The 2018 Edition of NFPA 70E - Standard for Electrical Safety in the Workplace may finally see Article 130.7(A) Informational Note 3 and the 40 cal/cm2 threshold eliminated. Based on Public Input Submittals for the First Draft and the corresponding NFPA 70 Committee Statement, it appears Informational Note 3 may be deleted since greater emphasis should always be placed on de-energizing regardless of the incident energy level.

In the past, the 40 cal/cm2 threshold was often associated with blast pressure. However blast pressure is not directly correlated with incident energy - it is more a function of short circuit current and the rate of energy release.

An article providing a bit more detail can be found here: 2018 NFPA 70E 130.7(A) Informational Note 3

The Public Input (proposals) regarding this informational note came from a few people including Arc Flash Forum Regular Contributor PaulEngr

So this week's question is simple.

What do you think about deleting the 40 cal/cm2 threshold Informational Note in the 2018 Edition of NFPA 70E?

Great to see it go
Should keep it
Something else

By the way, if you have concerns about the possibility of deleting Informational Note 3 and the 40 cal/cm2 threshold, there will be an opportunity to be heard during the public comment period for the first draft which is from March 7, 2016 to May 16, 2016.

If we are concerned about potential blast pressure, then perhaps that should be the parameter on which we are concentrating.

"it appears Informational Note 3 may be deleted since greater emphasis should always be placed on de-energizing regardless of the incident energy level. "

But greater than what?
I am thinking it should stay, but basically as a way of saying
"perhaps you should rethink what you are doing, before you do something stupid"

The simple fact is that if you are "always placing greater emphasis", you are also "NEVER placing greater emphasis".
As it is, just "placing greater emphasis" is a pretty low bar to performing the energized work, even if you take the informational note, as a regulation.

Hi Jim,

As it may appear from the converations that the 40 cal/cm2 may not be technically accurate with regards to bast pressure, perhaps it should be removed for the simple fact that is it not already emphasized to de-energize in all cases regardless of the incident energy level?

That pretty much sums it up.

a) Even though Blast Pressure wasn't specifically mentioned, it is brought up quite a bit when discussing Informational Note 3. It is not a direct function of total incident energy - and 40 cal isn't a magic number that defines it.
b) The emphasis is to always de-energize anyway, regardless of the incident energy.

If this makes it to the 2018 Edition (and I believe it will) then one more well intended article that began in the early days of NFPA 70E and later became questionable as we learned more - is about to be resolved.

In respect to the original question, I would propose that if we keep it, the rule should be changed to 1.2 cal/cm^2. That is the point defined as the arc flash boundary at which point we address concerns about arc flash in the first place. Outside that point we don't consider it worthy of addressing. Inside that boundary, we do. Thus greater emphasis is placed where it belongs, at the point where arc flash is considered a hazard.

If we were to propose ANY other boundary then we need to have some sort of additional steps that need to be taken. This is the reason that the prohibited approach boundary was eliminated.



Up until recently the test data was all over the map. As I understand it from the joint NFPA/IEEE test project from a single "where are we at" powerpoint that has subsequently disappeared and in which they didn't have any markings on it for scale, blast pressure seems to be more or less a constant.

This also makes sense from testing as well as modelling done on the subject in support of arc resistant gear by the CIGRE group. There are three key takeaways from their research. The first is that the arc blast pressure at the point of failure of the enclosure is dependent on the structural strength of the enclosure. In other words, arc blast has absolutely nothing to do with any properties of the arc itself. Voltage, current, phase angle, and all that other stuff don't matter. These are all simply heat sources that increase the pressure inside the enclosure by heating air. Knowing that pressure is directly proportional to temperature if we keep the volume constant, this should be obvious. The pressure before an initial rupture of either a purpose-designed rupture point (for arc resistant gear) or of the enclosure itself is dependent on the arc power but they all ruptured within 1 cycle in CIGRE's tests. CIGRE did a bunch of research on the size of the surrounding room and the impact on arc blast because there was some concern about making sure that the room was big enough when installing arc resistant gear but the upshot of that line of research is that it really doesn't matter. The resulting arc blast (pressure) falls off at a rate equal to the cube of the distance (^3) which makes sense since this is related to volume rather than area. CIGRE was also trying to predict the point at which an enclosure would fail but basically they couldn't computer model this and had to fall back on actual testing (blowing stuff up). I also carefully looked at every single test that CIGRE ran and never found anything that exceeded around 1-2 PSI in any of their pressure measurements and the rupture point was always within 1 cycle. As enclosures get larger the onset of arc blast increases but the pressure at which the enclosure ruptures is lower. Since surface area increases, strength over that same span goes down for a given construction technique.

