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#64363 - 04/06/06 12:08 PM New Transformers/New Short Circuit Current  
cgw  Offline
Member
Joined: Jul 2004
Posts: 133
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 05:33 PM Re: New Transformers/New Short Circuit Current  
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
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

#64365 - 04/06/06 08:25 PM Re: New Transformers/New Short Circuit Current  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
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.


#64366 - 04/06/06 08:43 PM Re: New Transformers/New Short Circuit Current  
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
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

#64367 - 04/06/06 09:11 PM Re: New Transformers/New Short Circuit Current  
SolarPowered  Offline
Member
Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
Tie the wire in a knot. [Linked Image]


#64368 - 04/06/06 09:16 PM Re: New Transformers/New Short Circuit Current  
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
White Plains, NY
Is the transformer something close to 3750kVA with a 480V secondary?


Ron

#64369 - 04/06/06 11:42 PM Re: New Transformers/New Short Circuit Current  
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
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

#64370 - 04/07/06 07:22 AM Re: New Transformers/New Short Circuit Current  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Perhaps you could just run each phase in its own conduit.... (*ducking*)

-Jon


#64371 - 04/07/06 08:10 AM Re: New Transformers/New Short Circuit Current  
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
White Plains, NY
Jon,
That would help, but be sure that PVC conduit is used.


Ron

#64372 - 04/07/06 11:52 AM Re: New Transformers/New Short Circuit Current  
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
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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