Main breaker trip

[JesseETG]

Does anyone know why an AC unit fed from a 480volt subpanel on the 3rd floor of a building would trip the main breaker on the 1st floor.

 

[jim dungar]

Yes. The two devices were never selectively coordinated. If the upstream device has an adjustable trip unit, which has been common for the past 20 years, it was shipped from the factory set to the minimum.

 

[JesseETG]

Am I able to possibly fix the issue by adjusting the breaker's Short-time Pickup?


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[winnie]

It is also very possible that the main has ground fault protection but the feeder and branch breakers do not.

IMHO you should not be adjusting the breaker pickup without understanding the implications for arc flash hazard at the facility in question.

-Jon

 

[JesseETG]

Thank you

 

[David Castor]

Most likely cause is a ground fault trip on the Main Breaker. 2nd most likely is probably an instantaneous trip on the main breaker.

 

[jim dungar]

Am I able to possibly fix the issue by adjusting the breaker's Short-time Pickup?


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Can't really answer without seeing the breakers' Time Current Curves for both overcurrent and ground faults.
Do you have any idea how much current was flowing when the trip occurred?

 

[hillbilly1]

Because of liability, I usually suggest to the customer to get an engineered coordination study, to get the proper settings on a ground fault breaker before making any changes. As others have said, they usually are set to the minimum settings from the factory, and rarely get changed until they start having problems.

 

[mayanees]

Am I able to possibly fix the issue by adjusting the breaker's Short-time Pickup?


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Give us breaker part numbers and I'll check that out.

 

[lordofthisworld]

Does anyone know why an AC unit fed from a 480volt subpanel on the 3rd floor of a building would trip the main breaker on the 1st floor.

Trip Settings are probably not set properly