It appears the 40 cal/cm2 footnote suggesting a greater emphasis be given to de-energizing is going to be deleted in the 2018 Edition of NFPA 70E. The reason is that an emphasis should always be given to de-energizing, not just above 40 cal/cm2.

40 is often the go/no-go value for energized work and Warning vs. Danger on labels.

So, let’s speculate about the future as a result of this deletion such as labeling, PPE, energized work etc.

Will you/your client(s) change safety practices for incident energy above 40 cal/cm2?

Yes/Probably
No/Probably not
Not sure/Doesn’t apply

If you answered Yes/Probably, please let us know what changes may occur as well as your thoughts!

When the 40 cal/cm2 threshold was first written into 70E it was a quantified guess for a threshold level where arc blast hazards would start to become more significant than thermal hazards. Additional testing has shown that arc blast is related to the rate of rise of the fault current, and arc blast has virtually no relationship to incident energy values. The challenge in re-educating folks is that there isn't much publically available test data on arc blast magnitudes. There are a few IEEE papers that provide limited data, and a couple IEEE papers will be presented on this topic at the 2018 Electrical Safety Workshop. Much of the research on arc blast has been in the area of arc-resistant equipment testing, and manufacturers consider most of this information proprietary.

In summary, we have enough data to refute the 40 cal/cm2 threshold, but we do not have enough data to establish thorough guidance on arc blast hazards. Nevertheless, the 40 cal/cm2 threshold has been so ingrained for over 10 years now that it will probably persist for quite a while, even once we have a better idea of how to quantify arc blast.

I believe that there will be a lot of resistance to changing >40 Cal/cm2 = Danger on the AF labels. This is due to the fact that many people do not like change. Changing this rule would mean changing facility's Electrical Safety Plans. It would also require changing the Danger labels to Warning labels. That will be very costly for facilities who do not have the staff in place to be able to print new labels and attach them to the equipment.

I believe that we should stick with the >40 Cal/cm2 = Danger on the AF labels until there is a consensus on how to calculate the Arc Blast pressure wave.

A few years a go my company decided to drop printing "Danger" labels all together. DANGER means if the danger is not avoided, it will cause death or serious injury. ..

I do not think the 40cal should have ever been there to begin with.

The software packages and plants that use the keyword danger are in violation of the ANSI Z535.5 and GHG standards for labels. The Danger signal word is reserved for use when there is an iminent danger of a fatality or life altering injury unless precautions are taken to avoid it. Under normal circumstances, electrical equipment does not pose any such hazard even if the incident energy is high (>40 cal/cm2). And if you believe the word "Danger" should be used (it shouldn't), then ALL the arc flash labels should say Danger irrespective of the incident energy because the hazard is the same across the board.

"then ALL the arc flash labels should say Danger irrespective of the incident energy because the hazard is the same across the board."

That is exactly the logic our company used when they decided that all of our labels should say "Danger".
Not because of "Arc Flash" but because of the "Shock" hazard indicated on the label.

Great conversation! I wanted to add to Bob's point (also an IEEE 1584 committee member) Part of the resistance to change might also still be based on liability. The perception that if someone is hurt (or worse) a legal team may fall back on an older edition of NFPA 70E and blow the dust off of this footnote to attempt to raise it as an issue. In reality as many now know, blast isn't so much a function of incident energy as it is short circuit current.

Just curious, does anyone know of any "serious" injury from the blast? It knocks people back and there can be a fall or hitting your head (i.e. famous 2003 black and white video of 3 guys at switchgear) but the idea that you will be smashed/crushed doesn't seem to have any supporting evidence.

Thanks for everyone's input here (and everywhere else) great discussions!
Jim Phillips

Agree with the group change will come slow. Blast was never correlated to cal/cm2. The better predictor would be fault current but it is more complicated. Current and containment is the key.

Our paper from 2016 IEEE-ESW called this into question. The original paper from Ralph Lee was totally theoretical and based on lightening and can't be correct.

There are a few papers which have good models. Our paper offers new data on blast which correlates with the Clark paper (cited in our paper).

This is free to IEEE-IAS members or available through IEEE Explore.

