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 Post subject: 2018 NFPA 70E Changes Article
PostPosted: Sun May 21, 2017 2:14 pm 
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The 2018 Edition of NFPA 70E is quickly approaching. Here is an article of mine that was recently published in the May Edition of Electrical Contractor Magazine. As usual, some changes were minor, others were quite major. There is also quite a bit of reorganization.

2018 NFPA 70E Changes Article

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Jim Phillips
jphillips@brainfiller.com


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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Mon May 29, 2017 8:21 am 
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What's New? What's Changed? 2018 NFPA 70E Update wrote:
Boundary, Arc Flash: The revised definition [..] no longer uses the term "second degree burn" but instead "at which incident energy equals 1.2 cal/cm2." The revised informational note references the Stoll skin burn injury model.


NFPA 70E year 2004 assumed that the incident energy requirement increases below one second. A quote from NFPA 70E year 2004 "For situations where fault-clearing time is 0.1 second (or faster), the Flash Protection Boundary is the distance at which the incident energy level equals 6.24 J/cm^2 (1.5 cal/cm^2)." This reference was removed in NFPA 70E year 2012 edition. NFPA 70E year 2012 stated that "a second degree burn is possible by an exposure of unprotected skin to an electric arc flash above the incident energy level of 1.2 cal/cm2 ( 5.0 J/cm2 )" and assumes 1.2 cal/cm2 as a threshold incident energy level for a second degree burn for systems 50 Volts and greater. NFPA 70E year 2015 explicitly prohibited using incident energy and PPE category together. The NFPA Handbook from 2015 showed a sample label with fields to be filled in for "available incident energy" and "level of PPE" while just half a page earlier states that "available incident energy" cannot be included with the "PPE category" in table 130.7(c)15(A)(b).

The revised definition of arc flash boundary in NFPA 70E year 2018 is even more misleading. It also contradicts the accompanying revised informational note referencing the Stoll skin burn injury model. A quote from A.Stoll "Heat Transfer in Biotechnology" summarizes the issue of using a critical thermal load approach in determining arc flash boundary. The quote reads:

"Serious misconceptions have crept into this field of research through adoption of rule-of-thumb terminology which has lost its identity as such and become accepted as fact. A glaring example of this process is the “critical thermal load.” This quantity is defined as the total energy delivered in any given exposure required to produce some given endpoint such as a blister. Mathematically it is the product of the flux and exposure time for a shaped pulse. Implicit in this treatment is the assumption that thermal injury is a function of dosage as in ionizing radiation, so that the process obeys the "law of reciprocity," i.e., that equal injury is produced by equal doses. On the contrary, a very large amount of energy delivered over a greatly extended time produces no injury at all while the same "dose" delivered instantaneously may totally destroy the skin. Conversely, measurements of doses which produce the same damage over even a narrow range of intensities of radiation show that the "law of reciprocity" fails, for the doses are not equal."

I would strognly encourage the NFPA 70E committee members to consult ASTM F1959/F1959M Standard Test Method for Determining the Arc Rating of Materials for Clothing and see what is says about skin burn injury determination. For anyone else who doesn't have a copy of ASTM F1969, the standard reads:

"12.1.4 Predicted Second-Degree Skin Burn Injury Determination (Stoll Curve Comparison) — The time dependent averaged heat energy response for each panel [..] is compared to the Stoll Curve empirical human predicted second-degree skin burn injury model:

Stoll Response, cal/cm2 = 1.1991 * ti^0.2901

where ti is the time value in seconds of the heat energy determination and elapsed time since the initiation of the arc exposure. A second-degree skin burn injury is predicted if either panel sensor heat energy response exceeds the Stoll Response value (at time ti)."


Incident energy alone has no impact on thermal damage and blast pressure. One can expose himself to any arbitrary incident energy and suffer no damage as long as the energy is delivered at slow enough rate. On the other hand, an exposure to only a fraction of 1.2 cal/cm2 may result in incurable burn provided that the energy has been delivered fast enough. Read Evaluation of onset to second degree burn energy in arc flash hazard analysis or this forum thread at https://www.arcflashforum.com/viewtopic.php?f=34&t=2221 for more information. The issue of using incident energy as a measure of damage alone and without regard to the rate of the energy release has been raised to NFPA 70E committee before year 2015 edition was published but unfortunately the group failed to address the matter back in 2015. It seems the new upcoming NFPA 70E is to obscure the matter even more instead of admitting the glaring mistake and fixing it.

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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Fri Jun 02, 2017 9:54 pm 
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IEEE 1584 is a theoretical only model. Really all the arc flash models are. It's pretty easy to poke huge gaping holes in them. But the model as it stands has withstood years of practical "tests". No need to switch from energy to flux unless it can be shown with direct evidence that it exceeds the 1584 track record.


