit and different types of tuning units for various configurations .This type dosent have impedenace bond for the application .Singapore Thomson East Cost Line use this type 8.5 CSEE UM71 (Hitachi) Track Circuit The operation and configuration is similar to other types of traction inunune jointless track circuits. It is not intended to repeat the details contained in earlier sections. Only the differences will be identified. Instead of the tuning unit, separate tuning and matching units are provided at the trackside for connection and track circuit separation. Resonating units for each frequency are available for use with impedance bonds. Maximum length is 600m and minimum length is 50m. Length may be extended up to 2km by the use of compensating capacitors connected at intervals between the rails. Two types of equipment are in use, the "T1" for most normal applications and the "T2" for extended connections in tunnel situations. IMPULSING TRACK CIRCUITS Rusty rails, as may occur on lightly used lines, will tend to cause wrong side failures whereby vehicles will remain undetected. To overcome this problem two basic methods are used. One is to weld a stainless steel strip onto the surface of such rails in critical areas; the strip will not go rusty. The other is to use impulsing track circuits. The principle of an impulse track circuit is to use high voltage "spikes" to be fed to the rails. If a train or single vehicle occupies the track the voltage of these spikes is enough to break down the resistance caused by the rust on the rails. The voltage spikes applied to the track have insufficient energy to electrocute personnel on the line, although their voltage is of the region of 150V. Frequency is of the order of 3 spikes per second. Although a higher ac. voltage on the rails would produce a similar effect, the power c:onsumption would be excessive. An impulse type track circuit generally consumes no more power than a conventional track circuit. Two types, the Jeumont and the Lucas track circuit, are in use on B.R. although they are not very common. TfNSW uses the Jeumont track circuit extensively. On the Jeumont track circuit, / a typical voltage waveform on the rails is shown on the following page. The receiver detects the correct polarity of the pulse (important in the event of a failed insulated joint) and the correct ratio of voltage and duration between the positive and negative parts of the waveform. It must also provide an output to the track relay in the interval between pulses. The arrangement of connections to the track is also shown. Figure 17 Jeumont Track Circuit -Typical Rail Voltage Waveform Figure 18 Jeumont Track Circuit -Typical Rail Voltage Waveform It is not intended to describe the construction and operation of Jeumont tracks in detail. They are generally considered to be traction immune due to the low frequency of operation (3Hz). Due to the high voltage on the rails, they can be used for long track circuits, up to 1500 metres on electrified lines and 3km on non-electrified lines. Double rail track circuits on d.c. electrified lines will be connected at each end via an impedance bond. The relay is peculiar to the Jeumont track circuit and is designed to operate with the receiver. RAIL CIRCUITS AND OVERLAY TRACK CIRCUITS Unlike a track circuit, which detects the absence of a train, a rail circuit detects the presence of a train. It may be used at automatic level crossings, for approach control of signals, or in place of treadles on high speed lines. It has also been used in situations such as bump marshalling yards to control the operation of points, due to its higher speed of operation when compared with the track circuit. To detect the absence of trains, a rail circuit is not fail safe. The simplest form of rail circuit consists of a standard track relay connected in series with its feed across the rails. A train or vehicle on the track will then complete the circuit, energising the relay. An alternative to the rail circuit is the overlay track circuit. It uses a much higher :frequency signal than conventional track circuits. The inherent impedance of the rail limits its extent of operation, so insulated joints are not required and it may be overlaid on most conventional track circuits. SHUNT ASSISTERS As mentioned earlier, increasing problems are being experienced with track circuits due to the introduction of lightweight vehicles with improved riding characteristics. Obviously, unreliability of track circuits puts the integrity of the whole signalling system at risk. Large scale replacement of existing track circuits with those that will operate satisfactorily with the new vehicles may not be practical or economic. If it can be shown that the problem is confined to certain types of train, it may be possible to deal with it by modifying the vehicles instead of the track circuits. On B.R. certain classes of one and two car diesel multiple unit vehicles are being fitted with shunt assisters. These operate by applying a high voltage (compared to most track circuits) signal to the rails. This is outside the frequency range of normal track circuit operation (50khz) and does not affect the operation of relays and/or receivers. It does however break down the resistance between rail and wheel and allow the track circuits to operate more reliably. Such a solution is not without problems. The on-board equipment must be monitored for correct operation and its failure could prevent further operation of the vehicle until the problem has been rectified. It is not therefore a substitute for ensuring reliable track circuit operation for normal traffic. ALTERNATIVES TO TRACK CIRCUITS In locations where track circuits cannot be installed or where a long track circuit is required, axle counters may be a possible solution. A set of axle counter equipment is generally more expensive than a single track circuit. If one set of axle counters can substitute for several track circuits, the overall cost may be cheaper. If required, axle counter sections can be designed to overlap, reducing the overall quantity of equipment. The track equipment may also be located remotely from the electronic logic equipment. Axle counters will be covered in more detail in a later section. Although not yet used in any working system, the use of Automatic Vehicle Identification transponders on vehicles has the potential for use as a train detection system. An esential requirement is, of course that all vehicles are fitted with transponders and that they can be proved to be functional.
