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 Post subject: Short Circuit vs. Fault Current
PostPosted: Tue Apr 29, 2008 6:35 am 
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Joined: Tue Mar 25, 2008 1:00 pm
Posts: 32
I've been given a list of "available fault currents" at a transformer secondary and I'm wondering if this is the same thing as the "secondary short circuit amperage". Am I splitting hairs here?


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PostPosted: Tue Apr 29, 2008 8:27 am 
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Joined: Sun Dec 23, 2007 1:44 pm
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Location: Charlotte, NC
Available Fault Duty

If these values were furnished by a utility, you will need to be sure that they did not furnish you with the infinite source values. That would assume no source impedance but the transformer. We get these requests all of the time and the SOP in the past was to use this method as worst case for service equipment sizing. We have modified our practice to furnish the actual available data for use in arc flash studies since the infinite source fault info may not be the worst case.

If the fault data furnished equals the full load amp rating of the transformer divided by the per unit impedance of the transformer, you got infinite source data from the supplier.

Example: 300 kVA unit, 480 volt secondary, %Z=2.0
Full load amps at 480 volts=361
Infinite Source Fault = 361/.02=18,050 amps

Otherwise, yes you are probably splitting hairs.

Hope it helps,
Alan


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PostPosted: Fri Jun 06, 2008 9:30 am 
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Also be careful if you are calculating the incident energy on the secondary. An arc flash between the transformer secondary and the first overcurrent device often does not clear right away, if at all, due to the large primary device.


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