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Practice GK/RC0752 Issue Two Date December 1998 Page D8 of 20 7 Track Circuit Interrupters 8 Length of Track Circuits Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits The interrupter (BRS-SM 374) is designed to be mounted on the stock rail, not the switch rail, and is electrically insulated from it. It is to be mounted as near as possible to the switch toe, at a position where the flangeway gap is not less than 70mm when the switch is closed. An interrupter may be either directly controlled in series with the track circuit or part of a separate circuit utilising an interrupter relay according to circumstances. The interrupter must be part of a separate circuit unless all of the following conditions apply: • The line is non–electrified. • The track circuit on which the interrupter is wired is a d.c. track circuit. • The operation of a track circuit by by the interrupter will place to danger any necessary signal on adjoining lines. Note: The bonding of the interrupter must be of opposite polarity to the rail on which it is mounted. In all other circumstances, the interrupter must be part of a separate circuit incorporating a track circuit interrupter relay. The interrupter relay is controlled directly by the interrupter itself and its front contacts are used to control all repeat relays of the required track circuit. Where necessary, an interrupter relay may control more than one track circuit or may be controlled by more than one interrupter. Minimum To cater for the longest wheel base vehicles, a standard minimum effective track circuit length of 18.3m must be provided for new and altered works. If this minimum length cannot be achieved, alternative safeguards must be provided, (eg. sequential clearance of the track circuits in the interlockings). Maximum The maximum lengths quoted in individual Codes of Practice within the Track Circuit Handbook are based on a ballast resistance of 3Ωkm for timber sleepered track and 5Ωkm for concrete sleepered track. If it appears likely that a track circuit will be required to operate at or near its maximum permitted length, tests should be made to ascertain whether ballast conditions etc, are satisfactory, particularly in wet weather, before the scheme design is finalised. The unpredictable effect of level crossings should also be borne in mind. On electrified lines, track circuits may have to be further restricted in length in order to limit the effects of interference from the traction system. Certain track circuit types require a greater minimum length due to their operating characteristics. See the relevant Code of Practice within the Track Circuit Handbook D8 RAILTRACK
<strong>General</strong> <strong>Information</strong> on Track Circuits 9 Track Circuit Gaps and Staggered IRJs 10 Selective Operation of Track Circuits 11 Bad Rail Surface Withdrawn Document Uncontrolled When Printed Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page D9 of 20 It is essential that all classes of vehicle, irrespective of wheelbase arrangement, are detected by the track circuiting, otherwise false track circuit clearance may lead to premature movements of points or irregular release of signals. Based on the dimensions of existing rolling stock and allowing for future developments, the following shall apply: a) The maximum dead section between two track circuits in areas of continuous track circuiting is 2.6m. This is the minimum wheelbase of vehicles working unattached. b) Opposite IRJs must be regarded as the ideal arrangement. If unavoidable, physically staggered overlaps between nominally opposite IRJs must not exceed the following limits: Non–electrified areas 2.6m Electrified areas if the traction rails overlap 2.6m Electrified areas if the insulated rails overlap 2.1m * Isle of Wight lines 1.7m * Note*: These distances are stipulated by the Electric Traction Engineer to prevent a motor bogie losing its negative return path (see Figure D12 for clarification). a) There must be at least 18.3m between the nearest joint of any physically staggered pair and an IRJ defining a clearance point at the end of the track circuit. Where this cannot be obtained, special sequential controls must be provided. b) If the physical stagger exceeds 1.6m, there must be at least 18.3m between the nearest joint of the staggered pair and the next IRJ. If the stagger is less than 1.6m, this distance may be reduced to 11m. Where the requirement cannot be met, special sequential controls must be provided. c) Full details of critical dimensions for train detection are contained in GK/RT0011. Operation of a portion of a track circuit by the selection of the positions of a set of points is not permitted. Such portions must be separately track circuited. At locations where oil film or rust is excessive, (eg. traction depots, terminal platform lines, etc), a stainless steel strip can be applied to the surface of the running rail by the Permanent Way Engineer. This must be shown on bonding plans, using the symbol specified in Part C. The vibration caused by the resulting uneven rail surface restricts its application to very low speed applications (5 mph maximum). Alternatively, the GEC Alsthom High Voltage Impulse (HVI) track circuit may be used (see GK/RC0756). 12 Emergency Crossovers Where a crossover is clipped and padlocked or worked from an adjacent ground frame and subject only to an emergency release, track circuiting is not required for movements over the crossover. RAILTRACK D9
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