40 cal/cm^2 Deleted – But Some Confusion Remains.

When the topic of incident energy above 40 calories per square centimeter (cal/cm^2) comes up, the discussion can be quite interesting.  People will sometimes refer to the high values in terms of a bomb or some other sensationalized description.  Although a higher calculated incident energy can be more hazardous, all is not as it appears to be. Is the large value due to a very strong source or is it simply due to a protective device possibly taking a long time to clear?    Each will behave differently.

When performing an arc flash study, if the calculated incident energy exceeds cal/cm^2 at any locations. people often just shake their head and ask, “Now what do we do?”
 We need to place the equipment into an electrically safe work condition but that in itself poses some risk.

When the 40 cal/cm^2 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/cm^2, arc flash labels may have the statement “No PPE Available.” This value also frequently triggers using the signal word “DANGER” on the label.  There may be comments made 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/cm^2 can be misleading.

What about PPE preserving the body? I won’t bother with that one, although I did ask a large group at one of my training programs recently to see 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, incident energy above 40 cal/cm^2 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 work condition.

The significance of 40 comes from the Informational Note 3 that was found in 130.7(A) of the 2015 Edition of NFPA 70E that stated: “When incident energy exceeds 40 cal/cm^2 at the working distance, greater emphasis may be necessary with respect to de-energizing when exposed to electrical hazards.”

However, this informational note was deleted in the 2018 Edition which I hoped would end the confusion surrounding 40 cal/cm^2? 
But it did not completely.  Many still look at this value as being a special threshold so let’s dig a little deeper.

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/cm^2 threshold. Unfortunately, the total incident energy is not always a good indicator of whether the arc flash would result in significant 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/cm^2, it is due to a longer arc duration rather than a large short-circuit current.

Electrically Safe


NFPA 70E always emphasizes placing electrical equipment in an electrically safe work condition rather than working on energized equipment. This is always the best option when an electrical hazard exists, regardless of whether the incident energy is above or below 40 cal/cm^2.

That is actually the justification for deleting the informational note – consideration should always be given to placing electrical equipment in an electrically safe work condition, not just because the incident energy is greater than 40 cal/cm^2.

Based on Jim Phillips’ article originally published in the March 2017 Edition of Electrical Contractor Magazine.