Big Red Button

Kill switch
A kill switch without a plastic cover
Classification	Mechanical component
Industry	Automotive, boating, energy, engineering, entertainment
Powered	Some are mechanical and others are powered

A kill switch, also known as an emergency stop (e-stop) or emergency power off (EPO), is a safety mechanism used to shut off a device or machinery in an emergency situation in which it cannot be shut down in the usual manner. Unlike a normal shut-down switch/procedure, which shuts down all systems in an orderly fashion and turns the machine off without damaging it, a kill switch is designed and configured to completely and as quickly as possible abort the operation (even if this damages equipment) and be operable in a manner that is quick and simple (so that even a panicking operator with impaired executive function or a bystander can activate it). Kill switches are usually designed so as to be obvious even to an untrained operator or a bystander.

Many kill switches feature a removable barrier or other protection against accidental activation (e.g., a plastic cover that must be lifted or glass that must be broken). Such a removable barrier is commonly called a Mollyguard. Kill switches are featured especially often as part of mechanisms whose normal operation or foreseeable misuse may cause injury or death; designers who include such switches consider damage to or destruction of the mechanism to be an acceptable cost of preventing that injury or death.

A similar system, usually called a dead man's switch (for other names, see alternative names), as its name suggests, is a device intended to stop a machine (or activate one) if the human operator becomes incapacitated, and is a form of fail-safe. They are commonly used in industrial applications (e.g., locomotives, tower cranes, freight elevators) and consumer applications (e.g., lawn mowers, tractors, jet skis, outboard motors, snowblowers, motorcycles and snowmobiles). The switch in these cases is held by the user, and turns off the machine if they let go. Some riding lawnmowers have a kill switch in the seat which stops the engine and blade if the operator's weight is no longer on the seat.

On railways, an emergency stop is a full application of the brakes in order to bring a train to a stop as quickly as possible. This occurs either by a manual emergency stop activation, such as a button being pushed on the train to start the emergency stop, or on some trains automatically, when the train has passed a red signal or the driver has failed to respond to warnings to check that he/she is still alert, which is known as a dead man's switch. A similar mechanism is the watchdog timer.