By Deepu Dharmarajan
Posted 3 years ago

CH20 | INTERFERENCE & IMMUNISATION-PART 1 (ELECTRICAL)

Signalling

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CONTENTS   

  1. Introduction
  1. A.C Traction Systems
  1. D.C. Traction Systems
  1. Mixed Traction Systems
  1. Protection Against Other Forms of Interference
  1. Future Developments and Problems

1.         INTRODUCTION 

Electric traction is being used as an economic means of operating a railway. Electric power is taken from the national or regional supplier. Due to the scale of production this can normally be produced at a lower price than with individual power units on each locomotive. Electric locomotives are much simpler and cheaper to control maintain and operate than their diesel equivalent.

Electric traction brings with it a number of additional problems for the signal engineer. Signaling equipment, in most cases, operates at relatively low voltages and currents compared to locomotives and multiple units. Unavoidably, traction circuits run alongside and even share conductors with signalling circuits. Interference, usually through direct contact or induction must be anticipated and equipment protected from its effects.

Even without electric traction, signalling equipment may still be subject to interference from adjacent power supply lines or lightning storms. Similar techniques are often employed to protect against the likely effects.

The objectives of protection (or immunisation) can be summarised as:-

a) Prevention of failures, particularly wrong-side failures.

b) To ensure the safety of staff working on the equipment under normal operating conditions and, as far as possible, under traction fault conditions also.

c) To prevent the effects of traction currents from damaging or destroying signalling equipment.

1.1.       Electrification Systems

 The effects of electric traction and the precautions which must be taken will depend on the type of traction supply.

The electrification system chosen for a railway will, in tum, depend on a number of factors: -

a) Type and frequency of traffic.

b) Availability of Power Supplies and the locations at which they can conveniently be brought to the lineside.

c) Local geographical and climatic conditions.

d) Structural clearances (for overhead wires  etc)

e) The overall length of the line.

f) Whether the line to be electrified is an extension to an existing electrified line or a completely new traction system

As a result many different systems are in use. Once a system has been selected, it is very expensive to change due to the investment in traction units and power supplies.

The systems can generally be divided into two groups: -

A.C. systems, normally employing a high voltage (typically 25kV) with overhead current collection.

D.C. systems, generally at much lower voltages (600 to 1500 volts). The method of current collection is either overhead (requiring a much heavier conductor than for a.c.) or third rail.

Low voltage systems tend to be d.c. because this makes the traction equipment on the trains much simpler. Early electrification did not have the benefits of modern power semiconductors to control traction motors so the power was at a voltage which could be applied directly to the d.c. traction motors.

The cost penalty is a large number of substations and heavy feeder cables.

The use of a.c. traction permitted substations to be spaced considerably wider apart and the overhead conductor to be used as the main feeder. No separate feeder cable is necessary. The equipment on board the train is generally more complex. The high supply voltage must be transformed to a usable voltage and rectified to d.c. for the traction motors. Modern variable frequency a.c. drives have now made the induction motor a viable alternative to the series wound d.c. motor.

1.2.       Modes of Interference

In most cases one or both running rails are used for the return current path. To some extent earth currents will also be present.

In all types of track circuits there is the risk of interference by direct contact, traction currents passing through the signalling equipment or between the signalling equipment and earth.

There is a high risk that traction currents could cause wrong side failures. The incidence of right-side failures could affect reliability to an unacceptable level.

Other circuits are generally not in direct contact but there is still the possibility of induced interference. insulation could also break down causing direct contact with the traction supply.

Protection must also be provided against dangerously high voltages (either induced or through direct contact) which could injure or kill personnel working on signalling and communications equipment.

2.         A.C. TRACTION SYSTEMS

2.1 Feeder Arrangements 

BR's 25KV 50 Hz AC. traction system is typical of those on many other railways and is usually derived from the 132 KV National Grid system. The supplies are taken as single phase through 132/25 Kv transformers provided by the Supplier. The supplies are taken to feeder stations at usually the same phase.

