The Definition of Kill Switch (Emergency Stop)

Definition of Kill Switch (Emergency Stop)

A kill switch, also called an emergency stop or e-stop, is a manually operated control that quickly stops a machine, vehicle or process when a person is in danger or equipment is about to be damaged. It is designed to be easy to find, easy to reach and simple to operate under stress, usually with a single push, pull or trip of a lanyard.

Kill switches are found on industrial machinery, conveyors, robots, presses, packaging lines, power tools, treadmills, personal watercraft, boats, lawn equipment and many other machines. On fixed equipment they usually take the form of a red mushroom-head push button. On boats and mobile equipment they are often a lanyard switch that shuts off the engine if the operator is thrown from the controls.

An emergency stop is a backup measure used when something has already gone wrong. It is not a replacement for machine guarding, safe design or lockout/tagout (LOTO), and it should never be used to make a machine safe for maintenance or servicing.

How Emergency Stops Work

Design requirements

Emergency stop functions on machinery are covered by several consensus standards. ISO 13850 sets principles for the design of the emergency stop function. IEC 60204-1 and, in the United States, NFPA 79 cover the electrical equipment of machines, including how stop circuits are built. Key design features include:

• Color and marking: the actuator is red, and the area behind it is yellow where a background is used, so the device stands out from normal controls.
• Shape: mushroom-head push buttons, pull cords, foot switches and bars are common, chosen so they can be operated quickly with a hand, foot or body.
• Latching: once pressed, the device stays in the stop position until a person deliberately resets it, often by twisting or pulling the button.
• Direct-opening contacts: electrical e-stop devices use contacts that are mechanically forced open, so a welded contact cannot keep the machine running.
• Priority: the emergency stop overrides all other functions and operating modes.
• Location: buttons are placed at each operator station and other points where a stop may be needed, and kept clear of obstructions.

Stop categories

IEC 60204-1 and NFPA 79 describe three stop categories. Only the first two are allowed for an emergency stop.

• Category 0: an uncontrolled stop, in which power is removed from the machine actuators immediately.
• Category 1: a controlled stop, in which power stays available long enough to bring the machine to a stop, such as by braking a motor, and is then removed.
• Category 2: a controlled stop in which power remains applied after stopping. This category is used for normal stops, not emergency stops.

The right category depends on the risk assessment. A Category 1 stop may be safer for a large rotating machine that would otherwise coast for a long time, while a Category 0 stop may be preferred where cutting power instantly is safest.

Reset behavior

Resetting an emergency stop device should only allow the machine to be restarted. It must not restart the machine by itself. A separate, deliberate start command is needed, so that a worker who resets a button cannot accidentally set equipment in motion while someone is still in the danger zone.

Lanyard Kill Switches

A lanyard kill switch connects the operator to the equipment with a cord or clip. If the operator moves away or falls, the cord pulls free and shuts off the engine. These switches are common on boats, personal watercraft, snowmobiles, all-terrain vehicles, go-karts and some fitness equipment.

• Boats: in the United States, a federal requirement that took effect April 1, 2021 requires operators of covered recreational vessels less than 26 feet long to use an engine cut-off switch when running on plane or above displacement speed. Covered boats are those with a cut-off device installed or built from January 2020 onward.
• Why it matters: an operator thrown overboard can be struck by a circling, unmanned boat and its propeller, which adds to the drowning hazard.
• Wireless options: some systems use a wireless fob worn by the operator that stops the engine when it moves out of range or is submerged.

Common Problems and Best Practices

• Not a substitute for LOTO: an emergency stop is a control circuit device. It does not isolate energy sources or dissipate stored energy, and anyone can reset it. Workers who service equipment must follow de-energization and lockout procedures.
• Not a substitute for guarding: standards treat the emergency stop as a complementary protective measure. Machines still need guards, interlocks and other safeguards.
• Blocked or hidden buttons: pallets, carts or tools left in front of e-stops can delay a stop. Keep access clear and mark locations.
• Bypassing: taping, disabling or rewiring an e-stop is a form of bypassing safety devices and should be treated as a serious violation.
• Testing: emergency stops should be tested on a regular schedule to confirm the machine stops as expected. Record the results and fix failures before the machine returns to service.
• Training: workers should know where every e-stop is, what it stops and how restart is authorized after use.
• Unworn lanyards: a lanyard switch only works if it is attached to the operator before the engine is started.

Example of Kill Switch (Emergency Stop)

At a packaging plant, an operator reached into a case erector to clear a jammed carton while the machine was paused. The machine cycled unexpectedly and caught the edge of the operator's glove. A coworker standing at the end of the line hit the nearest emergency stop button, which cut power to the drive motors and stopped the machine before the operator's hand was drawn in. The operator was not injured.

The investigation found that operators routinely cleared jams with the machine in a paused state, relying on the e-stop as protection. The plant retrained staff on lockout/tagout for jam clearing, installed an interlocked guard door at the jam point, added e-stop buttons at both ends of the machine and began monthly functional testing of every emergency stop on the line.

Frequently Asked Questions: Kill Switch (Emergency Stop)

Can an emergency stop be used instead of lockout/tagout?

No. An emergency stop is meant for emergencies. It does not isolate or lock out energy sources, it does not release stored energy and it can be reset by anyone. Maintenance and servicing require a proper lockout/tagout procedure.

Why are emergency stop buttons red and yellow?

Machine safety standards call for a red actuator, with a yellow background where one is used. The color combination makes the device easy to spot and distinguishes it from normal start and stop controls.

Should a machine restart when the e-stop is reset?

No. Resetting the device should only allow the machine to be restarted. A separate start command is needed so the machine cannot start unexpectedly.

What is a lanyard kill switch?

It is a switch connected to the operator by a cord or clip. If the operator falls or moves away from the controls, the cord pulls out and the engine shuts off. Lanyard switches are common on boats, personal watercraft and other vehicles.