It has been over 20 years since IEEE 1584, IEEE Guide for Performing Arc-Flash Hazard Calculations turns was first introduced to the world on September 23, 2002. All these years later there can still be confusion about arc flash studies and electrical safety.  I hear this in my training classes as well as see questions on social media.  I will attempt to clarify some of the more common misunderstandings in a Q&A format.

Two Second Rule

Q: The arc won’t last longer than 2 seconds, why is that?

A: 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 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 2 seconds.  The arc flash itself could possibly continue.

40 cal/cm² Arc Blast

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

A: 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 (PPE).  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 condition of the body after being exposed to a perceived blast pressure above this threshold. 

However, blast pressure is a function of the rate of delivery of the energy. 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

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

A:  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 and IF the equipment is still energized for some unknown reason (which can happen due to 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

Q: 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 no arc flash hazard, right?

A: I hear this quite often and have seen the words “No arc flash hazard exists” on labels. The simple answer is; 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.  There can still be a minor burn.  Also, the hazard is based on the working distance and 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

Q:  This will just take a minute; how dangerous could that be? 

A:  A common misunderstanding is believing the hazard is based on the time to perform the task.  The classic case is 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 which takes all of 2 minutes and “Boom”.  It can happen to anyone.