General Information on Track Circuits - RGS Online

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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B16 of 25 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits 11.2 Double and Single Rail Track Circuit Bonding Double rail track circuit arrangements have both rails fitted with IRJs to completely isolate a track circuit. Impedance bonds are used when a traction current return path is required. IRJs and impedance bonds are not required with Jointless Track Circuits. Single rail track circuit arrangements have only one rail fitted with IRJs to separate the track circuits. The other rail is electrically continuous. If this continuous rail is used for traction return purposes, the bonding arrangement is called Single Rail Bonding. If this continuous rail is not used for traction return purposes, the bonding arrangement is called Common Rail Bonding. Whilst some designs of track circuit can be used in either single or double rail mode, others are limited to double rail application. In some S & C areas and certain electric traction areas, it is necessary for one or more adjacent track circuit to share one common rail. This arrangement can lead to unsafe failure modes unless special steps are taken to ensure that elements of the common rail cannot become isolated from the remainder. This is achieved by creating a mesh of alternative diverse bonding paths and marking the associated bonds yellow as for the previous parallel bonding case. 11.3 Track Circuit Interrupters Track circuit interrupters are used at trap or run–back catch points on lines which are track circuited. The device is designed to interrupt the track circuit in the event of a rail vehicle leaving the track. This prevents automatic re– energisation 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 rail and usually insulated from it. It comprises a main body, a narrow neck and a head which is adjacent to the running edge and designed to break off when a rail vehicle passes over it. Connections are made to the head and the body such that 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 in Figure B12). It is not fixed to the switch rail. Bonding Connection Points Figure B11 B16 RAILTRACK
<strong>General</strong> <strong>Information</strong> on Track Circuits 12 Mutual Interference Between Track Circuits Withdrawn Document Uncontrolled When Printed Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B17 of 25 Note: In all cases, the interrupter is fixed to the stock rail (as shown in Figure B12). It must not be fixed to the switch rail. Figure B12 Stock Rail Switch Rail Correct Position Incorrect Position It is important to realise that where track circuits are connected together by a common rail, and detectors are used that are unable to discriminate between their own and other track circuit feeds, a degree of mutual interference between such 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 rail track circuits). The simplest way to describe mutual interference is to use the example of two d.c. single rail track circuits as shown in Figure B13 and Figure B14. It will be realised that this model also equates to a double rail insulated track circuit with a failed IRJ. Figure B13 shows the equivalent circuit in track circuit type format, whilst Figure B14 converts it to a standard electrical format for easier presentation of cause and effect. Consider the circuit in Figure B14 under conditions where VFB feed supply is disconnected. Clearly, a voltage will appear across RRB as a result of VFA, the value of which will depend on circuit parameters. The extent to which this voltage is of concern depends upon its value relative to the operating values of the relay. Provided that the bonding remains intact, an unsafe failure cannot arise from the mutual interference; either both track circuits will fail right side (occupied) or they will both show occupied when either one of them is legitimately occupied. However, there is the possibility of a wrong side failure where a bonding disconnection occurs. It is not appropriate to explain all possible scenarios here; the possibility is mentioned simply to convey the fact that some track circuit defects can be exceedingly difficult to understand and explain. Certain constraints are applied to various track circuit designs in order to limit the possibility of wrong side failure. Therefore, the design constraints described in the Track Circuit Handbook shall not be breached without expert advice. RAILTRACK B17
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