Locking device for a switch cabinet door, or of a door frame, of a multipolar electrical circuit breaker, in particular for low voltage, which is arranged in a switch cabinet on an extension means so as to be movable between an operating position, a test position and a disconnected position, - the locking device prohibiting access to the switch cabinet in the operating position and in the test position, and allowing access to the switch cabinet in the disconnected position, - and a securing means being provided for the locking device, which securing means allows access to the switch cabinet when an auxiliary release means (20, 30) is actuated from the outside of the switch cabinet door in the two positions operating position and test position, and also in the two locked positions (C, T), the securing means consisting of - a pivoting bolt which is provided with a latch and is fixed to a vertical side wall of the switch cabinet, - and a locking tab which is held on the inside of the switch cabinet door by a support and interlocks with the latch, thereby prohibiting access, the pivoting bolt being located in the door-securing position due to its own weight, - and the auxiliary release means consists in the locking tab in the support, which tab is in a position to be changed by means of an auxiliary release tool acting from the outside of the switch cabinet door, being fixed in the door-securing position, characterised in that the locking tab is rotatably held in the support as a rocker, in which support the locking tab can occupy a door-locking position in which it is interlocked with the pivoting bolt and prohibits access to the switch cabinet, and a door-release position in which it is not interlocked with the pivoting bolt and allows access to the switch cabinet, and the locking tab, after it is acted upon by the auxiliary release tool, occupies the door-release position as a result of rotation due to its own weight out of the door-securing position into the door-release position. - the locking device prohibiting access to the switch cabinet in the operating position and in the test position, and allowing access to the switch cabinet in the disconnected position, - and a securing means being provided for the locking device, which securing means allows access to the switch cabinet when an auxiliary release means (20, 30) is actuated from the outside of the switch cabinet door in the two positions operating position and test position, and also in the two locked positions (C, T), the securing means consisting of - a pivoting bolt which is provided with a latch and is fixed to a vertical side wall of the switch cabinet, - and a locking tab which is held on the inside of the switch cabinet door by a support and interlocks with the latch, thereby prohibiting access, the pivoting bolt being located in the door-securing position due to its own weight, - and the auxiliary release means consists in the locking tab in the support, which tab is in a position to be changed by means of an auxiliary release tool acting from the outside of the switch cabinet door, being fixed in the door-securing position, characterised in that the locking tab is rotatably held in the support as a rocker, in which support the locking tab can occupy a door-locking position in which it is interlocked with the pivoting bolt and prohibits access to the switch cabinet, and a door-release position in which it is not interlocked with the pivoting bolt and allows access to the switch cabinet, and the locking tab, after it is acted upon by the auxiliary release tool, occupies the door-release position as a result of rotation due to its own weight out of the door-securing position into the door-release position.