This week's question is the first of two parts and was submitted by Luc. - Thanks Luc.

As part of the arc flash study and now part of the arc flash risk assessment, special attention is given to reducing the incident using various mitigation methods such as device setting changes or other arc energy reduction methods. Some will attempt to reduce the prospective incident energy level to no more than some specific level such as 8 cal/cm^2

Some incident energy mitigation methods can be simpler and less expensive than others.

This week's question:

Do you have a target incident energy that is the maximum acceptable level for mitigation?

- No - we dress for whatever is calculated

- Yes - but we have no set maximum - we just reduce as much as possible

- Yes - we attempt to reduce all locations to less than 40 cal/cm^2

- Yes - we attempt to reduce all locations to less than 8 cal/cm^2

- Something else - please explain

We try to mitigate everything downstream of the main disconnect to < 8 cal/cm2. So far, we haven't developed a good way to decrease the main disconnect to below that in most installations. So we make that a stand-alone circuit breaker type so that you don't have to test fuses and also put remote trip and IR windows so that you can perform IR scans without exposing workers to higher incident energies.

If the Owner has primary distribution then we can use MV circuit breakers with relays that have a maintenance setting.

We first try to make sure everything is below 40 cal/cm^2.
though we try to get it below 8 cal/cm^2 or even 1.2 cal/cm^2 if we can.

In my present facility, the goal is to get the main switchgear down below 40 cal, and everything else down below 8 cal.

The switchgear that connects directly to our utility transformers is about 32 cal. Everything downstream of that I either have things I can adjust now (such as relay settings) or I have projects planned for the next few years to add things such as maintenance switches to allow me to get the working incident energy down.

At the moment we (as a company) do not have any policies regarding arc flash mitigation. Most are pushing for dress for whatever is calculated unless it is above 40 then reduce the incident energy.

I think this is a very hard question since there are no clear rules for what to do after the calculation is done. What a lot of people do seem to forget (at least here) is that PPE is getting better and better but the PPE calculated is meant to make you survive the arc flash. It says nothing about non lethal wounds or loss of limb. So personally I would go for option 2, no set maximum just as much as possible.

We have been successful at reducing all locations ( except incoming service entrances) to less then 8 cal/cm2 by changing fuses or CB settings.

In order to evaluate the need for reducing the incident energy, one need to know how fast the energy was delivered in first place. For example, the industry accepted 1.2 cal/cm2 threshold incident energy level for a second degree burn energy should be delivered within one (1) second time interval to cause 2nd degree to bare skin. The same amount of energy but delivered over longer than 1 second time interval won't cause the damage. Indeed, 1.2 cal/cm2 is the amount of incident energy delivered by exposure to sunshine for within one (1) minute. Alternatively, only a fraction of 1.2 cal/cm2 exposure is required to cause the damage when delivered within shorter than 1 second time interval. Check please this forum thread at http://arcflashforum.brainfiller.com/viewtopic.php?f=34&t=2221 for more information.

Also, it has been proven that the NFPA 70E tables will specify adequate arc flash PPE fifty percent (50%) of the time only (http://arcflashforum.brainfiller.com/viewtopic.php?f=2&t=3384&hilit=50%25). Therefore, you play a game of chance when relying on the NFPA 70E table method. The odds of specifying adequate arc flash PPE using the NFPA 740E tables are about equal to the odds of survival of placing a single round in a revolver, spinning the cylinder, placing the muzzle against ones head, pulling the trigger and repeating the procedure four (4) times in a row.

Though I disagree with your claims about arc flash injury below 1 second because of data to the contrary (from Prichard), and does not seem to correlate well with reality as so far, IEEE 1584 has worked 100% of the time even though the standard itself only claims 95% accuracy, and your probability math is a bit off not only because 4/6 is not 50% but also because of survivorship bias, one is not guaranteed a fatality due to exposure to an arc flash even if it exceeds the second degree burn threshold regardless of how that threshold is determined. The data put together by some of the Dupont folks show that a major injury involving hospitalization has a 50% chance when following the tables in 70E. However there are two changes to those tables. About 6 years ago, "2" disappeared. Second the 2015 tables no longer specify reduced PPE. The effect is that the study may soon be much less accurate. An arc flash by the way is rarely fatal. Perusal of the OSHA accident investigation data, though necessarily biased to some degree, shows on average better than an order of magnitude lower risk of a fatality compared to n injury requiring hospitalization due to arc flash. This surprising result has been cataloged by others as well. For this reason, your analogy is WAY off.

Not that electrical injuries are not serious but lets not go down the road of fear mongering and passing around false, questionable, or even unsubstantiated information. Making an emotional plea does help make the case when it is based on solid claims. But when it is obviously not, it simply destroys credibility and since we are dealing with rare events which create a "it can't happen to me" mentality, credibility is of the utmost importance.

Indeed, what you claim are "claims" correlate very well with the reality as the findings are based on a number of experimental studies referenced in Evaluation of Onset to Second Degree Burn Energy in Arc Flash as well as more studies including the ones listed below:

A. Privette, "Progress report for ASTM Burn Study", Duke Power Company, 1992
A. Brownell, et al., "Ocular and skin hazards from CO2 laser radiation". Joint AMRDC-AMC Laser Safety Team, Philadelphia, PA, 1972
A.Stoll, "Heat Transfer in Biotechnology", Advances in Heat Transfer, v.4. Academic Press. 1967

The quote from A.Stoll "Heat Transfer in Biotechnology" below summarizes the issue of using 1.2 cal/cm^2 as a threshold incident energy to 2nd degree burn while disregarding how fast the energy was delivered:



Would you mind returning a favor and sharing a copy of or the reference to Prichard's publication your disagreement is based on?



I highly regard the work done by IEEE 1584. Indeed, the 5% inaccuracy associated with the IEEE 1584 calculations is an order of magnitude less comparing to the reported 50% possibility of specifying inadequate arc flash PPE when using the NFPA 70E tables.



Here is how I come up with around 50% outcome you are questioning. It's a 6 shot revolver and you get to spin the cylinder each time you try. The chance of surviving the first trial is therefore 5/6. The chance of surviving two consecutive trials is 5/6 * 5/6, and in general the chance of surviving n trials is (5/6)^n. Therefore, the chance of surviving four trials is (5/6)^4 = 0.48, or 48%. Indeed, I was exemplifying the odds of being killed or receiving a major injury combined. My point is that the procedure claiming 50% success rate is no more convincing than tossing a coin and making decision based on the outcome.



I must be missing something but I expected the standard to get more accurate over the time contrary to what you are saying. I believe most folks relying on the NFPA 70E PPE recommendations for protection against electric arc expect avoiding incurable burns while surviving potential incident. Also, improperly sized or tested PPE may very well aggravate the injury.



I don't quite make a connection between the appeal and my post.

NESC and 1910.269 have a cutoff at 2. I try to keep it below that where possible.