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 Post subject: NFPA 70E D.8.1.1 DC Arc FlashPosted: Mon Nov 05, 2012 4:02 pm

Joined: Mon Nov 05, 2012 3:54 pm
Posts: 1
Looking at the equation given in D.8.1.1 (IEm=0.01*Vsys*Iarc*Tarc/D^2), the units are Volts*Amps*Time/area^2.

Volts*Amps = Watts
Watts*Seconds = Joules

It seems like the answer should be in J/cm^2, not in Cal/cm^2 as noted. That difference is 1 Joule = 0.239 calories.

Am I missing something? Seems like the result given is 4.187 times higher than it should be.

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 Post subject: Posted: Tue Nov 06, 2012 6:49 am
 Sparks Level

Joined: Tue Feb 24, 2009 7:05 am
Posts: 252
The answer lies in the 0.01 magic constant in front.
You don't see 1/Pi, you don't see 1/4, you don't see the other 1/2, you don't see 1/4.187.
Everything is hidden in 0.01.
Gotta love those Handbook style equations...

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