Introduction To Molded Case Circuit Breakers

Molded case circuit breakers (MCCBs) automatically disconnect current when the current exceeds the trip setting. The term "molded case" refers to the use of a plastic insulator as the outer shell of the device to isolate conductors from grounded metal parts. MCCBs typically contain a thermomagnetic trip unit, while larger MCCBs are equipped with solid-state trip sensors. Their trip units are divided into thermomagnetic trip and electronic trip units. Common rated currents include 16, 25, 30, 40, 50, 63, 80, 100, 125, 160, 200, 225, 250, 315, 350, 400, 500, and 630A.

MCCBs are also known as modular circuit breakers, where all parts are sealed within a plastic housing. Auxiliary contacts, undervoltage release, and shunt trip units are often modular. Due to their very compact structure, MCCBs are generally not maintenance-friendly. They are mostly operated manually, with electric opening and closing options available for larger capacities. Due to the application of electronic overcurrent trip units, molded case circuit breakers can be divided into Class A and Class B. Class B has good three-stage protection characteristics, but due to price factors, Class A products using thermal-magnetic trip units have a higher market share. A molded case circuit breaker houses the contacts, arc-extinguishing chamber, trip unit, and operating mechanism all within a plastic casing. Maintenance is generally not a concern, and it is suitable as a branch circuit protection switch. Overcurrent trip units are available in thermal-magnetic and electronic types. Generally, thermal-magnetic molded case circuit breakers are non-selective circuit breakers, offering only overload long-time delay and short-circuit instantaneous protection. Electronic molded case circuit breakers offer four protection functions: overload long-time delay, short-circuit short-time delay, short-circuit instantaneous, and ground fault protection. Some newly launched electronic molded case circuit breakers also feature area selective interlocking functions. Most molded case circuit breakers are manually operated, but some have motorized operating mechanisms.