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 Post subject: Determination of Incident EnergyPosted: Thu Feb 21, 2013 11:27 am

Joined: Thu Feb 21, 2013 10:56 am
Posts: 4
Hello Folks,

I have a few questions related to DC arc flash calculations. I am in the process of creating a MathCAD template for DC arc flash calculations. I have encountered some confusion with the incident Energy equations, in particular the variable units, for Stokes & Oppenlander and Paukert methods.

The equation for open air:

E_OpenAir = Earc / (4*pi*d^2) = Iarc^2*Rarc*tarc/(4*pi*d^2) = Varc*Iarc*tarc/(4*pi*d^2)

Correct me if I am wrong, but Varc is in Volts, Iarc in amperes, tarc in seconds; d (working distance, typical 18 inches = 45.72 cm) in centimeters. Now, I have conflicting information if where E_OpenAir is in J/cm^2 or cal/cm^2. ETAP
Arc Flash Session 3 at 00:58, which is from the ETAP tutorial, shows that E_OpenAir is in cal/cm^2. However, instinctively, I think that this is wrong and should be in J/cm^2 as V*I*t is in W*s which is Joules.....

Likewise, for the box/enclosure equation:

E_Box = k * Earc / (a^2 + d^2).

I think that "a", which is the reflectivity coefficient, should be in centimeters. Typical should be then 10 cm, 40 cm, 95 cm. Again, E_Box should be in J/cm^2.... Confirm please.

Lastly, the incident energy calculation for maximum power:

E_open = 0.01 * Vsys * Isys * Tarc / D^2;
E_box = 3*0.01 * Vsys * Isys * Tarc / D^2; D is the working distance expressed in cm.
Although Vsys*Isys*Tarc yields W*s = Joules, I reckon that E_open is in cal/cm^2 due to that unexplained "0.01" factor, which somehow converts that equation from Joules to calories??? If there is a source to clear this up, it would be great. Annex D.8.1.1 of NFPA 70E-2012 does suggest it is in cal/cm^2, however some backup rationale on this would really clear this up.

Could someone please give me some insight on this?

Thank you.

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 Post subject: Posted: Sat Feb 23, 2013 5:23 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2178
Location: North Carolina
These are all empirical equations. Thus units cannot be varied. The corrections are buried in the constant.

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