This cutaway view of a thermalmagnetic circuit breaker illustrates the typical internal workings of these circuit-protection devices.

This cutaway view of a thermalmagnetic circuit breaker illustrates the typical internal workings of these circuit-protection devices.


Thermal-magnetic circuit breakers combine the instantaneous trip ability of a magnetic breaker for shortcircuit currents with the overload tolerance of thermal breakers for normal motor-starting demands. When reset, the movable contact completes the electrical circuit between the stationary contact on the line terminal and the bimetallic strip. The pawl on the strip keeps the contacts engaged. Current flows from the movable contact through the bimetallic strip to the release arm and into the electromagnetic coil. The coil connects to the terminal supplying power to the load.

There are three ways to trip this circuit breaker. First, a prolonged high current flowing through the bimetallic strip creates heat. The strip begins bending from the heat because of the different coefficients of expansion between the two metals. When it bends enough the pawl releases the movable contact and spring tension snaps the contacts open breaking the circuit.

The second trip mechanism uses the electromagnetic coil as a solenoid. When current through the coil is high enough the solenoid core is pulled into the coil, pushing downward on the metal plate where the bimetallic strip is fastened.

As the metal plate is pushed down, the bimetallic strip is again pulled away from the movable contact popping it open.

The final trip mechanism places force on the solenoid core by pressing the manual release button.