Main Breaker Tripping

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HPESINC

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Recently I've had 2 situations where the main breaker trips. The first a 100 amp sub-panel that feeds a pump house intermittently trips. I've checked all the circuits in the pump house and cannot find any problems. Then for no reason the breaker feeding the sub-panel, located at the service, trips. The second, while checking a lighting panel in a store I attempted to reset a 1 pole 20 amp breaker the 200 amp breaker, at the service, tripped.
Any help would be appreciated.
 
Re: Main Breaker Tripping

If there is truly no problem, then the breaker is bad and needs replacement.

A fault downstream of a 20A breaker could possibly trip a 200A main if the amount of current exceeded 2000A. If it is a fault downstream on a 20A branch, since the wire is so small (probably #12), the fault has to be very close to the panel for the small wire not to have reduced the fault current significantly in just a couple feet.
 
Re: Main Breaker Tripping

Would you agree that a fault downstream of a 20 amp branch breaker much less than 2000 amperes could trip a 200 amp main that has been loaded to near 200 amps for many hours? I think that I've observed this happen on occasion. The main is just on the verge of tripping from a genuine overload or thermal condition anyhow, and it might not take much fault or disturbance on a branch breaker, even in a subpanel somewhere, to trip that main.

That's my best thoughts from a regular electrician.
 
Re: Main Breaker Tripping

Would you agree that a fault downstream of a 20 amp branch breaker much less than 2000 amperes could trip a 200 amp main that has been loaded to near 200 amps for many hours?
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Not really. It appears that we are talking about the instantaneous trip function of the main breaker. That is a magnetic trip and should not be affected by the load or overload. Those conditions are handled by the thermal element.
Don
 
Re: Main Breaker Tripping

I think the answer is simple enough. In both cases, you have a short circuit somewhere downstream (i.e., within the pump house, and on the 20 amp branch circuit). Now comes the hard part: finding it. On the 20 amp branch circuit, finding the fault point should be easier. On the sub-panel, you appear to have a "ghost problem," one that comes and goes as though by its own whim.

It should not be a surprise that an upstream breaker trips, instead of a downstream breaker, when there is a fault. Very few facilities have complete coordination of all of their breakers. The fault occurs, and the 20 amp breaker trips. You try to reset the 20 amp breaker, and this time the fault takes out the panel feed breaker. No surprise.
 
Re: Main Breaker Tripping

I had a similar problem with a 125a adj. trip breaker tripping in a GE distribution panel. I could not locate any problems in the branch circuit panel. No breakers were tripping there, nothing overloaded, and it had a balanced load. But when I turned on several loads (space heaters etc.) it tripped the 125a distribution breaker. I decided to replace the adjustible trip mechanism in the 125a breaker. Haven't had any problems since.
 
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