If you’re conducting an arc flash study, you’re probably using
one of the many software packages that can perform the analysis
for you — at the click of a button. However, as convenient as
they may seem, there are several pitfalls that can cause
erroneous results if you are unaware of the underlying premises
behind many of the calculations. The following five
considerations are important issues to keep in mind when you’re
doing this type of work.  Source Impedance  Fault current
sources like power generators cannot supply faults in a power
system indefinitely. The fault current magnitude is, in fact,
limited by the generator’s internal reactance, transmission line
impedance, transformer impedance, and series reactors (when
present). You can model the source either without these
impedances (infinite source) or with these impedances. When
undertaking a study for a large commercial or industrial site,
assuming an infinite source for the electrical model is a big
“no no.”  Assumption of an infinite source results