Comparison of Bolted to Arc energy question needs clarification

[mcmahanrf]

Question reads:

For a 480 V system, the arcing-fault magnitude relative to the available bolted- fault magnitude is approximately:
a. 25%
b. 50%
c. 75%
d. 90%

I don't have the IEEE but this

https://www.cedengineering.com/userfiles/Arc%20Flash%20Calculation%20Methods.pdf

References the calculation, using that calculation gives values of around 70% tracking down to 50% as fault current goes up. There are several variables that are not addressed by the question but I thought 50% would be a safe bet, I was wrong. anybody have any incites?

[bec51392]

I looked through my IEEE 1584... not one mention of anything close to what you are looking at but I though all the arc fault calculations came straight out of the NFPA 70E. this is what I found...

NFPA 70E INFORMATIVE ANNEX D.3 (PG60)

D.3 Doughty Neal Paper.
D.3.1 Calculation of Incident Energy Exposure. The following
equations can be used to predict the incident energy produced
by a three-phase arc on systems rated 600 V and below. The
results of these equations might not represent the worst case in
all situations. It is essential that the equations be used only
within the limitations indicated in the deƒnitions of the varia‐
bles shown under the equations. The equations must be used
only under qualiƒed engineering supervision.
Informational Note: Experimental testing continues to be
performed to validate existing incident energy calculations and
to determine new formulas.
The parameters required to make the calculations follow.
(1) The maximum bolted fault, three-phase short-circuit
current available at the equipment and the minimum
fault level at which the arc will self-sustain. (Calculations
should be made using the maximum value, and then at
lowest fault level at which the arc is self-sustaining. For
480-volt systems, the industry accepted minimum level for
a sustaining arcing fault is 38 percent of the available
bolted fault, three-phase short-circuit current. The high‐
est incident energy exposure could occur at these lower
levels where the overcurrent device could take seconds or
minutes to open.)

[bec51392]

Also found this? but all this data is extremely dated.


NETA Handbook series II - Arc Flash vol. 2:
• Available bolted fault current – The punch behind the arc fault
magnitude. Recall that the magnitude of a low-voltage arcing
fault is approximately 43-57% of the bolted fault value. This implies
that systems with significant bolted fault currents will have
elevated arcing current levels.