The circuit breaker is a popular and most important electrical device to ensure safety in the modern world. The circuit breaker is used to cut the power supply when an electrical wire has too much current flowing through it. Without these simple devices, household electricity would not be possible because of the chances for fire and short circuit resulting from simple wiring problems and equipment failures.

The device serves in the course of normal system operation to energize or de-energize loads. In case of conditions where excessive current develops, a circuit breaker opens for protecting equipment and surroundings from possible damage due to excess flow of current.

This automatically-operated electrical switch, unlike a fuse, can be reset (either manually or automatically) for resuming normal operation. These devices are made in varying sizes, right from small devices for protecting an individual household appliance up to large switchgear designed for protecting high voltage circuits feeding an entire city.

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Mechanical circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including:

• Lengthening of the arc
• Intensive cooling (in jet chambers)
• Division into partial arcs
• Zero point quenching
• Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.