Confused about 3 phase main tripping

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Sparkywise

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Location
Indiana
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Electrician
First time posting - I am an electrician on a college campus and had a problem: one of the maintenance workers turned off a 20 amp single pole breaker in a 100 amp 3 phase 277-480 sub panel feeding a 277 volt lighting circuit to replace a bad ballast. Once the ballast was replaced they turned the breaker back on not realizing the wire nut was not tight enough and came loose and instead of tripping the 20 amp breaker or the 100 amp main in the sub panel, it tripped the 1600 amp main feed to the building. Killed all the lights in the building! Some students were happy! Lol
My question is why did it trip the main?
 
The fault current caused by the loose nut traveled through the 20, 100, and 1600 amp breakers. All three detected the high current; all three initiated their internal trip mechanisms, creating essentially a "who will trip first" race. Any one of them could have won that race. Once that one tripped, the other two no longer saw fault current, so they terminated their attempt to trip.

If the building owner wants the system to force the breaker closest to the fault to trip first, then they must (1) Arrange for a fault coordination study to be performed, and (2) Arrange for the results of that study to be implemented in the field. An engineer does the first part; an electrician does the second part. It might not be cheap, but it should prevent future building-wide power losses.

It is likely that the 1600 amp breaker has possible adjustments that could be used to cause it to wait and give downstream breakers time to trip first. But to blindly make such adjustments (i.e., without proper calculations and well thought-out choices) can be dangerous in completely different ways.
 
Since the campus has an electrician, why are general maintenance workers changing ballasts? It seems like this entire situation could have been avoided.
 
Thank you for your information! Very helpful!
The general maintenance worker is an eager young worker and his zone is this building and another building. There is only one of me for the whole campus. So some of the “lighter” electrical work orders (light bulbs, ballast, and switches) have been given to them and I don’t see them.
 
As what Charlie b said, you need an engineered study to set it correctly. Usually they come from the factory set at the minimum settings, and may work for many years without a problem, until something like that happens.
 
retirede said:
Since the campus has an electrician, why are general maintenance workers changing ballasts? It seems like this entire situation could have been avoided.
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As long as the work is being done by a trained and qualified person, what difference does their job title make?.
 
In the Washington DC area, 90%+ of ballast in commercial office buildings are replaced by maintenance personnel. For me as long as anyone but me is changing them I am happy, I HATE changing ballast or most any work involving lighting.
 
brian john said:
In the Washington DC area, 90%+ of ballast in commercial office buildings are replaced by maintenance personnel. For me as long as anyone but me is changing them I am happy, I HATE changing ballast or most any work involving lighting.
Click to expand...

I don’t replace many ballasts anymore. I rewire the fixtures for LED bypass bulbs. 👍
 
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