Thermal trip unit with magnetic shield and circuit breaker incorporating

Abstract

A solenoid type magnetic trip assembly for a molded case circuit breaker includes an armature biased against an adjustable stop by a tension spring to set 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 magnetically permeable member mounted by a frame to slide longitudinally along a pair of guide rails. The frame defines a trip surface axially aligned with the elongated magnetically permeable member which engages a trip arm on a trip bar to trip the circuit breaker in response to a predetermined level of 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 magnetically permeable member of the armature extends. A radially enlarged slug on the free end of the elongated magnetically permeable member of the armature is subjected to a magnetic force opposite to the force generated by load current tending to pull the armature into the solenoid coil. This opposing 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 armature 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.