Adjacent feeder stations are not parallel (i.e. not necessarily from the same phase and usually not from the same source. This enables the Electricity Authority to balance the railway load between the 3 phases of its system.

The distance between feeder stations is dependent on traffic and power requirements. An electric locomotive accelerating with a full load may take a current of the order of min 270 amps. Neutral sections are provided between feeder stations to separate the supplies and track sectioning cabins (TSC) are located at intermediate points and the boundaries of feeding sections to provide alternative feeding and track isolation facilities in the event of failures or isolation for maintenance.

2.2  Return Paths for Traction Current 

2.2.1.Rail Return 

This is the simplest form of traction current return. In most cases, only one running rail is used. Modern Track Circuits allow dual rail traction return. On plain track this is normally the cess rail, but in complex layouts the location of return rail changes in order to meet track circuit requirements by cross-bonding. The problem with rail return is that the rail is not well insulated and return traction current will leak to earth over the length of the return rail.   This produces a large imbalance between the current in the overhead wire and that in the rail, which dramatically increases the level of induced voltages in lineside cables (see later sections for details).

2.2.2 Return Conductor

If the traction return path is well insulated, the current imbalance in the circuit is reduced, hence interference is minimised. A return conductor can therefore be provided to carry the traction return current. The current carried is in anti-phase to the current in the contact and catenary wires, and similar in magnitude, therefore the net electro-magnetic inducing field is reduced. The reduction over rail return, where signalling circuits are placed in the optimum position for minimum interference, is of the order of 45%. 

2.2.3 Booster Transformers

Some of the return current will still flow via earth instead of the return wire. This will be in inverse proportion to their relative impedances between the train and the substation. The cost of a conductor large enough to eliminate the earth currents would be prohibitive. To force the majority of the return current into the return conductor, transformers are provided in the traction circuit as shown on FIGURE1

The return conductors are bonded to the traction return rail midway between booster transformers. These are located at  3.2 Km intervals, the primary winding connected in series with the catenary and the secondary winding in series with the return conductor.

The effect of the booster transformer is to produce a current in the return conductor which approximates to that in the catenary but is 180â—¦ out of phase. Maximum suppression of induction is achieved with the traction load at the mid-point connector (95% reduction) and the minimum when at the booster transformers.

This alone does not immunise signalling and communications circuits. As the two conductors are widely separated, induction will still occur whenever a signalling circuit is nearer to one conductor than the other. The relative positioning of the overhead conductors, the return conductor and the signalling cable route must be such as to place the cable route equidistant from the traction conductors.

2.3 Modes of Interference and Immunisation Techniques 

Of the three possible modes of interference, conduction, electrostatic induction and electromagnetic induction, induction is the dominant mode in a.c. traction  areas.

Conduction, by direct contact between the power line catenary and the rails, equipment or cable conductors happens rarely. If unprotected, it is usually fatal to personnel and equipment. The only safeguard is the provision of contact circuit breakers in the traction supply.

Due to bonding of overhead structures direct to the traction return rail, the risk of a traction fault raising the rail potential, and that of track circuit equipment, to dangerous levels is small. Track circuits will however be subject to false operation due to the multiple return paths created by return conductors and earth bonding and extra precautions must be taken when setting up and adjusting track circuits to ensure safe operation.

Due to the distance between the traction equipment and the signalling equipment and the low supply frequency of 50Hz., electrostatic effect are usually negligible.

Electromagnetic induction is caused in one conductor by current flowing in another. This action is similar to a transformer, the traction conductors being the primary winding and the signalling conductors the secondary. The voltage is induced along the conductor (not between the conductor and earth). The effect is maximised when conductors run parallel to each other. The induced voltage increases with length of conductor and decreases with separation of the conductors.

As signal engineers we are concerned with the following aspects of a.c. induction into our lineside cables

a) voltages produced under normal operating conditions that could affect line circuits and tail cable circuits and result in the malfunction, failure or damage to equipment and/or hazard to staff working on the equipment.

b) Higher levels of AC. voltage induced in our cables as a result of traction system.

c) Any increase of these effects due to disconnection, earthing or other failures within the signalling equipment

Dangerous levels of induced voltage are possible if the traction current rises rapidly (short circuit or traction flash over).

