Circuit Breaker Coordination

[Martinelli21]

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.

 

[dpc]

If you are trying to coordinate two molded-case circuit breakers, the answer is no, they will not reliabliy coordinate, because both have instantaneous trip elements. If you can set the main breaker higher than the maximum fault current, it will coordinate, but that is not the normal situation. Normally, if both breakers see fault current that exceeds their instantaneous pickup settings, both will trip.

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.

 

[Martinelli21]

As I understand it, the electronic breaker has short-time delay & long time-delay feature where you set the breaker to hold the fault current for a period of time (for ex. 0.05 to 0.5 seconds) before the breaker trips. This allows for selective coordination because the breaker upstream will trip "instantaneously" in 0.015 seconds.

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.

 

[RalphChristie]

dpc is correct.

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.

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