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 Post subject: NFPA70E; Annex D.7.5-Arc Flash Boundary Calc problem
PostPosted: Wed Nov 24, 2010 11:26 am 

Joined: Tue May 18, 2010 6:52 am
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when i use the "theoretically derived equiation D.7.5B" to calculate AFPB distance and check my answer with SKM software value, my calculated distance is exactly one half the SKM value. anyone have a clue?


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PostPosted: Fri Nov 26, 2010 6:14 pm 
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Location: Maple Valley, WA.
Check the options settings for the SKM program. There are many settings that will affect the SKM calculations.

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Robert Fuhr, P.E.; P.Eng.
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PostPosted: Sun Nov 28, 2010 4:44 pm 
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rks wrote:
when i use the "theoretically derived equiation D.7.5B" to calculate AFPB distance and check my answer with SKM software value, my calculated distance is exactly one half the SKM value. anyone have a clue?


I am not sure why it is exactly 1/2 but I would expect the theoretical calculation to be higher. The empirically derived equation is from IEEE 1584 and is based on the normalized incident energy from a prior calculation step.

The normalized Ei uses ARCING current and gap distances. The boundary calculation also uses an X distance factor which is a variation in the theoretical "squared" factor used to account for the incident energy dropping off by the inverse distance "squared". The X is based on the type of equipment and will be less than squared i.e. less than "^2"

The theoretical calculation uses the BOLTED current and ignores most of the details described above.

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