General Info of TC

This document is the property ofRailtrack PLC. It shall not bereproduced in whole or in part withoutthe written permission of the Controller,Railway Group Standards,Railtrack PLC.

Published bySafety & Standards Directorate,Railtrack PLC,Floor DP01, Railtrack House,Euston Square,London NW1 2EE

Copyright 1998 Railtrack PLC

Railway Group Approved Code of PracticeGK/RC0752Issue TwoDate December 1998

GeneralInformation onTrack Circuits

SynopsisGeneral information to ensure that theIntegrity of Track Circuits ismaintained at all times.

Approved by

Keith TurnerStandards Project Manager

Authorised by

Richard SpoorsController, Railway Group Standards

Withdrawn Document Uncontrolled When Printed

jtrousdaleText BoxSignatures removed from electronic version

jtrousdaleText Box

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General Information on Track Circuits

R A I L T R A C K 1

Railway Group Approved Code of PracticeGK/RC0752Issue

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ContentsSection Description Page

Part AIssue Record A1Distribtuion A1Health and Safety Responsibilities A1Supply A1

Part B1 Purpose B12 Scope B13 Glossary of Terms B14 Limitations B55 Introduction B66 Electrical Behaviour of Railway track B77 Operation and Adjustment of the Simple Track Circuit B98 Train Shunt Imperfection B109 Detection of Lightweight Vehicles B12

10 Track Circuit Insulations B1311 Bonding B1612 Mutual Interference Between Track Circuits B1813 Detection of Rail Breaks B1914 Jointless Track Circuits B2015 Track Circuits and Electric Traction B2116 The Impedance Bonds B24

Part C Schematic Symbols1 Introduction C12 Drawing Symbols on Bonding Plans C13 Traction Return Bonding Symbols C84 Civil Engineers Scale Diagrams C10

Part D Planning and Design1 Introduction D12 Responsibilities for Bonding Design D13 Track Circuit Nomenclature D24 Choice of Track Circuit Type D35 Cut Sections D56 Operating Times D67 Track Circuit Interrupters D88 Length of Track Circuits D89 Track Circuit Gaps and Staggered IRJs D9

10 Selective Operation of Track Circuits D911 Bad Rail Surface D912 Emergency Crossovers D1013 Insulated Rail Joints and Bonding D1014 Track Circuit Equipment Positioning D1915 Layout and Wiring of Lineside Apparatus Housing D1916 Duplicate Rail Connections D2017 Communications D21



2 R A I L T R A C K

Railway Group Approved Code of PracticeGK/RC0752Issue TwoDate December 1998Page 2 of 2

Section Description Page

Part E Components and Installation1 Introduction E12 Responsibilities for Bonding Installation E13 Track Circuit Interrupters E24 Identification of Track Circuit Boundaries E35 Protection of Cross Track Cables E36 Mechanised track Maintenance E67 Rail Drilling E68 Rail Connections E79 Track Circuit Disconnection Box E17

10 Arrangement of Track Lead Rail Connections(Except Jointless) E19

11 Fishplate Bonding E2112 Jumper Bonding E2313 High Voltages E2414 Lineside Apparatus Housing Wiring E2415 Impedance Bonds E2516 Impedance Bond Installation E3017 Aluminium Busbars E3718 Side Leads E4619 Traction Negative Return Jumpers E52

Part F Instrumentation

Description and Use1 Introduction F12 Multimeters F13 The Universal Shunt Box F14 Rail Clip Insulation Tester F25 Track Circuit Fault Detector F46 Mark 4 Direct Reading Phase Angle Meter F5

Part G Testing and Commissioning1 Introduction G12 High Voltages G13 Lineside Apparatus Housing Inspection G14 Bonding Inspection G15 IRJ Inspection G26 Performance Test G2

Part H Maintenance1 Introduction H12 Routine Examination H13 Drop Shunt Test H14 Full Test H2

Part J Fault Finding1 Introduction J12 Categories of Failure J13 Intermittent Failures J14 Right Side Failures J25 Wrong Side Failures J4

References Ref 1



R A I L T R A C K A 1


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Part AIssue Record

This Approved Code of Practice will be updated when necessary by distributionof a replacement Part A and such other parts as are amended.

Amended or additional parts of revised pages will be marked by a vertical blackline in the adjacent margin.Part Issue Date Comments

Part A One August 1994 Original document.Part B One August 1994 Original document.Part C One August 1994 Original document.Part D One August 1994 Original document.Part E One August 1994 Original document.Part F One August 1994 Original document.Part G One August 1994 Original document.Part H One August 1994 Original document.Part I not used.Part J One August 1994 Original document.References One August 1994 Original document.

Part A Two December 1998 Revised document.Part B Two December 1998 Revised document.Part C Two December 1998 Revised document.Part D Two December 1998 Revised document.Part E Two December 1998 Revised document.Part F Two December 1998 Revised document.Part G Two December 1998 Revised document.Part H Two December 1998 Revised document.Part I not used.Part J Two December 1998 Revised document.References Two December 1998 Revised document.

DistributionControlled copies of this Approved Code of Practice should be made availableto all personnel who are responsible for the design, installation, testing,maintenance and faulting of Track Circuits.

Health and SafetyResponsibilities

In issuing this Approved Code of Practice, Railtrack PLC makes no warranties,express or implied, that compliance with all or any Railway Group Standards orCodes of Practice is sufficient on its own to ensure safe systems of work oroperation. Each user is reminded of its own responsibilities to ensure healthand safety at work and its individual duties under health and safety legislation.

SupplyControlled and uncontrolled copies of this Approved Code of Practice may beobtained from the Industry Safety Liaison Dept, Safety and StandardsDirectorate, Railtrack PLC, Railtrack House DP01, Euston Square, London,NW1 2EE.


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R A I L T R A C K B 1


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Part B1 Purpose

This Approved Code of Practice gives details of best practice in respect of trackcircuits in general, in order to achieve the requirements of GK/RT 0251.

2 ScopeThe contents of this Approved Code of Practice apply to all track circuits.

3 Glossary of TermsThe definitions of terms used by Signal Engineers vary depending on thelocation in which they were trained. The following terms will be used as standardthroughout this handbook.

BearerAn item of steel or concrete of nonstandard dimensions used to support thetrack in S & C areas (see Sleeper and Timber).BondingThe electrical connection from one rail or part of a track circuit to any other rail orpart of the track circuit.

Cross BondA traction bond cross connecting the traction rails of parallel tracks to form amesh of alternate paths for traction return current.

Fishplate BondProvided to ensure electrical continuity between two rails mechanicallyconnected by a steel fishplate.

Impedance BondSpecial device which presents a low impedance to traction current and a higherimpedance to track circuit current.

Parallel BondingIf any section of a track circuit is bonded in parallel to other sections of that trackcircuit, a disconnection will not cause the track circuit to indicate the presence ofa train. The actual presence of a train within that section may not be indicatedunder certain failure conditions. This method of bonding is defined as ParallelBonding and is the nonpreferred method of bonding. Where it cannot beavoided, special precautions must be taken (see individual Sections).Red BondA traction bond that has been designated by the Electrification Engineer as beingdangerous to staff if disconnected. It is coloured red for identification. TheElectric Control Room shall be advised whenever a disconnected red bond isobserved.

Series BondingSeries bonding is where the track is bonded together in series, so that if anyshort circuit or disconnection occurs, the track circuit will indicate the apparentpresence of a train. It is the preferred method of bonding.

Structure BondA bond that connects adjacent lineside metal structures to the traction return railsystem to ensure staff safety through equipotential zoning.

Traction BondA cable specifically provided for continuity of traction current return, although itmay additionally carry track circuit current.



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Traction Return BondingThe bonding required to carry the traction return current on both a.c. and d.c.electrified lines. Traction return bonding is generally parallel bonded.

Transposition BondA jumper cable where track circuit polarities and/or traction return rails areswitched across a pair of IRJs.

Yellow BondA jumper cable that has been designated by the Signal Engineer to be animportant part of the diverse bonding of common/single rail track circuit. It iscoloured yellow or identified by yellow tape. Damage must be reported andrepairs carried out as a matter of priority.

