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Changes to the definition of arc flash boundary in upcoming NFPA 70E

Changes to the definition of arc flash boundary in upcoming NFPA 70E

Just noticed there are changes, among many others, to the definition of arc flash boundary in the upcoming NFPA 70E Standard for Electrical Safety in the Workplace (see for the comprehensive list of changes). Specifically, the revised definition of arc flash boundary no longer uses the term "second degree burn" but instead "at which incident energy equals 1.2 cal/cm2.", and the revised informational note references the Stoll skin burn injury model.

NFPA 70E year 2004 assumed that the incident energy requirement increases below one second. A quote from NFPA 70E year 2004 "For situations where fault-clearing time is 0.1 second (or faster), the Flash Protection Boundary is the distance at which the incident energy level equals 6.24 J/cm^2 (1.5 cal/cm^2)." This reference was removed in NFPA 70E year 2012 edition. NFPA 70E year 2012 stated that "a second degree burn is possible by an exposure of unprotected skin to an electric arc flash above the incident energy level of 1.2 cal/cm2 ( 5.0 J/cm2 )" and assumed 1.2 cal/cm2 as a threshold incident energy level for a second degree burn for systems 50 Volts and greater. NFPA 70E year 2015 explicitly prohibited using incident energy and PPE category together. The NFPA Handbook from 2015 showed a sample label with fields to be filled in for "available incident energy" and "level of PPE" while just half a page earlier states that "available incident energy" cannot be included with the "PPE category" in table 130.7(c)15(A)(b).

The revised definition of arc flash boundary in NFPA 70E year 2018 is even more misleading. It also contradicts the accompanying revised informational note referencing the Stoll skin burn injury model. A quote from A.Stoll "Heat Transfer in Biotechnology" summarizes the issue of using a critical thermal load approach in determining arc flash boundary. The quote reads:

"Serious misconceptions have crept into this field of research through adoption of rule-of-thumb terminology which has lost its identity as such and become accepted as fact. A glaring example of this process is the “critical thermal load.” This quantity is defined as the total energy delivered in any given exposure required to produce some given endpoint such as a blister. Mathematically it is the product of the flux and exposure time for a shaped pulse. Implicit in this treatment is the assumption that thermal injury is a function of dosage as in ionizing radiation, so that the process obeys the "law of reciprocity," i.e., that equal injury is produced by equal doses. On the contrary, a very large amount of energy delivered over a greatly extended time produces no injury at all while the same "dose" delivered instantaneously may totally destroy the skin. Conversely, measurements of doses which produce the same damage over even a narrow range of intensities of radiation show that the "law of reciprocity" fails, for the doses are not equal."

Here is what ASTM F1959/F1959M Standard Test Method for Determining the Arc Rating of Materials for Clothing says about skin burn injury determination:

"12.1.4 Predicted Second-Degree Skin Burn Injury Determination (Stoll Curve Comparison) — The time dependent averaged heat energy response for each panel [..] is compared to the Stoll Curve empirical human predicted second-degree skin burn injury model:

Stoll Response, cal/cm2 = 1.1991 * ti^0.2901

where ti is the time value in seconds of the heat energy determination and elapsed time since the initiation of the arc exposure. A second-degree skin burn injury is predicted if either panel sensor heat energy response exceeds the Stoll Response value (at time ti).

Incident energy alone has no impact on thermal damage and blast pressure. One can expose himself to any arbitrary incident energy and suffer no damage as long as the energy is delivered at slow enough rate. On the other hand, an exposure to only a fraction of 1.2 cal/cm2 may result in incurable burn provided that the energy has been delivered fast enough. Read Evaluation of onset to second degree burn energy in arc flash hazard analysis for more information. The issue of using incident energy as a measure of damage alone and without regard to the rate of the energy release has been raised to NFPA 70E committee before year 2015 edition was published but unfortunately the group failed to address the matter back in 2015. It seems the new upcoming NFPA 70E is to obscure the matter even more instead of admitting the glaring mistake and fixing it.