Telecommunications circuits are susceptible to all normal levels of induced voltage on unprotected cables from the traction system. Therefore, special measures have to be adopted to protect these systems from the 50 Hz base and odd harmonics of this frequency.

2.3.1 Positioning of Cable Route

The simplest method to reduce the levels of induced voltage is the physical location of the cable route, as described earlier. The best position for the cable route is approximately equidistant from the two traction conductors (where booster transformers and return conductors are provided).

This distance will obviously vary for different designs of overhead equipment, for multiple track lines where two or more return conductors are mounted together, and for lines without booster transformers and/or return conductors.

On most circuits which form a loop (one conductor out and the other return), this provides an adequate level of protection for normal operation  provided  the signalling equipment  is in full working order. Both conductors are subject to the same electromagnetic fields which tend to oppose each other. Under fault conditions (earth faults and/or disconnections) further protection will be required.

2.3.2 Electro-Magnetic Screening 

With any wire installed parallel to an A.C. electrified line, a reduction in interference is obtained by the screening effect of earthed conductors in its vicinity. This includes cable sheaths, metal pipes and running rails.

All communications cables are provided with a screening sheath (usually aluminium) with up to a maximum of 4 steel tapes around the sheath. These are connected to a good earth of no more than 4 ohms located every 1000 m.

Signalling cables run in the same cable routes as screened telecomms cable and therefore benefit from the mutual screening effect between the cables.

Where existing cables, not to electrification standards are to be retained, it may be possible to immunise them by a separate screening conductor. This is a large cross-section copper wire run along the length of the cable route and earthed as above. Cable sheaths should also be earthed. 

2.3.3 Immunisation of Relays 

Relays for d.c. circuits can be designed to withstand substantial a.c. voltages without energisation.

The relays have copper slugs fitted over the cores,  near the pole pieces  and a magnetic shunt is fitted between the cores above the copper slugs.  When a DC voltage is applied to the winding the DC flux is produced as normal and attracts the armature.   Some of this flux is diverted via the magnetic shunt and therefore greater power has to be supplied. However, when an AC voltage is applied, the AC voltage has difficulty in establishing  itself  across the air gap due to the large copper slugs and it tends to short circuit the air gap via the magnetic shunt, thus the AC flux plays little part in the operation of the mechanism. Refer Figure 2 &3   showing the principle.

 

 

 

2.3.4 Choice of Operating Frequency 

Where d.c. circuits are not practical, equipment should be designed to operate at frequencies other than the mains frequency and its harmonics. Filters can be used to keep signalling and traction currents separate

Although the mains frequency is normally very accurate, fluctuations can occur and these must be allowed for in the design of systems. Fluctuations of ± 0.5% are typical so the design may allow for 1% maximum error to give a degree of margin for error.

It should therefore be evident that the available bandwidth between successive harmonics decreases by 2Hz each time and above 1kHz no bandwidth is available. In practice, harmonics of this order are very small and frequencies above 1.5 kHz are successfully used for track circuits. For additional safety, two or more frequencies are used together so that traction faults could not generate both together.

2.2 Practical Immunisation of Signalling Equipment

2.4.1. Limits 

Appreciable earth currents may still flow for some distance away from electrified lines and protection must be extended sufficiently far for their effects to become negligible. At the limits of electrification, or where a non-electrified line leaves the electrified lines, experience has shown that signalling equipment should be immunised for 800m from the electrified line.

2.4.2. Line Circuits 

These should be DC circuits using AC immunised line relays.

The length of line must be limited to 2km to ensure that the induced voltage from the traction system does not exceed limits for electrical safety (maximum induced voltage of 110v). Circuits required  to cover greater distances must be repeated  by means of a relay and new power supply.   Where circuits run along non-electrified branches, they must be cut 800m from the electrified line.

