The language found in an informational note of NFPA 70E 130.7 continues to be the subject of debate. The informational note states:

When incident energy exceeds 40 cal/cm^2 at the working distance, greater emphasis may be necessary with respect to de-energizing when exposed to electrical hazards. (2015 draft)

This note has lead to many interesting results, interpretations and procedures such as:

Arc flash labels that state "Danger - No PPE Exists"
This is incorrect. PPE exists with arc ratings much greater than 40 cal/cm^2.

Statements by people that at that level of incident energy the blast pressure will kill you.
Quite often, when the calculated incident energy is that great, it is because an upstream protective device has a long clearing time due to a low arcing current. Not exactly a blast.

A comment I hear is that even though informational notes are technically not part of the standard, the 40 cal/^cm2 language is still contained within the document so people fear liability as a result.

I have heard from a few people that the "40 cal/cm^2" informational note should be deleted. There are many mixed views about this. So, with a very long introduction, here is this week's question.

Do you believe the 40 cal/cm^2 NFPA 70E informational note should be deleted?

Yes
No

This is one of those sentences I never fully understand. The emphasis is always on working de-energized right? So how can there be a greater emphasis.
And what leads to confusion for me is the limited approach boundary part. If you are going to perform a task and the energy at working distance is greater than 40 cals/cm2 why would it matter if you are within the limited approach boundary or not.

Sometimes I see certain sentences and I just think ‘when it goes horribly wrong, the lawyers will have fun with interpreting these sentences’.

Until we have equations (from IEEE 1584) that will help us predict the actual arc blast pressure wave, I would suggest that no one works on energized equipment when the energy level is greater than 40 Cal/cm2. Although the Arc Blast pressure wave appears to be a function of the fault current and not the energy level. I suggest the we all wait until IEEE 1584 has released their test results.

Is IEEE even studying arc blast right now? Is there any data backing up 40 cal/cm2 as being the magic cutoff point? It would help with flexibility of work if we were able to do some tasks at incident energies above 40 cal/cm2. The only magical thing I know of of 40 cal/cm2 is that that's one of the typical cutoffs of PPE. But was that cutoff established before or after the 70E tables and notes?

The tables only go up to HRC 4. That's, I think, one of the main reinforcements behind using 40 cal/cm2 as the cutoff.

Yes, the latest testing done by the NFPA 70E and IEEE 1584 collaboration group has included arc blast pressure measurements.

Part of an article from our very own host (Jim Phillips) about the IEEE and the future of arc flash research (May 2013).


 - See more at: http://www.ecmag.com/section/codes-stan ... zsHPW.dpuf

I think I would have said ';within the Limited Approach Boundary or Arc Flash Boundary (the greater of the two) of the exposed electrical conductors or circuit parts"

I disagree, there is talk of how you should work de-energized, but the bulk of the effort in the standard, is all about what you do when you WILL not de-energize. To me that shows where the emphasis really is.



Truer words were never written.

Many great comments on this one. Bob's comment (He's also on the IEEE 1584 WG) about the pressure being a function of short circuit current than incident energy is at the heart of the problem. Just stating 40 cal/cm^2 as a limit does not address the correct variable (in my opinion)

An interesting and often overlooked issue with the language is that it doesn't even mention blast pressure, which seems to be a part of the story that has evolved around this language.

Regarding Larry's comment about the Limited Approach Boundary, I used the language from the 2015 Edition and the LAB reference was removed and it just references exposure to the electrical hazard.

Something I heard from a pretty reliable and well placed source earlier this year is the 40 cal limit really began back when 40 cal/cm^2 was the maximum rating for PPE. That might explain the language on some arc flash labels "No PPE Exists"

If the origin of the 40 cal/cm^2 language is correct, then this statement is really outdated. If this is not the case, someone please let us know.

Thanks everyone! Keep your comments coming!

The IEEE 1584 working group does have some limited pressure data (Thanks Luc for reminding of the article) however, it does not look like we will be producing equations this time around due to the limited amount of data.

Would it be beneficial to add a tA qualifier to the 40 cal/cm2.

Good suggestion Larry. I'm not sure what the answer is at this point. It is likely that until we have something better, it remains as is. I think language discussing that a long clearing time could lead to a large incident energy might be appropriate but that would likely confuse people since there would not be any guidance about what to do. That language would have a similar result like the "interaction" wording in 2009 - good intentions but not easily interpreted.

