Circuit Breaker Coordination
Circuit Breaker Coordination
(OP)
I have a 6600 to 380 VAC, 250 KVA transformer supplying a motor control center with a 600A breaker on the secondary side. If a bolted short circuit occurs at one of the motor terminals, will the breaker in the MCC bucket trip before the breaker at the transformer? Do I assume 9500A applied to both breakers instantaneously? Or since the motor circuit protector in the MCC is set to trip at 300A and the magentic setting of the transformer breaker is 3000A, will the MCC breaker trip before the transformer breaker? I would like to stay with thermal-magnetic breakers and I'd rather not use electronic breakers and the short time-delay feature.






RE: Circuit Breaker Coordination
Even going to electronic trip will not necessarily solve this problem, since molded-case breakers, at least in the US, must have instantaneous trip, regardless of the type of trip device.
RE: Circuit Breaker Coordination
Something else I've been wondering about, isn't a panelboard in your house similar to this and how does it work? I know I've shorted something in my house at some point and only that breaker to thaat circuit trips. The main breaker doesn't trip.
RE: Circuit Breaker Coordination
This is unfortunately one of the disadvantages of two instantaneous units in "series."
Your fault level at the MCC can be much lower than the fault level at the transformer breaker, depending on the distance between the two. If it is true, you can set your transformer breaker's setting higher than the faultlevel at the MCC. But, this is normally not the case. If you can, a time-delay would be ideal. A rule of thumb is a delay of 0.3 seconds between the two units. Also, the probability of a three-phase bolted fault is much smaller than an earth-fault.
In your house it also depends on the fault levels at the different locations. The wires (in your home) are small and have a big resistance, and are thus limiting the fault-current.
Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon
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