ARCFLASHSAFETY BY JIM PHILLIPS
When this value is exceeded, it is
often treated like an absolute go/no-go
threshold and can trigger many different responses and comments that are
not always correct. Above 40 cal/cm2,
arc flash labels may have the statement
“No PPE Available.” There may be comments such as, “Above that value, the
blast pressure will kill you.” My favorite
sensationalized comment that I have
heard is, “Above that level, PPE just preserves the body.”
Fact checking
Time for a little arc flash fact checking.
“No PPE Available?” The available arc
ratings of personal protective equipment
and protective clothing are continually
pushing the envelope. Some ratings can
now exceed 100 cal/cm2.
What about the blast pressure argument? An arc flash may result in a
pressure wave, and it could be significant
given the right conditions. However, the
blast is actually dependent on the short-circuit current and the rate at which the
energy is released. Simply stating it is a
function of an incident energy above 40
cal/cm2 can be misleading.
What about PPE preserving the
body? I won’t bother with that one,
although I did ask a large group recently
how many of them have heard that
statement. Everyone raised their hands
and chuckled.
So why has there been so much confusion surrounding the number 40?
For a better understanding, let’s back
up a few steps. For many years, inci-
dent energy above 40 cal/cm2 has been
widely considered a threshold where
energized work should be avoided. In
that case, just de-energize and place the
electrical equipment into an electrically
safe working condition.
The significance of 40 comes from
NFPA 70E and is currently found in
130.7(A) Informational Note 3 of the 2015
Edition: “When incident energy exceeds
40 cal/cm2 at the working distance,
greater emphasis may be necessary with
respect to de-energizing when exposed
to electrical hazards.”
Incident energy
The severity of an arc flash is defined
by the prospective incident energy and
is normally determined by performing
an arc flash study. The Standard IEEE
1584, IEEE Guide for Performing Arc-Flash Hazard Calculations, provides the
equations used for the calculations usually performed with arc flash software.
Two of the most important variables in
determining the incident energy are the
available arcing short-circuit current
and the duration of the arc flash, which
is normally defined as the time it takes
an upstream protective device to operate.
This means that a lower magnitude
arcing short-circuit current could result
in an unusually long protective device
clearing time determined from the
device’s time current curve. The result is
a very large calculated incident energy,
often exceeding the 40 cal/cm2 thresh-
old. Unfortunately, the total incident
energy is not always a good indicator of
whether the arc flash would result in sig-
nificant blast pressure.
Calculations based on a lower magnitude short-circuit current with a longer
clearing time could result in a calculated
incident energy that is greater than if
there was a large short-circuit current
with a short duration. More often than
not, when the results of an arc flash study
indicate locations where the incident
energy exceeds 40 cal/cm2, it is due to
a longer arc duration rather than a large
short-circuit current.
Electrically safe
There are many areas in the 2015 edition
of NFPA 70E that emphasize placing
electrical equipment in an electrically
safe working condition. This is always
the best option when an electrical hazard
exists, regardless of whether the incident
energy is above or below 40 cal/cm2.
Although the 2018 edition of NFPA
70E has not been finalized at this time, all
signs indicate the confusion surrounding 40 cal/cm2 is about to take a big step
towards being cleared up. How?
It appears the Informational Note
referencing 40 cal/cm2 that has been the
"line in the sand" for so many is about
to be washed away, i.e., deleted. Why?
Consideration should always be given
to placing electrical equipment in an
electrically safe working condition, and
not just because the incident energy is
greater than 40 cal/cm2.
Line in the Sand
The 40 cal/cm2 dilemma
IT’S THE SAME OLD STORY. An arc flash study was just completed, and the calculated incident energy exceeds 40 calories per square centimeter (cal/cm2) in many
locations. When this happens, people often just shake their head and ask, “Now
what do we do?”
PHILLIPS, P.E., founder of www.brainfiller.com and www.ArcFlashForum.com,
conducts training programs globally and is the author of “Complete Guide to Arc Flash
Hazard Calculation Studies.” He is secretary of the IEEE 1584 Arc Flash Working Group and
International Chair of IEC TC78 Live Working. He can be reached at jphillips@brainfiller.com
or 800.874.8883. I S T
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