a magnetically - locked switch assembly includes a switch unit having internal contact means positioned by an actuator arm between open and closed positions . unauthorized actuation of the actuator arm is prevented by a locking pin which is displaced to release the actuator arm only in the presence of an externally applied magnetic field . the actuator arm is mechanically actuated from outside the switch housing through a slip - clutch to prevent damage to the actuator arm by application of excessive force when the actuator arm is locked .