There is an important birthday celebration this month. IEEE 1584, “IEEE Guide for Performing Arc-Flash Hazard Calculations,” turns 20! “Born” on September 23, 2002, I have watched this standard mature and improve over the past two decades. However, I have also noticed that like some 20-year-olds, it is sometimes misunderstood, which results in confusion about arc flash studies and electrical safety. I will attempt to clarify some of the more common misunderstandings in a question-and-answer format.

Two-second rule

The arc won’t last longer than two seconds, why is that?

This is a common misunderstanding. IEEE 1584 contains language often referred to as the “two-second rule,“ which originated with the 2002 edition. When performing an arc flash study, sometimes there is not sufficient arcing fault current to trip an upstream overcurrent device in the instantaneous region. This can lead to a longer arc duration, sometimes lasting several seconds or more, resulting in an unusually large calculated incident energy. The two-second language is about exposure time, not arc duration. It is based on the premise that a person would quickly move out of the way in response to the arc flash and no longer be in the immediate area after two seconds. The arc flash itself could possibly continue.

40 cal/cm2 arc blast

I understand that incident energy above 40 cal/cm2 will create a large blast pressure, making me a “bug splat.” Is this correct?

Not exactly. The 40 cal/cm2 “stories” began with the 2000 edition of NFPA 70E when the hazard risk category (HRC) tables were introduced for selecting arc-rated clothing and personal protective equipment. The HRC tables only went up to 40 cal/cm2, which was the upper limit of most PPE at the time. An informational note was introduced for incident energy above 40 cal/cm2 that stated, “a greater emphasis may be necessary with respect to de-energizing.” IEEE 1584 incident energy calculations can sometimes exceed the 40 cal/cm2 threshold. This often led to many sensationalized stories about the body’s condition after being exposed to a perceived blast pressure above this threshold.

However, blast pressure is a function of the rate of energy delivery. Is the calculated incident energy above 40 cal/cm2 due to an arc duration of a few cycles or a few seconds? The first could have considerable blast pressure, but the second, although a lot of energy, is spread over a longer period of time.

This informational note was finally removed in the 2018 edition of NFPA 70E with a great justification: 40 cal/cm2 should not be the threshold where equipment is de-energized before a person is exposed to electrical hazards. It should always be de-energized and placed into an electrically safe work condition.

Electrically safe = no PPE required

We only work on systems placed into an electrically safe work condition, so I never need PPE, right?

Not quite. The process of creating an electrically safe work condition requires that conductors and circuit parts are treated as energized until absence of voltage can be verified. When using a test instrument for this process, the person is up close. If the equipment is still energized for some unknown reason, which can happen with conditions such as failed switches and missed sources, the hazard is still there. Arc-rated clothing and PPE is still required during this process, unless the incident energy is less than 1.2 cal/cm2.

No arc flash hazard

The incident energy on the label is less than 1.2 cal/cm2, which is the threshold where arc-rated PPE is required. This means there is no arc flash hazard, right?

I hear this quite often and have seen the words “no arc flash hazard exists” on labels. The simple answer is that there is still an arc flash hazard, just not one that is considered severe. This threshold is considered to be where the onset of a second-degree burn could occur. A minor (first-degree) burn could still happen. Also, the hazard is based on the working distance; if any part of the body is closer than the working distance, the incident energy will be greater, perhaps greater than 1.2 cal/cm2.

It can’t happen to me

This will just take a minute—how dangerous could that be?

A common misunderstanding is believing the hazard is based on the time needed to perform the task. The classic case is that the job has been completed, the PPE is put away in the truck and it is time to drive on. Then, the person remembers they forgot to perform one final task. With the PPE still in the truck, they go back inside, perform the task that takes all of two minutes, and “Boom!” It can happen to anyone.