Clearance PointThe minimum distance from switches and crossings at which track circuitshaving the function of proving clearance may be terminated to ensure a passingclearance of at least 457mm between vehicles in all circumstances.

Common Rail (CR)A track circuit arrangement where only one rail (the signal rail) is used with IRJsto separate the track circuits. The other rail (the common rail) is electricallycontinuous but is not used for traction return purposes.

Cut SectionA method of reducing the continuous length of a track circuit by the use ofindividual track circuits, each one controlling the same final TPR. These areindicated as one track circuit on the signalmans panel.

Double Rail (DR)A track circuit arrangement where both rails are fitted with IRJs, or tuned zonesare used to completely isolate a track circuit.

Dropaway TimeThe time between the application of a shunt to the rails and the front contacts oftrack relay (TR) fully opening (see also Pickup Time).Drop ShuntThe maximum value of noninductive resistance which, when placed across therails, will cause the track relay to fully open its front contacts.

FishplateMetal plates for joining rails together.Frequency RotationThe sequential application of specified frequencies.

Insulated Rail Joint (IRJ)A method of joining rails together whilst maintaining electrical insulation betweenthem.

Jointed Track CircuitsTrack circuits whose extremities are defined by the use of Insulated Rail Joints(IRJs).Jointless Track CircuitsTrack circuits whose extremities are defined by the use of tuned circuittechniques. The extreme limits of a jointless track circuit area are either definedby the use of IRJs or by the use of a tuned circuit between the rails.





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Joint HoppingWhere fast moving short vehicles pass from one track circuit to the next, thedifference between the pickup and dropaway times can cause the vehicle tomomentarily disappear.

Jumper CableUsed to electrically connect, for track circuit or traction purposes, two pieces ofrail that are not adjacent.Overlay Track CircuitA track circuit which can be superimposed over another, neither having anyeffect on the other and both operating independently.

Pickup ShuntThe minimum value of resistance between the two running rails at which thetrack relay will just close its front contacts.Pickup TimeThe time between the removal of a shunt from the rails and the front contacts ofthe track relay making (see also Dropaway Time).PlansFor the definition of all types of Plans, see SDH E11.

Selective OperationOperation of a portion of a track circuit by selection of the position of a set ofpoints. Selective operation of track circuits is no longer permitted.

Single Rail (SR)A track circuit arrangement where only one rail (the signal rail) is used with IRJsto separate the track circuits. The other rail (the common rail) is electricallycontinuous and is used for traction return purposes.

SleeperAn item of wood, steel or concrete of standard dimensions, used to support andgauge the track (see Bearer and Timber).SpurA section of running rail required to be electrically common to a series bondedrail, but which is not itself in series.

Stagger (Electrical)The phase or polarity difference between one track circuit and the next, orbetween the rails on either side of an IRJ within one track circuit.

Stagger (Physical)Occurs where two IRJs in a pair of rails are not exactly opposite each other,thus creating a dead section between track circuits or within a track circuit.

Switches & Crossings (S & C)Sections of track other than plain line.

Tail CableThis is a cable which connects the lineside apparatus housing to the tracksideequipment, but not direct to the running rails (see Track Cable).TimberAn item of wood of nonstandard dimensions, used to support the track in S & Careas (see Bearer and Sleeper).





Track CableThis is a cable which connects the track disconnection links/fuses or tracksideequipment to the rails.

Track JumpingOccurs when a fast moving vehicle passes over a very short track circuit (or ashort arm of a longer track circuit) and fails to deenergise the track relay.Track Circuit InterrupterA device that detects the passage of a vehicle by causing a permanentdisconnection within the track circuit until the device has been replaced.

Transposition JointAn IRJ where transposition bonds are used to transpose the traction and/ortrack circuit rails.

Catch or Trap PointA switch (ie. blades and tiebar only), inserted in sidings etc., to divert runawayrolling stock away from the main line, or on gradients to derail runaway wagonsetc.

CrossingThe inter section of two tracks on the level. Often known as a diamond crossingdue to the shape produced by the intersecting tracks. Not to be confused withthe crossover.

Switch ToesSwitch Rails Stock Rails

Heel Of Switch Rail

Closure Panels

Check RailClosure Rails

Wing Rails

Crossing Nose

Rail Joint

Crossing Angle

Crossing Back

Figure B1

Switch and Crossing TermsCrossoverA crossover consists of two points arranged to link parallel tracks. They areknown as facing or trailing, depending on whether a train proceeding in itscorrect direction along the line can run directly over the facing crossover, ormust reverse to cross the trailing crossover.

Double JunctionThe point of junction of two double track routes. It comprises two turnouts and acrossing.

Ladder JunctionA form of junction eliminating the crossing.

4 LimitationsWhere job titles are used within this Approved Code of Practice to reflect theanticipated functional splits of responsibility relevant to technical competence,they should not be interpreted as actual job titles. The specific split of





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responsibility will be governed by a contractual framework, to which referenceshould be made.

Catalogue Numbers shown within this document are not directly controlled byRailtrack and as such, will not be maintained and kept up to date. Althoughevery effort has been made to ensure that these were correct at the time ofpublication, it is therefore recommended that your supplier is contacted and acheck is made with regard to the accuracy of these catalogue numbers prior touse.

Where references are made to other documents, a comprehensive list of thesewill be contained within the Ref section of this document. The informationappertaining to these references was correct as of Issue 13 of the RailtrackCatalogue of Railway Group Standards.

5 Introduction5.1 The Purpose of Track CircuitsThe track circuit is a device designed to continuously prove the absence of atrain from a given section of track; it cannot absolutely prove the presence of atrain, since its designed failure mode is to give the same indication as if a train ispresent.

By proving the absence of a train, a clear track circuit can be used to confirmthat it is safe to set a route and permit a train to proceed.

5.2 Fundamental Design PrincipleA section of railway track is electrically defined by the provision of insulated railjoints (IRJs), or equivalent, in the rails at either end as shown in Figure B2. Asource of electrical energy is connected, via a series impedance, across the railsat one end and a detector, which is receptive to the particular form of electricalenergy, is connected across the rails at the other end.

InsulatedRail Joints

Detector(Relay)

Transmitter(Feed)

Figure B2

With no train within its boundaries, the detector senses the transmitted electricalenergy and energises the repeater circuit. This conveys the absence of a train tothe signalling system (ie. track circuit clear).A train within the track circuit will cause the rails to be short circuited such thatthe detector no longer sees sufficient electrical energy; it therefore changesstate and informs the signalling system (ie. track circuit occupied).It can be seen that an electrical short circuit between the rails, caused other thanby a train, or any disconnection within the circuit, will fail the track circuit andinform the signalling system that the track circuit is occupied. Such a circuitconfiguration incorporates a high degree of fail safe; it does, however, dependupon good electrical contact between the wheel sets of the train and the rails





upon which they run. It also depends upon a continuous low impedance pathbetween the steel tyres of each wheel via the connecting axle.

Track circuits apply this basic principle in a variety of ways for various reasons.The source of electrical energy may be d.c., a.c. at power frequencies, a.c. ataudio frequencies, or a series of impulses. The detector may be a simple relay,a more complex a.c. vane relay or a receiver tuned to a particular frequency orpattern of signals. Additional items may have to be added to overcome theproblems arising from sharing the rails with heavy currents created by an electrictraction system.

6 Electrical Behaviourof Railway Track

6.1 Ballast ResistanceBallast resistance is the resistance between the two rails of a track circuit andcomprises of leakage between the rail fixings, sleepers and earth. The value ofthis resistance is dependent upon the condition of any insulations, cleanliness ofthe ballast and the prevailing weather conditions. The ballast resistance isinversely proportional to track circuit length and is expressed as ohm kilometres,typical values being in the range 2 to 10km. Lower values may be obtained inwet conditions with bad drainage and/or contamination with conductive materials.Higher values may be obtained in dry/clean conditions or during frosty weather.A reliable track circuit must therefore be able to operate over a wide variation ofballast resistance.

Most simple explanations of track circuit operation portray ballast resistance as asingle resistance connected between the rails as shown in Figure B3. Whilstsuch a representation is useful in explaining the simple behaviour of d.c. trackcircuits, it is important to understand that the models limitations make itunsuitable to explain many of the more complex phenomena demonstrated bytrack circuits. For the types of track circuit used, the reactance of the ballastcan be considered as negligible.