Deepu Dharmarajan - Posted 3 years ago
CH10 | TRACK CIRCUIT BONDING
SignallingTRACK CIRCUIT BONDING CONTENTS Introduction Fouling & Clearance Points Positioning of Insulated Joints Jointless Track Circuits Bonding of Rails Track Circuit Interupters Other Information on Bonding/Insulation Plan NOTE: While these notes are based on the authors' understanding of railway signalling practice in New South Wales of Australia, they must not be taken to modify or replace any existing rules, instructions, or procedures of any railway administration. Where any apparent conflict exists, reference should be made to the appropriate documents produced by the administration of your Railway. This article will give fair bit of knowledge on bonding, there are numerous types of track circuits (FS2550, FS3000, CVCM, SDTC, TI21, MicroTrax). Bonding rules vary for these, and the respective manual shall be referred to. Bonding requirement for Axle counters is very limited (for Traction purpose only) and are out of scope for this article. 1. Introduction A modem signalling installation will use many track circuits. The limits of each track circuit must be precisely defined, and the track circuit must be connected to operate safely and reliably, even through the most complex points and crossings. In many cases this will involve the installation of insulated joints, although jointless track circuits are available to suit many applications. An insulated joint is relatively simple to provide in jointed track. The fishplate and its bolts are insulated from the rail and an "end post", a piece of insulating material of similar shape to the rail cross section, is inserted between the rail ends. In welded rail, this operation is much more complex. Either the rail must be cut and an insulated joint inserted or a length of rail is removed and a pre-assembled joint in a section of rail is substituted. The rail is then welded. Both operations involve the adjustment of the rails to allow for internal stresses. After installation an insulated joint will generally be weaker than the rail on either side. Therefore, although insulated joints are essential, their position must be chosen correctly, and the number of joints minimized. In addition, it may be desirable to avoid joints in positions of greatest wear, vibration, or stress on the rails. The positioning of insulated joints may often be a compromise between the requirements for an ideal track circuit and the practicalities of permanent way construction and maintenance. For correct operation, the two rails of each track circuit must both be electrically continuous between all extremities of the track circuit. The rails must also be insulated from each other. This requirement is just as important through points and crossings as on plain line. There should be no position within a track circuit where a vehicle can be totally undetected. Each track circuit must operate reliably and must as far as is practical fail safe. Any normal failure mode should result in the track indicating occupation by a train. In areas where electric traction is employed, one or both rails will be used for the return traction current. The track circuit arrangement must permit an adequate traction current return path while maintaining safe operation of track circuits. In addition to the signalling plan, it is customary to prepare a plan showing the arrangement of the track circuits. Instead of a single line for each track, it will show each rail individually. Its main purpose is to show the bonding and insulation arrangements for all track circuits. In addition, it may show other useful information such as position of cable routes and locations, overhead structures, traction power supply connections, earth bonding and further details of the track circuit equipment. 2. Fouling & Clearance Points Many insulated joints are positioned to prove clearance in the vicinity of points and crossings. The following terms will be used in these notes to describe the positioning of insulated joints. The fouling point is the position at which the extremity of a vehicle on one track is clear (by an adequate margin) of a movement on a converging line. The placing of an insulated joint at this position will not, however, ensure sufficient clearance. The clearance point is the position at which an insulated joint must be placed to ensure a vehicle stands beyond the fouling point. The distance of this from the fouling point will be determined by the longest overhang of all vehicles operating on the line. It follows that a joint which is intended to prove clearance must be positioned beyond the clearance point. although the signalling plan is not of adequate scale to show these joints accurately the track circuit bonding plan, or insulation plan, must be of a large enough scale to do so. If necessary, critical measurements must be taken from permanent way construction drawings. The limits of jointless track circuits cannot be precisely defined and cannot accurately determine clearance points. Even if jointless track circuit equipment is used through points and crossings, insulated joints will be needed to define clearance points and to electrically separate opposite running rails. 3. Positioning of Insulated Joints The positioning of insulated joints must fulfil all of the following requirements:- a) Within any track circuit, the two rails must always be of opposite. b) Unless adjacent track circuit signals employ different frequencies, the polarity (d.c. tracks) or phase (a.c. tracks) must be opposite on each side of all insulated joints. This is equally important whether the joint separates two track circuits or the two rails of the same track circuit. c) Where it is unavoidable to stagger block joints (i.e. they are not exactly opposite), the separation must be limited so that complete vehicles cannot remain undetected. d) Separation of a staggered pair of joints from an adjacent pair of joints (whether staggered or not) must not result in a vehicle being undetected. Critical clearance points cannot be defined by staggered. e) Minimum track circuit length must be greater than maximum vehicle wheel base. f) Maximum and minimum track circuit lengths must be within the specified range of operation of the type of track circuit. Most railways now employ a high degree of standardization of permanent way components. This will often restrict the position of insulated joints within points and crossings. Preferred positions must be used wherever possible to avoid additional cutting of rails and subsequent track maintenance. The use of these preferred positions will often result in joints being staggered. At a turnout, if there is a choice between joints in a high speed or low speed line, the low speed line is usually preferred. In cases where electric traction employs a single rail return, joints in plain line will usually be provided in one rail only. Pairs of joints will occur however in points and crossings and where it is desired to change the traction return rail from one side to the other. The above rules for staggering will still apply. In areas with double rail traction return, jointless track circuits must be used or joints must be provided in both rails, together with impedance bonds for continuity of traction current past the joint. 4. Jointless Track Circuits For adjacent jointless track circuits of the same type, no joints are necessary unless clearance points must be accurately defined. For replacement of signals and defining the end of an overlap, tolerances of 5-10 metres are usually acceptable. Adjacent jointless track circuits of the same type must be of different frequencies. Where a jointless track circuit adjoins a jointless track circuit of a different type, block joints will usually be needed due to different operating characteristics. A filter designed to operate with one type of track circuit is unlikely to discriminate correctly between signals of a different type of track circuit. Where any extremity of a jointless track circuit must be accurately positioned for clearance purposes, insulated joints must be provided. 