A molded case circuit breaker having a movable contact arm (12) pivotally supported on a operating handle (4) and driven into and out of engagement with a stationary contact (6) by an overcenter toggle spring (28) stretched between the movable contact arm (12a) and a latch lever (14) which is releasable to carry the line of action of the spring (28) overcenter of the movable contact arm (12) to cause automatic separation of the contacts (6,10) in response to operation of a bimetal thermal overload sensing unit (18) and which has a separate magnetic overload current sensing unit (38,40) disposed on the stationary contact support (8) and isolated from the thermal overload current sensing unit (18), the magnetic sensing unit (8) operating to release an insulating wedge member (44) for movement between the circuit breaker contacts (6,10) to separate the contacts independently of the latch trip operating mechanism.