Wrong-side failure
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A false clear or wrong side failure is a problem within a piece of railway signalling equipment that causes the output to show an indication more permissive than should be allowed. A typical example would be a signal showing a proceed aspect (say; green) when it should be showing a stop or danger aspect.
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[edit] Example of how a wrong side failure may occur
Consider a relay that has to energize to show a green light.
If a wire breaks, then the relay will show a red light, which is fail safe.
If a stray wire from another circuit touches the wire connected to the relay above, then that would be a wrong side failure, which is potentially dangerous.
This stray wire can be guarded against by ensuring that the insulation on the relay wire is of good quality, and that all terminals are locked away.
In addition, the relay may be double-switched, that is to say that it only energizes if a positive circuit and a negative circuit are both complete. That would then require two stray wires to cause a wrong side failure, which is much less likely than a single stray wire.
[edit] Accidents
While rare, they do occur and here are some examples.
- 1876 - Abbots Ripton rail disaster - signals frozen by blizzard in clear position. A signal arm that disappeared into a slot in the post was bound to get stuck, sooner or later.
- 1953 - Sydenham Rail Disaster - 5 killed
- 1979 - Invergowrie rail crash - signal was at "WRONG" yet taken by driver to be proceed signal.
- 1988 - Clapham Junction rail crash - single stray wire causes false green signal and collision killing 35. The signals affected had been behaving strangly for some time, but not enough to pin down a real problem.
- 1980s - Southern Region of BR - driver of stationary train sees signal ahead go to green, while train ahead still on line. Track circuit wiring fault somewhat like Clapham Junction rail crash. Alerts signallman. No accident.
- 2005 - Deelfontein rail crash - solder splatter bridges relay contact . Most testing is done with the relays unplugged which would not detect the splatter, while the solder splatter is likely to lead to a difficult to trace phantom problem.
[edit] False alarms
Railway authorities usually give the drivers and signalmen the benefit of the doubt, and investigate whether a wrong-side failure is the cause of the accident. This occurred with the Hinton train collision, but investigations soon showed that a wrong-side failure was not the cause.
[edit] See also
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