5. Bonding of Rails Each rail must be bonded to give electrical continuity throughout the track circuit. The arrangement of bonding may be dependent on traction power supply arrangements. For fail safe operation feed/transmitter connections must be at one extremity of the track circuit and relay/receiver at the other. Between the feed and relay connections, the bonding must as far as possible be fail safe. If the rails are not welded together or otherwise bonded (e.g. for traction current) the signal engineer must provide adequate bonding throughout the length of the track circuit. The safest way of bonding the rails together is in series. On plain line this is the only practical method of bonding so no problems will arise. Within points and crossings, all track circuits will have additional branches which must also be bonded. All sections of a track circuit should still be bonded in series, but this may not be possible in all cases due to traction requirements. A certain amount of parallel bonding may be necessary. Figures 2 & 3 below demonstrate the difference between series and parallel bonding. In Figure 2, the rails in the turnout are parallel bonded. A break in any bond or rail in this section of track could leave a vehicle undetected - the track circuit will indicate clear when a train is standing on part of it. If the connections are rearranged as in the Figure 3, only very short sections of rail are now parallel bonded. Other than in these short sections, a break in a bond or a rail will cause the relay to drop, indicating an occupied track. Even in non-electrified areas, there will always be short branches of a track circuit which. cannot be connected in series (e.g. where the switch and stock rail adjoin). When the arrangement of track circuit bonding and insulation is being designed, the length of these branches should be kept as short as possible. 5.1 Non-electrified Areas It is usual, although not essential , to install insulated joints in both rails where electric traction is not used. In this case, series bonding should be employed throughout. To maximise the amount of series bonding, certain portions of rail through points and crossings may be common to two adjacent track circuits. In general, this should not cause a problem but in complex track layouts, the bonding and insulation must be checked very carefully to ensure that the use of a common rail between several nearby pairs of track circuits does not provide an electrical path for false operation of a track circuit and does not cause any track circuit to be shunted by trains outside its limits. Refer FIGURE 4 for a Track Circuit Sharing a common rail. 5.2. Single Rail Traction Return On many electrified lines, particularly those with a high voltage a.c. supply; traction return is via one rail only. The signal engineer still has exclusive use of the other rail. The two rails are normally designated the signalling rail and the traction rail. Of course, the signalling equipment must always use both rails. Track circuits connected in this way are described as single rail track circuits. The signalling rail will be series bonded. The traction rail must be connected to give the lowest impedance path back to the feeder station. Track circuits should be connected so that as much as possible of the traction rail bonding is in series. Often, however, parallel bonding must be accepted in the traction rail. A typical example of bonding for a single rail track circuit is shown on Figure 5. The signalling rail is connected to provide the maximum amount of series bonding. The traction rail shows a significant amount of parallel bonding. 5.3 Double Rail Traction Return On Some lines, particularly those with d.c. traction, a lower supply voltage increases the traction current. This will often require both rails to be used for traction return. Track circuits must therefore operate safely and reliably while sharing both rails with the much larger traction currents. To allow traction currents to pass conventional insulated joints, impedance bonds are used. Where two double rail tracks adjoin, the centre connections of the two impedance bonds are joined together. The ends of each coil are connected to the rails on either side. Where a double rail track circuit is of a jointless type, impedance bonds are not normally needed where tracks of the same type adjoin. At the end of a section containing a number of jointless track circuits, insulated joints are usually required and an impedance bond (resonated to the track circuit frequency) will be needed to pass the current around the insulated joint. Impedance bonds are also needed wherever the traction return must be connected to the supply at a feeder point and where adjacent roads are bonded together to reduce the impedance of the return path. Plain line track circuits are inherently series bonded. Through points and crossings, however, if the double rail traction return is to be continuous, a proportion of parallel bonding is required. Although it prevents proof of continuity of the track circuit and its bonding, parallel bonding must be accepted as the only means of providing a low impedance traction return. Additional security is usually given by duplicating some or all of the rail-to-rail bonds (which of course will all be traction bonds and must consist of a suitably sized conductor). Even in series bonded sections of rail, bonds or jumpers are often duplicated for reliability reasons. It must be remembered that maintenance routines must include regular checking of the integrity of these bonds because the disconnection of a single bond will not become evident as a track failure. Refer Figure 6 for Double Rail Track Circuit with Parallel bonding. 5.4. Transition Between Single & Double Rail At the end of double rail sections, where they adjoin single rail track circuits, insulated joints will be required in both rails. An impedance bond will generally be needed for the double rail track. Its centre connection in this case will be bonded to the traction rail of the single rail track circuit. Refer Figure 7 for Typical Connections Between Single and Double Rail Track circuits. Where double rail tracks are in general use, even a small section of single rail return will increase the impedance of the traction return path. Single rail tracks in this situation are kept as short as possible. In some cases, two separate track circuits may be provided where a long track includes both points and plain line, even if one single rail track circuit could perform adequately over the total distance. 6. Track Circuit Interrupters Track circuit interrupters are used to detect that a train has been derailed by a set of catch (trap) points by maintaining the track in the occupied state. The reason for this is that a derailed vehicle may be completely clear of the rails while still in a position which would be foul of other movements. If track circuit interrupters are provided, the following rules will generally apply to the track circuit bonding: - a) The track circuit interrupter will be insulated from the rail upon which it is mounted. b) It will be bonded in series with the opposite rail to the one upon which it is mounted. c) Traction current should not pass through the track circuit interrupter. If mounted on a double rail track circuit, the interrupter must be connected in a separate circuit. A contact of the interrupter repeat relay must be included to cut the TPR circuit of its associated track. 7. Other Information on Bonding/Insulation Plan The bonding or insulation plan will often show other additional information such as:- a) Position of overhead electrification structures. b) Bonding between overhead structures and traction return rails. c) Positions of locations, cable routes and signal structures. Note : Please comment in the query section ,if you wish to discuss about Bonding for Track Circuits like FS2500/FS3000, CVCM ,SDTC,TI21 etc on real layout .
Deepu Dharmarajan - Posted 3 years ago
CH11 | SIGNALS AND TRAINSTOPS
SignallingComing Soon....
