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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B14 of 25 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits 10.3 Concrete Sleepers Early forms of concrete sleeper were fitted with chairs for bullhead rail in similar fashion to those fitted to timber sleepers. The chair was usually secured to the sleeper with a through bolt from the underside. These did not present any widespread 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 ballast resistance is usually quite low and subject to more severe weather related swings. It is also now known that damp concrete behaves as an electrochemical secondary cell which can give rise to residual voltage problems with d.c. track circuits. Modern concrete sleepers incorporate a rubber pad under the rail foot and moulded insulations where the fixings bear on the top of the foot, as shown in Figure B9. The effect is to increase ballast resistance to levels significantly higher than those obtained with timber sleepers. However, the insulations do erode due to the vibration of passing traffic and, consequently require periodical replacement. Lack of attention to insulation usually results in gradual degradation of the ballast resistance rather than sudden failure. Centre Leg Figure B9 Front Arch Heelseat Rear Arch Insulation Rail Pad Rail Foot 10.4 Steel Sleepers Steel sleepers are equipped with insulations similar to modern concrete sleepers and, provided they are subject to an effective preventative maintenance programme, 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 than is the case with modern concrete sleepers. B14 RAILTRACK
<strong>General</strong> <strong>Information</strong> on Track Circuits 11 Bonding Withdrawn Document Uncontrolled When Printed Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B15 of 25 Bonding describes the means by which the individual components of the railway track are connected together electrically for track circuit purposes. The term also includes the additional electrical connections necessary for the proper operation of electric traction. Symbols used on bonding plans are shown in Part C and various terms are explained in Section 3. Refer also to GK/RT0252. 11.1 Series and Parallel Bonding In order for a track circuit to fail safe (to show occupied) in the event of a bonding disconnection, it is necessary to bond all elements of the track circuit in series. However, in S & C areas, it may not be physically possible to arrange total series bonding of both rails. Examples of series and parallel bonding are shown in Figure B10. Provided that a spur is very short, it is permissible to bond it in parallel without additional safeguard. However, where the spur is long, or in other cases where necessary, parallel bonding may be resorted to provided that steps are taken to ensure that vehicles are not lost due to disconnection of part of the parallel system. This is achieved by creating a mesh of alternative diverse bonding paths between parallel elements, and clearly identifying the associated bonds by their yellow colour. It is necessary to ensure that such yellow bonds are repaired quickly before other bonds in the mesh have time to fail in a manner likely to cause an unsafe failure. Because of the additional complication of significant rail impedance with parallel bonding, audio frequency track circuits are generally unsuitable in all but the simplest of pointwork. Figure B10 Track Relay Preferred Series Bonding Track Feed Track Relay Track Feed Non-preffered Parallel Bonding RAILTRACK B15
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