I have addressed the 40cal/cm^2 threshold from several different perspectives over the years. This number can still cause some interesting discussions, especially when NFPA 70E 130.7(A) Informational Note 3 suggests a greater emphasis should be given to de-energizing when this value is exceeded.

Having a calculated incident energy above this level can trigger responses such as: No energized work, Danger - No PPE available on the arc flash label as well as others.

This week's question is about your concerns (or lack of) regarding the 40 cal/cm^2 value.

What are the reasons an incident energy above 40 cal/cm^2 is big concern? (Select all that apply)

It is referenced in NFPA 70E
Liability
Blast Pressure
Too much thermal energy
We don't have PPE above that level
Not sure if it is really a concern
None of the above

It is a myth that sufficient PPE ca not exist. The 40 cal was the cut-off for HAZARD-RISK categories and the task tables only. Annex H does not appear to have an upper cut-off value. Is 48 cal/cm^2 really more dangerous than 39.9?

It is also a myth that above 40 cal, the problem is blast pressure more than it is heat. Most arc blast events occur within a few milliseconds, while many >40 cal events are because the clearing time is in seconds.

Just last week I was asked to identify all areas in our mill "greater than category 4", i.e. >40 cal/cm2. I made sure in my report to mention that this is an artificial threshold.

At our company we decided to remediate all Hazards that were identified above 25cal/cm^2. We decided that 40cal/cm^2 was too high of a risk and wanted to better protect our employees. We as an electrical safety team decided that no matter what the reasoning it is better to engineer safeguards whenever possible. What we have found once we started was with a little work most of our hazards are being lowered to less than 8cal/cm^2. That is a significant decrease and well worth the money to help our technicians work in a safer environment. When we cannot find a way to lower the risk, we have installed remote operating devices or make the technicians open the next upstream device to remove the technician from the Hazard completely. We have also implemented a No Energized work policy, obviously we have to operate electrical equipment. It has cost us a great deal of money to put these safeguards in place, but we are trying to change the culture at our company.

Other than the references, does anyone have any facts as to why 40cal was picked? Was it simply that was the clothing available at the time or is there another reason?

To me the NFPA note is confusing. It seems to me that if exceeding 40 calories is a deadly risk, the NFPA should say so unambiguously with language like “where the hazard exceeds 40 calories, the work must be done de-energized.� With this other language, it implies there could be exceptions. I believe a creative person will find a way to get around the limit and I don’t think that’s a good idea.

I believe the people at the NFPA consider themselves to be and are experts in their respective field. If they’re experts and want to set (and sell) safety standards, they should define the limits and back it up with science. But this language implies there's wiggle room. Plus, they don’t define what “additional emphasis� means. How do we know how much additional emphasis is enough and what is it? If 40 calories is the correct number, then it should be mandatory to de-energize above that number.

I don’t think the NFPA can have it both ways. Either 40 calories is the limit, or it’s not. If the experts are unwilling to tell us, are there other limits they’re also unsure of?

The National Electrical Code uses the word “shall�. I agree with that use because the word “shall� is unambiguous. But working energized openly exposes a person to several risks, including arc flash. Yet NFPA is unwilling to say you shall de-energize when the arc flash hazard exceeds 40 calories. This equivocation tells me they’re unsure 40 calories is the right number. Could it be 50 or 60 or 70 calories? I’d ask that they set a standard based on the research and use language that can’t be misinterpreted. If research proves the level needs to be changed, then change it.

If we look at today's arc flash clothing market, there are arc rated clothing (ATPV) of up to 106 cal/cm2. The reason we cannot recommend arc rated clothing & PPE beyond 40 cal/cm2 is because of liability. NFPA 70E (2015), Article 130.7(A) foot note No.3 clearly mentions to de-energize the equipment when incident energy exceeds 40 cal/cm2. Foot note No.1 mentions that PPE does not provide protection against physical trauma injuries due to arc blast pressure. So far we do not have mathematical equation for blast pressure and I think the decision of making 40 cal/cm2 as threshold came from collective experience of NFPA 70E committee. But with advanced scientific research and many knowledgeable engineers such as on this forum, blast pressure equation may not be too far. Whether it is as scary as it sounds like, or not as big threat as it sounds like, only time will tell.

Just to clarify. Footnote 3 does not say clearly that you HAVE to de-energize equipment at 40cal or more. It clearly says that you should give GREATER EMPHASIS to de-energize.