Deepu Dharmarajan - Posted 3 years ago
CH12A | CABLES
SignallingCONTENTS Introduction Signalling Cables Telecommunication Cables Power Cables Selection of Cable Type Methods of Termination Cable Routes Cable Construction Electrical Properties Cable Testing Data Cable Fiber Cables INTRODUCTION Railway signalling now involves a wide range of equipment and techniques to transmit information, ranging from simple d.c. to carrier and data transmission. Associated with these is an equally wide range of interconnecting cables. Some are peculiar to railway signalling. Others are generally produced for telecommunications or general electrical purposes and have found applications within railway signalling. Refer Article Signalling Cable Standards on Rail Factor for a comprehensive list of cable standards, user of this article shall make effort to cross refer their current local guideline/standard in this regard. 2. SIGNALLING CABLES Most lineside signalling circuits are d.c. or mains frequency a.c. Voltages are low, typically 24 - 120 volts. In some cases point machine can be 3 phase ,415/400 Volts ,3 wire system. Cables designed for conveying d.c. or low frequencies are generally far simpler than those for a.c. use. This is because the transmission of d.c. is far less demanding on the type and dimensions of the insulating materials and also on the construction of the cable. Under d.c. conditions the voltage drop along the cable conductor is the product of the current flowing and the resistance of the conductor. As long as the insulation material and its thickness is chosen such that its resistance is very high then it will have little or no effect on the voltage drop along the conductor. Although the electrical characteristics of most signalling cables will be similar, cable construction will vary according to the environment in which the cable is to be installed. The most common variants are:- 2.1 Internal wiring This is usually flexible (stranded conductor) for easy installation along relay racks ,cable frame ,interlocking cabinets and cable ducts. The cable will generally be installed in a controlled environment so the sheath will not have to withstand such great changes in temperature, humidity and mechanical stresses as those installed outside. Many countries now have quite stringent requirements for the sheath material to satisfy fire regulations, the objective being to avoid emission of harmful gases in the event of a fire. Many existing internal cables have a PVC sheath. EVA (Ethylene Vinyl Acetate, otherwise known as Cross Linked Polyethylene) is often preferred now as it generally satisfies fire regulations. Some railways insist to use double sheathed insulation. Requirements for emission of smoke /and hazardous gas can be referred to IEC 61034-1/-2 and IEC60754-1 respectively. Similarly, IEC 60332-1 & IEC 60332-3 for flame test on Single wire and bunched wires respectively. UL1581 is an American Standard for electrical wires. Standard Size of wires are defined by AWG (American Wire Gauge)/European Or British Standards. Similar requirements exist for cables in tunnels and EVA sheathed cables are also used on underground railways. Internal cables are generally required as multicore cables (e.g. wiring between racks) and single core (individual circuits in interlockings and locations). Annealed Copper wires are used for electrical conductor compliant to IEC 60228(Australian AS/NZS 1574 & section 1&2 of 1125 ) 2.2 Lineside Cables Although these may carry similar circuits to the internal cables, they must withstand a more hostile environment. Typically, they will be installed in lineside troughs or buried and will be subject to changes in temperature and humidity, often lying-in waterlogged ground. Often UV Rays, oil, rodent and vermin will also be a problem. Individual conductors are insulated by an ethylene propylene rubber (EPR) compound/ while the outer sheath is polychloroprene (PCP) to give an oil resistant cable which will also withstand abrasion. If the cable requirement ask for UV Protection, Low Smoke Zero Halogen, Flame/Fire Retardant, carbon/iron/ wash plant liquid protection, outer sheath shall be selected accordingly . HDPE (High Density Polyethylene) cross linked polythene cover majority of such requirements. Most cables are multicore, to carry many separate signalling circuits. A single conductor will normally be adequate as the cable will not be subject to significant vibration once installed. Power cables are of similar construction but generally consist of two cores (for d.c. or single phase a.c. distribution and will generally have a stranded conductor due to the cross-section required. Some railways favour some form of armoured sheath (e.g. steel wire) for added mechanical protection. 2.3 On-track Cables Trackside electrical equipment is generally connected by cables across or under the ballast. Such cables must be strong and capable of withstanding considerable vibration. British Railway uses a flexible, multi-strand cable. Materials and construction are as for the lineside cables above, but the sheath is thicker, and the conductor is composed of a larger number of smaller strands (50/0.50mm). Again, many railways prefer an armoured cable but this can also present problems with earth faults where the risk of damage to the cable is high. 3.TELECOMMUNICATION CABLES Under a.c. conditions the inductance and capacitance of a cable can have a considerable bearing on the voltage drop and these factors become of major importance as the frequency increases. The capacitance of a cable pair or conductor will depend on the type and thickness of the insulation material. For a given material the thickness required for ac. transmission will generally be greater than for d.c. transmission. Suitable insulation materials for a.c. cables are dry air, paper and polythene. P.V.C. and rubber are not considered to be satisfactory, except in the case of low frequency cables such as 50Hz power. With multicore cables there is the additional problem of interference in one circuit due to the current in a neighboring circuit, commonly called "Crosstalk". The degree of crosstalk which may be encountered can depend on a number of different factors e.g. a) The frequency of the disturbing signal e. crosstalk increases with frequency (square waveforms, because of their high harmonic content are particularly troublesome). b) The magnitude of the current flowing in the disturbing i.e. crosstalk increases with the current. c) The position of the conductors in the cable relative to each To satisfy the latter problem, cables are manufactured with cores twisted together to form pairs, and adjacent pairs may be twisted at different rates. A circuit should always utilise the two conductors of a pair for out and return. Common returns should not be used in such cables. The effect of twisting the conductors together in such a fashion is that cancellation of any induced voltage occurs across any load terminated at the ends: of the cable pair. Often cables are made up in multiples of quads instead of pairs. In such cases a pair is formed by utilising the diametrically opposite cores. The resultant cable is smaller in diameter than the equivalent multi-pair cable, since there is less wasted space within the cable. With the use of very high frequencies on carrier circuits and for broad bandwidth applications such as closed-circuit television, the problems of crosstalk and attenuation increase with frequency. Eventually, twisted pair cables become unsatisfactory. Coaxial cable, effectively a cable pair consisting of a central conductor surrounded by and insulated from an outer metallic sheath, is employed. The fields created by the high frequency signals are contained in the cylindrical space between the inner and outer conductors, thus alleviating the crosstalk problem. A larger spacing between conductors also reduces the high frequency attenuation. 