So if we use CIGRE data which is published and apply it to non-arc resistant gear cases we can conclude:
1. Arc blast occurs at around 1 cycle, maximum.
2. Arc blast is dependent on enclosure design but it is pretty much fixed regardless of the enclosure.
3. As enclosure size increases arc blast onset can go either way because the enclosure will rupture at a lower pressure but take longer to get to the same pressure.
4. Arc blast will occur quicker with higher arc power (increasing voltage or current) but we are arguing over milliseconds.

Irrespective of any of this, a fatality due to overpressure occurs somewhere around 20-100 PSI according to military research on pressure effects. Ear drums are blown out at 1-2 PSI and people are knocked down at around 5-10 PSI. So we can expect an arc blast to result in blown ear drums and perhaps knock people around but no fatalities. This is exactly what has been reported from incident investigations.

Sp...I hear you about the research but I'm afraid that it isn't going to be very valuable other than to provide a data point and move on.

What I saw was the whole note was to be eliminated. The comment about a greater emphasis should always be placed on de-energizing regardless of the incident energy level was just that - a comment. I don't believe it was part of a proposed text. I believe it was simply based on the overall philosophy of electrically safe. If some form of text did remain, you bring up a great point that NFPA should consider. I have heard from another good friend on the NFPA 70E committee that would like to see 1.2 cal/cm^2 remain with the note.

Thanks!

I think there is a real issue here but I have trouble finding a good way to define it. As an example say that I'm working on a 15 A circuit off a 120 V lighting panel. I think we can agree that the incident energy at anything less than a couple inches away is less than 1.2 cal/cm^2. But I think we can also agree that the current modelling efforts fail us with the cases that are below 250 V given your own efforts at trying to further define up the "exception" in IEEE 1584. What we don't want to do is enter a world where we are saying that residential electricians should wear arc flash PPE especially because there is no evidence for this and being unrealistic leads to a lack of compliance. But right now if I use 70E-2015 as written and I don't do an incident energy analysis (how to do that?) for anything that wouldn't require an incident energy analysis (the old pesky "exception" here), I end up on the table method. And guess what those don't have? There simply is no "No" for PPE. It starts at PPE 1 (ATPV 4). This is the one place where losing H/RC 0 was a loss.

Similarly there is no place in the Code where it states succinctly what to do outside 1.2 cal/cm^2. We have an arc flash boundary but really very little guidance on what to do (or not to do). Previously we sort of always defaulted to "H/RC 0" for everything unless there was a clear arc flash boundary. I think what we need to consider or define somewhere is that there are a total of 3 necessary and sufficient conditions for an arc flash hazard to occur:
1. The equipment needs to be capable of producing an arcing fault. Especially some solidly insultaed switchgear, some underground switchgear, and a whole range of intrinsically safe equipment for example is incapable of this.
2. The incident energy at the "working" distance exceeds 1.2 cal/cm^2. This needs to be defined outright as the distance to the face/chest area to eliminate confusion about arms and legs so that it is clear what an arc flash boundary means especially if it is say only 12".
3. The worker has to be engaging in an activity that is likely to cause an arcing fault and thus the likelihood of injury is less than other potential hazards.

If any of these three conditions is invalidated, an arci flash hazard does not exist. With item #1, it can't exist. With item #2 even if it exists the severity does not reach an actionable level. With item #3 even if the severity is great enough the likelihood is not.

Somehow just as with the old goofy definition (pre 2009) where we only considered arc flash if we were inside the limited approach boundary, the new definition has similar deficiencies. At a minimum restating the arc flash hazard definition may be where this needs to go.

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.

Likewise, the PPE and any other fabrics ignition or melting characteristics are not at all a function of incident energy but strongly depend on the rate of energy release and exposure time. Read Behavior of apparel fabrics during convective and radiant heating for more information including free time to ignition or melting vs. thermal heat flux calculator.