H. Hoagland, C. Maurice, A. Haines and A. Maurice, "ARC flash pressure measurement by physical method, effect of metal vapor on ARC blast," 2016 IEEE IAS Electrical Safety Workshop (ESW), Jacksonville, FL, 2016, pp. 1-9.
doi: 10.1109/ESW.2016.7499702

keywords: {arc furnaces;copper;electrical safety;fault currents;pressure measurement; vaporization;Cu;arc blasts;arc flash pressure waves;door ejection speed;electrical safety industry;electrode materials;fault current;metal vaporization effect;thermoacoustic blast;vapor expansion;Arc discharges;Copper;Electrodes;Hazards;Mathematical model;Predictive models;Pressure measurement;arc blast;arc flash hazard assessment and work practices;arc flash pressure wave;copper electrode effect on arc blast;physical method to measure arc blast;tungsten electrode effect on arc blast.},

URL: http://ieeexplore.ieee.org/document/7499702/

I can't even come close to Hugh's knowledge and experience on anything related to electrical injuries but I can offer this...

I don't honestly believe both with doing some of my own purely theoretical calculations as well as reviewing Hugh's group's work, plus both knowledge, training, and accident investigations of actual industrial explosions, never mind working for a long time with a variety of gas, liquid, and solid fuel industrial burners that arc blast is capable of causing a serious injury, and I even doubt it can knock someone over, or possibly even off balance. By this I mean it can get up to around 5-10 PSI in an electrical enclosure at most based on data from multiple sources that actually measured it and as soon as all that pressure is released, the "shock wave" dissipates so quickly that the overpressure is so low that it is doubtful that ruptured ear drums (1 PSI) are even very likely at typical working distances. In addition the arc blast pressure wave is moving very fast. Maybe not at supersonic (>1100 FPS) speeds but certainly something approaching that. With that in mind people and equipment affected by the shockwave should see the peak almost instantly (under 1 "cycle" propagation time). Couple this with the fact that almost all the studies show arc blast occurring within 1 cycle when one does occur and we can pretty much conclude that anyone "knocked around" or "thrown" from the arc blast will be propelled roughly 1 frame later (1 cycle) in video and that the effect is over within 1 more cycle. Even at 60 FPS, this all occurs in 2 frames of video. What I've seen in the videos though shows the movement but with a delay to it that then lasts for several frames.

The fact is that the arc flash videos that are out there obviously show that people are physically "propelled" away from the arc flash. Not only that but there is a significant delay (a lot more than 1 cycle) before movement starts. So it can't be denied that there isn't some sort of reason that people are "thrown" from the area. Given that human reaction times are around 250-350 ms or roughly 15-20 cycles, my contention is that the "thrown back" that we see in videos is not in fact a physical force at all but simple human reactions, whether cognitive or some sort of instinctual fear response ("flight") is what is actually being seen. People and animals naturally jump back and react swiftly and immediately to loud bangs, bright lights, and sudden releases of heat. When they do, they frequently run into each other or walls or anything else in the area in an attempt to escape. So it stands to reason that exposure to an arc flash is going to cause the same reaction. If they are on a ladder or on a scaffold, or there is a wall in the path of escape at the time, then we can expect indirect injuries from the arc flash itself. This explanation seems much more believable given the evidence compared to "arc blast injuries".

I agree. I've never seen a fatality from arc blast. I have seen a broken (parallell bus in South Africa, Door blown off the building from oil filled switchgear blast, not JUST arc blast but basically a fuel-air bomb ignited by an arc) and a concussion (the guy fell down) from 100,000A blast with door open.

As a follow up/suggestion it would seem that there are still some valid recommendations:
1. Recommend securing personnel and tools. In other words be aware of your surroundings. If you are on scaffolding you need hand rails and toe boards. If you are on a ladder or a manlift, you need to be tied off. Tools need to be either secured from falling (strapped) or make sure that others are not located below where they can be injured from falling tools. In other words the same practices that go for working at heights in the first place.
2. Be aware of the possibility of debris being ejected from the enclosure and by doors which may be blown off the enclosure due to an arc blast. Stand to one side when operating disconnects with the doors closed.
3. An arc blast may rupture ear drums. Wear ear plugs.

All of these are equally valid almost without regard to the incident energy (cal/cm2). I can make a case for calculating some kind of minimum threshold current for items 2 and 3 but really these requirements are so trivially achievable and have been a part of best practices recommendations even several decades ago it seems like a huge waste of time to do calculations for such things.