A dead man's switch (for other names, see alternative names) is a switch that is automatically operated if the human operator becomes incapacitated, such as through death, loss of consciousness, or being bodily removed from control. Originally applied to switches on a vehicle or machine, it has since come to be used to describe other intangible uses like in computer software.
These switches are usually used as a form of fail-safe where they stop a machine with no operator from potentially dangerous action or incapacitate a device as a result of accident, malfunction, or misuse. They are common in such applications in locomotives, aircraft refuelling, freight elevators, lawn mowers, tractors, personal watercraft, outboard motors, chainsaws, snowblowers, tread machines, snowmobiles, amusement rides, and many medical imaging devices. On some machines, these switches merely bring the machines back to a safe state, such as reducing the throttle to idle or applying brakes while leaving the machines still running and ready to resume normal operation once control is reestablished. When the switch is an electrical one, it is usually wired as part of a series circuit.
Dead man's switches are not always used to stop machines and prevent harm. These switches can also be used as a fail-deadly. A spring-operated switch can also be used to complete a circuit when it is no longer held down. This means that a dead man's switch may be used to activate a harmful device, such as a bomb or IED. The user holds down a switch of some sort in their hand which arms the device. The device will activate when the switch is released, so that if the user is knocked out or killed while holding the switch, the bomb will detonate. The Special Weapons Emergency Separation System is an application of this concept in the field of nuclear weapons. A more extreme version is Russia's Dead Hand program, which allows for automatic launch of nuclear missiles should a number of conditions be met, even if all Russian leadership were to be killed. A similar concept has been employed with computer data, where sensitive information has been previously encrypted and released to the public, and the "switch" is the release of the decryption key, as with WikiLeaks' "insurance files".
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