4. POWER CABLES For both signalling and telecommunications applications it is necessary to distribute power to the lineside. For most purposes this is best distributed as a.c. and transformed and/or rectified locally to the equipment served. A wide range of power cables is manufactured to supply the electrical industry, and these are generally employed within signalling and telecommunications systems. The sheath and/or insulation material may have to be modified to suit the specifications for signalling cable. As most power distribution is single phase, 2-core power cables are generally used. The conductors may be stranded copper or solid aluminium. 5. SELECTION OF CABLE TYPE 5.1 Lineside Signalling Circuits Signalling circuits will be taken to mean d.c. or mains frequency a.c. circuits between an interlocking and a lineside location (or between locations) to convey signalling information. They are normally laid in main cable routes and may be very numerous in areas of complex signalling. Cables for signalling circuits normally utilise conductors with only one strand, as mechanical strength is not of major importance. Typical cross sectional area is 0.5 to 2.5mm 2. To economize on installation costs, a smaller number of cables with a higher number of cores is preferred. 5.2 Tail Cables On-track cables (tail cables) to lineside equipment (e.g. signal heads, point machines etc.) from relay rooms or location cupboards must withstand physical damage and vibration. The B.R. standard cable has 50 strands to give it flexibility and a heavy-duty sheath for protection. The following sizes are commonly used: 5.3 Power Distribution Standard industrial sizes of 2-core power cable are used. The size of the conductors will depend on the power loading and the consequent voltage drop along the line. For most types of signalling equipment, a voltage drop of 10% is usually the maximum acceptable. It is therefore important to perform an estimate of power consumption of all equipment before deciding upon the size of cable to install. Allowance should be made for possible future additions to the electrical load. Renewal of power cables at a later stage can be expensive. 5.4 Internal Relay Room/Location Wiring Due to the diverse termination points of internal circuits, the only means of installing internal circuits is to use individual wires. Cables may be useful for wiring between racks or between different rooms/floors of a building to simplify installation by allowing factory pre- wiring. Alternatively, armoured (e.g. steel wire) cable could be used. This has fallen in popularity in recent years due to cost, difficulty of handling and difficulty in performing a satisfactory repair in the event of damage. 6 . METHODS OF TERMINATION All cables and wires must be terminated to connect on to other equipment. Each individual conductor in the cable will normally be terminated, even though some cores may be spare. In addition to terminating the individual cores the cable sheath is often clamped to a fixed bar in the location or equipment room. This is to avoid the weight of the cable placing a strain on the individual terminations which could lead to breakage of the conductors. The following are the most widely used forms of termination: - 6.1 Crimped Termination This method is mainly used for stranded cables. It is unsuitable for small cross-section single conductors as it weakens the conductor mechanically. It is a mechanical method which involves compressing a metal termination on to the wire. Various types of crimped connector are available:- a) A "ring" type connector to fit over a screw terminal. This is secured by nuts and washers. b) "Spade" type connectors which can be inserted into equipment terminals without the need to completely remove nuts and washers. c) A special type of spade connector for use in BR930 relay bases. These are simply inserted into the relay base. The crimps are constructed so as to spring apart slightly and lock the wire in position. A special tool is required to extract this type of connector d) "Shoelace " type connectors which can be inserted into equipment terminals without the need to completely remove nuts and washers. There are terminals available capable of inserting wires without the need of ferrules. Its recommended to use ferrules for multi stranded wires, however single wire cores can be inserted without a ferrule. For all types of crimped connection, it is important to choose the correct size to suit the terminal, the size of conductor and the size of insulation. Its also important to ensure the terminal can withstand the current carrying capacity of the wire. 6.2 Screw Terminals These are used in conjunction with crimped terminations or to directly terminate the conductors of solid cored cables. The cable is held tightly by a screw and/or nuts and washers to provide mechanical strength and electrical continuity. Where screw terminals are used, it is common to provide disconnection links to enable portions of the circuit to be isolated. 6.3 Plug Couplers Where modular equipment requires to be unplugged it is common to connect all wiring to a plug coupler. Wires may be soldered or wire-wrapped for security. This allows very quick disconnection and reconnection and avoids the need to check and/or test the wiring when a module is replaced. Plug couplers are often electrically or mechanically coded to ensure that they are only connected to the correct equipment. 7. CABLE ROUTES A safe route must be determined for the cables, both within buildings and externally. Within buildings, suitable ducts may be provided as part of the building or the equipment racking. If the cable route is shared with other than signalling cables, it must be ascertained that there is no hazard from electrical interference. Outside buildings there are several methods of cable installation used. These are described below. 7.1 Troughing Troughing is laid on the surface. It is usually inset into the ground for stability and the safety of staff walking along the track. Cable installation is simple. The lids are removed, the cables placed in the trough and the lids replaced. Two types are now in common use:- a) Ground Level Sectional Concrete Troughing. b) Ground Level Plastic Troughing. Plastic troughing is easier to handle but requires more accuracy in installation. Lids must be clipped on rather than being held in place by their own weight. 7.2 Ducts The following types of cable duct are in general use:- a) Earthenware duct b) Thick wall Rigid V.C. duct c) Thin wall V.C. duct. This duct must be laid in concrete d) Asbestos Cement duct - not generally available but has been used in the past in large quantities. e) Steel duct f) Flexible plastic pipe - this is corrugated along its length to allow the pipe to bend A common problem with all ducts is that over a long period of time they tend to accumulate debris which is either washed or blown into them. After a long period of time has elapsed, this may make further cable installation more difficult. 7.3 Cable Racks or Trays Slotted steel or plastic trays may be fixed to lineside structures, retaining walls etc. Cables may be fixed to this using plastic cable ties. The cable route is easily accessible for installation purposes but is also exposed to the environment and may be unsatisfactory in areas where vandalism is a problem. A similar method is to use cable hangers - hooks on to which the cable is placed and held in position by its own weight. This method is particularly useful in tunnels where there is little risk of vandalism and clearances are limited. 7.4 Buried Cables Provided the cable sheath is suitable cables may be buried direct in the ground. This avoids the cost of providing an expensive cable route. Cables are normally buried in one of two ways:- a) Laid in a trench with protective tapes or tiles and backfilled. b) Mole ploughed using a mechanical mole ploughing machine. In both cases warning markers should be provided on the surface at regular intervals. Buried cables suffer from several problems:- a) It is difficult to install further cables at a later stage without risk of damaging existing cables. b) Cables are vulnerable to excavations by others as they are not visible. Cable markers may eventually become obscured. c) Fault location and rectification is more difficult. d) Over a long period, ground movements and the growth of tree roots etc. may stretch and damage the cables. When specifying the type of cable installation, the engineer should take all costs, risks and benefits into account. 