Thermal-magnetic circuit breakers

Feb. 9, 2006
Thermal-magnetic circuit breakers contain two different switching mechanisms, a bimetal switch and an electromagnet.

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.

About the Author

Robert Repas

Robert serves as Associate Editor - 6 years of service. B.S. Electrical Engineering, Cleveland State University.

Work experience: 18 years teaching electronics, industrial controls, and instrumentation systems at the Nord Advanced Technologies Center, Lorain County Community College. 5 years designing control systems for industrial and agricultural equipment. Primary editor for electrical and motion control.

Sponsored Recommendations

The entire spectrum of drive technology

June 5, 2024
Read exciting stories about all aspects of maxon drive technology in our magazine.


May 15, 2024
Production equipment is expensive and needs to be protected against input abnormalities such as voltage, current, frequency, and phase to stay online and in operation for the ...

Solenoid Valve Mechanics: Understanding Force Balance Equations

May 13, 2024
When evaluating a solenoid valve for a particular application, it is important to ensure that the valve can both remain in state and transition between its de-energized and fully...

Solenoid Valve Basics: What They Are, What They Do, and How They Work

May 13, 2024
A solenoid valve is an electromechanical device used to control the flow of a liquid or gas. It is comprised of two features: a solenoid and a valve. The solenoid is an electric...

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!