Double switching
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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 insulation.
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[edit] Accidents
[edit] Clapham
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).[citation needed] The signal relay in question was switched ("cut") in only the "active" 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.
It would not have cost a great deal to have made this signal of double switched circuits. If the signal relay had been double-switched, then the likelihood of having two stray wires would have been greatly reduced.
[edit] Baggage door
A similar accident on the United Airlines Flight 811 was caused in part by a single-switched safety circuit for the baggage door mechanism.[citation needed] 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 short term value for money.
[edit] Faulty compass
Big airplanes have three independent attitude indicators, one for the pilot, one for the co-pilot, and a third one to resolve disputes between the first two. A Peruvian airplane apparently had a faulty wire in one of the indicators.[citation needed] Unfortunately, the indicators for the pilot and co-pilot were switched to "common mode", so they both displayed the same wrong attitude indications. In the dark, it was not possible to tell the true horizon in any way other than the attitude indicator, and so the plane crashed into the sea.
