Small, lightweight circuit breakers consist of an operating mechanism, contacts, protective devices (various trip units), an arc-extinguishing system, and other components. The main contacts are closed either manually or electrically. Once the main contacts are closed, a free-trip mechanism latches them in the closed position. The coil of the overcurrent trip unit and the thermal element of the thermal trip unit are connected in series with the main circuit, while the coil of the undervoltage trip unit is connected in parallel with the power supply.
When a short circuit or severe overload occurs in the circuit, the armature of the overcurrent trip unit is attracted; this actuates the free-trip mechanism, causing the main contacts to disconnect the main circuit. When the circuit is overloaded, the thermal element of the thermal trip unit heats up, causing the bimetallic strip to bend and trigger the free-trip mechanism. When an undervoltage condition occurs in the circuit, the armature of the undervoltage trip unit is released, which also actuates the free-trip mechanism.