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, or the battery is drained, 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 that 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, also known as double-cut, 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.

Accidents

While accidents from the problem are rare, they do occur:

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 an accident. This occurred with the Hinton train collision, but investigations soon showed that a wrong-side failure was not the cause. The 2008 Chatsworth train collision was determined to be human error, not wrong-side failure as a few initially said.

Wrong-side failures to be reported

In the United States there is a rule that wrong-side failures are to be reported to the Federal Railroad Administration.