The 40 cal/cm^2 limit found in an NFPA 70E 130.7 Informational Note 3 continues to give people heartburn.

Informational Note No. 3: When incident energy exceeds 40 cal/cm2 at the working distance, greater emphasis may be necessary with respect to de-energizing when exposed to electrical hazards.

Although it is only an informational note, many take this value as an absolute go/no go when it comes to energized work and prohibit work at locations where the incident energy exceeds 40 cal/cm^2. Often when there is a large prospective incident energy it is because of a long arcing time i.e. a protective device may take a long time to operate. There are many opinions about this number so here is this week's question.

Does your company/client permit energized work where the incident energy exceeds 40 cal/cm^2?
Yes
No
It depends
We don't perform energized work

Your thoughts and opinions are strongly encouraged!

Intentional live work is never allowed.
Testing for dead, means assuming the conductor are live, so technically live work is being performed above 40cal/cm^2.

JBD, thanks for bringing this up - It has always been a concern/problem and the reminder is certainly appreciated. I wrote an article around 5 years ago about people not working above 40 cal/cm2 but in making the circuit "safe" a hazard must be introduced for checking voltage.
http://www.ecmag.com/section/safety/catch-22/
I've been surprised with a "dead" circuit being proven energized during this test (only once) and many others have similar experience where they test for absence of voltage. Thanks for your comment!

This clause causes nothing but trouble and there is not a shred of technical support for it. As I understand the history back when the "pickle suits" were in use, 40 cal/cm^2 was something of a physical limitation. Now we can get Oberon suits rated up to 100 cal/cm^2. There is also something of a myth that you'd die on the spot or something like that from arc blast, again with no technical justification whatsoever because arc blast is dependent on the mechanical strength of the enclosure only. Basically pressure builds up in the enclosure and is then vented all at once. An theoretical equation developed by Lee is invalid based on test work that for instance has resulted in CIGRE guides on developing arc resistant equipment.

That leaves us with the last problem. "Greater emphasis" is simply undefined. No further guidance is given.

The problem here of course is that the greater emphasis that we should be using is embodied in ANSI Z10 aka "ALARP" or "ALARA". As of 2015, this term is mentioned but as yet it is still not required. The principle works like this:
1. Perform a risk assessment.
2. If a risk (likelihood and hazard considered jointly), then the hazard must be eliminated or reduced to an acceptable level. For most operations this will be somewhere in the neighborhood of 1.2 - 8 cal/cm^2 depending on what the local site defines as standard work wear.
3. If the hazard is reduced to as low as practical (further reductions become cost/technically difficult) then the next step is to apply an administrative control and signage. The arc flash labels (required by NEC) meet the signage requirement. The next step is to reduce the exposures to only when absolutely necessary. This is already done as it is embodied in 70E, Article 130.1 which requires an energized work permit.
4. Finally if somehow step 3 reduces the frequency of exposures to an acceptable level (it doesn't/won't) then we get to step 4 which is when then and only then, PPE must be specified.

Right now we seem to have the cart before the horse because 70E mentions step 1 as of 2015 (finally!), then jumps straight to step 3 and then since we don't really even go further with the reduction, we're right into step 4 and the PPE comes out.

I'd recommend dumping the "greater emphasis" clause entirely or changing it from 40 to 1.2 cal/cm^2, to put the emphasis squarely where it belongs.

Our company does not allow energized work. With that being said when our arc flash reports come back with energy over 40cal/cm^2, it is our policy to remediate that hazard below 25cal/cm^2. When we calculate different strategies of course it usually goes lower than that limit. Before the remediation work is completed our technicians are required to de-energize before doing any work at all to the system.
This strategy was implemented because the equipment at our facilities is older and we wanted to make sure our technicians are protected from any potential hazards.

In my career (160 accident investigations of arc flash) I have never seen anyone even injured by arc blast. Thermal is always the issue. Not saying it isn't real or something to consider but it definitely has nothing to do with 40 cal/cm2 level. It is based on current but the 40 cal warning is OK as long as it is read. It does not prohibit work. It is a warning about arc blast and OK as long as folks understand. We are planning a paper on arc blast with some more real data for 2016 ESW. Working on the beginnings of a model with Wei-Jen Lee and will donate the data to the IEEE 1584 committee if the method is fruitful. I'll keep the group posted.

