Such guard locking devices are for instance key-operated mechanical interlocks, also called trapped-key systems. They are based on the easy-to-understand premise that a key cannot be in two places at the same time - it can for instance be inserted in a switch OR be used for opening a guard. Another safety-relevant feature is that a key can only become free in a safe condition, when there are no hazards - a switch is turned off, a guard interlock is closed and locked.
