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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page E26 of 52 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits * WBS style M2/5 impedance bonds have been serviced as ”Universal” since 1984. Notes N. (New) - In production and serviced. S. (Serviced) - Out of production, but still serviced. O. (Obsolete) - Out of production and not serviced. Type All new impedance bonds are manufactured to specification BR863, the current rating of which is shown in Figure E23: Bond Type Type 3 3000A continuous, 4500A for 2 hours Type 2 2400A continuous, 3600A for I hour Type I 950A continuous, 1100A for I hour (Not to BR863) Type 0 (Not to BR863) Figure E23 On high current dc. traction lines, ie. where classes 92, 373, 465 and 471 locomotives and multiple units run in revenue–earning service, only Type 3 impedance bonds fitted with six side leads (three on each side) are to be used. On other lines, any type of impedance bond may be used and need only be fitted with two side leads. 15.3 Sleeper Spacing The sleeper spacing required for impedance bonds is shown in Figure E24. Bond Style Type Sleeper Centres (mm) WH3 3 650* MR or S 2 750** P3 1 635 DE 1 585 M 0 700 M2/5 0 700 M6 0 700 DD 0 725 B 0 710 Figure E24 * The fixing centres for Type 3 impedance bonds is 650mm. However, any spacing between 570mm and 730mm will be satisfactory as the impedance bond is not fixed down at the terminal box end (see Figure E26). * * S and MR impedance bonds are also only fixed at one end, therefore the spacing of the sleepers is not critical E 26 R A I L T R A C K
Withdrawn Document Uncontrolled When Printed 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 E27 of 52 15.4 Impedance Bond Interchangeability When an impedance bond is changed for another type, it is usually necessary to change some or all of the associated connections. Figure E25 is included as a guide to the items affected. The preferred replacement is shown shaded. As well as physical interchangeability, the replacement bond will need to be sufficiently rated for the peak and continuous traction currents that occur on that line. Imbalance in traction return currents will tend to apply a proportion of any interference content to track circuit receivers, which under particular conditions could lead to a wrong side failure. One mechanism whereby imbalance is detected, is due to saturation of the impedance bonds. As the difference between the traction currents flowing through the impedance bond’s half windings increase, the transformer action of the impedance bond becomes less efficient as it is driven into saturation, thereby suppressing the track circuit voltage. The operation of the track circuit will become unreliable, but some protection against wrong side failure of the track circuit is provided. The level of imbalance current required to saturate impedance bonds is roughly proportional to their rating, with higher rated type 3 bonds able to withstand greater imbalance current levels than type 2 bonds for example. This saturation of impedance bonds leading to right side failure of the track circuit, can provide useful protection against wrong side failure due to traction interference. Therefore, care should be applied when replacing one type of bond for another, as the tolerance to imbalance may be increased, but at the expense of detection of situations where track circuits become exposed to traction interference which could lead to wrong side failure. Additionally, certain bond/relay combinations should be avoided as they may create problems in correctly setting up 50Hz a.c. double rail track circuits. In particular, G4 relays should not be used with type 2 or type 3 bonds. If the track relay is a style CE391, the recommended replacement impedance bond is style DE, but if this is not available, it will usually be necessary to change the relay and base to a VT1. It is also recommended that the impedance bond at the other end of the track circuit is changed in order to get a satisfactory adjustment, particularly if a style B impedance bond is being displaced. R A I L T R A C K E 27
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