Rail

Rail

BallastResistance

Figure B3

When considering other than the simple case, a more accurate model wouldrepresent the ballast resistance as a series of resistances between each rail andearth as shown in Figure B4. Although there is a further component ofresistance between the rails independent of earth, it is high compared to the railearth resistance and can be discounted for most calculations.





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Rail

Earth

Rail

Figure B4

6.2 Rail ImpedanceThe d.c. resistance of rail is very low, around 0.035/km, although this isincreased to approximately 0.25/km by the relatively higher resistance ofgalvanised iron bonds in jointed track. The inductance of rail can raise theoverall impedance per rail from approximately 0.3/km (50Hz) to, in the case ofreed track circuits, 2.5/km (400Hz) and for TI21 track circuits, 10/km (2kHz).These impedance values may be increased further by large traction currents,due to the rail being driven toward saturation.

When considering a.c. track circuits, rail inductance must be taken into accountby application of the further complex model including rail inductance as shown inFigure B5. Although of little consequence at power frequencies, audio frequencytrack circuits exhibit a steep decline in rail voltage as distance from thetransmitter increases. Since the ballast resistance is now distributed throughoutthe length, detailed calculation requires the use of hyperbolic functions.

These effects can usually be ignored when considering the operation of a.c.power frequency track circuits, where rail voltage can be expected to declinevery little between the feed and relay ends.

Rail

Earth

Rail

Figure B5

6.3 Rail to Rail CapacitanceAlthough an even more complete picture would include railtorail capacitance,this is very small and of marginal significance relative to track circuit operation ataudio frequencies.

6.4 Workable Lengths of Track CircuitsIt can be seen that the workable length of a track circuit is limited by threefactors:

the declining value of ballast resistance; the increasing value of rail impedance;





Immunisation / Electrification requirements, including electromagneticcompatibility with trains.

As the various types of track circuit feed/transmitter produce differing poweroutputs, and as rail impedance is frequency related, it follows that the maximumworkable length will vary with design type and with the minimum ballastresistance at which the track circuit is expected to remain functional.

7 Operation andAdjustment of the

Simple TrackCircuit

Consider the simple d.c. track circuit depicted in Figure B6.

FeedResistance

CableResistance

CableResistance

CableResistanceCableResistance

Rail

Rail

BallastResistance

TrainShunt

TR

Figure B6

7.1 Track Circuit ClearThe ballast resistance forms an additional load in parallel with the relay. As theballast resistance falls due to wet weather, the current drawn from the feedincreases. This will cause the voltage across the feed resistor to increase, soreducing the rail and relay voltages. If this reduction causes the relay voltage tofall below the relay pickup value, the track circuit will not clear after anoccupying train has departed. A further reduction of the relay voltage to belowrelay dropaway value will fail the track to the occupied state without thepassage of a train.

Reducing the value of feed resistance has the effect of increasing the currentfed into the rails and raising the rail/relay voltage.

Long feed end leads insert additional nonadjustable feed resistance andthereby reduce the effectiveness of the adjustable feed resistance. Long relayend leads reduce the ratio of relay voltage to rail voltage by potential divideraction; the effect is to cause the track circuit to indicate occupied at a higherballast resistance. It therefore imposes a shorter maximum workable length.

7.2 Track Circuit OccupiedWhen the track circuit is occupied by a train, a short circuit current will flow fromthe feed end equipment, which is limited by the value of the feed resistance andthe characteristics of the feed end equipment itself. The feed end equipment isdesigned to cope with this worst case power dissipation.

The train shunt resistance is in parallel with the ballast resistance. With anygiven value of feed resistance, the relay will operate at particular values ofcombined ballast/train shunt resistance. Thus, higher ballast resistance willrequire a lower value of train shunt resistance to operate the relay and viceversa.





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The minimum permitted drop shunt resistance is 0.5 (0.3 on certainimpedance bond track circuits). During very dry weather or severe frostconditions, the ballast resistance increases towards its natural maximum and willoffer only a small contribution towards the overall shunt. Thus, when a 0.5(0.3) shunt is placed across the rails, it must still reduce the relay voltage tobelow dropaway value.

It should also be noted that the track relay is dropped by short circuit rather thandisconnection. Therefore, the dropaway time of the relay is increased due tothe inductive circuit prolonging the decay of the coil current.

7.3 Principles of Basic AdjustmentThe difficulty with adjusting track circuits (where such adjustment is provided) isknowing the prevailing value of ballast resistance. Details entered on the trackcircuit record card provide a useful history. These vary with track circuit typeand the appropriate Code of Practice within the Track Circuit Handbook shouldbe consulted.

Assuming average conditions, the feed resistance is adjusted to obtain a relayvoltage in the range 25% to 75% above the pickup value whilst maintaining thedrop shunt resistance at a value greater than the minimum required. If the trackcircuit fails due to wet weather, it may be possible to remedy the situation byreducing the feed resistance. It is important that the track circuit is retestedafter it has dried out.

8 Train ShuntImperfection

The energy seen by the relay with a train on the track circuit will depend uponthe resistive value of the train shunt (see Figure B6). This energy will be zeroonly when the train shunt is zero. Whilst the ohmic resistance of an axle andwheels is virtually zero, there are a number of factors that can make theeffective train shunt much higher. Since some factors are track based, whilstothers are vehicle specific, the precise mixture of factors applying to a particularvehicle at a particular place can be very variable.

8.1 Rust FilmsLight rust film on the rail head and/or tyre tends to act as a semiconductor, inthat it exhibits high resistance until the voltage exceeds a particular thresholdvalue when it breaks down completely. The breakdown voltage rises insympathy with the extent of the contamination; very heavy rust films, resultingfrom prolonged disuse, render many track circuit designs incapable of detectingvehicles. Figure B7 gives an approximate characteristic of such films.

Current VeryGood

Good Poor Bad

DampLight Rust

DryLight Rust

Heavy RustOr

Leaf Residue

Voltage0.05V 0.1V 0.3V 0.6V - 200V

0.01V

Figure B7



B 1 0 R A I L T R A C K


The mechanical strength of light rust films is much reduced by the presence ofmoisture when the contaminant tends to be squeezed out from the wheelrailcontact patch and doesnt cause any shunting problems. Therefore, lightlyrusted rails will only be a problem when dry. This problem is most severe whenconditions are:

showery weather accompanied by drying wind; or prolonged periods without trains.

Special precautions need to be taken after relaying operations, when trackcircuits must not be restored to full operation until a reasonable surface hasbeen created.

8.2 Leaf ResidueThis problem is confined to particular areas where the extent of linesideafforestation is significant. It is also limited to autumn when trees are sheddingtheir leaves. Leaves are drawn into the wheelrail interface by the passage of atrain, where they are squashed into a pulp which contaminates both rail and tyre.The severity of this problem in particular years is connected to the generalweather situation. In simple terms, reasonably dry weather with little wind willcause the leaves to fall gradually over a long time period and to be reasonablysap free when they do fall. Conversely, gale conditions will lead to a suddenmassive fall of sap laden leaves. It is the latter situation which gives rise to theworst conditions.

In terms of track circuit operation, the electrical characteristics of severe leafresidue are similar to very heavy rust. Fortunately, the sites suffering suchproblems are generally known and special arrangements can be made.

8.3 Coal Dust and SandProblems with coal dust on the rail head tend to be confined to colliery areas,where coal deposited on the wagon chassis after loading/unloading issubsequently shaken off. Sand contamination is usually associated with slowmoving locomotives using their sanders excessively.

In each case, the effect is similar to heavy rust films.

8.4 Composition Tread Brake BlocksCertain types of rolling stock are fitted with a composite type of tread brake blockinstead of the traditional cast iron variety, the intention being to improve brakeperformance. This is found to deposit a contaminant film on the steel tyre, whichtends to insulate the train from the rails.

8.5 Tread and Disc BrakesWhen considering the electrical contact between two pieces of metal separatedby a thin film of insulation, it can be appreciated that surface roughness of themetal can permit high spots to penetrate the film. Where this occurs, theinsulation will be ineffective.

