How Molded Case Circuit Breaker works

Molded Case Circuit Breakers (MCCBs) work based on the combination of thermal and magnetic trip units. The trip units are responsible for sensing overcurrent conditions and initiating the opening of the circuit breaker contacts. Here's how an MCCB typically works:

1. Normal Operation: During normal operation, the MCCB allows the flow of current through its contacts without interruption. The contacts are closed, providing a low-resistance path for the current to pass through the circuit.

2. Overload Protection: If an overload condition occurs, where the current flowing through the circuit exceeds the rated current of the MCCB for a sustained period, the thermal trip unit comes into play. The thermal trip unit consists of a bimetallic strip that is sensitive to temperature. As the current exceeds the predetermined threshold, the bimetallic strip heats up due to the resistive heating caused by the current flowing through it.

3. Trip Operation: When the bimetallic strip reaches a specific temperature, it bends or deforms, causing the trip mechanism to release. The release of the trip mechanism results in the opening of the circuit breaker contacts, interrupting the flow of current. This operation protects the circuit and connected devices from the damaging effects of sustained overcurrents.

4. Short Circuit Protection: In the event of a short circuit, where a sudden surge of current occurs due to a fault in the system, the magnetic trip unit within the MCCB operates to provide rapid tripping. The magnetic trip unit consists of a solenoid coil that generates a magnetic field when high short circuit currents pass through it. The magnetic field exerts a force on the trip mechanism, causing it to release and open the contacts.

5. Resetting: After tripping, MCCBs need to be manually reset to restore the normal operation of the circuit. The reset process involves closing the contacts by operating the reset mechanism or handle provided on the MCCB. This manual operation ensures that the circuit breaker is intentionally reset after the fault condition has been addressed and resolved.

It's important to note that the specific operation and characteristics of MCCBs may vary depending on the manufacturer, product design, and the specific trip units used. Different MCCBs may have adjustable trip settings, different trip curves, and additional features such as selective coordination and monitoring functions to enhance their functionality and protection capabilities. Following the manufacturer's guidelines and standards is crucial for the proper installation, operation, and maintenance of MCCBs.