Dead man's switch

A dead man's switch (or dead man's handle) is a device intended to take some action if the human operator becomes incapacitated in some way, a form of fail safe practice. While such devices have been used in suicide bombing, to trigger the explosive if the bomber is shot, they have many less exceptional uses, most of which are intended to promote safety.

For example, most freight elevators and lawn mowers use a dead man's switch or a similiar mechanism, causing them to respectively stop closing the elevator door or shut down if the switch is released. If a person faints while mowing the lawn, their hands will relax and the mower will shut off.

A very common use is in a locomotive, especially those used on underground railway systems or other rapid transit systems. The driver is required to hold down the dead man's handle to keep the train running - if he is unable to do so, the train will brake and come to a stop. The recent inquiry into the Waterfall train disaster in Sydney, Australia found that it was common practice to wedge the dead man's handle (in fact a footplate) down using an umbrella or signalling flag, since the drivers found it tiring to apply pressure continually. Naturally this compromised safety, and has been one of the factors found to have caused this crash.