A dead man's switch is a switch that is designed to be activated or deactivated 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, as in computer software.
These switches are usually used as a form of fail-safe where they stop a machine with no operator from a 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, treadmills, 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.
Dead man's switches are not always used to stop machines and prevent harm; such switches can also be used as a fail-deadly, since a spring-operated switch can be used to complete a circuit, not only to break it. This allows a dead man's switch to be used to activate a harmful device, such as a bomb or improvised explosive device. The switch that arms the device is only kept in its "off" position by continued pressure from the user's hand. 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 is the handwritten letters of last resort from the Prime Minister of the United Kingdom to the commanding officers of the four British ballistic missile submarines. They contain orders on what action to take if the British government is destroyed in a nuclear attack. After a prime minister leaves office the letters are destroyed unopened.
This 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 Vault 7.
Related devices are called dead-man's vigilance device and kill switch.
Background
Interest in dead man's controls increased with the introduction of electric trams (streetcars in North America) and especially electrified rapid transit trains. The first widespread use came with the introduction of the mass-produced Birney One-Man Safety (tram) Car, though dead-man equipment was fairly rare on US streetcars until the successful PCC streetcar, which had a left-foot-operated dead man's pedal in conjunction with the right-foot-operated brake and power pedals. This layout has continued to be used on some modern trams around the world. In conventional steam railroad trains, there was always a second person with the engineer, the fireman, who could almost always bring the train to a stop if necessary. For many decades two people were assigned to electric and diesel locomotives as well, even though a single person could theoretically operate them.
With modern urban and suburban railway systems, the driver is typically alone in an enclosed cab. Automatic devices were already beginning to be deployed on newer installations of the New York City Subway system in the early 20th century. The Malbone Street Wreck on the Brooklyn Rapid Transit system in 1918, though not caused by driver incapacitation, did spur the need for universal deployment of such devices to halt trains in the event of the operator's disability. According to a Manhattan borough historian, there have been at least three instances where the dead man's switch was used successfully – in 1927, 1940, and 2010.
The status and operation of both vigilance and dead man's switch may be recorded on the train's event recorder (commonly known as a black box).
Types
Handle
Many dead man's switches are mounted in the control handle of a vehicle or machine and engage if the operator ever loses their grip.