A dead man's switch is a safety feature integrated into many machines which works to turn the machine off if the operator experiences a problem. As the name implies, it will turn a machine off even if the operator dies, but such devices will also turn off the engine if the operator becomes incapacitated. In many cases, such switches can also function when the operator is harassed or disrupted, as might be the case with a train operator being bothered by passengers.
There are a number of different styles of dead man's switch, but all of them require some sort of input from the operator. When the operator stops interacting with the switch, a signal is sent to cut the engine, thereby turning the machine off and applying the brakes, if applicable. While not an ideal fail-safe, this measure can prevent loss of life and property damage.
In a classic version, the switch is located on a lever or bar that the operator must maintain constant contact with. On a lawnmower, for example, users must hold the handle of the lawnmower in position to keep the engine on. A dead man's switch can also be a button or pedal which must be held down to keep the machine turned on, and some advanced systems use touch sensors which rely on very light pressure of hands or feet.
Dead man's switches can be found on a wide variety of machines, including trains, tractors, boats, and heavy lifting equipment. They can also be found on machines designed for personal use, like treadmills and speedboats. A common form on some devices is a key tether, which pulls the key out of the ignition if the user falls off or slumps back, cutting the power.
You may also hear a dead man's switch referred to as a kill switch, dead man's handle, dead man's brake, or dead man's control. In the case of a machine that is used to transport large amounts of people or cargo, the switch is commonly linked with a data recorder, so that in the event of an accident, investigators can determine whether or not the safety feature worked as it should have.
One unexpected bonus of the dead man's switch is that it can make machinery very difficult to steal, except by experienced operators. Such switches are often subtle or confusing to use for people who have not been trained on a particular machine and, as a result, thieves may find it difficult or impossible to start the machine to steal it.