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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page D16 of 20 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits of the 50Hz system. Immunisation of track circuits on the a.c. electrified system and non-electrified lines against d.c. interference, will usually involve significantly greater distances (up to 20 km). The extent of such a zone is dependent upon track layout, position of feeder stations and traction load etc, and requires specialist assessment and subsequent verification. 13.12 Buffer Stops Rail mounted buffer stops in double rail track circuited areas must be fully insulated. In the case of single rail track circuits in a.c. electrified territory, they should be insulated from the signal rail. The type of IRJ provided close to buffer stops must be of a design which offers similar tensile strength to conventional steel fishplates (see GK/RT0031). 13.13 Electrical Stagger Where the electrical energy of one track circuit is capable of operating the adjacent track circuit due to IRJ failure, the polarity (d.c.) or phase (a.c.) of each must be arranged so that they oppose rather than reinforce each other. The intent being that IRJ failure will not result in false operation. The following methods are available to counteract lack of proper electrical stagger: • Provide an additional transposition to restore correct electrical stagger. • Abut feed ends. • Provide a feed end relay (d.c. track circuits only). • Convert one track circuit to a non–interfering form of energy. 13.14 Fouling and Clearance Points Where tracks cross or diverge etc, it is necessary to define track circuit limit dimensions which ensure that traffic passing along one route is not obstructed by vehicles standing on the other. The two critical dimensions are referred to as the fouling point and the clearance point, as shown in Figure D12. Figure D12 1970mm (Between Running Edges) 4880 MM Crossing Nose Fouling Point Clearance Point D16 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Withdrawn Document Uncontrolled When Printed 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 D17 of 20 13.14.1 Fouling Point This is a position a short distance away from the point of running line divergence (crossing nose). Should any part of a vehicle on one track be between the crossing nose and the fouling point, it will make physical contact with any vehicles passing on the other route. Where the angle of divergence is less than 45°, the fouling point occurs where the distance between the running edges of the two rails is 1970mm, measured at right angles from the track for which the fouling point is being determined. Where the angle of divergence is greater than 45°, the fouling point occurs at 1970mm from the crossing nose, measured along the track for which the clearance point is being determined. In the case where tracks become parallel with a running edge separation of less than 1970mm, the fouling point occurs where the tracks first become parallel. 13.14.2 Clearance Point As track circuits detect the wheelsets of vehicles which are inboard of bodyshell limits, the boundary of any track circuit designed to give assurance of clear passage along the other route must be some distance beyond the actual fouling point. This is defined as the clearance point. In the absence of protecting trap points, the clearance point is 4880mm further from the crossing nose than the fouling point. Where trap points are provided, the clearance point is defined as the switch tips of the trap points. In both cases, the IRJ defining the track circuit limit is positioned at the first suitable rail joint beyond the clearance point. All clearance points shall be shown on the bonding plans. 13.14.3 Minimum Length of Track Circuits To prevent a vehicle bridging a short track circuit and consequently providing a false clear condition, the minimum length of a track circuit is 18.3m. 13.14.4 Staggered IRJs Where practical, IRJs should be positioned in each rail so that they are opposite to each other. Where a physical stagger between opposite IRJs is unavoidable, the maximum physical stagger must not exceed 2.6m. This is the minimum wheelbase of vehicles which can work unattached. To ensure that motor bogies do not become insulated from the traction return path, the physical stagger for a signal rail overlap in electric traction areas must not exceed 2.1m. On the Isle of Wight, vehicles used have smaller bogies and as such, the maximum physical stagger is reduced to 1.7m. In order to avoid loss of detection of a single car, four wheel vehicle, the permitted distance between inner joints of staggered pairs of IRJs must not be less than 11m, where the IRJs are staggered at a distance of less than 1.6m (the minimum bogie wheelbase). If however, either pair of IRJs are staggered at a distance greater than 1.6m, or provide a clearance point (see Clause 13.14.2), then the distance between inner joints of staggered pairs must be not less than the 18.3m minimum track circuit length. Note: When consulting other documents, care should be taken to ascertain whether “running edges” or “outside edges” of rails are being referred to. RAILTRACK D17
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