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Article 430 in the National Electrical Code (NEC) recognizes two types of molded case circuit breakers (MCCBs) - instantaneous and inverse time - for motor branch circuit short circuit and ground fault protection, in addition to fuses. There's an important difference between these two types of circuit breakers.
Instantaneous trip breakers do exactly what the name implies. When the current in the circuit reaches a value that the breaker trip mechanism is set to, it will trip off instantaneously with no time delay. Instantaneous breakers are magnetic trip only. Instantaneous breakers are also referred to as motor circuit protector breakers or MCPs.
Inverse time breakers, on the other hand, have a time delay mechanism that will allow the breaker to delay tripping off. The amount of time delay decreases as the current increases. Inverse time breakers are thermal-magnetic. Many in the industry use the slang term "thermal-mag" breakers for inverse time breakers.
More often than not, in industrial locations, MCPs are used in combination starters for motors. The MCP breaker is used for the...