A dead man switch is a switch or control which, if not continuously held in place (either by a human being or mechanically), will cause a drastic event to occur, such as an explosive detonating or a reactor's control rods dropping into place. The idea is that if the operator should be killed (or, in the case of automation, deactivated), the switch would set the event in motion, in order to prevent a more critical result. 

A simple example of a dead man switch is the safety lever on certain types of hand grenade; once the restraining pin is removed, the lever acts a dead man switch, setting the fuse on the grenade once released.

More generally, a dead man switch is any security system which requires an active response within a period of time; if the response does not occur, or is incorrect, then it initiates some kind of emergency response (setting off an alarm, calling the police department, initiating a lockdown of high-security areas, etc.). In extreme cases the response may be a form of ScorchedEarthPolicy, in which the secured equipment or data are rendered unusable.

It this the same as a Dead Man's Handle (in the UK anyway) which is a safety device on all trains which has to be held continuously in place or it will cause the train to put on emergency brakes. --AndrewCates

The DeadManSwitch is now archaic. In many if not most situations, the systems using a switch has been replaced with a DeadManControl System in which the carrying out of a proper manual action is required upon automatic warning of a hazardous condition. In the case of a train in motion, absence of an acknowledgement results in an AutomaticTrainStop.

''Sorry, but such an assertion is simply incorrect. There remain (and probby will forever) many instances of systems that require a "system fatal" dead man switch configuration simply because no graceful error recovery mechanism exists. To say that an approach is "archaic" just because ''you'' can fit a recovery path into ''your'' system is somewhat shortsighted.''
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See: DeadManControl 

CategoryIdiom, C''''''ategorySafetySystems