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 Post subject: Incident energy vs ATPV value for clothing
PostPosted: Thu Mar 15, 2018 6:01 am 

Joined: Wed Nov 01, 2017 6:09 am
Posts: 27
Hello,
It is probably a stupid question but when you select clothing for safe work on a electrical installation is it just look at the incident energy calculation(for instance 12 cal/cm2) and choose clothing that has higher the ATPV rating?
Or is it more to consider when you select the ATPV rating for clothing?


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 Post subject: Re: Incident energy vs ATPV value for clothing
PostPosted: Fri Mar 16, 2018 10:10 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Yes but the goal is head to toe protection and there are some "category" items. As I've pointed out before, the math doesn't really stand up to strong scrutiny but in cases where PPE is worn as you describe, the injuries have been relatively minor if there is an injury at all.

Underclothing should be nonmelting. Wearing an Under Armour shirt under the arc rated clothing for instance can negate the benefits. This means underwear should be cotton, wool, silk, etc.

Leather work gloves work up to 12 cal/cm2. Rubber voltage rated gloves with leather protectors work up to 40 cal/cm2 or more. So if you need both shock and arc flash protection, the rubber gloves work just fine without something extra.

Safety glasses and ear plugs need to be worn with everything else. Even in a hood the face shields have been known to melt so the safety glasses are there for that reason. Ear plugs are for the noise. Pretty much any one of the molded kinds don't melt. A few brands of the cheap foam ones can melt.

Other hazards override arc flash PPE. For instance if faced with need for a respirator, the respirator comes first. Although I've found that I can usually wear a respirator under a hood or slip a balaclava around one anyways.

Leather boots is what 70E calls for. This is obviously a problem in winter or wet conditions. I believe Arcwear has done some testing on boots and qualified some others since mostly they pass pretty well.

There was a big "thing" for a while about belts and belt buckles. 70E kind of gives this a pass, along with underwear and sock elastic bands.

When it comes to fall protection there are arc rated fall protection harnesses. Be aware that there have been lots of major injuries and fatalities not so much from a shock or arc flash but that the worker was so startled by the incident that they fell off a ladder or off a platform or out of a manlift. Need to have some sort of work positioning to prevent this.

When it comes to head/face protection there are basically 3 directions. At one time 70E allowed it up to 4 cal/cm2. The more recent editions require either a balaclava and a face shield or a hood up to 12 cal/cm2, then a hood for anything above 12 cal/cm, and the requirement kicks in at 1.2 cal/cm2. OSHA increased this to starting at 9 cal/cm2 for generation, transmission, and distribution equipment in 30 CFR 1910.269. The assumption here is that all of the equipment is overhead lines so the rate that incident energy falls off is much faster. Working distances for the calculation are also lower so the calculated value is higher than following IEEE 1584 strictly so this may make a difference. Also OSHA has a lower cutoff of 2 cal/cm2 in the same section while 70E has a lower cutoff of 1.2 cal/cm2 but again due to the working distance difference the two are not directly comparable.


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 Post subject: Re: Incident energy vs ATPV value for clothing
PostPosted: Mon Mar 26, 2018 10:49 am 
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Joined: Thu Jan 10, 2008 8:49 pm
Posts: 499
Location: New England
The other consideration is what 'safety factor' you desire. Some of this is from memory so look it up, but I recall the ANSI rating on clothing has multiple conditions, one being internal heat that passes through the garment, and the second is that the garment doesn't 'rip open'. The internal criteria is a temp of 1.2 cal/cm2, so a 12 cal blast lets interior temps get to 1.2 cal - which is burn territory. Also, I recall that only 50% of the identical garments had to reach the rated threshold. So again, identical garments should have very close responses, but some 12.0 garments might test at 11.7 and some 12.3. So yes, you only have to use clothing rated to the risk, but the safety margin at that rating is also small. Going with higher rated clothing makes the risk difference smaller.


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 Post subject: Re: Incident energy vs ATPV value for clothing
PostPosted: Mon Mar 26, 2018 11:04 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 540
I think Haze10 is referring to the energy break-open threshold (EBT). Either APTV or EBT or both are on the label.


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 Post subject: Re: Incident energy vs ATPV value for clothing
PostPosted: Fri Mar 30, 2018 4:51 pm 
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ATPV is the lower of either the thermal or breakopen value. When you start looking deeply into both IEEE 1584 and ASTM 1959 you will find a lot of issues with the models but let's back up to a higher level for a moment and consider the bigger picture.