This one causes problems frequently with us. Since incident energy is a function of both short circuit current and duration, long duration arcs lead to high incident energy but it is a low heat flux i.e. low energy per time. This can really be a problem on transformer secondaries.

Hopefully we'll have a good idea of blast at some point. But IF the arc in not in an enclosure it should never be a blast issue. Inside a secondary transformer could cause a blast but it will be a buiild-up of pressure in the container until the break open and typically that is the door of if the worker isn't that location, it is unlikely to be a blast hazard. Just have to think it through until we have a good formula for this.

Didn't mean to deviate into the arc blsat territory...and by the way, the "40 cal" rule doesn't even mention arc blast, BUT...

Hugh, I think the arc resistant gear folks already did a lot of research specifically on calculating pressures within an enclosure. Their specific interests were in developing a mathematical model of pressures within the equipment for use in designing arc resistant gear to reduce or avoid testing costs. I think in the end they still didn't eliminate all of the adjustable parameters but they got it down to the point where you can do just a small handful of tests rather than dozens for designing equipment.

Most interesting to me though is peak pressure. This occurs when we have a relatively sealed enclosure such as before the "blast doors" open. They did enough modelling and testing to show that:
1. The enclosure is ruptured within 1 cycle almost irrespective of any other parameters. Higher power generally turns into larger enclosures which means more air volume to heat but since there is a higher arcing power it takes less time to heat up, and as equipment gets physically larger unless the basic materials such as sheet metal thickness scale up, the maximum pressure decreases...so the net result is that it appears that there is almost a constant here.
2. Maximum pressure occurs due to the physical withstand of the equipment. It is independent of parameters of the arc. All that the arc parameters will do is govern the onset to rupture (if it ruptures).
3. The CIGRE modelling didn't address the pressure outside the enclosure. I thought I saw some data from Wei-Jen Lee or Ammerman early on in the joint IEEE/NFPA study in a fuzzy slide JPEG that suggested that measured arc blast was relatively a constant and based on the CIGRE data I'd believe that this is the case. Either way, we have a pressure wave that dissipates into a relatively open space. That won't carry much pressure/energy for very long.
4. There is a lot of physiological data on the effect of pressure on the human body. Ear drums rupture at 1 PSI. People are knocked down in the 10-15 PSI range. And fatal organ damage starts (depending on data source) somewhere between 20-35 PSI and increases to 100% death probability around 35-50 PSI. But the fuzzy slide I was referring to suggested it never gets above 2 PSI. Just wish I could find that slide again.

The original intent on 40 cal was blast. Thermal can easily to 100 or more. The blast is the issue. But that is not clearly stated in the note. Just in the meeting and I think in the handbook.

All the research is great but most of that isn't published and there is no model you can use on existing equipment to determine if the blast is "too much". That's what we need.

I love the PSI statements you made about humans but it is far more complicated than that in application.The pressure wave is directional. 1 PSI IN the ear will rupture the ear drum but HOW do I get 1PSI in there. When they measured the arc blast in IEEE 1584 version 1 they used a microphone but according to a DOE researched I questioned trying to get a new method of measuring, that mic washed out at about 1 PSI so our measurements weren't meaningful. Everything I have seen on blast is still theoretical. Hoping with Dr. Lee we can find a way to realistically quantify for worker protection instead of equipment design. The work I have done does blow off the door in 1 cycle so I agree with the research. Just need to go further to practical application so people are properly protected but everyone isn't scared to operate anything over 1 year old. The AR gear is great BUT it isn't law so MANY have not and will not switch.

If I hear of one more equipment salesperson telling someone that arc flash suits are just body bags, i think I will throw up. I have NEVER seen anyone die in an arc flash suit but I have seen "properly installed, properly maintained equipment with no sign of impending failure" kill a few people and injure many others.

I haven't been putting in arc resistant gear because the employees most at risk (electricians) are mostly hampered by it, NOT helped. To address the risk to them, putting it in a blast resistant box is not the answer. Incidentally controlling the energy release helps ALL involved, not just someone operating a breaker.