Dead man's switch
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 as locomotives, aircraft refuelling, freight elevators, lawn mowers, tractors, personal watercraft, outboard motors, chainsaws, snowblowers, treadmills, snowmobiles, amusement rides, and many medical imaging devices.
A more extreme version is Russia's Dead Hand program, which allows for either automatic or semiautomatic launch of nuclear missiles should a number of conditions be met, even if all Russian leadership were to be killed.
[1] 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.
Interest in dead man's controls increased with the introduction of electric trams (streetcars in North America) and especially electrified rapid transit trains.
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.
[5] 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.
The handle springs up if pressure is removed, releasing the rod's contact with the internal switch, instantly cutting power and applying the brakes.
Another method used, particularly with some lever-type controllers, which are rotated rather than pushed or pulled, requires that the handle on the lever be turned through 90 degrees and held in that position while the train is in operation.
This is done because the Autopilot system is not capable of full-self driving, and requires that the driver be able to take over operation of the vehicle without warning, should the car encounter a problem it does not know how to solve.
If the user loses grip of the saw, the springs in the throttle trigger will push it back out to the off or idle setting, stopping the blade from spinning.
A backup dead-man's switch button is provided on the side of the controller for use in the case of a failed touch sensor or if it is too cold to remove gloves.
In the Waterfall train disaster, south of Sydney, Australia, in 2003, the driver suddenly died of a heart attack, and his slumped body kept the pedal depressed.
Should the rider fall off the vehicle or the operator at least move away from the controls, the cord will be pulled out of the dead man's switch, turning off the engine or setting the throttle position to "idle".
In the case of treadmills, the dead man's switch usually consists of an external magnet attached to a cord that clips to the user.
Once armed, the system would detonate the onboard nuclear weapons if the aircraft dropped below a predetermined level,[17] typically due to being shot down.
The main safety failing with the basic dead man's system is the possibility of the operating device being held permanently in position, either deliberately or accidentally.
Vigilance control was developed to detect this condition by requiring that the dead man's device be released momentarily and re-applied at timed intervals.
[18] Software versions of dead man's switches are generally only used by people with technical expertise, and can serve several purposes, such as sending a stored message, a notification to friends, or deleting and encrypting data.
System components on a spacecraft that put it into a safe mode or cause it to execute default behaviors when no command is received within a predefined time window can be considered a dead man's switch, but hardware or software that attempts to receive a command from human operators through an alternate channel is an auto-recovering or adaptive communications system, not a dead man's switch.
More recent safety standards do not consider this to be adequate, as the driver may slump over the dead man's handle and continue to hold it down even though they are not capable of controlling the train.
If they do not respond by moving a controller, or releasing and then re-applying the dead man's handle, the system will automatically initiate an emergency brake application.
This automation capability has been made possible by advancements in computing, control, and navigation technologies and is of particular importance in a general aviation setting since private aircraft are often flown by only a single pilot.
The term "dead man's switch" is sometimes used to describe a form of defensive blackmail or insurance file in which the release of damaging material is threatened if anything happens to a person.
