Key switch

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Switching lock from an RCD (formerly known as FI switch)

A switch lock is a mechanical device similar to the pawl in electrical components, in particular in residual current circuit breakers and line circuit breakers or automatic circuit breakers , which preferably enables a circuit to be switched off with very little force with a small path. For this purpose, the force of a spring is released, which was tensioned by a spring mechanism when switched on, and the switch snaps back into its rest position.

The typical triggering criterion is a current that releases the lock through the action of an electromagnet (rapid triggering e.g. in the event of a short circuit or a defined fault current) or a heated bimetal strip (slow triggering due to overcurrent). The triggering criterion for a fault voltage circuit breaker is the voltage on the winding of the electromagnet (typically 42 V).

A similar technical solution includes staircase time switches and short -time switches . Older automatic staircase lights contained a clock mechanism that was wound up by the stroke of an electromagnet.

Switch locks of electrical fuses must be provided with a trip-free release in accordance with DIN VDE 0660 .


  • Wilfried Knies, Klaus Schierack: Electrical systems engineering. 5th edition, Carl Hanser Verlag, Munich and Vienna, 2006, ISBN 3-446-40574-7


  1. Circuit breakers with screw bases are also called permanent fuses.