Double switching

Double switching, double cutting, or double breaking is the practice of using a multipole switch to close or open both the positive and negative sides of a DC electrical circuit, or both the hot and neutral sides of an AC circuit. This technique is used to prevent shock hazard in electric devices connected with unpolarised AC power plugs and sockets. Double switching is a crucial safety engineering practice in railway signalling, wherein it is used to ensure that a single false feed of current to a relay is unlikely to cause a wrong side failure. It is an example of using redundancy to increase safety and reduce the likelihood of failure, analogous to double insulation. Double switching increases the cost and complexity of systems in which it is employed, for example by extra relay contacts and extra relays, so the technique is applied selectively where it can provide a cost-effective safety improvement.

A landslip or washaway detector is buried in the earth embankment, and opens a circuit should a landslide occur. It is not possible to guarantee that the wet earth of the embankment will not complete the circuit which is supposed to break. If the circuit is double cut with positive and negative wires, any wet conductive earth is likely to blow a fuse on the one hand, and short the detecting relay on the other hand, either of which is almost certain to apply the correct warning signal.

The Clapham Junction rail crash of 1988 was caused in part by the lack of double switching (known as "double cutting" in the British railway industry). The signal relay in question was switched only on the hot side, while the return current came back on an unswitched wire. A loose wire bypassed the contacts by which the train detection relays switched the signal, allowing the signal to show green when in fact there was a stationary train ahead. 35 people were killed in the resultant collision.

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