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A track circuit is a simple electrical device used to prove the absence of a Train on a Railroad Track , used to inform signallers and control relevant signals. OPERATION A track circuit typically has power applied to each rail and a Relay coil wired to each rail. When no train is present, the relay is energised, causing the front contacts of the relay to connect (or 'make'). When a train is present its axles short the track circuit and the track relay coil is de-energised, causing the front contacts to break. SAFETY The Fail-safe design of a track circuit means that, should a fault such as power failure occur, the track circuit will show the track to be occupied, to prevent another train from being given movement authority onto the track. Wrong Side Failure s can occur when the rail head (contact between rail and wheel) no longer conducts, for example when there are leaves, sand or rust on the rails. Accidents It is usually the lack of track circuits that cause accidents, though very rarely there are equipment failures. Caused by lack of track circuits
Caused by faulty track circuits
HISTORY The track circuit was invented by William Robinson , and this enabled the automatic block signal. Robinson's block signal was first demonstrated in Model form in 1870 and subsequently installed on the Philadelphia And Erie Railroad at Kinzua , Pennsylvania . It consisted of electrically-operated discs located on top of small signal huts, and was based on an open track circuit. He improved on this with the closed track circuit, replacing the earlier installation in 1872 Ph93 . The "open" operation of the first track circuit installation meant that the short circuit made by the train powered the signal to stop. This was not Fail-safe , and was soon changed. The United Kingdom was rather slow in adopting track circuits, partly perhaps because they were an American invention, and partly because many carriages had wheels with wooden hubs, which would not operate track circuits. SUITABILITY DC track circuits have DC track relays and supply, and insulating block joints on each track isolate each circuit from the next along the track. This is unsuitable for areas where the train receives a DC electrical supply, eg. in Third Rail areas, because the track circuits must be insulated from each other and also would be subject to stray currents. An alternative is to use AC track circuits. These use AC supply and track relays, and track circuits are isolated from each other with inductors or 'spiders' that isolate the AC track circuits but allow the traction return current to pass. NON-ELECTRIFIED LINES The track is divided into electrical sections, using insulated rail joints. A power supply is connected to the rails at one end, with a series resistor. A relay (or other detector) is connected to the rails at the other end of the section. With no trains in the section, the electric circuit is complete and the relay energises (picks up). When a train enters the section, its wheels short-circult the track circuit current, and the relay de-energises (drops). Should a rail or wire break, the relay drops in a Fail-safe manner. A small section of rail near the blockjoint is not protected against rail breaks. Track circuits need insulated sleepers, and good (high) ballast resistance. Wooden sleepers are fine, except in tropical countries where termites are a problem. In the UK, wheels with wooden hubs that defeated the track circuit put off the installation of track circuits for many years, until they could be replaced. The power supply might output 3 V DC, while the relay might receive 1 V DC. Reliability While in theory it would be enough to insulate only one of the two rails between adjacent track circuits, there is a risk that the power supply of track circuit A might have a false feed to the relay of track circuit B. This risk can be reduced by having insulated rail joints on both rails. The polarity of adjacent track circuits is reversed wherever possible as an additional precaution (called "stagger"). The track circuit relays can be biased so that they require the correct polarity of voltage in order to pick up although this is unusual. Other dangers Tracks through damp and muddy sections, such as tunnels, can suffer from potential false feeds caused by the mud acting as a battery across the rails. Such problems can be tackled by Tunnel stick circuits that allow the track relay to pick up only if a train is occupying the next track circuits. The track relay itself has to be made as fail safe as possible, so that for example a surge of current does not weld any contact of the relay with the relay in the up or green position. Ideally, all track circuits should be indicated to a signal box, so that if they fail, there is some chance that the signallers will notice. The track circuits on Cowan Bank affected by sand on the rails were not indicated to a signalbox. ELECTRIFIED RAILWAYS Most electrified railways use the running rails to return traction current to the substations. Traction currents of the order of 1000 A must co-exist with track circuit currents of 1 A. Track currents can be dealt with by
JOINTLESS TRACK CIRCUIT Jointless track circuits use audio frequency tuned circuits to create what amounts to a blockjoint. Frequencies of the Aster SF 15 type track circuit are 1700 Hz and 2300 Hz on one track and 2000 Hz and 2600 Hz on the other. TI21 type track circuits use the following frequencies; A 1699 Hz Down line B 2296 Hz Down line C 1996 Hz Up line D 2593 Hz Up line E 1549 Hz Down line F 2146 Hz Down line G 1848 Hz Up line H 2445 Hz Up line A to D are used in two-track areas, while E to H are additional frequencies for use in four-track areas. Jointless track circuits eliminate most of the impedance bonds that electrified railways would otherwise require. CAB SIGNALLING The track circuit current may be modulated at different frequencies, which can be detected by equipment on the train to provide the driver with a signal indication inside the cab. This is called Cab Signalling . SEE ALSO
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