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 Post subject: transformer secondary side arc flash
PostPosted: Fri Feb 04, 2011 11:14 am 
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Hello: I have a question the use of primary transformer side fuses to control the secondary side IE before the main overcurrent protective device.
1. I have current limiting fuses on the primary side of my 2400V/480V transformer
2. My calculated short circuit on the secondary is the transformer is 15kA.
3. The primary would see 15kA * (480/2400) amperes.
4. I want use this the 'relfected' value in the primary tcc curve to when the fuse would clear and thus reduce the IE.

This is used in withstand curves so why not use in arc flash calculations?

Thanks for anyone's help here.


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PostPosted: Fri Feb 04, 2011 11:57 am 
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Is the 15 kA the three phase arcing short circuit current? If it is then you can reflect it to the primary. Often the primary current in that situation is quite low. With "typical" oversized fuses or primary relay settings, the Ei becomes quite large - because of the long clearing time.

If it is a line-ground fault/arc flash that does not escalate into three phase, the primary current will only be 57.7% of its reflected value on delta wye transformers. Similar situation with delta-delta and line-line faults (same concept used when shifting the ANSI C57 thru fault curves for protection)

Hope it helps - let me know if you any other info.

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Fri Feb 04, 2011 1:08 pm 
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Thanks for info

Your answer helped me a lot. Thanks


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PostPosted: Fri Feb 04, 2011 2:40 pm 
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Just make sure you know that arcing fault current, and calculated fault current are two different things. So that you are both talking about the same terms, as my initial impression was that you were taking 'fault current' on the secondary and not arcing fault current.


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PostPosted: Sat Feb 05, 2011 9:45 am 
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arc flash current through transformer

got it. Thanks,


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