Where circuits from the same supply feed in opposite directions, care must be taken to ensure that the total length of parallel circuits is less than 2km.

All vital line circuits are double cut to reduce possible false operation and hazard to staff where an earth fault is present.

2.4.3.Track Circuits 

The traction return current can usually be carried satisfactorily by only one of the running rails. Single rail DC track circuits, immune to the highest AC voltage that could occur, are used. It has been found desirable to limit the track circuit length to a lower value than on non-electrified lines to prevent the combined effect of a return conductor and a broken rail providing an alternative path to the train shunt.

The track feed set must be designed to prevent a significant DC voltage being applied to the rails as a result of rectification of AC from the traction current. The track relay must also be immune to AC in the same manner as those used for line circuits.

Reed track circuits operate at frequencies clear of 50Hz harmonics and may also be used in AC electrified areas as may most modulated audio frequency track circuits such as the TI21. When little used and therefore rusty rails require track circuiting there are two solutions - a welded stainless steel strip on the top surface of the rail or a high voltage impulsing type of track circuit.   The Jeumont track circuit is immune to false operation by AC traction current, but its length is severely limited in a.c. traction areas. This may not be a serious disadvantage, as it is mostly used for point and crossings into sidings and loops.

There are few types of track circuits mentioned here, Signal Engineer must check with the product owner and manual for other types of track circuits.

2.4.4 Signals 

Colour light signals use tungsten filament lamps, which operate readily on both AC and DC. Therefore the only method of preventing induced AC lighting the lamp is to limit the length of the circuit between control relays and lamps and employ as high a voltage as practical.

The usual practice is to have a transformer for each aspect in the signal head to reduce the voltage to the 12 volts or less required to operate the lamp and supply 110V AC over the contacts of the controlling relays. In this case the maximum parallelism allowed is 183m, i.e. a signal head must be less than 183m from its controlling relays.   If the overall maximum distance between signals fed from the same location or relay room supply exceeds 183 m, then an isolating transformer is required to feed signals on one side of the location or relay room. By limiting the length of signal lamp circuits, it is unnecessary for them to be double cut.

Shunt signals generally operate on 110-volt lamps so no transformer is necessary.

Searchlight signal operating mechanisms must be immune to false operation. This immunity is  achieved by the use of chokes mounted as close as possible to, and in series with, the d.c. searchlight mechanism. An a.c. searchlight signal could be operated at a different frequency (e.g. 83.3Hz)

For LED Signals (current practice), signal head has AC/DC rectifier unit and few types are mentioned below

1)Aldridge RL 400 ,120V Main Line Signal 212mm /127mm.Cable to signals shall be less than 750m for single cut circuits .Between 750m to 1500 meter for single cut circuits with two bleed resistors fitted in the signal head .Fit a 130VAC ,20mm varistor across each LED unit.

2) Aldridge Tunnel Signal 120V AC 127mm.Cables to Signal less than 1200 mm for    single cut circuits and less than 2000m for double cut circuits .No varistors required for surge protection.

3) Alstom Mark 2 120V AC Mainline out door signals 212mm.Cable to signal less than 750m for single cut circuits ,between 750m and 1500 m for single cut circuits with two 4K7 6W bleed resistors fitted in the signal head .Less than 2000m for double cut circuits .No varistors required for surge protection ,but must have some surge protection to earth on each leg of 120VAC supply at power supply locations .

2.4.5.Points 

Electric point machines are immunised by using permanent magnet machines. These ensure that, even if there is an AC voltage at the terminals, the motor's field will remain uni-directional and although there will be considerable vibration, there is no resultant torque and therefore the motor will not move.

Electro-Pneumatic point machine valves must be immune.  These valves are immunised  in a similar manner to that outlined for relays.

Electro-Hydraulic (clamp lock) point mechanisms were found to have inherent immunity and can be used with no special measures being taken.

Mechanical Points should have insulation inserted in the point rodding at the ends adjacent to the lever frame and the points.   This is to prevent stray voltages causing electric shock to personnel. Detection of all types of points is by polarised relays. These relays are fully immunised in the manner already described.