7.5 Aerial Cable Where an existing pole route is in good condition, it may be economic to install aerial cable. Aerial cable is however vulnerable to damage (gunshot, falling branches etc.) and the effects of lightning. Ordinary cable is unsuitable for aerial installation. Aerial cable has a steel strainer wire installed to provide the necessary tensile strength for hanging on poles. 7.6 Application In general the application of the above methods are as follows:- 7.6.1 Track-side cable routes Ground Level Concrete Troughing and Plastic Troughing is usually preferred where a large number of cables is required. Burying may be employed for small cable routes (i.e. single lines). Aerial cable could be considered if an existing pole route is still in good condition and the risk of damage or interference is low. 7.6.2 Under Track Crossings Practices vary but in general steel or asbestos cement ducts are preferable because of their ability to withstand vertical impact. In some cases thick wall P.V.C. duct has been used. Ducts should be installed sufficiently far below the track to be clear of track maintenance equipment (tamping machines, ballast cleaners etc.). 7.6.3 Platform Routes In general, platform routes should be provided with cable ducts and associated manholes for access to joints and pulling through of cables. Usually, it is more convenient to use earthenware ducts for platform routes. Surface troughing or hangers along the platform edge may interfere with track maintenance. 7.6.4 Tunnels The method used will depend upon the construction and cross section of the tunnel. It is often difficult to install a ground level route which will be clear of track maintenance machinery. Cable trays or hangers on the tunnel wall are therefore the most suitable. It is of course possible to design new tunnels to accommodate a cable route. Whether this is best located on the tunnel wall or floor will be determined by the type of track and tunnel drainage requirements. 7.6.5 Tail Cables Opinions vary on the best method. It is unlikely that there is a best method which suits both the signal engineer and the permanent way engineer. Cables may be simply laid across the top of the ballast. These are visible and easily removed and replaced. They are also vulnerable to damage by track maintenance and trailing objects on trains. Use of surface level ducts provides added protection but interferes with track maintenance. Placing the ducts below ballast level increases installation costs. Ducts can also become blocked with ballast and other debris. Surface cables secured to the top surface of the sleepers have greater protection than those laid loose across the ballast. This method may not always be acceptable to the track engineer. Tracks laid on concrete sleeper can use hard hats to cover the cable where hard hats are fastened on the concrete. 8. CABLE CONSTRUCTION A vast range of cables is now available. it is only possible to cover some of the main features of modem signaling cables in this section. There are many other cables widely in use for different application including control voltage to a Point Machine. Refer Figure 2 for the construction of a modern quad cable with Water Resistant property. Even though below section is based on Quad Cable Construction, we use this opportunity to discuss other alternative options available for armour, screen, internal & external sheath and the conductor insulation. Selection of PVC materials (XLPE -Cross Linked Polyethylene, LDPE -Low Density Polyethylene HDPE -High Density Polyethylene) and the addictive included for UV, Pest, Carbon /Iron /Copper/Mineral Dust, Acid /Salt, Industrial Cleaning solution shall be selected according to the operators need and the type of environment for the intended application. It is signal engineers’ responsibility to ensure that cable will not fail and select the property in case operator doesn’t specify detailed requirement. 8.1 Conductor Materials Copper is the most widely used conductor material due to its very low resistance and excellent mechanical properties. It is also available in sufficient quantities at an acceptable price. The copper is manufactured as wire of a consistent cross-section by repeatedly drawing the copper through holes in dies of reducing cross-section until the desired size is reached. As this process tends to harden the copper wire, it is usually annealed to restore its ductile properties. Annealed copper wires are complied to EN60811-203 /IEC 60228 .Refer RailFactor Article “Standard for Signalling cables “ as well for more details. Where rubber insulations are employed, the copper is generally coated with tin to prevent a chemical reaction between them causing corrosion of the copper and a change in the mechanical properties of the rubber. Where a large cross-section is required, aluminium may be used in place of copper (e.g. for power cables). Although Aluminium is more resistive, it is lighter and cheaper and has greater mechanical strength. Aluminium cables generally employ a solid conductor and are therefore more rigid than the copper equivalent. 8.2 Insulating Materials Each core of a cable must be insulated from all others and must therefore be surrounded by an insulating material throughout its length. Signalling cables use thermoplastic (P.V.C. or Polyethylene) or elastomeric (natural rubber or polychloroprene (P.C.P.)) insulations. The addition of P.C.P. to rubber improves its resistance to weathering. In very wet conditions, however, it has a tendency to absorb water over a long period of time. This will adversely affect its electrical characteristics. With telecommunications cables, the capacitance between cores becomes significant with a.c. signals. Dry air is the best insulator but impractical for cable construction. In older cables paper was widely_ used. This effectively consisted of a mixture of organic fibres and a large proportion of air spaces. As paper insulation is no longer used, polyethylene ie: HDPE (High Density Polyethylene-Solid) insulation is common these days with Low Smoke, Zero halogen property especially in the tunneled application. 8.3 Formation of Conductors Quad formation of conductors are the most recent trend for multicore cables .They are constructed in 1 Quad (4 Core ) ,3Quad (12 Core) ,4 Quad ( 16 Core ) , 5Quad (20Core ), 7Quad( 28Core) and 10Quad ( 40 core) .Each quad is arranged in the form of a Star (Diamond) and twisted together to get the best electrical property in terms of mutual Capacitance ( <45nF/Km) and Electromagnetic Compatibility compared to other formation which makes higher mutual capacitance .Lightly twisted pair formation is also available for better Electro Magnetic Compatibility. German DB(DB416.0115-Standard) defined Quad cable is popular for CBTC /ETCS application. Quads are helically stranded in concentric layers and cables more than 7Quad include two extra conductors with perforated insulation for surveillance. Signal Engineer shall select the compatible cable with respect to their signalling solution. 