Tread brakes cause the tyres to be roughened at each brake application,whereas disc brakes allow the tyres to be rolled into a very smooth surfacecondition. This can be observed visually as tread braked tyres have a mattappearance, whilst disc braked tyres show a mirrorlike quality.

Therefore, tread braked vehicles provide a better train shunt than disc brakedvehicles.

8.6 Axle Weight and Suspension DesignThe pressure applied to any contaminant film is proportional to the downwardforce of the wheel on the rail and this is proportional to vehicle axle weight.



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In practice, wheels do not roll smoothly and friction free. There is a guidanceforce continually pulling the wheelset into the correct trajectory and this guidanceforce is associated with microscopic slippage between wheel and rail. Advancesin bogie design have tended to reduce this guidance force and slippage, giving asmoother ride for the passenger as well as reducing the wear rate of both railand tyre.

Unfortunately, these qualities reduce the ability of the tyres to penetrate any film,as well as reducing their ability to clean the rail by abrasion.

8.7 Track GeometryVehicle guidance force and wheel rail slippage are increased in curved track.Therefore, train shunt will be improved when the vehicles are travelling oncurved track.

9 Detection ofLightweight

VehiclesIf all rails and tyres were clean and wheelrail contact was perfect, any type ofvehicle would satisfactorily operate any type of track circuit. However,secondary lines in particular have suffered a fall in traffic leading to regularformation of light rust films. At the same time, the vehicles using such lines havebeen increasingly of the modern DMU variety, which magnify the train shuntdifficulties because of their suspension design, brake type, weight (which,although still heavy, is relatively light) and small number of vehicles in a train.When a vehicle is static on a light rust film, the track circuit voltage will usuallybreak it down and the track circuit will occupy. This is because the track clearrail voltage is higher than the film breakdown voltage. However, when thatvehicle is moving, the wheels are continually rolling onto new film which requiresto be repeatedly broken down. Consider the following sequence of events:

When a wheel first enters the track circuit, the track clear rail voltage ispresented across the film. The film breaks down resulting in the rail voltagecollapsing towards zero.

As the wheel moves on to new surface, there is insufficient voltage available tobreak through the new film. The train shunt is removed and the rail voltage risestowards the clear value.

When the rail voltage attains the breakdown level, the film is punctured, the trainshunt reapplies and the rail voltage once again plummets toward zero.

The result is a high frequency noise voltage across the rails which can beobserved with a suitably sensitive instrument.

Where the threshold breakdown voltage is less than the rail voltage at which therelay drops away, the noise will not result in track circuit malfunction. Thisparameter is used to assess the performance of various track circuit typesrelative to their ability to detect lightweight vehicles.

To assist vehicles to shunt track circuits, a device known as the Track CircuitAssister has been fitted to modern diesel multiple units.





10 Track CircuitInsulations

10.1 Insulated Rail JointsInsulated rail joints (IRJs) are required to join rails together mechanically but notelectrically. The Permanent Way Engineer is responsible for the installation andmaintenance of all IRJs.

10.2 Point EquipmentApart from the IRJs, used to electrically separate sections of rail, the reliableoperation of track circuits requires the provision of other insulations in particularcircumstances.

Any direct metallic connection between the two rails will be interpreted as a trainand will cause the track circuit to fail occupied. At a set of points, there aremany of these connections, which therefore need to be fitted with insulations, asshown in Figure B8, which is a typical example; there are, however, someregional variations.





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Insulation Insulation

Insulation

B

C

D

A

E

A SoleplateThe soleplate is formed from two metal plates secured together by a bolted connection at an intermediateposition between the rails, which includes insulated ferrules, washers and plates to maintain electricalseparation.Where the soleplate is extended to one side, as required for point machine operation, a second insulatedconnection is provided between the point machine and the nearest rail.

B Permanent Way Stretcher BarsThese connect the two point switches together and are formed from two separate pieces connectedtogether with two bolts. The bolted connection includes insulation ferrules, washers and plates tomaintain electrical separation.

C FPL Stretcher BarInsulation ferrules, washers and plates are fitted where the stretcher bar is connected to one of the pointswitch blades; usually that furthest from the drive mechanism. The design is such that the insulation canbe fitted at either end of the stretcher bar, but should not be fitted at both ends.

D Point Drive RodInsulation is provided either separately, or is incorporated into the drive rod jaw connection onto the pointmachine.E Lock & Detector RodsInsulated bushes are fitted where the screwed end connections are attached to the switch extensionpieces.

Figure B8





10.3 Concrete SleepersEarly forms of concrete sleeper were fitted with chairs for bullhead rail in similarfashion to those fitted to timber sleepers. The chair was usually secured to thesleeper with a through bolt from the underside. These did not present anywidespread problem since track circuits were not common in the rural areas,where concrete sleepers were seen to be advantageous.

Although short track circuits can be made to work over such sleepers, the ballastresistance is usually quite low and subject to more severe weather relatedswings. It is also now known that damp concrete behaves as an electrochemicalsecondary cell which can give rise to residual voltage problems with d.c. trackcircuits.

Modern concrete sleepers incorporate a rubber pad under the rail foot andmoulded insulations where the fixings bear on the top of the foot, as shown inFigure B9. The effect is to increase ballast resistance to levels significantlyhigher than those obtained with timber sleepers. However, the insulations doerode due to the vibration of passing traffic and, consequently require periodicalreplacement. Lack of attention to insulation usually results in gradualdegradation of the ballast resistance rather than sudden failure.

Centre Leg

Heelseat Rear Arch

Rail Foot

Rail Pad

Front Arch

Insulation

Figure B9

10.4 Steel SleepersSteel sleepers are equipped with insulations similar to modern concrete sleepersand, provided they are subject to an effective preventative maintenanceprogramme, track circuits will operate satisfactorily.

However, as the sleeper is in more intimate electrical contact with general earth,much higher levels of track circuit unreliability will result from poor insulation thanis the case with modern concrete sleepers.





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11 BondingBonding describes the means by which the individual components of the railwaytrack are connected together electrically for track circuit purposes. The termalso includes the additional electrical connections necessary for the properoperation of electric traction. Symbols used on bonding plans are shown in PartC and various terms are explained in Section 3. Refer also to GK/RT0252.

11.1 Series and Parallel BondingIn order for a track circuit to fail safe (to show occupied) in the event of abonding disconnection, it is necessary to bond all elements of the track circuit inseries. However, in S & C areas, it may not be physically possible to arrangetotal series bonding of both rails. Examples of series and parallel bonding areshown in Figure B10.

Provided that a spur is very short, it is permissible to bond it in parallel withoutadditional safeguard. However, where the spur is long, or in other cases wherenecessary, parallel bonding may be resorted to provided that steps are taken toensure that vehicles are not lost due to disconnection of part of the parallelsystem. This is achieved by creating a mesh of alternative diverse bondingpaths between parallel elements, and clearly identifying the associated bonds bytheir yellow colour. It is necessary to ensure that such yellow bonds arerepaired quickly before other bonds in the mesh have time to fail in a mannerlikely to cause an unsafe failure.

Because of the additional complication of significant rail impedance with parallelbonding, audio frequency track circuits are generally unsuitable in all but thesimplest of pointwork.

Track Feed

Track Relay

Track Relay Track Feed

Non-preffered Parallel Bonding

Preferred Series Bonding

Figure B10





11.2 Double and Single Rail Track Circuit BondingDouble rail track circuit arrangements have both rails fitted with IRJs tocompletely isolate a track circuit. Impedance bonds are used when a tractioncurrent return path is required. IRJs and impedance bonds are not required withJointless Track Circuits.

Single rail track circuit arrangements have only one rail fitted with IRJs toseparate the track circuits. The other rail is electrically continuous. If thiscontinuous rail is used for traction return purposes, the bonding arrangement iscalled Single Rail Bonding. If this continuous rail is not used for traction returnpurposes, the bonding arrangement is called Common Rail Bonding.

Whilst some designs of track circuit can be used in either single or double railmode, others are limited to double rail application.

In some S & C areas and certain electric traction areas, it is necessary for oneor more adjacent track circuit to share one common rail. This arrangement canlead to unsafe failure modes unless special steps are taken to ensure thatelements of the common rail cannot become isolated from the remainder. This isachieved by creating a mesh of alternative diverse bonding paths and markingthe associated bonds yellow as for the previous parallel bonding case.

