Working Principle Of Molded Case Circuit Breakers

The main contacts of the low-voltage circuit breaker are closed manually or electrically. After the main contacts are closed, the free-trip mechanism locks the main contacts 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, causing the free-trip mechanism to operate and 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 pushing the free-trip mechanism to operate, thus disconnecting the main circuit.

When the circuit is undervoltageed, the armature of the undervoltage trip unit is released, also causing the free-trip mechanism to operate and the main contacts to disconnect the main circuit.

When the shunt trip button is pressed, the armature of the shunt trip unit is attracted, causing the free-trip mechanism to operate and the main contacts to disconnect the main circuit .