Dead man's switch
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- For other uses, see Dead man's switch (disambiguation).
A dead man's switch (for other names, see Alternative names) is a safety measure intended to take a specific action in case the human operator becomes incapacitated in some way, a form of fail-safe. A very common use is in a train locomotive.
In some countries, most freight elevators, lawn mowers, jet skis and snowblowers use a dead man's switch or similar mechanism that causes them to slow down, stop moving or shut down if the switch is released. Thus, if an accident occurs and causes somebody to faint or become separated from the vehicle, it will not continue down its path, avoiding a potential worsening of the situation.
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Background
Interest in dead-man's controls increased with the introduction of electric streetcars and especially of electrified rapid transit trains, though dead-man equipment was quite rare on the former until comparatively recently. In conventional steam railroad trains, there was always a second person in the cab with the engineer, the fireman, charged with shoveling coal to heat the engine's boiler, and/or otherwise look after the tasks required for a running engine other than its actual operation. For many decades, this was also the practice on electric or diesel engines, which could theoretically be operated by a single person. In the event something should happen to the engineer, the fireman was always available to assist and bring the train to a stop.
With urban and suburban railway systems, the operator, called the motorman, was typically in an enclosed cab or otherwise remote from other personnel in the event an emergency occurred. Though automatic devices were already beginning to be deployed on newer installations of the New York City Subway system in the early 20th century, the disastrous Malbone Street Wreck on the Brooklyn Rapid Transit system in 1918 pointed up the need for universal deployment of devices to halt trains in the event of an operator's disability.
Types
Pneumatically or electrically-linked dead-man's controls, still used today, involve relatively simple modifications of the controller handle, the device that regulates the flow of electricity to the train's motors. The main requirement is that the train's brakes be placed in emergency condition if live pressure is not maintained on the controller.
Typically, the controller handle was a horizontal bar that is 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. In the event pressure is removed from the handle it springs up, releasing the rod's contact with the internal switch and instantly cutting power and applying 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 part of the handle the motorman grasped. This button also had to be pressed continuously, typically with the palm of the hand so that the button's top 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 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 lever must be continually held in place by the train operator; because the lever is also used to both accelerate and stop the train, it must remain in constant use).
Vigilance control
Vigilance control is similar to a dead man's switch, the difference being that a vigilance control system requires that the operator press a button at specified regular intervals. If the operator fails to operate the vigilance control, a warning sounds, and should the driver still not operate the vigilance control the machinery will stop.
A limitation of the system is that if the vigilance control cycle time of a train locomotive is for example a minute, the train could travel for that minute into potentially dangerous territory, such as passing a signal at stop.
Because there is no connection between the vigilance control and an Automatic Warning System, the vigilance control will let some accidents slip through.
The vigilance control described above has the disadvantage of requiring mindless repetition of the vigilance control operating function, something that can be done "with your eyes closed." One way of avoiding this fault is to reset the timer every time evidence can be taken that the driver is alert and on the job. For instance every time drivers operate the traction/braking controller or audible warning.
Something that could be considered an early vigilance control device is the one-legged stool issued to watchmen.
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.
Accidents due to insufficient vigilance control
Software uses
Software versions of dead man's switches are used generally only by people with technical expertise, and 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 for a week consecutively, 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 locking up his or her data securely (or deleting it), while others may just wish to alert friends or the authorities by e-mail that something undesirable might be going on.
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
- Dead man's pedal
- Dead man's handle
- Dead man's control
- Dead man's pedal
- Dead man's brake
- Deadman's
Penalty switches are known as Vigilance controls on locomotives, railcars and multiple units
Event recording
The status of the vigilance may be recorded on the trains event recorder (aka black box)de:Totmanneinrichtung es:Pedal de hombre muerto ja:デッドマン装置 sv:Dödmansgrepp