11.3 Track Circuit InterruptersTrack circuit interrupters are used at trap or runback catch points on lineswhich are track circuited. The device is designed to interrupt the track circuit inthe event of a rail vehicle leaving the track. This prevents automatic reenergisation of the track circuit after the removal of the train shunt.

The interrupter is a metal device attached to the four foot side of the stock railand usually insulated from it. It comprises a main body, a narrow neck and ahead which is adjacent to the running edge and designed to break off when a railvehicle passes over it. Connections are made to the head and the body suchthat electrical continuity is provided between them until the interrupter is broken.The arrangement is shown in Figure B11.

The interrupter is fixed within the track circuit to the stock rail (as shown inFigure B12). It is not fixed to the switch rail.

BondingConnectionPoints

Figure B11





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Note: In all cases, the interrupter is fixed to the stock rail (as shown in FigureB12). It must not be fixed to the switch rail.

Correct Position

Incorrect Position

Stock Rail

Switch Rail

Figure B12

12 Mutual InterferenceBetween Track

CircuitsIt is important to realise that where track circuits are connected together by acommon rail, and detectors are used that are unable to discriminate betweentheir own and other track circuit feeds, a degree of mutual interference betweensuch track circuits is inevitable. This condition may be introduced by design(single rail track circuits on electrified lines) or by failure (IRJ failure of double railtrack circuits).The simplest way to describe mutual interference is to use the example of twod.c. single rail track circuits as shown in Figure B13 and Figure B14. It will berealised that this model also equates to a double rail insulated track circuit with afailed IRJ. Figure B13 shows the equivalent circuit in track circuit type format,whilst Figure B14 converts it to a standard electrical format for easierpresentation of cause and effect.

Consider the circuit in Figure B14 under conditions where VFB feed supply isdisconnected. Clearly, a voltage will appear across RRB as a result of VFA, thevalue of which will depend on circuit parameters. The extent to which thisvoltage is of concern depends upon its value relative to the operating values ofthe relay.

Provided that the bonding remains intact, an unsafe failure cannot arise from themutual interference; either both track circuits will fail right side (occupied) or theywill both show occupied when either one of them is legitimately occupied.However, there is the possibility of a wrong side failure where a bondingdisconnection occurs.

It is not appropriate to explain all possible scenarios here; the possibility ismentioned simply to convey the fact that some track circuit defects can beexceedingly difficult to understand and explain. Certain constraints are appliedto various track circuit designs in order to limit the possibility of wrong sidefailure. Therefore, the design constraints described in the Track CircuitHandbook shall not be breached without expert advice.





Common Rail TC A TC B

RCARCB

RSA RSB

Earth

VFA VFB

RFA RFBRRBRRA

Figure B13

Notes for Figure B13 and Figure B14:

TCA TCB

OPEN CIRCUIT FEED VOLTAGE VFA VFBFEED RESISTANCE RFA RFBRELAY RESISTANCE RRA RRBSIGNAL RAIL EARTH RESISTANCE RSA RSBCOMMON RAIL EARTH RESISTANCE RCA RCB

Common Rail

VFA

RRA

RSA

Earth

RFB

VFBRSB

RCBRCA

Further Track Circuits

RFA RRB

Figure B14

13 Detection ofRail Breaks

Where rails are series bonded, a completely broken rail will be immediatelydetected as a right side track circuit failure (ie. occupied).Where the rails are not series bonded, a broken rail will not be detected by thetrack circuit.





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14 Jointless TrackCircuits

Insulated rail joints can be expensive both to install and to maintain, especially ontracks subjected to high speed, high axle weight traffic or where there is anintensive service.

The use of audio frequencies permits the physical limits of an individual trackcircuit to be defined by tuned short circuits between the rails rather than byinsulation in the rails themselves. Consider two jointless track circuits abutting ata tuned zone as shown in Figure B15. Nontrack mounted equipment has beenomitted for clarity.

TunedZone

Feed F1 Feed F2TuningUnit F1

TuningUnit F2

Figure B15

The tuned zone comprises a measured length of track with a tuning unit acrossthe rails at each extremity. The track circuits operate at different audiofrequencies and each tuning unit is designed to its own track frequency, suchthat the following criteria are obeyed:

a) Consider frequency F1:The F2 tuning unit behaves as a short circuit between the rails, due to seriesresonance of its inductive and capacitive components.The F1 tuning unit tunes the two rails (inductive) and the F2 tuning unit shortcircuit to parallel resonance, thus presenting a significant impedance tofrequency F1.

b) Consider frequency F2:The F1 tuning unit behaves as a short circuit between the rails, due to seriesresonance of its inductive and capacitive components.The F2 tuning unit tunes the two rails (inductive) and the F1 tuning unit shortcircuit to parallel resonance, thus presenting a significant impedance tofrequency F2.

A wheelset proceeding along track circuit F1 will shunt the track circuit, but whenit enters the tuned zone its effectiveness will reduce until, having passed tuningunit F2 (short circuit at frequency F1), it will no longer shunt track circuit F1.Similarly, the wheelset would not shunt track circuit F2 as long as it remained ontrack circuit F1, due to tuning unit F1 presenting a short circuit to frequency F2.As the wheelset passes F1 tuning unit, it commences to shunt frequency F2,becoming more effective as it progresses towards the F2 tuning unit and beyondinto F2 track circuit proper.





By careful design of components, it is possible to arrange a short overlap in thecentre of the tuned zone where both track circuits are effectively shunted.

Since the design of individual tuning units must take account of both frequencies,it is necessary to specify the exact frequencies involved. Such equipment istherefore produced for a fixed set of frequencies and those frequencies areused in pairs alternately along the track.

15 Track Circuits andElectric Traction

Beyond the boundaries of electrified areas, track circuit type and configurationcan be selected on the basis of train detection and economic criteria alone.However, track circuit arrangements in electrified areas are constrained by theneed to ensure safe and reliable operation of both signalling and tractionsystems. This means that the track circuit must be immune to both falseoperation and damage by the flow of traction currents through the rails.

Parallel tracks are cross-bonded at regular intervals, such that the traction returncurrent from an individual train will have a number of different parallel paths backto the supply. This minimises the impedance to the traction supply and hencethe volt drop, whilst it also limits the amount of current which can flow through anindividual track circuit.

Although permitted track circuits will be inherently immune to false operation(wrong side failure) from the presence of traction currents flowing in the rails, insome circumstances these can be of a magnitude sufficient to cause damage toequipment, or right side failure of the track circuit. The levels of traction currentthat the track circuit is subjected to can generally be sufficiently limited by wellmaintained bonding, track circuit length restrictions (single rail track circuits) andbalance of traction currents between rails (double rail track circuits).Specific restrictions related to interference are contained within the individualtrack circuit type Codes of Practice.

15.1 D.C. Electrified AreasIn d.c. electrified areas, the relatively low supply voltage results in high currentsreturning to the sub-stations via the running rails. In order to minimise voltagedrop in the d.c. traction supply, wherever possible, all running rails are used forthe return of traction currents and therefore double rail track circuits are used.However, in S&C areas, it is not usually possible to bond the track in double railform, therefore single rail track circuits have to be installed.

Traditionally, all track circuits in d.c. electrified areas, were operated with 50Hza.c. current, using phase sensitive vane relays. Double rail track circuits, withimpedance bonds providing traction current continuity, were provided on plainline and single rail track circuits in S&C areas. More recently, jointlessmodulated audio frequency track circuits have been introduced, reducing thenumber of IRJs and impedance bonds required in plain line areas.





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15.2 A.C. Electrified AreasIn present 25kV a.c. electrified areas, traction currents are lower than in d.c.systems and in most cases, single rail traction return is sufficient forelectrification purposes. Increased traffic levels and alternative feedingarrangements, may however, increase the need for both running rails to be usedfor traction return.

Traditionally, all track circuits in a.c. electrified areas, were operated with d.c.current, although feed and relay components are specifically modified to provideprotection from damage and immunity to interference.

