A wrong side failure
describes a failure condition in a piece of railway signalling
equipment that results in an unsafe state.
A typical example would be a signal showing a 'proceed' aspect (e.g. green) when it should be showing a 'stop' or 'danger' aspect, resulting in a "false clear". (The converse is a right side failure
, where even with any reduction the resulting state is safe overall).
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 de-energize and the signal will show a red light, which is fail-safe.
If a stray wire from another circuit touches the wire connected to the same relay, 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.
While accidents from the problem are rare, they do occur:
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.