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A kill switch without a plastic cover
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A kill switch, also known as an emergency stop or e-stop, 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, 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.
In large ships, an emergency stop button pulls the countershaft for the fuel pumps to the stop position, cutting off the fuel supply and stopping the engines. With a controllable-pitch propeller, the stop button may declutch the engine from the propeller.
NASCAR (National Association for Stock Car Auto Racing) requires all their stock cars to be equipped with a steering wheel-mounted kill switch, in case the accelerator pedal sticks and the driver needs to shut down the engine.
Kill switches are also used on land vehicles as an anti-theft system and as an emergency power off. Such devices are often placed in bait cars and configured so that observing police can trigger the switch remotely.
A related concept is the dead man's switch, where the operator must be holding a button or lever any time the vehicle is operating. A common example of this would be the kill switches used by boaters wherein a cord connects the kill switch to the operator (usually by the operator's life jacket), and if the operator is thrown overboard in an accident, the cord will pull the switch and immediately shut down the vessel's engine. This prevents it from becoming a run-away vessel that could impose a danger to other vessels or swimmers at sea, and allows the operator to swim back to the vessel and re-board it. A similar device is featured on most lawnmowers: a lever on the handle blocks the engine's fuel supply and applies a brake to the blades as long as it is not held down.
Monster Truck Racing Association requires all of their monster trucks to be equipped with kill switches (either remote or in cab), in case the monster truck loses control and the driver needs to shut off the engine. Monster trucks' kill switches are tested before races.
Pioneer era planes & World War I aircraft
Early aviators using rotary engine-powered aircraft from the beginnings of their use in 1908, up through the end of World War I in 1918 had what could be called a reversed functionality version of the "dead man's switch" for cutting the ignition voltage to the spark plugs on such a powerplant, to give a degree of in-flight speed control for a rotary engine. This was often called a "blip switch" or "coupe switch" (from the French term coupez, or "cut") and when not being pressed, allowed the high voltage from the engine's magnetos to operate the ignition with normal engine operation in flight — pressing the "blip switch" cut the flow of high voltage from the magnetos, stopping the combustion process in the cylinders. When such a "blip switch" was intermittently used on landing approach, this allowed a limited degree of engine speed control, as rotary engines generally did not have a conventional throttle in their carburetors to regulate engine speed, but only for governing the fuel-air ratio for startup and full-speed operation.
On large industrial machines, an emergency stop button is typically located on the panel, and possibly in several other areas of the machine. This provides a rapid means to disconnect the energy source of the device to protect workers. For fail-safe operation, the emergency stop button is a normally closed switch which ensures that a broken wire will neither accidentally activate the emergency stop nor prevent it from being activated. On machinery controlled by a Programmable Logic Controller, the emergency stop is designed in a way that it overrides the output of the controller.
In the European Union, most types of machinery are required to be equipped with an emergency stop according to the Directive 2006/42/EC. Exceptions apply for machinery in which an emergency stop would not lessen the risk as well as for portable hand-held/hand-guided machinery.
A kill switch is also used for gasoline pumps or any other device that pumps large amounts of explosive or flammable chemicals. Most vehicles nowadays also have a kill switch that cuts power to the fuel pump if the vehicle is overturned. There is commonly a single kill switch for all pumps at a pumping station. The kill switch is also used on such things as industrial band saws and belt sanders. Kill switches are also found on school-use electric powered tools such as drills and wood/metal lathes.
Elevators and escalators
Elevators often have a red two-way button on the control panel which is either marked "Emergency Stop" or "Run/Stop". Normally, the button is in the "up" or unpushed position, allowing the elevator to "run" in normal service. When the button is pushed, the elevator comes to an immediate stop. When the button is pulled back out, it resumes normal service, thus the reason for the use of the phrase "Run/Stop". Escalators will typically have a key-operated control that will turn the escalator off, or change its direction to up or down. Next to the key switch will be a red "Emergency Stop" button, which is used in the event of equipment failure, or where there is a potential for injury, such as when someone's shoe gets stuck in the "comb" at the top or bottom of the escalator and there is a risk of serious injury. The key switch is used to return the escalator to service after it has been stopped.
