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 Post subject: PPE at less than the working distance
PostPosted: Thu Jun 13, 2013 2:24 pm 

Joined: Wed Aug 25, 2010 7:15 am
Posts: 24
Location: St. Paul, MN
130.5(B)(1) says, "The incident energy exposure level shall be based on the working distance of the employee's face and chest areas from a prospective arc source..." and, "Recognizing that incident energy increases as the distance from the arc flash decreases, additional PPE shall be used for any parts of the body that are closer than the distance at which the incident energy was determined".

Does anyone have policies that address the PPE for the body parts that are closer then the working distance? If I have a 1.2cal/cm2 at 18" working distance, what PPE is needed on the arms and hands when they are closer than 18"? If the electrician is wearing voltage rated gloves and protectors, their hands would be protected, but do they need to wear a arc rated shirt because their arms may be closer than 18" even though they are working at a category 0 panel? Is so, how do you communicate what the rating of their shirt needs to be?

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PostPosted: Fri Jun 28, 2013 2:31 pm 
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Joined: Mon Jan 31, 2011 6:21 pm
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Location: Salt Lake City, Utah
We have our whole staff wear arc rated shirts and pants no lower than HRC 2, so they will always have at least that minimal protection. Then they donn higher protection according to the task.

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PostPosted: Mon Jul 01, 2013 8:01 am 
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Location: Charlotte, NC
I read in another thread that NFPA 70E is concerned primarily with survivability - hence the working distance of 18" to the face and chest.

So, bearing that in mind, it would seem that anything closer than the face and chest would regretably be deemed an acceptable loss - as in the workers life is preserved.

At a first impression, this seems harsh, but remember the goal seems to be survivability.

PPE is designed to protect a worker from an arc flash - How much protection is there from the consequences of an arc blast, though? The force of the blast may be sufficient to knock a worker back 20 feet, but there is a wall 10 feet behind him. What is going to happen? Crush injuries.

I know it seems like a rambling point, but there are only certain -requirements- given by NFPA 70E. IF you want additonal protection, you would have to consider the arc blast for crush injury and calculate the cal/cm^2 at closer than the working distances. I have done some calculations, mostly out of curiosity, as to the curves - if you look at the graph in the attached file, you can see the incident energy rises rapidly as you encroach closer to the source.


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PostPosted: Sat Jul 13, 2013 5:36 pm 
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Location: North Carolina
I can't point to a specific source but the various probabilistic risk assessment methods consider a fatality as an acceptable risk if the probability is less than around 10^-5 to 10^-6 per year. A serious injury (lost time accident) is acceptable with an order of magnitude more frequent occurrence. Arc flash fatalities are occurring with an incident rate right at the fatality threshold according to ESFI data. I am not aware of any data source collecting serious injury data. For tasks where the likelihood exceeds around the 10^-5 to 10^-6 threshold such as removing or inserting breakers in drawout gear while energized or making circuit modifications to energized circuits, that pretty much requires appropriate PPE.

Alicia Stoll's data is for a 2nd degree burn threshold. There are other data sources for 1st and 3rd degree burns. Unfortunately the effect tends to be exponential due to the nature of incident energy so small changes in distances result in huge changes in affected surface area of an object intersecting a sphere. Most of the data on fatalities indicates that a small amount of body surface area in the critical area (face/chest) with 2nd degree or greater burns (around 10-25% depending on the source) results in a significant risk of fatality. There is not a lot of data relating burn extents to say lost time injuries, and there is very little data showing the effect of say 3rd degree or 1st degree burns. So unfortunately until the burn modeling gets better it appears that it is hard to quantitatively describe the risk at any point other than the 2nd degree burn threshold. So it becomes a binary decision and considers fatalities only.

This is not the purview or even the fault of 70E. 70E does not make recommendations as to how to calculate arc flash hazards at all except with the tabular method. By far the vast majority of practitioners following 70E use IEEE 1584 but that's not the only method out there.

If/when the clinical data on burn injuries extends further, then we can probably use that data. However right now if you spend any time at all looking at the medical data, even the 2nd degree burn threshold is both extremely conservative and it is virtually impossible to set a more objective threshold. I have submitted one request for a change to the 2015 edition to quantify what "incidental amount of material" for non arc resistant material means and to specify it as a specific surface area. This is very reasonable and required me to find the best possible literature describing a reasonable cutoff but this is actually very hard to do. I was specifically interested in whether the reflective striping on otherwise ordinary cotton shirts for visibility reasons is acceptable instead of spending lots of money on FR shirts even when FR rating (<1.2 cal/cm^2) is not necessary but that nonmeltable clothing is the guiding requirement.


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