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Thermal-Magnetic Principles

Normal Load Operation

Thermal (Inverse Time) Trip Mechanism.

The thermal trip mechanism operates in response to overload conditions. The mechanism includes a bimetal element located behind the trip bar. The bimetal element is part of the current carrying path. When there is an overload, the increased current flow heats the bimetal and causes it to bend. As the bimetal bends, it touches and rotates the trip bar causing the circuit breaker to trip. The time needed for the bimetal to bend and trip the circuit breaker varies inversely with the current.

Short Circuit Operation

Magnetic (Instantaneous) Trip Mechanism.

The magnetic trip mechanism operates when there is a high current (short circuit) in the current path. The mechanism includes an electromagnet and an armature. When high level current passes through the conductor, the magnetic field strength of the electromagnet rapidly increases and attracts the armature. As the top of the armature is drawn to the electromagnet, the armature rotates the trip bar causing the circuit breaker to trip.


  • The Fuchs-type thermal-magnetic circuit breakers are ideal for upstream cascading breaker systems.
  • Thermal-magnetic breakers can be mounted in any position. The tripping characteristic does not change.
  • Magnetic only tripping circuit breakers are available, ideally suitable for motor short circuit protection.
  • Specially designed breakers for class 2 co-ordination with contactors and overload relays.