15.3 Dual Electrified AreasWhere tracks may be subject to the flow of both a.c. and d.c. traction currents,the choice of track circuits is limited to those that are immune to both and do notuse frequencies (including harmonics) contained in the traction supply.15.4 Single Rail Track CircuitsWhere traction return current flows through a single rail track circuit, the majorityof the current will flow in the traction rail, resulting in a voltage drop along itslength. This voltage drop is proportional to the current, the track circuit lengthand the impedance of the rail. With a train shunt applied toward the feed end ofthe track circuit, this voltage drop can be presented across the signal rail andtrack receiver in series. Dependent upon the relative impedance of the signal railand the receiver at the frequencies of interest, a proportion of this voltage will beapplied across the receiver.

If the traction supply contains some voltage disturbance at a frequency to whichthe track circuit is sensitive, then this will be conducted through trains and flowas current through the running rails. If this is of sufficient magnitude, form andduration, then with a train shunt at the feed end, a wrong side failure couldoccur.

In addition to conducting the voltage ripple present on the traction supply,modern traction units employing active control methods (such as three phasedrives) can actively generate currents at other frequencies and superimposethem onto the supply. Whilst the traction control systems can be designed so asto avoid critical frequencies as far as possible, some interference content atfrequencies used by track circuits may be produced. Depending upon the typeof traction unit, the magnitude of this interference content may be limited by theuse of an Interference Current Monitor Unit (ICMU) on the train, which will isolatethe traction unit from the supply if sufficient interference flowing through thetrain, is detected. These ICMUs however, take a finite time to operate, andwhilst the operate delay, due to the use of slow operating repeat relays, isgenerally sufficient to cope with transient interference, it may be necessary tomodify the track circuits before the train can reliably operate.





15.5 Double Rail Track CircuitsWhere both running rails are used for traction current return, the electrificationarrangements, using impedance bonds, are designed to keep the currentsflowing in each rail balanced. Under such conditions, any interference contentwithin the traction current should similarly be balanced and little or nointerference applied to the receiver. However, due to a number of reasons,interference currents flowing through the track circuit may be, or become,imbalanced:

presence of check rails; track curves; earthing of one rail; bonding differences; asymmetric position of conductor rail / catenary; broken rails; disconnected impedance bond sideleads

When the track circuit becomes unbalanced any interference in the tractionreturn current due either to disturbances in the supply, or generated by tractionunits, will result in interference being presented to the receiver. The magnitudeof this interference is largely independent of the length of the track circuit, but isproportional to the imbalance of currents flowing through the receiver end of thetrack circuit (either an impedance bond or tuned zone). Therefore, with a trainoccupying the track circuit, interference can be applied to the track receiver,which if of sufficient magnitude, form and duration, will cause a wrong side failureof the track circuit.

15.6 Rolling Stock CompatibilityMeans of providing compatibility between rolling stock and track circuits, withoutthe use of ICMUs, is preferable and modern traction units may be acceptable foruse with the existing track circuits, if it can be demonstrated that the predictablelevel of interference which may be generated, is insufficient to interfere withcorrect track circuit operation. Such an assessment will need to makereasonable assumptions as to the proportion of traction current that can flowthrough an individual track circuit, the resulting magnitude of interference whichwill be presented to the track receiver and the minimum response time of thereceiver and interlocking. Therefore the validity of such assessments relies uponthe following:

cross bonding between parallel tracks; track circuit length limitations; prevention and detection of imbalance; integrity of rails and bonds; operating times.

CAUTION: Although general precautions and limits that providecompatibility between rolling stock and track circuits, have been includedin the Train Detection Handbook Codes of Practice, these are notcomprehensive and special conditions may apply to certain routes topermit the operation of rolling stock.

Where new types of rolling stock are to be introduced, existing constraintswill require reassessment, as to their adequacy.





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16 Impedance Bonds16.1 OperationImpedance bonds are devices which allow traction current (d.c. or a.c.) to passthrough, whilst limiting the track circuit current. They are necessary whereverdouble rail traction return and IRJ dependent track circuits coexist.

An impedance bond is configured to provide a very low impedance path todouble rail a.c. or d.c. traction return currents (typically less than 0.4m per coil)whilst presenting a high enough a.c. impedance between the rails (typicallygreater than 15) to allow the operation of track circuits. It also provides acentre connection for cross bonding, which minimises the passage of trackcircuit current between circuit currents.

The winding connected between the rails is comprised of heavy gauge copper,fitted with a centre tap connection and wound on a heavy iron core. Providedthat each running rail carries equal amounts of traction return current, thecurrent from each rail passes in opposite directions through the coil from the railto the centre tap connection. The net flux in the iron circuit will be zero and theimpedance to traction current (d.c. or a.c.) will be very small, as shown in FigureB16.

Cross Bond Cross Bond

IRJ IRJ1/2 Traction Current

1/2 TractionCurrent

TrackTransmitter

TrackReceiver

Figure B16

The a.c. track circuit current attempts to flow between the two rails and istherefore in the same direction through the two halves of the winding, resulting inthe track circuit current seeing a larger, albeit still relatively small, impedance.

Tracktotrack cross bonding on double rail track circuits must be provided viathe centre taps of impedance bonds on electrified lines, as shown in Figure B17.

Cross Bonds

Figure B17





16.2 Auxilary WindingThe impedance available from the simple impedance bond remains a handicap.It is therefore usual to enhance the impedance by parallel resonance of thetraction winding, use being made of a stepup (approximately 50:1) transformerto reduce the value of the necessary capacitance to realistic levels. Anothersolution is to connect the resonating winding to form an auto transformer, referto 16.2.2.

16.2.1 Resonated Impedance BondsThe induction of the traction winding is tuned to resonance at or near the trackcircuit operating frequency by use of a parallel capacitor, which raises the rail torail impedance at the track circuit frequency, and thereby reduces the bondseffect on the track circuit.

For power frequency track circuits (eg. 50 Hz), the value of capacitance requiredto attain resonance is reduced to an achievable magnitude, by applying thecapacitance via an auxiliary winding, as shown in Figure B18.

To Next Bond

Auxiliary FluxWindingIRJ

IRJ

Or Cross Bonding

Figure B18

The value of capacitance required to achieve resonance depends on thefollowing:

a) Traction winding inductance, which may differ between designs. Thecapacitance required will vary inversely to the inductance.

b) Auxiliary turns ratio, which may differ between designs. The capacitancerequired will vary inversely as the square of the turns ratio.

c) Track circuit frequency, where the capacitance required will vary inverselyas the square of the frequency.

Resonated impedance bonds are used at the feed and relay ends of jointedaudio frequency track circuits and for all intermediate bonds associated withtraction cross bonding.





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16.2.2 Auto Coupled Impedance BondsThe method used to couple the feed and relay ends of certain designs of trackcircuit into the track as shown in Figure B19. At the feed end, the reducedvoltage appearing across the the traction winding is applied to the rails whilst atthe relay end, the current from the track circuit passing through the tractionwinding is usefully employed to drive the relay.

To Next Bond

IRJ

IRJ

Or Cross Bonding

To Track Relay OrFeed

OR

To RailsTo Track

Relay Or Feed

Figure B19

A double rail A.C. track circuit with autocoupled impedance bonds is shown inFigure B20.

Control

110V

110V

Resonant Bond

Local

Figure B20

If the traction current in each rail is not equal, the imbalance results in a net fluxin the iron circuit, and if that flux is sufficient to saturate the iron core, the trackcircuit current will be presented with a short circuit. It is therefore important tomake the bond as tolerant as possible of traction current imbalance and this isdone by creating an air gap in the magnetic circuit. Such bonds will tolerate 20%imbalance before saturation.


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R A I L T R A C K C 1


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Part CSchematic Symbols

1 IntroductionThe schematic symbols described here apply to bonding plans. Symbolsdepicting track circuits on signalling plans are to be in accordance withGK/RT0004.

2 Drawing Symbols onBonding Plans

2.1 BondingBonding plans must show connections which require traction voltage warninglabels, as shown in Part E.

Single rail traction return: In non-electrified areas one railshall be drawn bold, this shall be the series rail for CRBonding and the positive or BX rail for DRDS Bonding.

Signal rail insulated by IRJs

Continuous traction return rail

Electrified lines: Rails bonded, but not track circuited.

Electrified lines: Only one rail continuity bonded.

Non-electrified lines (In electrified areas): Rails not bonded.

DescriptionSymbol

Stainless steel strip on rails.



