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#57196 - 10/07/05 03:15 AM Tripping main breaker
poorboy Offline

Registered: 06/02/05
Posts: 128
Loc: Central Maine
I will be going to a job we did 2 yrs ago to investigate a problem that was explained to me by phone yesterday. I can't remember the precise specifics of this service but the general scope is this:

We installed a GE service with a high voltage switch (this is indoor and replaced an old transformer vault) with 4160 volts coming in, going thru a dry type transformer to a 208/120 section with a main breaker (can't remember the Amperage...prob 600 or 800 Amps).

When the "campus" electrical crew has shut down the 4160 on the site for various maintenance reasons and then restored power, the 208 volt main breaker in our installation tripped. I will be visiting the site soon and get the particulars on the type of breaker, whether it has any adjustments, etc.

Any thoughts on what I may be in for?

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#57197 - 10/07/05 03:57 AM Re: Tripping main breaker
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
It sounds as though it might be the combined surge currents of all the loads being re-energized simultaneously. Are there a good many motors for A/C compressors etc. on site?

I'd try opening some of the sub-feeder breakers, then turn on the power, then close the sub-feeds one at a time to minimize the surge currents.

#57198 - 10/07/05 10:40 AM Re: Tripping main breaker
Bob Offline

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
I agree with Paul. You did not say what the size of the transformer is. Also need to check the actual load. The transformer inrush plus the load pickup can be too great and trip the main breaker.

[This message has been edited by Bob (edited 10-07-2005).]

#57199 - 10/07/05 11:09 AM Re: Tripping main breaker
IanR Offline

Registered: 12/06/04
Posts: 326
Loc: Palm Bay FL USA
I have to agree with Paul. It may be just that simple.(itemized power up) At my company we(not I, others I work with) have actually damaged panels and destroyed breakers because the power was restored to everything at once. Instead of gradually one item at a time. The inrush current at a system wide level can be staggering, say the least.

[This message has been edited by IanR (edited 10-07-2005).]

[This message has been edited by IanR (edited 10-07-2005).]

#57200 - 10/07/05 01:41 PM Re: Tripping main breaker
poorboy Offline

Registered: 06/02/05
Posts: 128
Loc: Central Maine
Looked at it today...I was wrong, it is a fused switch, 1200 Amp GFI, the kind you have to pump the handle twice to turn on.(high pressure contact switch) The rep I called is looking at the job file to see if it has been programmed (or calibrated?) properly. They will have a record of it if it was. I was not involved if it was, but will report back what I find out. Must be related to this feature, as the load is nowhere near 1200 Amps and there is very little motor load. It has no transformers on the load side of it, just the one for 4160 to 208 which feeds it.

[This message has been edited by poorboy (edited 10-07-2005).]

#57201 - 10/07/05 02:48 PM Re: Tripping main breaker
CTwireman Offline

Registered: 02/07/02
Posts: 839
Loc: Connecticut, USA
I "third" the inrush current motion.

#57202 - 10/07/05 07:58 PM Re: Tripping main breaker
rj Offline

Registered: 01/23/03
Posts: 23
Loc: Skidmore, Missouri, USA
Kind of sounds like a problem I got called back to work for this week. They lost power to a building. It was a 1200 amp GFI main that tripped. I turned off all the breakers in the dist. panel and reset it. I turned the breakers on one at a time but when I came to a 350 amp. breaker, boom the main would trip. Come to find out we had a bad contactor for a motor.
Do they just reset the main or do they shut all sub panels off and bring them on one a time? That could be a problem if they just reset the main and try to start everything up all at once.

#57203 - 10/08/05 05:54 AM Re: Tripping main breaker
poorboy Offline

Registered: 06/02/05
Posts: 128
Loc: Central Maine
I am handicapped here by not knowing yet what the "programming" that was/wasn't done to the switch is all about. If it is supposed to have been done in response to a detailed load survey and things have changed in the 2 yrs since, then reprogramming may be in order. I am covering for people (who were involved in this job) who were out of town for a week when this call came in. They will know if the initial start up was done per spec. I am eager to learn more about that aspect of it, and at the same time open to the possibility of the inrush theory,too.

Always good to be challenged by these things, and its also good to have a large cross section of electricians to run it by. Thanks, and I'll keep you posted.


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