RE: Changes to the definition of arc flash boundary in upcoming NFPA 70E

I'm concerned since as you told, they making changes so that the criteria is not based anymore on second degree burn threshold but on an incident energy level which we believed was the threshold.

Being a trainer on arc-flash matters, I always explained what represent the 1.2 cal/cm². So to tell that the standard accept a level of injury. To tell workers that they might get burned even wearing recommended PPE.

With this definition, the criteria represent nothing that could be easy to explain.

I would have to explain that the standard is not based anymore on burn level, which is a concept easy to understand by workers compared to Stoll researches and why, based on that, the standard committee thinks it's a safe practiced. The problem is that I have hard time to understand it myself (after a first reading of ArcAdvisor paper).

It's already a pain to explain that we cannot state anymore a PPE Category defined by the standard when we do an Energy incident analysis during retrained as to explain why there is two different selection methods of PPE for those who get their first training.

But worst, I will have to explain the variability of second degree burn. My Ethic Code states that I have to warn my clients about any additional hazards that I discover. Also, I could be held criminally responsible if I'm not diligent about that matter.

Even worst, following my ethics, I would have to warn my own father who also gives an Arc-Flash training designed by others than him. (He has not the academics but he is my reference on how to deal with regulations on field since he was maintenance and project manager, planner, etc.)

I understand that the definition changes are to keep the way we do arc-flash analysis, but it's not promoting better safety.

I don‘t like that.

RE: Changes to the definition of arc flash boundary in upcoming NFPA 70E

My greatest contribution to this thread, and the handling of this issue by a group we simply call "the NFPA, would simply be to encourage the 2 prior contributors, and as many others as possible, to pursue this. Perspective is crucial. We certainly realize that the true goal of even having an NFPA, and certainly the 70E is for the safety (and protection) of people, as opposed to measured levels or language of previous standards.

Both of your posts show your recognition of that. Now we (a far larger group than posts on this thread indicate) must continue our work toward our goals, in this instance as well. The 70E committee certainly has not produced anything better, or as good as, Stoll for determining harmful levels of energy transfer. Perhaps they shouldn't even be attempting to specify on that level, but instead be at the level of "not permitting any flesh burn, and satisfying such conditions and levels as specified and determined by "Stoll", et-al.". ...or whatever.


(Me,,,wrong?, just fine-tuning my sarcasm!)

RE: Changes to the definition of arc flash boundary in upcoming NFPA 70E

The definition has been 1.2 cal/cm2 for quite a while, so not much is really changing. The concept of the "onset of a 2nd degree burn" is subjective to some extent and I think the committee is trying to remove ambiguity. The question is generally: How is standard improved by including this description? In this case, it really doesn't add anything to the discussion except for a qualitative explanation of the 1.2 cal/cm2 value. The NEC doesn't explain the basis for their required ground wire sizes, but there is some fairly solid science behind it. I think this is a similar situation.

If you're unhappy at this point, when IEEE 1584 is updated, you're going to be really miserable.

And as always: NFPA 70E is not a legally binding standard. It's a consensus standard.



RE: Changes to the definition of arc flash boundary in upcoming NFPA 70E

Maybe I overeact there or maybe I was anticipated overeaction from my trainees when I would have to debunk the "second degree burn" case. Even if NFPE 70E or the canadian equivalent CSA Z462 are not legaly binding, we have a criminal law that my clients have to comply too and showing that they didn't follow Z462 can be legally messing. There's a statement a my Order Ethical Code, which is legally binding, that I have to care about keeping informations that could affect public safety.

So for know I stand by : "Some statements in the standards are continuously questionned, debated and reavaluted but the standard we have here is the best consensus available and it's efficiency is documented." I will ensure to back myself with the "the documented efficiency" statement


RE: Changes to the definition of arc flash boundary in upcoming NFPA 70E

I can't speak to legal issues related the CSA Z462, and it's certainly advisable to comply with the standard. However, nothing about arc flash calculations or PPE requirements is high precision. Too many variables. And as you point out, it is a moving target.

I don't see why you say you have to "debunk" the 2nd degree burn case. That's still the intent of the determination of the arc flash boundary.


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