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Double switching is the practice in Railway Signalling in particular of cutting the power to a relay in both the positive and negative sides, so that a single false feed of current to that relay is unlikely to cause a wrong side failure.
It is analogous to Double insulated.
The Clapham Junction rail crash of 1988 was caused in part due to the lack of double switching. The signal relay in question was switched ("cut") in only the "active" side, while the return current came back on a 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 lost their lives in the resultant collision.
In the scheme of things, it would not have cost a great deal to have made this signal of double switched circuits.
A similar accident (shown on cable TV) involving an aircraft was caused in part by a single switched safety circuit for the baggage door mechanism. Failure of the insulation of the wiring in that circuit allowed the baggage door to be unlocked by a false feed, leading to a catastrophic de-pressurisation.
A small difference in the train and aircraft examples, is that weight is critical on an aircraft, while it is not critical with a railway signal. Double switching also incurs the cost of extra relay contacts and extra relays, so double switching is necessarily applied selectively to get value for money.
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