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Interlocking is a method of preventing undesired states in a state machine, which in a general sense can include any electrical, electronic, or mechanical device or system.

In most applications an interlock is a device used to help prevent a machine from harming its operator or damaging itself by stopping the machine when tripped. Household microwave ovens are equipped with interlock switches which disable the magnetron if the door is opened. Similarly household washing machines will interrupt the spin cycle when the lid is open. Interlocks also serve as important safety devices in industrial settings, where they protect employees from devices such as robots, presses, and hammers. While interlocks can be something as sophisticated as curtains of infrared beams and photodetectors, they are often just switches.

Trapped key interlocking (commonly called Castell Interlocking) is a method of ensuring safety in industrial environments by forcing the operator through a sequence using a defined selection of keys, locks and switches.

In microprocessor architecture an interlock is hardware that stalls the pipeline (inserts bubbles) when a hazard is detected until the hazard is cleared. One example of a hazard is if a software program loads data from the system bus and calls for use of that data in the following cycle in a system in which loads take multiple cycles (a load-to-use hazard).

Interlocks may be strictly mechanical, as in the internal safety of a firearm that prevents release of the firing pin unless the chamber is properly closed.

In the operation of a device such as a press or cutter that is hand fed or the workpiece hand removed, the use of two buttons to actuate the device, one for each hand, greatly reduces the possiblilty of operation endangering the operator. No such system is fool-proof, and such systems are often augmented by the use of cable–pulled gloves worn by the operator; these are retracted away from the danger area by the stroke of the machine.

• Poka-yoke
• Fail-safe
• Breath alcohol ignition interlock device