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#64363 - 04/06/06 09:08 AM New Transformers/New Short Circuit Current
cgw Offline
Member

Registered: 07/29/04
Posts: 133
Loc: Rochester NY
In an existing building the utility is replacing transformers (for an addition which will have a new separate main disconnect). The new transformers have a much higher short circuit let through current. The original building service is a 50 year old ITE switchboard.
The let through current is over 80,000A.
The short circuit rating of the main circuit braker is 50,000A. (the transformers are about ten feet away from the switchboard indoors in a room next to the electric service room)
Any suggestions (other than replacing the entire switchboard)??? The main circuit breaker and switchboard are rated at 1600A.

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#64364 - 04/06/06 02:33 PM Re: New Transformers/New Short Circuit Current
earlydean Offline
Member

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
http://www.ferrazshawmut.com/products/pdf_107/A4BQ.pdf

try current limiting fuses in a appropriate enclosure between the transformer and the main.
_________________________
Earl

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#64365 - 04/06/06 05:25 PM Re: New Transformers/New Short Circuit Current
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Anything put between the transformer and the existing equipment would have to have a tested series rating. It is all but impossible to get a calculated series rating unless the existing equipment are "power circuit breakers".

Have you thought about running the incoming cables around the building a few times? This will add impedance and lower the available fault current.

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#64366 - 04/06/06 05:43 PM Re: New Transformers/New Short Circuit Current
Ron Offline
Member

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
You only have a few choices. By mearly adding a current limiting device between the transformer and the 50kAIC breaker doesn't help. They must be series rated together, which they generally are not for older equipment.
You can add a reactor downstream of the transformer, and it will limit fault current, but be careful of voltage drop problems.
Validate the fault current that you are being told, at least with an infinite primary calculation, and don't forget to add contribution from downstream motors.
Wire can reduce current significantly, so run lots of wire as mentioned by JBD.
_________________________
Ron

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#64367 - 04/06/06 06:11 PM Re: New Transformers/New Short Circuit Current
SolarPowered Offline
Member

Registered: 07/05/04
Posts: 615
Loc: Palo Alto, CA, USA
Tie the wire in a knot.

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#64368 - 04/06/06 06:16 PM Re: New Transformers/New Short Circuit Current
Ron Offline
Member

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
Is the transformer something close to 3750kVA with a 480V secondary?
_________________________
Ron

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#64369 - 04/06/06 08:42 PM Re: New Transformers/New Short Circuit Current
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
The Circuit Breaker can be the first or second element in series....

Install L or T type current limiting fuses with an appropriate rating: have an EE design it.

If the bussing permits, install a different breaker.

With such an old piece of gear: scrap it.... (I can't believe that old breaker is still passing its tests.)

Alternately, feed the existing service from the new installation. Re-engineer it.

Blame the extra on the Poco. Not your fault.
_________________________
Tesla

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#64370 - 04/07/06 04:22 AM Re: New Transformers/New Short Circuit Current
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Perhaps you could just run each phase in its own conduit.... (*ducking*)

-Jon

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#64371 - 04/07/06 05:10 AM Re: New Transformers/New Short Circuit Current
Ron Offline
Member

Registered: 03/13/02
Posts: 577
Loc: White Plains, NY
Jon,
That would help, but be sure that PVC conduit is used.
_________________________
Ron

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#64372 - 04/07/06 08:52 AM Re: New Transformers/New Short Circuit Current
Bob Offline
Member

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
Check the fault current as given. If you were given a fault current with an infinite source the actual value will be less. You will need to recaculate the fault including the utiltiy system impedance. The utility may be reluctant to give this information.

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