Abstract: The disclosed instantaneously tripping device includes a stationary iron core encircling a conductor and responsive to a flow of shortcircuiting current through the conductor to attract a movable iron core to open contacts. A locking and releasing plate are operatively coupled to the movable core to transfer the movement of a closing latch to the latter. After the completion of the closing operation effected by a switching mechanism, the locking plate locks the movable core while only during the closing operation of the switching mechanism does the releasing plate release the movable core to permit it to be attracted by the stationary core.