ADJUSTABLE TRIP UNIT AND CIRCUIT BREAKER INCORPORATING SAME

摘要

A solenoid type magnetic trip assembly for a molder case circuit breaker includes an armature biased against an adjustable stop by a tension spring to se t the initial gap for the magnetic trip, so that the spring bias remains constant for the full range of the initial gap. The armature includes an elongated armature element mo unted by a frame to slide longitudinally along a pair of guide rails. The frame define s a trip surface axially aligned with the elongated armature element which engages a trip arm on a trip bar to trip the circuit breaker in response to a predetermined level o f overcurrent. A bimetal providing a thermal trip function is cantilevered from a support spaced from the trip bar by the armature, but has a terminal portion at the free end projecting toward the trip bar and through which the elongated armature element of the armature extends. A radially enlarged slug on the free end of the elongated arma ture element of the armature is subjected to a magnetic force opposite to the force g enerated by load current tending to pull the armature into the solenoid coil. This opposi ng force increases as the initial gap increases, placing the slug closer to the magnetic frame, so that a greater range of trip currents can be selected despite limited room for a rmature travel. A gap in the magnetic frame prevents short circuiting the magnetic field where the few turns of a large gauge coil wire produce an unsymmetrical winding. A magnetic shield protects the bimetal from deformation during high current short circuits. A non-magnetic spring clip firmly retains the magnetic frame in a rece ss in the circuit breaker housing.