MCB tripping


Thread Starter


We had a problem in our plant in which the MCB was tripping after some period of operation of machine.

The machine was drawing the normal current. There was no problem in the machine. We replaced the MCB and the problem got solved.

Later, my boss was asking about the curve of the previous MCB. Guys, can you please educate me about the curve of a MCB.What is the difference of a C - curve MCB with respect to other types ?
I will need more information regarding the main circuit breaker to answer all parts of this question. The basic question is - what is a circuit breaker curve?

The curve refers to the tripping characteristics of the circuit breaker; how fast and at what current magnitude the breaker will trip. The curve varies based on size, type, make, and model. The curve is adjustable on many circuit breakers larger than 400A. The curve is generally in a log-log time current scale; the time is on the y axis and the current in amps is on the x axis.

The circuit breaker curve is needed to determine the selective coordination of an electric system. A properly coordinated system will clear a fault (open a circuit breaker) at the first upstream over current protection device.

>Later, my boss was asking about the curve of the previous MCB.

Your boss might ask this question to verify proper coordination between the main circuit breaker, downstream branch circuit breakers, and upstream feeder circuit breakers. He is concerned if the curves are not properly coordinated the upstream circuit breaker may open first, resulting in a larger outage than necessary to isolate the fault.