The concerns that were voiced about Arc Flash exceeding 40 Cal/Cm^2 has generally been about the debris and energy released. However, an Arc Flash blast could occur over a short or long time period, For example a the forces from an Arc Flash operating from a circuit with fast-acting fuse could have forces greater then that from a circuit breaker that taking a couple seconds to trip. The blast pressure and amount of flying projectiles has not addressed in the standard. blast from 40 cal.cm^2 that occurs in milliseconds will certainly be stronger then the blast from a 40 cal.cm^2 that occurs over 2 seconds. Although the burns would be the same the injuries could be more. Therefore wearing a 70 Cal/Cm^2 suit for a major event may be appropriate. However, ever with a Arc Fault suit a person could very well be injured by falls or other injuries then burns during an ARC fault event, I am aware of an accident where a large 4160kv, 1600 ampere non-load break disconnect completely blew up and hurled all its parts several feet. The person operating the switch was in ordinary Work clothes and was un-injured, since he was standing to the side of the switch and facing away.

Again I must point out the paradox of limiting work to such systems to de-energized only. Since any circuit must be assumed to be energized until it is tested, the testing itself would be prevented by such a limit. A strict limit would effectively prevent all work.

I suggest:
Isolate
Put on appropriate PPE for testing a live circuit
Test to prove circuit is de-energized
Apply grounds
Remove extra PPE
Perform work de-energized.

I believe this meets the spirit of the footnote in question.

You have not addressed the OP question concerning what level of PPE is appropriate.

What would be your recommendation for PPE to 'verify de-energized' a location where there is a hazard of 50cal/cm^2 with a protect device clearing time of 3 cycles?

An arc flash suit rated for at least 50 cal/cm^2.

John 1960 wrote: an Arc Flash blast could occur over a short or long time period, . Isn't the blast just a millisecond event as the arc bridges the gap? Even if the arc lasted 2 seconds the blast would not. Am I wrong?

Using incident energy exposure as a measure of arc flash hazard (or any other thermal hazard) is meaningless unless the heat intensity is taken into account. An incident energy can produce very different damage effects to bare skin or any other material including PPE. It's all about how fast the energy has been delivered. As an example, a fraction of 1.2 cal/cm^2 can produce 2nd degree burn to bare skin and make damage to many other materials when delivered fast enough while 40 calc/m^2 or more energy won't produce any damage at all when delivered within long enough time interval.

Indeed the issues arising of using incident energy thresholds both for bare skin exposure and rating the PPE have been addressed on this forum. Please find below links to two IAEI publications addressing the matter and featuring free online Time and Threshold Incident Energy for Bare Skin Exposure vs Thermal Radiation Level and Time to Non-Piloted Ignition or Melting vs. Thermal Radiation Level calculators respectively:

"Evaluation of onset to second degree burn energy in arc flash hazard analysis." by Furtak, M.; Silecky, L. IAEI, July-August 2012
http://arcadvisor.com/faq/threshold-incident-energy-second-degree-burn

"Behavior of apparel fabrics during convective and radiant heating." by Furtak, M. IAEI, May-June 2015
http://arcadvisor.com/faq/ignition-melting-characteristics-fabrics-textiles

I thought the OP was actually a poll asking us our concerns. I chose the 6th answer. As to your question, I agree with Voltrael.

It was.

My response was to ask for additional information based on a reply to the original posting.

doesn't OSHA have their safe work cutoff at 40cal?

Hi All,

The understanding that I always had was that incident energy levels above 40 cal/cm2, there was more of a concern of blast pressure (answer 3 above). However it seems in recent that this is a myth and the science may not be accuate.

Going out on a limb here, if NFPA 70E and CSA Z462 are going to reference a particular incident energy level where greater emphasis placed on de-energizing, should we not make this level 1.2 cal/cm2 to be consistent with a worker receiving a 2nd degree burn to bare skin based on the Alice Stoll experiments??

I am not sure where this thread is going but I understand the clothing (PPE) rating and protection scheme fairly well. The rated PPE is NOT guaranteed to protect you 100%. I am in the camp of Reduce Reduce Reduce. I don't care if it is pressure or heat, It is very bad. Find a way to visible check by review of drawings, physically looking at switches and locate someplace else(lower incident energy) to confirm zero energy. I once had a utility in In NY tell me they couldn't provide information on a downtown bus (Syracuse) with multiple transformers and we postponed our work until the utility was able to schedule the entire downtown outage.

Rule Number 1 is the drawings are wrong. Unless you can physically determine the relationship between the zero energy verification location and the working location, you are taking a leap of faith here that the drawings are correct when you verify elsewhere. I would suggest verifying at both locations, beginning with the lower IE one.