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of Practice GK/RC0752 Issue Two Date December 1998 Page B8 of 25 7 Operation and Adjustment of the Simple Track Circuit Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits • Immunisation / Electrification requirements, including electromagnetic compatibility with trains. As the various types of track circuit feed/transmitter produce differing power outputs, and as rail impedance is frequency related, it follows that the maximum workable length will vary with design type and with the minimum ballast resistance at which the track circuit is expected to remain functional. Consider the simple d.c. track circuit depicted in Figure B6. Feed Resistance Figure B6 Cable Resistance Cable Resistance Train Shunt Cable Resistance Cable Resistance B8 RAILTRACK Rail Ballast Resistance 7.1 Track Circuit Clear The ballast resistance forms an additional load in parallel with the relay. As the ballast resistance falls due to wet weather, the current drawn from the feed increases. This will cause the voltage across the feed resistor to increase, so reducing the rail and relay voltages. If this reduction causes the relay voltage to fall below the relay pick–up value, the track circuit will not clear after an occupying train has departed. A further reduction of the relay voltage to below relay drop–away value will fail the track to the occupied state without the passage of a train. Reducing the value of feed resistance has the effect of increasing the current fed into the rails and raising the rail/relay voltage. Long feed end leads insert additional non–adjustable feed resistance and thereby reduce the effectiveness of the adjustable feed resistance. Long relay end leads reduce the ratio of relay voltage to rail voltage by potential divider action; the effect is to cause the track circuit to indicate occupied at a higher ballast resistance. It therefore imposes a shorter maximum workable length. 7.2 Track Circuit Occupied When the track circuit is occupied by a train, a short circuit current will flow from the feed end equipment, which is limited by the value of the feed resistance and the characteristics of the feed end equipment itself. The feed end equipment is designed to cope with this worst case power dissipation. The train shunt resistance is in parallel with the ballast resistance. With any given value of feed resistance, the relay will operate at particular values of combined ballast/train shunt resistance. Thus, higher ballast resistance will require a lower value of train shunt resistance to operate the relay and vice versa. Rail TR
<strong>General</strong> <strong>Information</strong> on Track Circuits 8 Train Shunt Imperfection Withdrawn Document Uncontrolled When Printed Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page B9 of 25 The minimum permitted drop shunt resistance is 0.5Ω (0.3Ω on certain impedance bond track circuits). During very dry weather or severe frost conditions, the ballast resistance increases towards its natural maximum and will offer only a small contribution towards the overall shunt. Thus, when a 0.5Ω (0.3Ω) shunt is placed across the rails, it must still reduce the relay voltage to below drop–away value. It should also be noted that the track relay is dropped by short circuit rather than disconnection. Therefore, the drop–away time of the relay is increased due to the inductive circuit prolonging the decay of the coil current. 7.3 Principles of Basic Adjustment The difficulty with adjusting track circuits (where such adjustment is provided) is knowing the prevailing value of ballast resistance. Details entered on the track circuit record card provide a useful history. These vary with track circuit type and the appropriate Code of Practice within the Track Circuit Handbook should be consulted. Assuming average conditions, the feed resistance is adjusted to obtain a relay voltage in the range 25% to 75% above the pick–up value whilst maintaining the drop shunt resistance at a value greater than the minimum required. If the track circuit fails due to wet weather, it may be possible to remedy the situation by reducing the feed resistance. It is important that the track circuit is re–tested after it has dried out. The energy seen by the relay with a train on the track circuit will depend upon the resistive value of the train shunt (see Figure B6). This energy will be zero only when the train shunt is zero. Whilst the ohmic resistance of an axle and wheels is virtually zero, there are a number of factors that can make the effective train shunt much higher. Since some factors are track based, whilst others are vehicle specific, the precise mixture of factors applying to a particular vehicle at a particular place can be very variable. 8.1 Rust Films Light rust film on the rail head and/or tyre tends to act as a semi–conductor, in that it exhibits high resistance until the voltage exceeds a particular threshold value when it breaks down completely. The breakdown voltage rises in sympathy with the extent of the contamination; very heavy rust films, resulting from prolonged disuse, render many track circuit designs incapable of detecting vehicles. Figure B7 gives an approximate characteristic of such films. Current 0.01V Figure B7 RAILTRACK B9 Very Good Damp Light Rust Dry Light Rust Heavy Rust Or Leaf Residue Good Poor Bad 0.05V 0.1V 0.3V 0.6V - 200V Voltage
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