CIRCUIT BREAKER WITH ADJUSTABLE LOW MAGNETIC TRIP

A circuit breaker has a magnetic trip assembly which can be adjusted to trip the breaker for low level overcurrents in the range of five to ten times rated current. The magnet trip includes for each pole of the breaker a helical torsion spring having one torsion arm which biases an armature against an adjusting bar to form a gap between the armature and a stationary magnetic structure in which magnetic flux strong enough to attract the armature and trip the breaker is induced by overcurrent. A second torsion arm of the spring has a first portion which bears against and slides along a pivot member carried by the adjusting bar to adjust the bias force applied to the armature by a given amount per unit travel of the adjusting bar over a low trip current portion of the range of travel of the adjusting bar. A second terminal portion of the second torsion arm of the spring extending at an angle from the first portion engages and slides along the pivot member on the adjusting bar to provide a greater change of bias force per unit travel of the adjusting bar at the higher trip current settings. Movement of the adjusting bar adjusts the spring bias for all poles of a multiphase circuit breaker simultaneously. The gaps between the armatures and the fixed magnetic structures of all the poles can also be adjusted by caroming surfaces on the adjusting bar against which the springs bias the armatures. In an alternative embodiment, the gaps can be adjusted separately for each pole by screws carried by the adjusting bar which bear against ramming surfaces formed by twisted tabs on the armatures.