Thermal-magnetic circuit breakers contain two different switching mechanisms, a bimetal switch and an electromagnet. The bimetal serves as a means of handling overcurrents. The bimetal typically sits behind a trip bar and is part of the currentcarrying path. Electrical current exceeding the breaker-overload rating heats the bimetal enough to bend it toward the trip bar. As the bimetal bends, it touches and rotates the trip bar to open the circuit. The time the bimetal needs to bend and trip the circuit varies inversely with the current.

The magnetic portion of the breaker consists of an iron core with a wire coil around it, forming an electromagnet. Load current passes through the electromagnet coils so the electromagnet responds to short-circuit currents. A high-level of current thus makes the electromagnet generate enough field strength to attract a nearby armature. As the top of the armature moves toward the electromagnet, the armature rotates the trip bar to trip the breaker, open the current path, and deenergize the electromagnet coils.

Thermal-magnetic breakers are often employed where it is important to quickly limit shortcircuit current. This is because the electromagnet in these devices can extinguish the arc between breaker contacts in as little as 4 msec. This compares favorably to the speed of interruption available from other types of breakers, such as hydraulic-magnetic, which generally energize a solenoid to interrupt short-circuit currents. It may take hydraulic-magnetic breakers 10 msec or more to completely stop current flow. One point to note is that thermal-magnetic breakers are sensitive to temperature. In sufficiently warm ambients, their normal current-handling capacity must be derated according to manufacturer recommendations.

Circuit Breaker Industries Ltd. ( provided information for this article.