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 and released but unfortunately the group failed to address the matter.

With reference to what you've published from an academic point of view there seems to be some merit to it. But simultaneously I can make very simple physical arguments that ultimately the critical parameter is a temperature at which a particular chemical reaction occurs which is irreversible and which is known as a burn. Knowing that there is a cooling rate which definitely matters over a much longer period of time, (heat input - cooling output) * time = thermal energy and thermal energy can be converted into a temperature knowing some other parameters of the system. Thus BOTH the heat input (your argument) and time matter over a long enough period of time. Over a very short period of time though if we are going to get to enough thermal energy to cause a temperature which causes a burn, I no longer care about cooling output because it's not big enough, and this factor drops out. Thus I end up with a very simplified form which is (heat input)*time = thermal energy. Thus whether we use energy (cal/cm^2) or power (W/cm^2) is simply two different ways of expressing the same thing except at an academic level.

The various data sets do seem to suggest an exponential shape to the curve but whenever anyone shows data on a log scale, this immediately throws up red flags. It is quite well known that practically everything whether exponential or not looks like a straight line on a log-log scale. Thus this is reason to be suspicious and to look at the arguments about WHY it should be a log scale before just accepting it as factual. I'm seeing a lot of curve fitting to data, not necessarily starting from a theoretical basis, even with the cited references. I'd even go so far as to suggest for instance that Stoll's data doesn't include the "cooling rate" parameter which could easily alter the shape of the data to look more like a curve and not a straight line which is what we would expect.

Your argument should not be made with the 70E Committee. They are simply following the lead of standards promulgated by for instance IEEE and ASTM. What you are suggesting is that 70E establishes a new standard independent of the committees who are charged with doing the research and creating the technical standards. I'm sure if IEEE changed the way of determining arc flash potential to heat flux (W/cm^2) then it wouldn't be long before ASTM and 70E followed suit. But not the other way around unless there is clear demonstrated evidence of where the current standard is not working.

Even if the academic argument is valid, we don't have any cases showing where either the cal/cm^2 metric is absurd or it doesn't work. And I'm not talking about theoretical arguments here. Most likely the only way you're changing it is if someone is burned due to a high heat power flux (W/cm^2) while the energy flux (cal/cm^2) metric indicates conditions are safe.

So it's not that the 70E Committee is ignoring the argument. It's that there's not enough convincing evidence to move in a direction counter to not only their purpose but counter to other standards as well that are better researched. It is after all a consensus safety standard, and going in that direction without a strong argument for doing so is not consensus.

I have never seen thermal energy conversion into a temperature. I wondered if you could show an example of converting the energy into a temperature? Your proposed expression for thermal energy is indeed very simple and does not adequately describe the reality. Alice Stoll[1] has nicely summarized the issue of using critical thermal load for evaluating the damage to the skin:

"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."

[1]- A.Stoll, "Heat Transfer in Biotechnology", Advances in Heat Transfer, v.4. Academic Press. 1967



I see no problem using logarithmic scale. The log-log scale is very popular and commonly used in electrical engineering. I wondered if published time-current, trip characteristics also are also throwing up red flags for you too? Exponential function as well as most of the functions do not look at all straight on the logarithmic scale. Power function of the form y=ax^k is rather exception to the rule as it does indeed appear as a straight lines in a log–log graph, with the power and constant term corresponding to slope and intercept of the line. Also, I don't quite understand the problem you have with Stoll's test data.



I've seen and already demonstrated a lot of evidence proving the NFPA 70E methodology of adopting the 1.2 cal/cm^2 critical thermal load threshold for a second degree burn by an exposure of unprotected skin to radiated heat is plain wrong. Also, it was shown that the NFPA 70E tables will specify adequate arc flash PPE only 50% of the time. It doesn't surprise me because the PPE and any other fabrics ignition or melting characteristics are not a function of incident energy but strongly depend on the rate of energy release and exposure time.

Consensus works in marriage, it may or may not work in social life but as a matter of fact it does not work in science. The notion of a flat Earth was one of scientific consensus in old days too.

There is plenty of folks sitting on IEEE, ASTM, NFPA committees and reading this forum. I'd love to hear their opinion on the matter of using cal/cm^2 metric for evaluation of arc flash potential.