200 lbs. over a 1.5 square foot area is close to 200 square inches. Thats less than 1 PSI. Organ ruptures are at 20+ PSI according to military data looking at the damage from nuclear bombs and other concussive blasts that I was able to find. We are nowhere near that. Knocked off your feet...yes. It takes surprisingly little force to surprise someone. Think cow tipping. But kill you outright, totally ridiculous. I saw a powerpoint where I could not see the data from the joint study that was saying a maximum of 2 PSI as a sort of magic threshold they did not exceed. Still gives us 400 lbs. of force but again does not cause any direct damage beyond flying objects during the event. Distances and volume of air propelling this would also rapidly fall off at a rate of inverse cubed distance since its a volume expansion and not radiant energy. I'm thus dubious about the purported killer Lee derived results.

Is the force from an explosion expressed in pounds - or pounds per square inch?

The equation that I have gives the result in PSI, not pounds.

This was taken from my Arc Flash spreadsheet:
Based on 480VAC, 33.6kA Fault Current, 34.783 kA Bolted Fault, tA .033


Torso = W 15" x H 25" = 375 Sqin
@ ----12"------18"------24"-----36"------60"------120" Distance
IE---47.57---24.46---15.25----8.00 ----3.39------1.09 cal/cm2
------2807----1949---1504----1044------659------353 lbs Force
----- 7.49-----5.20----4.01----2.78------1.76------0.94 psi
PSI = (144*(11.5*4.52/1000)/((distance/12)^0.9))

That equation is the Lee theoretical equation. The presentation I saw was based on measured data and was an update on progress of the joint IEEE/NFPA study. The chart was not readable but the bullet point said 2 PSI maximum. The concussion results that I found were also in PSI. Don't get me wrong here. 2 PSI on a 16x16 size 1 starter MCC door is still 2048 lbs., enough to knock the door off almost any electrical enclosure. It will toss people around if close enough.. But it is not the enormous level of force that the Lee theoretical calculation produces for even moderate fault currents and is flat out not the case that "no PPE is available" due to huge concussive forces as some have concluded from the "40 cal rule".

Just like the Lee incident energy equation, it is way oversimplified and does not match reality. I have never seen evidence of pressure waves approaching " Lee" levels. For large enclosures especially (switchgear) if it were accurate, the enclosure would fly apart at ALL seams.

Hi JIm

Greetings from Canada. Being the President of a local electrical contractors chapter, I am very often confronted by fellow electricians on simply the term cal/cm2. This is not exactly a familiar term to allot of electricians in oppose to temperature in either degrees C or F scales. Now when we throw the 40 cal/cm2 at them it can become more confusing. If I have it correctly the term cal/cm2 goes back to the Alice Stoll experiments in the late 1950's and early 1960's? Regarding the "more emphasis on de energizing". I agree and like the comment above that, is that not what NFPA 70E and CSA Z462 are all about? De energizing as the first choice. If the 40 cal/cm2 is going to remain in both standards, perhaps we should focus our efforts on inserting some information in an annex on conversions to PSI or other terms that are more familiar to the maintenance worker. Some real to life examples of how much energy 40 cal/cm2 really is and why it is so dangerous. Something similar to as what we know that 1.2 cal/cm2 for 1 second is the same as holding your finger an inch away from the hottest part of the flame of a cigarette lighter.

Leonard,

There is a chart I use in presentations to put it in perspective:

1.0 cal/cm2 = Equivalent to a finger tip exposed to a cigarette lighter flame for one second
1.2 cal/cm2 = Amount of energy that will instantly cause 2nd degree burns to bare skin
4.0 cal/cm2 = Amount of energy that will instantly ignite a cotton shirt
8.0 cal/cm2 = Amount of energy that will instantly cause incurable 3rd degree burns to bare skin

Hi Larry

Thanks allot for the chart here. Very helpful and I will certainly utilize

Best regards

For blast pressure effects on the body:

Pressure
psi........Effect
1-5 ......Possible tympanic membrane (eardrum) rupture
15 .......50% incidence of tympanic membrane rupture
30........Possible lung injury
40 .......Concrete shatters
60 .......Threshold for being potentially lethal
75 .......50% incidence of lung injury
100 .....Probable fatal injuries
200 .....Death more likely than not

I think the blast danger is much more along the lines of a worker being propelled into something or thrown off of something and being injured, than from direct blast injury.

Hi All

Thanks for the information and comparisons. Good information and reference material

All the best

Len

Yes. A calorie is a measure of energy (heat) and cm2 is a measure of area. Cal/cm2 is the amount of energy per square centimeter (an indication of heat density).