jorgepaz wrote:

I'm performing an Arc Flash study on a 7200 v system but the arc flash boundary value is around 21 feet. What could be wrong in the calculation that is giving me that big value?

Is it normal?

Thx,

Jorge

It depends on a few details. What is the incident energy? Also, I'm assuming this is for equipment since an AFB is being calculated which implies IEEE 1584 equations are being used.

If IEEE 1584 is being used and this is MV equipment, at medium voltage, a value known as the distance exponent is quite low which is likely the reason assuming everything else is correct.

What I mean by this is the incident energy theoretically decays as the inverse distance squared. i.e. double the distance and the incident energy drops by (1/2) squared or 1/4 of the value.

However, based on IEEE 1584, for low voltage equipment, it is not really a squared value. It is 1.641 for distribution equipment and 1.473 for low voltage switchgear - smaller rate of decay with distance. For medium voltage switchgear the exponent is 0.973 which means the incident energy decays very slow with distance thus requiring a greater distance to hit 1.2 cal/cm2 (the energy at the AFB)

Hope that helps.