A tripping device (1) for circuit breakers comprising a thermally influenceable wire-shaped tripping element (4) is presented, one end of which is attached to a rotatably supported tripping lever (3) which is under spring force and the other end of which is fixed in location. The thermally influenced lateral movement at one end of the tripping element (4) causes the unlatching of the circuit breaker. At the free end of the tripping lever (3), a locking element (11) is provided which engages a cam (10) of a contact lever (2) which is under spring force, in such a manner that the circuit breaker is switched off both with contraction and with expansion of the tripping element (4).