50 amp motor tripping main 2000 amp breaker

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boone

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What could be the reason that a 50 amp motor that shorted to ground would tripped the main 2000 amp breaker? (It also tripped the motor circuit breaker that is 150 amps.) The motor circuit breaker has a Instantaneous Overide Setting. The info I have states that the setting should be 8 to12 times higher than the circuit breaker rating(150 amps). It is only set to 750. Would the overide setting set too low allow the main breaker to trip?

Thanks,
boone
 
Under short circuit conditions, with the current being in the instantaneous region of one or more breakers, every breaker in line between the main and the branch circuit will see the same high current at the same time. It is entirely possible that any one of them will trip before the others, or that two or more will trip. It is a random event. In order to avoid tripping the main on a branch circuit fault, you would have to perform a selective coordination study, and set each breaker according to its results.

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boone said:
What could be the reason that a 50 amp motor that shorted to ground would tripped the main 2000 amp breaker? (It also tripped the motor circuit breaker that is 150 amps.) The motor circuit breaker has a Instantaneous Overide Setting. The info I have states that the setting should be 8 to12 times higher than the circuit breaker rating(150 amps). It is only set to 750. Would the overide setting set too low allow the main breaker to trip?

Thanks,
boone
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Does the 150A motor circuit breaker have ground fault protection?
If it doesn't and the main breaker does, then you shouldn't be surprised that it trips on a ground fault.
And, even if the 150A breaker does have ground fault protection, you'd need to ensure correct discrimination between that and the main breaker.
 
The main service GFP was doing what it is suppose per design, depending on the motor OCP and the setting of the GFP it may not be possible to coordinate the two devices.

What size and type fuses?
what are the GFP settings?
 
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