Dead man's switch

A dead man's switch (for other names, see alternative names), as its name suggests, is a device intended to stop a machine in case the human operator becomes incapacitated, and is a form of fail-safe. They are commonly used in train locomotives, freight elevators, lawn mowers, tractors, jet skis, outboard motors, snowblowers and snowmobiles.

Background
Interest in dead-man's controls increased with the introduction of electric streetcars and especially electrified rapid transit trains, though dead-man equipment was quite rare on US streetcars until comparatively recently. In conventional steam railroad trains, there was always a second person with the engineer, the fireman, who could bring the train to a stop if necessary. For many decades this practice continued on electric & diesel locomotives, even though a single person could theoretically operate them. With modern urban and suburban railway systems, the driver was typically alone in an enclosed cab. Though automatic devices were already beginning to be deployed on newer installations of the New York City Subway system in the early 20th century, it took the disastrous Malbone Street Wreck on the Brooklyn Rapid Transit system in 1918 to point out the need for universal deployment of such devices to halt trains in the event of an operator's disability.

Types
Handle

Pneumatically or electrically linked dead-man's controls, still used today, involve relatively simple modifications of the controller handle, the device that regulates traction power. The main requirement is that the train's emergency brakes are applied if pressure is not maintained on the controller.

Typically, the controller handle is a horizontal bar, rotated to apply the required power for the train. Attached to the bottom of the handle is a rod which, when pushed down, contacts a solenoid or switch inside the control housing. The handle springs up if pressure is removed, releasing the rod's contact with the internal switch, instantly cutting power and applying the brakes.

Though there are ways that this type of dead-man's control could conceivably fail, they have proven highly reliable.

On some earlier equipment, pressure was not maintained on the entire controller, but on a large button protruding from the controller handle. This button also had to be pressed continuously, typically with the palm of the hand so that the button was flush with the top of the handle. Another method used, particularly with some lever-type controllers, which are pushed or pulled rather than rotated, requires that the handle on the lever be turned through 90 degrees and held in that position while the train is in operation.

Some dead-man's controls require the motorman to hold it in the mid-position rather than apply full pressure (see pilot valve).

In many modern New York Subway trains, for example, the dead man's switch is incorporated into the train's speed control. On the R142A the train operator must continually hold the lever in place. This was depicted in the movie and book The Taking of Pelham 123, in which a group of men hijack a New York City subway train for ransom, but because of the Dead-man's feature, cannot escape while the train is moving.

Every lawn mower sold in the US since 1982 has an "operator-presence" device, which by law must stop the blades within 3 seconds after the user lets go of the controls.

Touch Sensor

On the Nottingham Express Transit vehicles, the tram's speed controller is fitted with a capacitive touch sensor to detect the driver’s hand. If the hand is removed for more than a short period of time, the track brakes are activated. Gloves, if worn, have to be finger-less for the touch sensor to operate. A back up 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.

Pedal

A pedal can be used instead of a handle. In the Waterfall train disaster, it appears that the driver, who was overweight, slumped on his seat, keeping the pedal pressed when he died.

Seat Switches

On tractors, the switch is beneath the seat, and will cut the engine if the operator gets off the tractor while the transmission is engaged or the power take-off is spinning.

Key Switches

On recreational water vehicles, such as jet skis and wave runners, the user has the ignition key attached to his wrist or waist by a leash. The key will be removed from the ignition switch if the rider falls into the water, thus turning off the engine.

Vigilance control
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.

Something that could be considered an early vigilance control device is the one-legged stool issued to watchmen: If the watchman were to fall asleep on his stool, he would fall over and be woken.

There has also been a proposal to introduce a similar system to automotive cruise controls.

A hybrid between a dead man's switch and a vigilance control device is a dead-man's vigilance device.

Software uses
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 notification to friends or deleting and encrypting data. The "non-event" triggering these can be almost anything, such as failing to log in consecutively for a week, not responding to an automated e-mail ping, a GPS-enabled telephone not moving for a period of time, or merely failing to type a code within a few minutes of a computer's boot. Motivations vary, depending on the individual's needs. For example, somebody in a police state may be concerned about the security of their data (or deleting it), while others may just wish to alert friends or the authorities by e-mail that something undesirable might be going on.

Miscellaneous uses
Dead man's switch devices have also been used in suicide bombing, to trigger the explosive if the bomber is shot or overpowered. This is a fail-deadly mechanism, rather than a fail-safe mechanism.

Alternative names

 * Driver's Safety Device (DSD) is the official UK railway term.
 * Dead man's pedal
 * Dead man's handle
 * Dead man's control
 * Dead man's brake
 * Kill Switch
 * Vigilance control
 * Live-man switch

Event recording
The status and operation of both the vigilance and dead-man's may be recorded on the train's Event recorder (aka black box).