In electrical engineering, miniature circuit breakers (MCBs) and molded case circuit breakers (MCCBs) are two common types of circuit protection devices. Their primary function is to promptly disconnect the current in cases of overload, short circuit, and other abnormal conditions to protect the circuit and equipment. However, the distinction between C curve and D curve for MCBs, which does not apply to MCCBs, might indeed cause some confusion. Below, we explain this in a Q&A format, hoping to provide some reference for electrical personnel.

Why are there C curves and D curves for MCBs, but not for MCCBs?

It's important to clarify why MCBs have C and D curves, which are set according to the different nature of loads and the requirements for instantaneous protection of the circuit breaker. Besides C and D curves, there are also B curves, K curves, etc. MCCBs, on the other hand, are also categorized based on the nature of the load into distribution type and motor type; it's just a different nomenclature.

D curve MCBs are used for motor type loads and also have overload protection; why don't motor type MCCBs have overload protection?

Typically, motor type loads require a thermal relay for overload protection. If a thermal relay is set, then the overload protection inside the MCB becomes somewhat redundant; however, some small-capacity motors might not need a thermal relay for overload protection, but the cables still require overload protection. In such cases, overload protection inside the MCB is necessary.

MCBs are used for end-load protection and are not very precise in their protective value, hence there's no need to differentiate which need overload protection and which don't. Moreover, to prevent incorrect use by non-professionals, equipping them with overload protection does not affect usage while enhancing safety and has a minimal impact on cost, so why not?

For motor type MCCBs, which carry a larger rated current for loads, thermal relays are usually installed for overload protection. If overload protection were also included inside the MCCB, it would be redundant and wasteful. Furthermore, the overload protection inside the circuit breaker has a large margin of error, making it difficult to provide effective overload protection for motor type loads, thus making it somewhat meaningless to include it.