Numerically regardless of all the modelling issues IEEE 1584 did a numerical calculation that calculated the likelihood of a second degree or more severe injury using the incident energy calculated via IEEE 1584 and applying PPE with an ATPV based on ASTM 1959. Numerically the injury will not be a second degree burn 90% of the time. This is NOT the "50%" threshold in the calculations, estimations, etc. This is 90%. That's based on pure mathematical modelling. Yes you can poke great gaping holes in every aspect of IEEE 1584 and ASTM 1959. Sorry but these are purely theoretical models. This is the Wizard of Oz here....do NOT pull back the curtain.

Second let's digress a little bit and consider something else. If you look closely at the raw data (ignore the computer generated sigmoid curve from the ASTM 1959 tests) you will find that at around 0.5 to 1 cal/cm2 above and below the ATPV calculated value, we get either 100% passing or 100% failure of the fabric samples. So realistically at least considering the PPE mathematical risk (and again...it's not really much) with say a 10 ATPV shirt, at 9 cal/cm2 or less the protection goes to 100%. So the only time that we even get into the "90%" factor is in the small window of say from 9 to 10 cal/cm2. Anyone using a 10 ATPV shirt is probably wearing it for everything from 1.2 or 2 all the way to 10 cal/cm2 because comopanies aren't going to issue (and can't buy) shirts rated 1, 2, 3, 4....10 ATPV. So numerically VERY few conditions are even going to approach the limit of the PPE.

Third, Neal and others put together a report a few years ago in which they collated roughly 50-60 cases of arc flash incidents covering a little over 100 exposures. It's the most comprehensive collection of data out there. Some interesting information I found from it is that if we look at only cases where workers wore no arc flash PPE at all, they had minimal (less than second degree burns on face/chest) injuries 15% of the time. With arc rated PPE but the incident energy exceeded the PPE rating, they still had minimal injuries 50% of the time. Finally if the PPE met or exceeded the incident energy and was worn properly, it worked 100% of the time. So far I know of no case where PPE in use met or exceeded the IEEE 1584 calculated incident energy. So in practice, IEEE 1584/ASTM 1959 and related standards works 100% of the time. We can apply fudge factors to try to improve on it but I'm not sure how much further we can improve over 100% success. And personally since I'm a field service engineer and go to a lot of plants with less than stellar maintenance practices, that I've got somewhere between a 50 and 100% chance of walking away even when I don't know the incident energy. I'll take that over the 15% chance any day or the 0.0% chance that the consulting firms are trying to scare people into believing.

The reality is that IEEE 1584-2002 relies on test results of suspending 3 bus bars in a completely open enclosure and setting off an arc flash with a piece of #14 copper wire. If there is equipment in the enclosure it acts as an arc absorber and absorbs some of the energy. If the arc happens at the back of the cabinet like in the case of stabs on the back of draw out equipment like breakers and MCC buckets, the incident energy is much lower because the thermal radiation is obscured/blocked. Testing on actual equipment by EPRI shows MUCH lower actual incident energy values compared to IEEE 1584 models. And IEEE 1584 itself shows that arc flash tends to be a bimodal distribution with a long tail...most of the time the arc flash is much less than predicted. Thus for all these reasons we can expect that IEEE 1584 probably represents a worst case scenario. Actual real world scenarios rarely reach this level of incident energy. Hence the reason that it works 100% of the time when PPE exceeds the incident energy, and still works a majority of the time when it doesn't.

So getting back to the OP question...yes you match the ATPV on the PPE with the calculated incident energy. It's really quite that simple. There is a lot of argument about whether or not that is the right way to do things but it's hard to argue with a 100% success rate.


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 Post subject: Re: Incident energy vs ATPV value for clothing
PostPosted: Sun Apr 08, 2018 11:45 am 
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Location: Louisville, KY
Arc Rating is the critical number. it is relatively conservative (contrary to the definition in the standard test method, it does not mean at the arc rating the worker has a 50% chance of second degree burn).

These two articles are helpful. I do most of the arc flash testing in my company ArcWear BUT we do not sell clothing.

https://www.e-hazard.com/blog/arc-ratin ... explained/

https://www.e-hazard.com/blog/why_cant_ ... ash_study/

Categories or Calculations there will always be tradeoffs. Wet clothing (overdressing and causing a worker to sweat) can lower arc ratings.

We recommend dressing to the level in the calculation or category depending on what method your company uses.

The one thing we ALWAYS recommend is AR daily wear for all electricians. Having a $1000 flash suit at the office or in the truck when something blows up gives that worker a 50% chance of living if clothing ignites. Wearing AR dailywear almost always gives the worker a 95% chance of living.

Hugh Hoagland
ArcWear.com


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