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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B2 of 25 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits Traction Return Bonding The 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 Bond A jumper cable where track circuit polarities and/or traction return rails are switched across a pair of IRJs. Yellow Bond A jumper cable that has been designated by the Signal Engineer to be an important part of the diverse bonding of common/single rail track circuit. It is coloured yellow or identified by yellow tape. Damage must be reported and repairs carried out as a matter of priority. Clearance Point The minimum distance from switches and crossings at which track circuits having the function of proving clearance may be terminated to ensure a passing clearance 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 IRJs to separate the track circuits. The other rail (the common rail) is electrically continuous but is not used for traction return purposes. Cut Section A method of reducing the continuous length of a track circuit by the use of individual track circuits, each one controlling the same final TPR. These are indicated as one track circuit on the signalman’s panel. Double Rail (DR) A track circuit arrangement where both rails are fitted with IRJs, or tuned zones are used to completely isolate a track circuit. Drop–away Time The time between the application of a shunt to the rails and the front contacts of track relay (TR) fully opening (see also Pick–up Time). Drop Shunt The maximum value of non–inductive resistance which, when placed across the rails, will cause the track relay to fully open its front contacts. Fishplate Metal plates for joining rails together. Frequency Rotation The sequential application of specified frequencies. Insulated Rail Joint (IRJ) A method of joining rails together whilst maintaining electrical insulation between them. Jointed Track Circuits Track circuits whose extremities are defined by the use of Insulated Rail Joints (IRJs). Jointless Track Circuits Track circuits whose extremities are defined by the use of tuned circuit techniques. The extreme limits of a jointless track circuit area are either defined by the use of IRJs or by the use of a tuned circuit between the rails. B2 RAILTRACK
Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B3 of 25 Joint Hopping Where fast moving short vehicles pass from one track circuit to the next, the difference between the pick–up and drop–away times can cause the vehicle to momentarily disappear. Jumper Cable Used to electrically connect, for track circuit or traction purposes, two pieces of rail that are not adjacent. Overlay Track Circuit A track circuit which can be superimposed over another, neither having any effect on the other and both operating independently. Pick–up Shunt The minimum value of resistance between the two running rails at which the track relay will just close its front contacts. Pick–up Time The time between the removal of a shunt from the rails and the front contacts of the track relay making (see also Drop–away Time). Plans For the definition of all types of Plans, see SDH E11. Selective Operation Operation of a portion of a track circuit by selection of the position of a set of points. 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 IRJs to separate the track circuits. The other rail (the common rail) is electrically continuous and is used for traction return purposes. Sleeper An item of wood, steel or concrete of standard dimensions, used to support and gauge the track (see Bearer and Timber). Spur A section of running rail required to be electrically common to a series bonded rail, but which is not itself in series. Stagger (Electrical) The phase or polarity difference between one track circuit and the next, or between 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 Cable This is a cable which connects the lineside apparatus housing to the trackside equipment, but not direct to the running rails (see Track Cable). Timber An item of wood of non–standard dimensions, used to support the track in S & C areas (see Bearer and Sleeper). RAILTRACK B3
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