C 2 R A I L T R A C K

Railway Group Approved Code of PracticeGK/RC0752Issue TwoDate December 1998Page C2 of 11

Insulating rail joints: Separate track circuits on both sides.

DescriptionSymbol

Insulating rail joints: Track circuit on left, none on right.

Insulating rail joints: Track circuit on right, none on left.

Insulating rail joints: Between different sections of the sametrack circuit.

Standard jumper bond.

Traction bond.

Yellow bond.





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Structure bond.

DescriptionSymbol

Signal engineering equipment to rail bond.

Structure to Earth Wire.

Rail to rail bond (cross bonds) - (one example).

Return conductor or earth wire to rail bond.

Track circuit interrupter.

Connections for dc track circuits.

Connections for ac track circuits.





Connections for HVI track circuits.

Description

Guard boarding The provision of guard boarding will beindicated by a thin line on whichever sideof the conductor rail it is required.A suitable note may be added if required.

Insulated buffer stops.

Non-insulated buffer stops.

Insulated points.

Non-insulated points.

Symbol

2.2 Track Circuit Actuator Interference Detector (TCAID)

TCAID-N or TCAID (MC)

TCAID-D, detection to the right

TCAID-D, detection to the left





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2.3 Impedance BondsIf an impedance bond contains an internal resonating capacitor, the symbol mustbe shown filled in.

Double rail to double rail track circuits.

Description

Double rail to single rail track circuits.

Double rail track circuits to non-track circuited line.

Intermediate impedance bond.

Cross Bonds (Track to track bonds): Using impedance bonds in double rail areas.

Tuned impedance bond.

Symbol





2.4 Symbols For TI21 Jointless Track Circuits

Tuned zone with a transmitter and a receiver.

Description

Transmitter of centre fed track circuit at an end tuning unit (ETU).

Receiver at an IRJ with an end tuning unit (ETU).

Low power: Show at transmitter only.

Example of a TI21 track circuit bonding plan.

Symbol





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2.5 Symbols For Reed Track Circuits

Transmitter.

DescriptionSymbol

Receiver.

Intermediate simple loop (loop symbol points towards TX).

Compound loop (loop symbol points towards TX).

Example of a jointed reed track circuit bonding plan electrified.

Note: Track circuit frequency is indicated in ( ) brackets following the track circuit name.

Example of a jointless reed track circuit bonding plan.





3 Traction ReturnBonding Symbols

Separate Traction Return Bonding Plans are only used on the former SouthernRegion.

Running rail bonded at each rail joint for dc electric traction.

Description

Insulated rail joint.

Resonating bond.

Impedance bond.

Track circuit cut section.

To specify that more than one bondis required, indicate as shown.

Symbol





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Single protective boarding.

Description

Double protective boarding.

Insulated rail joint and continuity cableto eliminate magnetisation of points.

Aluminium advance plate: Only installed for attachment of traction return bonds in areas of single rail traction return.

Symbol

Cable Identification Codesa Single 500mm sheathed copper cable (soldered lugs) or single 800mm

sheathed aluminium cable (crimped aluminium or Cadweld aluminium lug).c Single 161mm sheathed copper cable (gas weld heads).d Single 161mm sheathed copper cable (soldered lugs) or single 240mm sheathed

aluminium cable (Cadweld aluminium or copper lug or crimped aluminium lug).f Single 161mm bare copper cable (gas weld heads) or single 150mm aluminium

cable (crimped 20 bi-metal bond heads).g Single 161mm copper cable (soldered 20 copper bond heads).h Single 161mm sheathed cable (one gas weld head and one soldered lug).j Single 161mm sheathed cable (one soldered lug and one soldered 20 copper bond

head) or single 150mm sheathed aluminium cable (one crimped 20 bi-metal bondhead and one crimped aluminium lug).

m Single 1,000mm sheathed aluminium cable.

N Single 630mm sheathed copper cable.n



C 1 0 R A I L T R A C K


4 EngineersScale Diagrams

4.1 General SymbolsThese symbols are to be used on the 1:100 and 1:200 scale diagrams whenrequesting IRJs and point insulations to be installed by the Permanent WayEngineer. The symbols are to be coloured red on plans returned to thePermanent Way Engineer.

Note: On Permanent Way Engineers Plans, dimensions are to the insideedges of rails.

DescriptionSymbol

Insulated rail joint required.

Insulated soleplate and stretcher bars required (position of insulationto be shown).

Drilling of insulated soleplate for facing point lock required, toMD 82017 (position of insulation to be shown).

Extended sleepers and soleplate required for the installation ofcombined type machines with left hand drive (show in reverse for righthand drive). Standard facing points (not clamp locks), drilled toBRS-SM 318.

Extended sleepers and soleplate required for the installation ofcombined type machine. Right hand drive for single or double slips (notclamp locks), drilled to BRS-SM 319.



R A I L T R A C K C 1 1


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DescriptionSymbol

Extended sleepers and soleplate required for the installation ofcombined type machine. Left hand drive for single or double slips (notclamp locks), drilled to BRS-SM 320.

Indicates switch rail, stock rail and soleplate to be pre-drilled forhydraulic clamp locks with multiple drives and soleplate, in accordancewith BRS-SM 2200, 2228, 2240, 2244 or 2260, as appropriate. Ifconcrete sleepers are to be used, they are required to be drilled inaccordance with BRS-SM 622.

Stainless steel strip welded to rails required.


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R A I L T R A C K D 1


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Part DPlanning and Design

1 IntroductionThe principles laid down here apply to the planning, layout and design of alltypes of track circuits and track circuit bonding. Special requirements forindividual types of track circuit are given in subsequent Codes of Practice withinthe Track Circuit Handbook. Signalling design requirements are contained inGK/RT0201.

Many of the parameters affecting track circuit design are related to the physicaland electrical characteristics of the trains operating over the track circuits.

Dimensions of track sections which are critical for achieving safe and reliabledetection are contained in GK/RT0011 Appendix A. If the accuracy quotedcannot be attained, dimensions should be rounded up unless otherwise stated(ie. if a maximum is given).For a description of terms and definitions used in track circuit design andoperation, see Part B.

For details of symbols used on scheme plans and bonding plans, see Part C.

2 Responsibilities forBonding Design

2.1 GeneralBonding requirements are contained in GK/RT0252.

2.2 Nonelectrified LinesThe design of all track circuit bonding on nonelectrified lines is the responsibilityof the Signal Engineer.

This requires the production of detailed scale bonding plans for all track circuitingin switch & crossing work, usually based on the Permanent Way Engineerstrack layout plan. On plain line a detailed bonding plan need not be produced,as long as sufficient detail of feed and relay leads is provided in linesideapparatus housing diagrams.

2.3 Electrified LinesBoth Signal and Electric Traction Engineers require connections to the runningrails, so compatibility between them is essential. In addition, there are a numberof connections upon which there is common reliance. It is therefore necessaryto have common plans/records that show in detail the track bondingarrangements. There must be agreement between both parties before any newwork or alterations are carried out. Design Standards are contained inGM/TT0126 and GM/TT0129.

The Signal Engineer is responsible for the design of:

a) All fishplate bonds in nontraction rails.b) All fishplate bonds in traction rails of a.c. only electrified areas.c) The position of all insulated rail joints.d) All jumper bonds between separate sections of nontraction rails.



D 2 R A I L T R A C K

Railway Group Approved Code of PracticeGK/RC0752Issue TwoDate December 1998Page D2 of 20

e) In a.c. electrified areas (excluding the former Southern Region), all rail toimpedance bond connections and connections between impedance bonds onthe same track.

f) In dual a.c./d.c. and d.c. electrified areas (excluding the former SouthernRegion), the responsibility for impedance bond connections is subject tospecial arrangements between the Signal and Electric Traction Engineers.

g) On the former Southern Region, impedance bond connections for trackcircuit only purposes.

h) All track circuit rail connections.i) Identifying the need for Yellow Bonding and specifying which bonds are to

be yellow.

