Abstract: The present invention relates to circuit breakers of a particular class, namely, the type in which a toggle is manually erected to drive a "heart" as a unit for closing the circuit breaker. The "heart" includes a contact arm, an actuator, and overcurrent releasable latch means which collectively form a normally latched unit. Under excess-current conditions, the latch means releases the actuator, and contact arm then opens the circuit.The latch means includes a conventional bimetal fixed at one end to the contact arm. The bimetal carries a pivoted latch lever, and is surrounded by a core-and-armature unit adjacent its secured end, for operating the latch lever in response to high overcurrents. High magnetic trip sensitivity is achieved in a construction wherein the whole electromagnet moves with the contact arm, without resort to a current-carrying coil for the electromagnet.