Abstract: The present invention relates to a multi-pole circuit breaker with an interchangeable magneto-thermal tripping unit electrically and mechanically connected to the breaker unit by means of connecting screws. The connecting lugs of the breaker unit are positioned with vertical play in transverse slots in the breaker unit casing and are electrically connected to the movable contacts by flexible connections. When the connecting screws are tightened the breaker unit connecting lugs are shifted to bear on the associated tripping unit lugs irrespective of any positioning defects. A reset spring of the electro-magnetic tripping device is anchored on one end on the armature and on the other end to the tripping device adjusting bar.