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#34147 - 02/02/04 07:02 PM tie breaker question  
seattleman  Offline
Joined: Feb 2004
Posts: 17
Seattle, WA, USA
I was reading about how a tie breakers prevents paralleling of busing from two transformers to supply two loads. So normally these each feed their own load, but if one transformer fails, you can feed both loads from one transformer.

But what would be the result of paralleling bussing, like combining A phases of two different souces? I dont see any direct short happening. Any ideas on what would happen?

Work Gear for Electricians and the Trades

#34148 - 02/02/04 07:10 PM Re: tie breaker question  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
As long as the cycles are in sync you would have twice the current.

This happens when you parallel gensets.

EDIT: changed "sink" to "sync". [Linked Image]

[This message has been edited by iwire (edited 02-03-2004).]

Bob Badger
Construction & Maintenance Electrician

#34149 - 02/02/04 09:49 PM Re: tie breaker question  
Roger  Offline
Joined: May 2002
Posts: 1,716
There is another situation that could occur also. If the transformers primaries are different sources, and one of these sources were to be turned off for servicing, you would have a reverse angle and some one working on the source of the supposedly dead side would be subject to full primary voltage.

I know this would be unlikely, just wanted to throw it out on the table.


#34150 - 02/03/04 12:21 AM Re: tie breaker question  
seattleman  Offline
Joined: Feb 2004
Posts: 17
Seattle, WA, USA
OH,that's a good point, I was looking for the safety issue at hand.

#34151 - 02/03/04 12:22 AM Re: tie breaker question  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Hopefully, the installation was engineered and commissioned to accommodate phasing and synchronization of the two sources, whatever they are. There should be construction drawings and acceptance test reports that bear this out. If the tie is at low voltage, a phasing check can be conducted with a power-rated voltmeter. Post again if you need more information.

Another aspect that should have been considered, if all breakers are simultaneously closed—a “closed transition”—the available fault current on the buses roughly doubles, and the gear should be so intended.

#34152 - 02/04/04 04:22 PM Re: tie breaker question  
Ichabod  Offline
Joined: May 2003
Posts: 26
Statesboro, GA, USA
Electric utility substations quite often have such a scheme. We normally have low side bank breakers at the output of the two transformers and a bus tie breaker between the buses. If either transformer fails its low side breaker is opened and the bus tie breaker is closed. There are interlocks to make sure that a failed transformer is not energized from the low side.

#34153 - 02/05/04 01:12 AM Re: tie breaker question  
Big Jim  Offline
Joined: Jul 2003
Posts: 394
Denver, CO USA
If you lose one source and everything is paralleled, you find yourself in the position of trying to backfeed the down source by passing current through your equipment. Not at all healthy for your stuff.

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