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 Post subject: 63 kVA Transformer Secondary - Arc-Flash Hazard
PostPosted: Thu Aug 04, 2011 4:18 pm 
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Joined: Sun Apr 24, 2011 5:00 pm
Posts: 12
Location: Stratham, West Australia
A 415V switchboard is supplied from a 63 kVA, 22/0.433kV transformer ( %z = 3.2% ).
A 250A MCCB is installed at the transformer secondary terminals and provides the only primary protection for the switchboard.
The bolted three phase initial symmetrical rms short-circuit current at the switchboard is 2.26 kA ( due to Utility impedance in addition to the 63 kVA transformer impedance).
Based on the IEEE 1584 -2002/2004a formulas, 25mm gap ( see attached spreadsheet ) the arcing current through the 250A MCCB is 1.76 kA which would cause the MCCB to trip in approx. 20 sec.
After 20 sec, the resulting incident energy at a working distance of 500mm would accumulate to a “dangerous” incident energy of 87.7 Cal / cm². How realistic is this ?


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PostPosted: Thu Aug 04, 2011 4:21 pm 
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Location: Stratham, West Australia
Incorrectly sized MCCB on 63kVA transformer

I forgot to mention that the 250A MCCB is too large to provide either overload or short-circuit protection, however my question concerns the arc-flash hazard.


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PostPosted: Thu Aug 04, 2011 8:02 pm 
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Location: Spartanburg, South Carolina
Unless there are restrictions on the worker's movements, he is likely to move away from the arc in less than 20 seconds and not receive the full 87.7 cal/cm² energy. IEEE Std 1584 and NFPA 70E suggest that a 2 second limitation on the time if movements are not restricted.


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PostPosted: Fri Aug 05, 2011 5:33 am 
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jghrist wrote:
IEEE Std 1584 and NFPA 70E suggest that a 2 second limitation on the time if movements are not restricted.


I'm curious to know which part of NFPA 70E talks about the 2 second cutoff.
I know it's in IEEE 1584 (subject to appreciation by the person doing the study), but I wasn't aware it was in NFPA 70E.


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PostPosted: Fri Aug 05, 2011 9:48 am 
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Vincent B. wrote:
I'm curious to know which part of NFPA 70E talks about the 2 second cutoff.
I know it's in IEEE 1584 (subject to appreciation by the person doing the study), but I wasn't aware it was in NFPA 70E.

You're right, it isn't in NFPA 70E. NFPA 70E doesn't specify how to calculate incident energy, but IEEE 1584 is one of the methods used in Annex D (which is not a part of the NFPA 70E requirements).


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PostPosted: Mon Aug 08, 2011 7:05 pm 
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I think 20 seconds is more in the area of welding isn't it?? In electrical time, that is HUGE!!


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