The Electric Traction Engineer is responsible for the design of:

a) All fishplate bonds in d.c. or dual a.c./d.c. traction rails.b) All jumpers bonds between separate sections of traction rails and between

the centre connection of impedance bonds in different tracks.

c) On the former Southern Region, rail to impedance bond connections fortraction purposes.

d) All other permanent traction related bonding.2.4 Adjacent LinesIn all cases where lines run adjacent to or cross each other, but are notphysically connected, all these lines must be represented on the bonding plansand the bonding plans cross referenced to each other.

3 Track CircuitNomenclature

Identification of individual track circuits is to be in accordance with StandardSignalling Principle No. 54 and must be shown on plans at convenient intervalswithin the respective track circuit.

To avoid confusion with other plan annotation, the following Track Circuitdesignations are to be avoided:

B, F, I, N, O, R, Q, T, CL, OL, HVI, RB, RN, RR, RT, RX, TB, TC, TF, TFR, TI,TJ, TN, TO, TR, TX, VT, IBJ, IRJ, OCC.





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4 Choice of TrackCircuit Type

There are a number of design features of track circuits which constrain choicefor a given application:

a) The need to detect vehicles on poor rail surfaces.b) The need or otherwise to avoid insulated rail joints.c) The need for immunity to a.c. and/or d.c. traction interference.d) The need to achieve maximum reliability at economic cost.e) The need to track circuit through complex S & C.The Figure D1 summarises the key attributes and limitations of each type oftrack circuit.

CAUTION: Although general precautions and limits that providecompatibility between rolling stock and track circuits, have been includedin the Train Detection Handbook Codes of Practice, these are notcomprehensive and special conditions may apply to certain routes topermit the operation of rolling stock.

Where new types of rolling stock are to be introduced, existing constraintswill require reassessment, as to their adequacy.





Type Suitable ForLightly Used

Lines

IRJsRequired

Immunity From SuitableFor S & C

a.c.50Hz d.c.

D.C. Medium VoltageA.C. Immune #1

Yes Yes Yes No Yes

D.C. Diode #1 Yes Yes No No NoTI21 #1 No No Yes#4 Yes NoHVI #1 Yes Yes Yes Yes YesD.C. Low Voltage Plain No Yes No No YesD.C. Low VoltageA.C. Immune

No Yes Yes No Yes

D.C. Medium VoltagePlain

Yes Yes No No Yes

D.C. Medium VoltageA.C. Immune/D.C.Tolerant

Yes Yes Yes Yes#2 Yes

A.C. WR QuickRelease

No Yes No No Yes

A.C. 50Hz Vane No Yes No Yes Yes#3A.C. 83.3Hz Vane No Yes Yes Yes Yes#3Reed No Yes Yes Yes YesAster/SF15 No No No No No

Notes:# 1 Preferred track circuits for new works.# 2 Limited dc immunity. Used in an area of ac lines close to ac/dc duallines not fitted with any means of isolating the traction rail systems. Usemust be subject to a proper immunisation evaluation exercise.# 3 Single rail type has restricted length but adequate for S & Capplication. Double rail type is difficult in complex S & C but permits longlength in plain line.# 4 The use of TI21 on ac electrified lines requires the earthing oflineside structures to be to a separate conductor rather than to the rail.This precludes the use of TI21 on ac lines unless part of a major newelectrification scheme.

Figure D1





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5 Cut SectionsDesigning cut sections into a track circuit is a method of reducing the continuouslength. The track circuit is split into individual track circuits, each one controllingthe same final TPR. They are indicated as one track circuit on the signalmanspanel. Special care must be taken where an individual section of such a trackcircuit is used separately for control purposes (eg. level crossing timing).The cascading of cut sections (ie. controlling the feed to a track circuit by therelay of the next track circuit) is not permitted. The individual cut sections shouldbe either returned individually to the interlocking or summated in the TPR linesidecircuit or, in the case of SSI, summated in the data.

Cut sections must be identified in accordance with GK/RT0009 (ie. AA1, AA2,AA3, etc) in the direction of normal running. The two portions of a centre fedjointless track circuit are treated separately for this purpose (eg. AA2 and AA3 inFigure D2).

Relay Feed

AA1 AA2 AA3 AB

AA2 AA3 ABRX RX RXAA2/3TX

(50HZ)Track Circuit

(Centre Fed)Jointless

Track Circuit

Figure D2

Where a monitoring device is provided, it must indicate to the technician thelocation at which the failed relay/receiver is housed, irrespective of the lineaffected. With reference to Figure D3, an example of the labelling for anindividual display would be Loc 10 (AA2, AA3, BC2, BC3).

AA3 AA4

BC2 BC1

AA3AA2RX RX

AA2BC3

BC3 BC2RX RX RX RXTX

BC1/2

TX RXAA4

BC1 BB6

BB6

AB1

AB1TR

AA3/4

LOC. 10 LOC. 11 LOC. 12

Figure D3

Track circuits must not consist of more than two nonmonitored cut sections(where track circuits are centre fed, four receivers (Rxs) may be nonmonitored).





The monitoring device will usually be housed at the nearest interlocking, but thiswill largely be governed by the routine and outofhours fault finding coverarrangements which exist in the vicinity.

The transmission of information to the monitoring device may be achieved byadditional FDM channels, a lowcost FDM system approved for use in signallingor telecommunications cables or direct wire circuits.

6 Operating Times6.1 Time DelaysThere can be significant differences between the dropaway and pickup timesof different types of track circuit, such that the rear track may register clearbefore the forward one registers occupied. The detection of the vehicle istherefore momentarily lost, resulting in a wrong side failure, which could permitthe irregular release of vital interlocking. To overcome this, additional timedelays must be built into the pickup time of track repeaters, the preciserequirement being dependent upon the combination of track circuit typesinvolved.

The indication circuits to the signalman may be transmitted via a TDM or FDMlink. Therefore the transmission system reaction times must also be consideredto ensure that the signalman does not observe an apparent loss of traindetection.

6.2 Operating Categories And ConditionsIn order to simplify the number of possible permutations, track circuits areassigned to operating categories as follows:

Track Relay Operating Characteristics Operating Category

Slow to Pick Up - Quick to Drop Away AMedium to Pick Up - Medium to Drop Away BQuick to Pick Up - Slow to Drop Away C

Track Circuit Type Operating Category

TI21 AUM71 (French AD.C. (all types) BPhase sensitive a.c. (50Hz, 75Hz & 83.3Hz) BAster BReed with adjustable track filter BWestern Region Quick Release (a.c./d.c.) BDiode BCoded BGEC Alsthom High Voltage Impulse (HVI) CReed without adjustable track filter C





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With Geographical systems, the differing combinations of abutting categories oftrack circuits need to be examined and dealt with specially, according to theoriginal design principles. With free wired relay interlocking and SSI, they mustbe dealt with as follows:

Category AWhen used with a free wired relay interlocking, these track circuits do not requirea slow to pick up TPR. Therefore, the TR may be used directly in controls.

When used with an SSI, standard track circuit data must be used.

Category BWhen used with a free wired relay interlocking, these track circuits require oneslow to pick up TPR, in accordance with Figure D4

When used with an SSI, standard track circuit data must be used.

Category C and Category B abutting Category CWhen used with a free wired relay interlocking, these track circuits must beprovided with two slow to pick up TPRs, in accordance with Figure D5. The TRand TPR must be in the same location case or equipment room. The T2PRmust be controlled directly by contacts of both TR and TPR to prevent the dropaway of T2PR from being unnecessarily delayed whilst still achieving the delayedpickup required.

When used with an SSI, track circuit data with extra delay must be used.

Where the time delay is achieved by relay cascade, it is important that othercontacts of the TR and any intermediate repeater relays are not used for controlindication purposes. To prevent inadvertent subsequent use, a suitable notemust be made on the Contact Analysis Sheet.

A schedule must be provided listing all TPRs, the individual sections repeated byeach TPR and the type of track circuit (including the frequency in the case of ajointless track circuit).

B50

N50

BR 933

EG TPR

EG TR

EG TR

Figure D4

EG TPR

B50

N50

EG TR

BR 933

EG TPR

BR 933

EG T2PR

EG TPREG TR

Figure D5





7 Track CircuitInterrupters

The interrupter (BRS-SM 374) is designed to be mounted on the stock rail, notthe switch rail, and is electrically insulated from it. It is to be mounted as near aspossible to the switch toe, at a position where the flangeway gap is not less than70mm when t

General Info of TC

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