Circuit breakers are specialized switches designed to automatically disconnect and stop the flow of electricity in the event of overcurrent. Small circuit breakers, such as those used in residential, commercial, and light industrial services, are thermal-operated. They contain a bimetallic strip that bends when heated by the circuit current.

When the bimetallic strip produces enough force, the trip mechanism is activated and the circuit breaker will open. Larger circuit breakers are automatically initiated by the magnetic field generated by the current-carrying conductor inside the circuit breaker, or can be triggered by an external device that monitors the circuit current.

Because circuit breakers do not fail when subjected to overcurrent conditions - instead, they simply open and can be reset by moving the lever - they are more likely to be permanently connected to the circuit in a way that is more permanent than a fuse.

While it may appear to be just a switch from the outside, it can be used as such. However, its true function is to operate as an overcurrent protection device.

It should be noted that some cars use inexpensive devices called fusible links for overcurrent protection in the battery charging circuit, due to the cost of properly rated fuses and holders.

Fusible links are a primitive form of fuse, consisting of a short rubber insulated wire designed to melt open in the event of overcurrent, without any hard protective sheath.

This crude and potentially dangerous equipment has never been used in industrial or even residential electrical applications, primarily due to encountering higher voltage and current levels. For this author, their use even in automotive circuits is questionable.

When a short circuit occurs, a large current (generally 10 to 12 times) produces a magnetic field that overcomes the force of the return spring, pulling the tripping mechanism to actuate the switch, which instantaneously trips.

When overloaded, the current increases, and the heat generated intensifies, causing the bimetallic strip to deform to a certain extent and push the actuating mechanism (the larger the current, the shorter the action time).

The main contact of the low-voltage circuit breaker is closed by manual operation or electrically closed. After the main contact is closed, the free tripping mechanism locks the main contact in the closed position. The overcurrent tripping coil and the thermal tripping element of the thermal tripping mechanism are connected in series with the main circuit, and the undervoltage tripping coil is connected in parallel with the power supply.

When a short circuit or severe overload occurs in the circuit, the armature of the overcurrent tripping mechanism is attracted, causing the free tripping mechanism to actuate and disconnect the main circuit. When the circuit is overloaded, the thermal element of the thermal tripping mechanism heats up, causing the bimetallic strip to bend and push the free tripping mechanism to actuate.

When the circuit is undervoltage, the armature of the undervoltage tripping mechanism is released, causing the free tripping mechanism to actuate. The shunt tripping mechanism is used as a remote control. During normal operation, its coil is de-energized. When remote control is required, the start button is pressed to energize the coil, and the armature drives the free tripping mechanism to actuate and disconnect the main contact.

Nowadays, there are electronic types that use current transformers to collect the size of each phase current, compare it with the set value, and when the current is abnormal, the microprocessor sends a signal to the electronic tripping mechanism to actuate the operating mechanism.