Building main tripped again

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tom_jlr

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I have four story office building, being serviced by a 1600 amp switchboard with protected by a 1600 amp circuit breaker with ground fault protection. The main tripped for the third time this year. Twice because of a faulty compressor in a roof top unit and once from a light switch.

What is bothering me is the last tripping of the main. The facility maintenance man was "fixing" a switch that had become loose. He was working on it hot and was tightening the terminals (that's his story). Anyway..."poof" the whole building goes down again.

Building owner is looking for solutions to eliminate the main from tripping. He was told that the GFP is doing exactly what its suppose to do. But that is not good enough, he fails to believe that a light switch can take down the whole building.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
I have four story office building, being serviced by a 1600 amp switchboard with protected by a 1600 amp circuit breaker with ground fault protection. The main tripped for the third time this year. Twice because of a faulty compressor in a roof top unit and once from a light switch.

What is bothering me is the last tripping of the main. The facility maintenance man was "fixing" a switch that had become loose. He was working on it hot and was tightening the terminals (that's his story). Anyway..."poof" the whole building goes down again.

Building owner is looking for solutions to eliminate the main from tripping. He was told that the GFP is doing exactly what its suppose to do. But that is not good enough, he fails to believe that a light switch can take down the whole building.
If the GFP has been left at factory settings than it is extremelylikely that a fault, to ground, on a 20A branch circuit will cause the main to trip.

Even if the GF settings have been cranked up to maximum, it is still probable that a fault to ground on any circuit greater than 100A will still cause a trip.

The solution to stopping main device tripping is very simple, add GF to the branch circuits.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Why are we talking about the GFP tripping mechanism? This can be a simple phase to ground fault, and it could be the instantaneous device on the main breaker, and not the GFP device, that caused the trip. If the main breaker is not selectively coordinated with the other breakers between itself and the branch circuit breaker that serves the lighting, then there is no reason to believe that a fault would trip the 20 amp breaker before the 1600 amp breaker. Both breakers will receive the command to trip at the same time, and one of them will win the race. You have to go through a design and calculation process, and then you have to adjust the trip setting on the various breakers, in order to ensure that the breaker closest to the fault will trip first. That is a requirement for certain emergency and legally required circuits, but not for normal power supply circuits. If the facility has not gone through a selective coordination process, then it should surprise nobody that the main has tripped on three occasions. I predict it will happen again and again, until the owner has the trip setpoints revised.
 

zog

Senior Member
Location
Charlotte, NC
What they said.110% +.

Me three.

And, if they were not doing energized work, which I am guessing was not per 70E, this would not happen either. If the owner did not want these sort of things to happen they should have done a little engineering before hand. :)
 
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