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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Sun Jun 04, 2017 11:31 am 
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PaulEngr wrote:
IEEE 1584 is a theoretical only model. Really all the arc flash models are. It's pretty easy to poke huge gaping holes in them. But the model as it stands has withstood years of practical "tests". No need to switch from energy to flux unless it can be shown with direct evidence that it exceeds the 1584 track record.


Just to be clear, I've never mentioned IEEE 1584 in my post and I've never intended to "poke huge gaping holes" or critise the IEEE theoretical model in any way. Actually, the IEEE model provides for solving IEEE P1584 equations (5.8) and (5.9) based other than 1.2 cal/cm2 threshold incident energy technically allowing to use variable threshold incident energy to 2nd degree as a function of heat flux.

Of course, NFPA 70E is free to hold to its erroneous claim that the 1.2 cal/cm2 is the threshold incident energy level for a 2nd degree burn on a bare skin but the new NFPA 70E reference to Stoll model is appauling and misleading as A.Stoll had personally bewared of using the critical thermal load when determining arc flash boundary as cited above.

I also believe that the ASTM F1959/F1959M has been developed way prior to NFPA 70E adaptation of arc flash hazards. Hence it seems that NFPA 70E has switched from flux to energy, and not the other way around.

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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Tue Jul 18, 2017 8:47 am 
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It may be interesting to note that the 2018 Edition of NFPA 70E has changes the definition of Arc Flash Boundary and will no longer uses the "second degree burn" language. Instead the definition is

Arc Flash Boundary: When an arc flash hazard exists, an approach limit from an arc source at which incident energy equals 1.2 cal^cm2

The Stoll Curve is referenced in an information note.

This may be a better definition since the arc flash boundary is a calculation based on 1.2 cal/cm^2.


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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Fri Jul 21, 2017 7:43 am 
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Jim Phillips (brainfiller) wrote:
It may be interesting to note that the 2018 Edition of NFPA 70E has changes the definition of Arc Flash Boundary and will no longer uses the "second degree burn" language. Instead the definition is

Arc Flash Boundary: When an arc flash hazard exists, an approach limit from an arc source at which incident energy equals 1.2 cal^cm2

The Stoll Curve is referenced in an information note.

This may be a better definition since the arc flash boundary is a calculation based on 1.2 cal/cm^2.


Does it mean that the NFPA 70e group has finaly recognized that the 1.2 cal/cm2 has nothing to do with the second degree burn except it is a single point from the Stoll's curve indicating the amount of energy required to cause the damage when the energy is delivered in a one (1) second time interval? Did NFPA provide with any reasoning for the change in definition of arc flash boundary? Do they keep insisting to calculate the arc flash boundary based on 1.2 cal/cm^2 only because that's the value that was used for the calculations historically albeit incorrectly? Also, how one would determine "When an arc flash hazard exists" before even bothering to calculate the boundary?

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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Wed Sep 06, 2017 9:18 am 
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If at some point IEEE 1584 updates its formulas to reflect the Stoll model, I imagine that the AFB would be based on a variable 2nd-degree burn threshold (i.e. 1.2 cal/cm2 for 1 sec exposure, but lower for a shorter exposure and higher for a longer exposure).

http://arcadvisor.com/faq/threshold-incident-energy-second-degree-burn scroll to the last table for an example of how the AFB should be different depending on exposure time despite equal cumulative IE.

However, I'm wondering how it would impact PPE selection. For very high fault currents with very short fault clearing times, the calculated IE according to current equations may not provide sufficient protection to prevent 2nd-degree burns. Would PPE selection then be based on the rate of energy delivery (cal/cm2 per second)? How would that work? Would it make a big difference in the practical world? Can someone enlighten me?

Thanks.


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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Wed Sep 06, 2017 10:59 am 
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PPE selection really wouldn't change significantly because of how ASTM 1959 actually works. Kinetrics actually creates a consistent arc and mounts fabric samples at various distances from the arc. I'm not sure how the next part works but effectively they take readings from copper calorimeters that are mounted behind the fabric samples. All of the various tests are rated pass/fail depending on whether or not it it above or below the Stoll curve. So they have at the end a bunch of samples at various incident energy ratings (raw unprotected) and whether or not the fabric sample passed or failed at that incident energy. Then they take all those pass/fail readings (about 30) and use a nonlinear curve fit program to fit a sigmoidal curve (S-shaped) to the data. The 50% (midpoint) on the S-shaped curve is given as ATPV. All of the raw data as well as the model fit data is on a detailed one page report that quite often the PPE manufacturers will put out either publicly or on request.

So the big difference as I see it if we went to a thermal flux model is effectively for PPE, we have a change in units but that's about it.

As to IEEE 1584 itself, energy vs. flux is only the tip of the iceberg. There are a whole host of problems and issues with the model as it stands. However what is clear to me and many others is that IEEE 1584 "works". You might get burned but you won't get a second degree or greater burn on the face/chest area and you will survive.


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 Post subject: Re: 2018 NFPA 70E Changes Article
PostPosted: Tue Sep 19, 2017 7:21 am 
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When does 2018 Edition get adopted/go into effect?


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