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With modern urban and suburban railway systems, the driver is typically alone in an enclosed cab. For many decades two people were assigned to electric and diesel locomotives as well, even though a single person could theoretically operate them.
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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. This layout has continued to be used on some modern trams around the world. 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. Interest in dead man's controls increased with the introduction of electric trams (streetcars in North America) and especially electrified rapid transit trains.
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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 WikiLeaks' "insurance files". After a prime minister leaves office the letters are destroyed unopened. They contain orders on what action to take if the British government is destroyed in a nuclear attack.
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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.Ī 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. The Special Weapons Emergency Separation System is an application of this concept in the field of nuclear weapons. 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 switch that arms the device is only kept in its "off" position by continued pressure from the user's hand. This allows a dead man's switch to be used to activate a harmful device, such as a bomb or IED. 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.ĭead 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. 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. 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. 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. A pedal acting as a dead man's switch in a bucket lift truckĪ dead man's switch (see alternative names) 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.