2.4.6.Level Crossings 

Where lifting barriers are used it is desirable to position the barrier so that, if it is knocked over no part of it shall come closer than 150 mm to the overhead line equipment. If other positioning requirements make this impossible, then the barriers should be made of metal or have a continuous metallic strip of adequate section along its length. The barrier or metallic strip should be bonded to a traction return rail or cable.

Control circuits must be immunised as already described.

Where closed circuit television is used special precautions must be incorporated into the design.

2.4.7. Remote Control 

Immunisation of remote control systems is dealt with in the separate notes covering remote control. This mainly involves line isolation at regular intervals. 

2.4.8.Power Supplies 

Although the safety precautions in force for higher voltage power supplies would cater for the possibility of induced voltages at the levels expected, care must be taken when working on supplies which are switched off to avoid the possibility of high induced voltages. As the cable may not be sectionalised in the same way as vital signalling circuits, dangerous voltages could occur on long power feeders.

3.   D.C. TRACTION SYSTEMS

Because of the low traction voltage, traction currents are high, typically, 2,000 - 3,000 amps per train during acceleration.   Due to the large DC return currents present in the earth near to and in the return rail, there are problems caused by corrosion, and problems caused when insulation, equipment and cables break down.

A d.c. supply will normally produce no inductive interference effects other than from switching transients and ripple at harmonics of the mains frequency from an unsmoothed power supply. The main hazard is from conduction (to which track circuits are particularly exposed) by direct contact or via earth faults.

The main precautions therefore comprise operation of equipment from a.c. supplies, effective insulation arrangements and earth leakage detection.

3.1   Limits 

Earth currents from d.c. supplies are much more troublesome than for a.c. They may propagate over much longer distances and immunisation should be provided for at least 3km from a d.c. electrified line.

 3.2 Line Circuits

DC circuits are used which are not sectionalised other than for volt drop purposes. It might initially seem unwise to. use d.c. but the reasons for accepting d.c. line circuits on B.R. in most situations are as follows:-

a) All cables have non-metallic sheaths and are therefore less likely to pick up DC potentials along their routing.

b) All cables are tested to a rigid specification 

c) Additional insulation is provided by terminating cables on non-metallic materials

d) All line circuits are double cut to ensure that an earth fault in one leg of the circuit cannot cause false operation

e)Additional earth leakage detection equipment is used

f) In most dc. traction supplies there is a significant a.c. ripple from the rectifiers. Use of a.c. circuits would not provide immunity from the effects of this ripple

3.3         Track Circuits 

AC track circuits use vane relays which have self immunity to the effects of DC. Even where the traction supply may contain a.c., the two element vane relay will only operate if the interference signal is at the correct phase relationship. The relay will not respond to higher harmonics unless these are contained in the correct proportions in the supply to both coils.

Reed and TI21 track circuits and several other audio frequency track circuits are also immune to the effects of DC.

3.4  Signals 

No special precautions are taken for signal lighting circuits. These are usually a.c. fed with signal head transformers. Search-light signals are operated by a.c. vane type mechanisms, which, like track relays, have self immunity to DC.

3.5  Points 

No special precautions are taken in the choice of machine for the control of points. Although a.c. machines might be considered to have better immunity, d.c. operation is normally acceptable at higher voltages (110-130 volts) together with adequate earth leakage detection.

Clamp locks, having separate valve and motor circuits can only be falsely operated by two simultaneous faults.

Detection of points may be achieved with a.c. circuits using vane type three position relays. Alternatively, 110 volts a.c. may be used from the detector to the location where each individual circuit is transformed and rectified to operate a 50 volt d.c. line relay.

4.         MIXED TRACTION SYSTEMS

 There are some areas where both traction systems are in use, either using the same tracks or an adjacent track. This means that the signalling system has to be immune to both AC and DC traction supplies. Circuits used must operate on a frequency distinct from the mains frequency and its harmonics. 

