The movable contact arm of a circuit breaker is mounted on a pivot pin by a slot which extends generally transverse to the longitudinal arm of the contact arm, so that when the contacts are welded, initial motion of the circuit breaker operating mechanism to open the contacts is translated into pivoting of the contact arm about, and thereby breaking the weld. In a circuit breaker in which the contact arm is coupled to the operating mechanism through a cam mechanism which allows the contact arm to blow open in response to very high overcurrents, the slot extends generally longitudinally so that the cam mechanism translates initial movement of the operating mechanism into longitudinal movement of the contact arm to apply a shear force tending to break the weld.