Trackside timing11/24/2023 In some railway electrification schemes, one or both of the running rails are used to carry the return traction current. A series resistor limits the current when the track circuit is short-circuited. The relays and the power supply are attached to opposite ends of the section to prevent broken rails from electrically isolating part of the track from the circuit. Circuits are powered at low voltages (1.5 to 12 V DC). To prevent one circuit from falsely powering another in the event of insulation failure, the electrical polarity is usually reversed from one section to an adjacent section. These sections are separated by insulated joints, usually in both rails. Circuits through the relay contacts therefore report whether or not the track is occupied.Įach circuit detects a defined section of track, such as a block. The current to the track relay coil drops, and it is de-energised. When a train is present, its axles short ( shunt) the rails together. When no train is present, the relay is energised by the current flowing from the power source through the rails. (series resistor next to battery not shown)Ī track circuit typically has power applied to each rail and a relay coil wired across them. (series resistor next to battery not shown) Schematic drawing of occupied track circuit The basic circuit Schematic drawing of track circuit for unoccupied block They help prevent dispatchers and operators from causing accidents, both by informing them of track occupancy and by preventing signals from displaying unsafe indications. Track circuits allow railway signalling systems to operate semi-automatically, by displaying signals for trains to slow or stop in the presence of occupied track ahead of them. On the other hand, false occupancy readings are disruptive to railway operations and must be minimized. Hence the circuit is designed to indicate the presence of a train when failures occur. Since this is a safety appliance, fail-safe operation is crucial. This circuit is monitored by electrical equipment to detect the absence of the trains. The basic principle behind the track circuit lies in the connection of the two rails by the wheels and axle of locomotives and rolling stock to short an electrical circuit. An alternative to track circuits are axle counters. Illustration of track circuit invented by William Robinson in 1872 Track circuit transformer on the right, new axle counter on the left (Slovenia).Ī track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals.
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