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Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page J2 of 5 Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits Traction Interference This is usually associated with d.c. traction railways and arises because of the high currents necessary with the low supply voltage. If a failure occurs with trains in certain positions or during times of heavy traffic, the traction return system should be investigated. Earth Faults Due to track circuits being very “earthy”, especially during wet weather, trains at certain positions on other lines may affect the rail to rail voltage of a track circuit, causing intermittent failures. An earth fault is usually noticed with a second fault caused by a defective continuity bond. Broken Rails/Bonds These can cause intermittent continuity problems. 4 Right Side Failures When called to a fault, it is first necessary to determine whether the cause lies in the track circuit itself or its associated repeater circuits. This can be resolved by proceeding directly to the relay end and examining the track relay (having confirmed that the track is supposed to be clear !). This section is concerned with fault location in the track circuit itself. 4.1 Types of Fault Since the various track circuits used have differing types of feed units, ranging from a single d.c. cell to complex transmitters, the methods for checking these different feed units are described in the relevant Code of Practice within the Track Circuit Handbook. If the transmitter or feed is found to be functioning correctly, it can then be used to determine the general nature of the fault (ie. short circuit or disconnection). This can be done by taking voltage and current measurements at the feed end as shown in Figure J1 . Figure J1 Remove Link For Current Measurement J2 R A I L T R A C K A V Feed
Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits A disconnection in the series bonding will: a) Reduce the current being fed into the track circuit. Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page J3 of 5 b) Reduce the voltage normally dropped across the feed impedance. c) Increase the rail voltage at the feed end. A short circuit will: a) Increase the current fed into the track circuit. b) Increase the voltage dropped across the feed impedance. c) Reduce the rail voltage at the feed end. 4.2 Locating a Disconnection The higher rail voltage can be measured at all positions along the track circuit from the feed end up to the point of disconnection. On the relay side of the disconnection the rail voltage will be very low. Fault location therefore entails walking through and checking the rail voltage to identify the position of the step change in value. The faulty bond etc, can be confirmed by measuring a voltage across it. 4.3 Locating a Short Circuit 4.3.1 <strong>General</strong> The extent of the change in feed end voltages and currents will depend upon the type of track circuit and the physical position of the fault along the length of the track circuit. Consult the relevant section concerned with the specific type of track circuit, as appropriate. The general position is as follows: a) At d.c. and power frequency a.c., the rail impedance is negligible, and the resulting electrical circuit is constant, wherever the short circuit is located within the track circuit. Thus, any voltage across the rails, permitted by an imperfect short circuit, will be constant throughout the length, giving no clue as to its physical position. b) With audio frequencies and impulses, the rails have significant impedance, and the effect of a short circuit will vary depending upon its physical position. The closer it is to the feed end, the more it will increase the feed current and decrease the rail voltage. In either case, it is not possible to precisely locate the short circuit by simple observation of rail voltage along the track circuit. Other methods available are as follows: 4.3.2 Visual Inspection and Provocation This is particularly useful where track circuit equipment is not responsible. Examples of such a fault include signal wire or point rods touching the rails, and faulty insulation in point connections etc. Connect a meter across the rails and observe its reaction when the item of equipment is provoked (eg. waggle the signal wire or stand on the rods, having first ensured that they will not be operated). In the case of faulty insulations, a hammer blow to the metalwork adjacent to the insulation will often produce sharp changes of rail voltage. Alternatively, the insulation should be carefully dismantled and reassembled. R A I L T R A C K J3
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