8.4 Core Identification Cores are identified in a number of ways, for a DB defined Quad Cable as shown in Figure 2 on light brown conductors black bracelets are printed in different combination .Eg: Two black circles printed together and repeated on fixed length apart ,One circle printed and repeated on certain length etc . As a Designer I am not a big fan of such identification as its confusing for the technicians for the first time. Methods of identification can be classified into 5. a) Coloured Tape wrapped around each core b) Coloured Insulation (Especially for Signalling Application Power Cables) c) Numbered Tape d) Numbering Printed on the insulation. e) Identification concentric rings (as mentioned above on DB defined Quad Cable) When numbering is used care must be taken to avoid confusion between 6 and 9. The easiest method is to write the number (six or nine) as well as or instead of the numerals. Various systems of colour coding are used depending on the size, type and manufacturer of the cable. In a paired cable only one conductor may be colour coded. The numbering of cable cores/pairs always starts from one at the centre and increases towards the outside of the cable. Each end of the cable is identified - the A end is the end at which the numbering of each layer runs in a clockwise direction, the opposite end is the B (or Z) end. 8.5 Core Wrapping, Screen, Inner Jacket, Swellable Tapes & Armour Plastic tapes are overlapped around cores which collectively hold all the cores together, on top of it, a copolymer coated aluminium tapes are wrapped. Tinned copper wire of 0.5 mm. run along the cable making in contact with the aluminium tape. This arrangement provides the functional earthing (EMC) option for the cable. This continuity wire needs to be earthed along with the armour for earthing the induced current generated due to parallelism (double sided for AC Traction line and singe sided for DC Traction). This arrangement is protected with Black coloured Zero Halogen Low Smoke compound PVC Inner jacket. Steel Tape armour is taped around the inner jacket. Swellable water blocking tapes are wrapped around the Inner jacket and Steel armour to avoid longitudinal water penetration. This arrangement along with inner and outer sheath is making the cable compliant to moisture barrier requirement per BS-EN 50288-1, EN 50288-7 or equivalent, water immersion test complying to IEC 61156-1. This construction is ensured to pass the Transversal water tightness and armour long water tightness test according to EN50289-4-2/A and water absorption for conductor insulation and outer jacket according to EN60811-402. 8.5.1 Types of Armour Armour provides additional mechanical heavy-duty protection, such as crushing, and resistance from pest such as rodent penetrating into inner conductor. Some cables will have nylon wrapping beneath outer sheath to protect from Termites and steel tape armour for rodent protection. Metallic armour not only provide mechanical protection it can also offer EMC protection but dose not replace the need for screen but lines less than 25kV can consider avoid screen under specific conditions. 1) Steel Tape Armour Steel Tape armour is sandwiched between water blocking tapes for DB cable. These are used for buried cables. According to American Railway Engineering and Maintenance of way Association (AREMA) states that tape armouring provide high degree of shielding protection than shield wires. 2) Steel Wire Armour Steel wire surround lead sheath for some cable design and are used for buried cable. This will surround the braided sheath and such sheath are used for high frequency emc protection 3) Corrugated armour Corrugated steel /copper surround the cable lengthwise beneath the outer sheath which is used for lines less than 25kv .This is mainly used in Optical Fiber Cable for optimum flexibility and recommended to replace with Fibers Reinforced Plastic (FRP) for electrified territory more than 25kV due to chance for high induced voltage. 8.5.2 Types of Shield/Screens Screening /shielding is used for reducing the effect of electromagnetic interference (EMI) or electrical noise which can disrupt the transmission performance in some environments. This noise may be because of external interference from other electrical equipment or because of interference generated within the cable from adjacent pairs (cross talk). 1) Aluminium /polyester tape with a tinned copper drain wire DB 416.0115-Standard Quad Cable referenced in Figure 2 have aluminium foil with attached tin plated copper wire . 2) Copper /polyester tape with a tinned copper drain wire This solution can provide better screening effect compared to aluminium foil. 3) Bare copper braid This is good for electromagnetic interference when the cable is subject to movements 4) Tinned copper braid Good for electromagnetic interference in presence of corrosive atmosphere or high temperature 8.6 Outer Jacket In addition to insulating individual cores, the entire cable must be contained within a sheath for both mechanical strength and environmental protection. Cross Linked Polyethylene (High Density Polyethylene Otherwise known as HDPE) are the most widely used. Low Density Polyethylene-LDPE) are also used depends on area of usage. Some older cables used lead, but its expense and associated health risks have led to its disuse. The choice of sheath material should consider the environment in which the cable will be used. Factors such as moisture, exposure to light and heat, the presence of oils and solvents, presence of carbon/iron dust, Train washing plant solutions, temperature, water immersion (IPX7) /submersion (IPX8) and the required level of resistance. In nutshell outer jacket is one of the most important elements for mechanical protection from external damages such as chemical (oxidation acid, oil), Mechanical (Abrasion), Environmental (Heat, Sun exposure, moisture, water), Fire exposure etc. Thermoplastic or Thermoset polymers are widely used where Thermoset have excellent properties against threats. 8.6.1 Ingress Level- Mechanical Properties. Mechanical shock severity shall be shared with the cable supplier whether its Low (Energy shock of 0.2 J, mainly for household installation hence not applicable) or Medium (Energy shock of 2J-standard industrial application) or high severity (Energy shock -5J) 8.6.2 Ingress Level- UV Resistance. Designer shall share the UV intensity requirement to the cable supplier based on the regional severity and exposure levels whether its Low (AN1 – Intensity ≤500 W/m²) or Medium (<500 W/m² intensity ≤700 W/m²) or High (<700 W/m² intensity ≤1120 W/m²). Refer Rail Factor Article “Standards for Signalling Cables” for more details. 8.6.3 Ingress Level- Water 8.6.3.1 Water Environment 8.6.3.2 Water Penetration The factor defines the water penetration in cables and to prevent the entry and migration of moisture or water throughout the cable. Water ingress can happen through Radial due to sheath damage and in this case, water enters in the cable by permeation through protective layers or due to any mechanical damage. Once water enter the cable, it travels longitudinally through out of the cables core. Where as longitudinal penetration moisture or water enters inside cables core due to ineffective capping or poor cable joint /termination .Please note water proofing and water absorption tests are different .There are no specific test for longitudinal water penetration for power cables .Radial water penetration test shall only be applied .Separate water penetration barrier are applied below the armour (or metallic screen layer ) and along conductor .Refer cable construction above in Figure 2 8.6.3.3 Moisture Protection Resistance offered by the jacket and the additional chemical used are ensuring the protection. However, the material with highest degree of water resistance is often not flame resistant, hence a tradeoff must be made between these two contradictory requirements. Choice of sheath material, make use of chemical moisture barrier and water blocking tapes can protect the cables from moisture. 8.6.4 Flame & Fire 8.6.4.1 Low Smoke Zero Halogen (LSOH) This is the property of cable to emit very low smoke and zero halogen and ensuring low corrosivity and Toxicity. Even though normal PVC cable ensure better mechanical and electrical properties, its poor in fire retardancy, corrosivity and low smoke capability. 8.6.4.2 Smoke Density Smoke can prevent fire fighters’ visibility and evacuation, especially in tunnel, work areas, control room and public areas. 8.6.4.3 Flame Retardant Flame Retardant property is vital, during a fire flame spread shall be retarded to limit to a confined area thus eliminating fire propagation. 8.6.4.4 Fire Retardant Property which when ignited do not produce flammable volatile products in required amount to give rise to a secondary outbreak of fire. 8.6.4.5 Fire Resistant Fire resistant cables are designed to maintain circuit integrity of emergency services during fire. Please note that Fire resistant cables are super expensive and normally considered for very vital cables (Eg; Fire cabinet ,depend on contractual requirement .Please note that Fire Resistant and fire retardant are different property and fire resistant is more stringent requirement. The individual conductors are wrapped with a layer of fire resisting mica/glass tape which prevents phase to phase and phase to earth contact even after the insulation has been burnt away. The fire-resistant cables exhibit same performance even under fire with water spray or mechanical shock situation. 