4.1.       Limits

Although the d.c. immunity is required for a greater distance, it is normal to dual immunise for approximately 3km.

4.2  Line Circuits 

Circuits are usually d.c. with precautions taken as for a.c. and d.c. lines.

4.3 Track Circuits 

Formerly, the most popular method was to use a.c. two-position vane relays but operated from an independent power supply at a frequency different to the mains supply and its harmonics (normally 83.3 Hz for 50 Hz mains). The additional power supplies were a significant added cost. Modern audio frequency track circuits such as the TI21 are immune to both d.c. and a.c. traction.

4.4  Signals

The circuits are identical to those used in AC traction areas. Searchlight signals are immunised by using an AC vane type mechanism with a maximum length of 55 metres for the feed circuit. If signals are more than 55meters  apart on either side of the relay room or location an isolating transformer must provided in the feed circuits of all signals on one side of the relay room/location.

4.5 Points

The methods of operating points are identical to those used in areas of AC traction.

Detection of all types of point is either by a two element vane relay from an independent supply at a separate frequency, or by a filtered circuit such as the GEC vital Reed system. Frequencies 477.5 Hz and 414.75 Hz (type RR 4000) have been allocated for use with point detection circuits on B.R. and are employed for Normal and Reverse detection respectively.

5.  PROTECTION AGAINST OTHER FORMS OF INTERFERENCE 

Apart from traction, adjacent power supplies and lightning are the main sources of electrical disturbance.

Power transmission lines generate the same type of interference as a.c. traction and protection is therefore similar.

Lightning protection is a very specialized area and will only be covered in outline.

A direct lightning strike produces voltages far higher than the worst traction fault conditions. It is virtually impossible to provide effective protection. When lightning finds a path to earth, it will however raise the voltage at the point at which it enters the earth. Currents will flow which, although of extremely short duration, can induce large voltages in adjacent equipment. The danger is that the voltages may be large enough to find a path through the signalling equipment or break down insulations in cables etc.

Fortunately, protection can be provided against this effect by the prov1s10n of surge arrestors. The most common type is a gas discharge tube which has three electrodes, one connected to each leg of a loop circuit. and the other to earth. For the normal operating voltages of the signalling circuit, it will appear as an open circuit. When the voltages induced by lightning are sufficiently high, an arc will form in the gas discharge tube which will provide a low impedance, high current capacity path to earth for the duration of the lightning strike. When the current is insufficient to sustain the arc, the surge arrester will revert to its open circuit state.

Semiconductor devices are also available to operate in a similar manner. Their switching time is much faster than a gas discharge tube, giving quicker protection, but their current capacity is much lower. They provide better protection against moderate strikes but may not be able to handle the current caused by severe strikes.

Both types of device are also suitable for protecting vulnerable equipment (e.g. electronic track circuits) from traction fault conditions where required.

6.  FUTURE DEVELOPMENTS AND PROBLEMS 

We have concentrated so far on interference caused by the supply. This is always at the mains frequency or harmonics of it and these frequencies can therefore  be filtered or avoided as appropriate to the type of equipment.

Electric traction units have in the past invariably employed d.c. traction motors. These do not produce any different interference to that already produced by the power supply.

Many railways are now employing traction units either with d.c. motors and variable frequency thyristor chopper controllers or more recently, ac. variable frequency induction motors. The control equipment for these types of traction unit can produce interference at many (often continuously variable) frequencies over a broad spectrum.

It is impossible to detail specific problems but the signal engineer must be very careful in the future as new traction units are introduced that he is aware of their possible effects on the signalling equipment. In Part 2 we will cover Electronic Interference

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CH1 | THE PURPOSE OF SIGNALLING

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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.

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Deepu Dharmarajan - Posted 3 years ago

CH2 | BASIC SIGNALLING PRINCIPLES | PART 1

SIGNALLING 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...........

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Deepu Dharmarajan - Posted 3 years ago

CH3 | SIGNALLING A LAYOUT | PART 1

SIGNALLING 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...........

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