Electric bass and electric guitar
An electric musical instrument, such as a electric guitar or bass guitar, may have a kill switch, also known as a "stutter switch". They are typically used by heavy metal music and hardcore punk guitarists. A kill switch works by switching between the hot signal from a pick-up and the ground signal, and does not break the circuit. A typical way of achieving this is (on a guitar with a volume control for each pick-up) by turning down the volume on one of the pick-ups then alternating the pick-up selector switch between that pick-up and one with the volume turned up. They are sometimes used when a singer doubles as a guitar player and wants to mute their guitar from sounding during a vocal solo. Kill switches are also used as a special effect during a song, which enables a guitarist to play a chord or note and then have that chord or note "stutter" on and off in a rhythmic fashion. Kill switches are rarely a feature in consumer-grade, mass market instruments, and are usually installed as an after-market modification, either by the guitarist themselves or by a specialist instrument workshop/store. The kill switch circuit can also be built into an effects pedal and controlled by a foot switch.
Well-known users of the kill switch circuit are Tom Morello of Rage Against the Machine and Audioslave, Victor Smolski of Rage, Josh Elmore of Cattle Decapitation (most notably on "The Ripe Beneath the Rind"), Buckethead (on, for example, "Jordan", "Soothsayer", and "Circarama"), Ace Frehley of Kiss, and Jonny Greenwood of Radiohead. Jack White uses a kill switch during some of his performances and in songs like "Icky Thump". Greg Ginn of Black Flag achieved the notorious feedback scream which opens most of the tracks on Damaged with a kill switch. More recently, Eddie Van Halen has been using a custom EVH Wolfgang guitar on which he has had a kill switch installed.
MIDI keyboards sometimes have a "panic switch" or a kill switch which stops an unwanted sustained note from continuing. On rare occasions, a MIDI keyboard may get "stuck" on a held note, which can disrupt the performer's show. The reason that MIDI notes can get stuck is because a MIDI cable does not carry a sound signal, like an electric guitar or electric bass patch cable. A MIDI cable carries digital instructions regarding a note, such as its pitch, when to start the note, and when to end the note. If the "end the note" message is not received, then that note may continue to sustain.
Treadmills often use a safety key (magnetic based) that the runner clips to his or her waist. The safety key is pulled out if the runner falls down, and the treadmill stops immediately. In other cases treadmills have a more traditional kill switch, often mounted towards the rear of one of the hand-railings.
The emergency stop on an amusement ride is similar to that on industrial equipment. Typically brakes on a ride are designed to be disengaged when power is applied; disconnecting power will cause all brakes to engage. Most amusement rides have a computer that can, similar to the rail example provided above, engage the emergency stop when such a ride is determined to be out of operating specification.
The concept of a "kill switch" is also found in computer software. The practice of software applications connecting to a remote server as part of their normal operation is ubiquitous in computer software. This can be done to facilitate automatic software updates, and to gain data regarding the usage or users of the software. Notably, this provides a means for the software developer to impose access restrictions on the client's use of the software, effectively functioning as a remote kill switch. The client software may be instructed to halt operation or even disable or remove itself permanently upon certain condition being satisfied. For example, the software may be required to authenticate itself with the remote server, with a failure resulting in degraded or outright disabled functionality. One notable example of this is the anti-piracy features in Windows Vista, which reduce the functionality of the product if they determine that the installation is not genuine. In the example of Microsoft Windows, the company developed a verification tool named Windows Genuine Advantage, that originally activated a kill switch, or reduced functionality mode, on what Microsoft's mandatory software deemed to be an unlicensed copy of the operating system. Software kill switches have been shown to have varying degrees of success, as false positives have been known to occur, prompting some vendors like Microsoft to "turn off" the software kill switch in response to market pressure.
There is some anecdotal evidence that some software vendors install kill switches in their software to enforce planned obsolescence, also known as a forced update. This can cause considerable disruption in said customer's business functions.
With the shift towards application stores for software distribution, particularly on mobile devices, Apple, Google, and Microsoft maintain software kill switches which allow software to be remotely removed from devices, most prominently to guard users against security threats.
There have been increasing calls for remotely operated kill switches to be enabled on smartphones  so that a phone owner can remotely disable a lost or stolen phone. This prevents a thief from being able to use the stolen phone to make long distance calls or accrue other charges.
- Dead man's switch
- Hardware Trojan
- Internet kill switch
- Blue Light Station
- SawStop a fast kill switch for table saws triggered by electrical conductivity of user's finger
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