8.6.5 Pest Resistant Depends on the intended area of the project ,there could be various threats such as ants, termite ,rodents , squirrels ,wood peckers ,other birds ,beetle and larva where cable contact with any plants to mention some .Various chemical compounds added on to the sheath ,depends on pest chemical and armours are protecting the cable .It may not be practical to have armoured cable for indoor application due to flexibility issue and outdoor environment have more threats . 9. ELECTRICAL PROPERTIES SIGNALLING CABLES 9.1 Voltage Rating of Cable Signalling control cables are normally rated for 600v/1000V. Voltage range classification for LV, HV, AC & DC according to IEC 60038 are as shown in Table 3 Maximum for High Voltage for IEEE is 35kV and in some countries its 45kV, which is country specific. Refer Table 4 for maximum permitted voltage Vs Rated voltage. 9 .2 Resistance of Cable This is the resistance of wire which increases with distance and normally included in the cable data sheet from the supplier. Its measured Ω/kM @ 50Hz (or 60Hz) and 20°C. This is the main parameter to calculate the voltage drop of cable. Voltage received at the end gear shall not be less than 10% of the source voltage. This means when you feed 130V from the SER, signal at the track side should at least get 117V. Cable conductor size shall be selected based on voltage drop calculation and shall cross check with field gear data sheet that the 10% voltage drop allowed will still fall in the minimum required voltage Important Note: Signal Engineer shall ensure that the resistance value (Ω/kM) provided by the supplier in the data sheet is loop resistance or wire resistance. Loop resistance means it’s the value for two conductors. While calculating voltage drop, number of loops used for the respective circuit and its distance shall be used. 9.3 Reactance of Cable This is defined in Ω/kM @50Hz (or 60Hz). This is important parameter for MV cables which need to be asked from the supplier but for LV, designer can define the allowed limit for LV 9.4 Capacitance of Cable Measured in µF /KM which is mandatory parameter for MV cable and shall be requested from supplier. As mentioned above Quad formation have less than 40µF /KM. Lower the capacitance better the cable property. 9.5 Maximum Short Circuit Current (Conductor and Screen) Maxum short circuit current in kA for conductor and screens for 1.0 seconds and 0.5 seconds respectively shall be requested and obtained from supplier. 10 CABLE TESTING It is not the purpose of this article to give detailed instructions on the procedures for testing and maintenance of different types of cable. However, the general principles of cable testing are described here. In general, whenever a cable is installed, repaired, re-terminated or jointed and at regular intervals during the life of the cable, tests must be made to ensure that: - a) Each core is continuous and of the correct resistance. A rise in the resistance of a core could indicate a potential fault. b) Each core is insulated from all other cores. It is normal for the insulation resistance to fall slightly during the life of a cable. Serious deterioration must, however, be detected before it causes any safety hazard. c) Each core must be adequately insulated from earth. Unwanted connections to earth are a potential danger to all signalling circuits and must be avoided. Where the cable has a metallic sheath, the insulation tests must include the sheath. Where the sheath is earthed and/or bonded for reasons of safety or noise immunity, the continuity of the sheath is also important. The continuity tests may be made using a suitable digital or analogue multi-meter set up to measure resistance. All tests will require the cooperation of persons at each end of the cable. A telephone circuit between the ends (using the cable to be tested if convenient) is essential to carry out an efficient test. The simplest method is to put a loop between one conductor and each other conductor in turn at one end of the cable. The loop resistance is measured at the other end using the meter. Any variation between individual readings (and changes since the previous test) should be investigated and resolved before the cable enters (or re-enters) service. Insulation and earth tests should use a suitably rated insulation tester (1000-volt Megger or similar for signalling cables). Tests should be performed between each core and each other core in turn. The acceptable value of resistance for a cable will depend on the circuits connected through it. However, as a general guide, a new signalling cable should give readings better than 10MΩ (when terminated). Readings less than 1MΩ could potentially be dangerous and require urgent investigation. The earth test may initially be carried out between earth and all cores connected in parallel. Only if this test is unsatisfactory need individual cores be tested to earth. Although a new cable is always completely tested before being brought into use, a complete test of a working cable is not always practical without serious disruption. In this case, routine tests are often carried out on a sample of cable cores (spare cores if available). Previous readings should be retained for comparison. 11. DATA CABLES Ethernet cables falls under this category. They are classified into different category Cat 1 to Cat 9, whereas Cat 1-4 are not suitable for modern day rail application and above Cat7 is not yet came into application while preparing the article. Refer Table 4 for category classification. Data cables with twisted pairs have different construction depends on the purpose, cable shall be selected. Refer Table 6 for various construction. 12. Fiber Cables Although many signalling applications must use metallic cables, the availability and cost of fiber optic cables is rapidly improving. Instead of electrical signals, they transmit information by passing light signals along the length of a glass fiber. Internal reflection contains the light signal within the fiber. Although not specifically employed in conventional signalling systems, fiber optic technology has the following advantages and necessary for modern communication-based train control system: - a) An extremely high capacity and bandwidth. b) Immunity from all types of electrical It is therefore of great use for communications purposes on electrified lines. Conventional jointing techniques are not applicable to glass fiber cables. Instead, the two ends must be cut squarely, butted up to each other and fused together by the application of heat. This is a very precise operation as any irregularities in the fiber will cause attenuation of the signal. Much of the work of jointing fiber cables can now be done automatically by sophisticated (and usually very expensive) fusion splicers. The action of cutting the two ends squarely, aligning them for a parallel joint and fusing for the correct period of time is largely automatic. Even with the high degree of automation, fusion splicing is not always 100% satisfactory each time. It is therefore usual to provide additional spare fiber at the joint. This must be accommodated within the joint closure. Category cables have limitation to transfer data more than 100meter, Fiber has significance in this case. There are two types of fibers: Single Mode: Long Distance Application Multi-Mode: Short Distance Application Single mode Fiber must be complied to G652-D type as per ITU-T standard and multimode with IEC 60793-2-10 There are two types of construction Loose Tubes used in cable concrete trough, direct buried and other harsh environment Micro Tubes for less harsh environment The END NOTE :- Please comment if you wish to include Cable Voltage Drop Calculation , Stanadard conductor sizes and a sample cable plan
Deepu Dharmarajan - Posted 3 years ago
CH13 | POINT CONTROL AND OPERATION
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