The miniature circuit breaker is composed of operating mechanism, contacts, protection devices (various releases), arc extinguishing system, etc. The main contact is manually operated or electrically closed. After the main contact is closed, the free trip mechanism locks the main contact in the closed position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and the coil of the undervoltage release is connected in parallel with the power supply.

When the circuit is short-circuited or severely overloaded, the armature of the overcurrent release pulls in, causing the free tripping mechanism to act, and the main contact disconnects the main circuit.

When the circuit is overloaded, the heating element of the thermal trip unit will bend the bimetal and push the free trip mechanism to move. When the circuit is undervoltage, the armature of the undervoltage release is released. It also makes the free trip mechanism act.