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 Post subject: Evaluation of Onset to Second Degree Burn Energy in Arc Flash
PostPosted: Tue Apr 24, 2012 1:25 pm 
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Our interest in determining accurate onset to second degree burn energy and its significance in computing the arc flash boundary is focused on the prevention of injury to the skin of a human who might be exposed to an arc-flash. During the last two decades different formulas have been proposed to calculate incident energy at an assumed working distance, and the arc flash boundary in order to determine arc rated personal protective equipment for Qualified Electrical Workers. Among others, the IEEE Standard P1584 Guide for Performing Arc-Flash Hazard Calculations [1584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society. September 2002] and formulas provided in Annex D of NFPA 70E [NFPA 70E Standard for Electrical Safety in the Workplace. 2012.] and CSA Z462 [ CSA Z462 Workplace electrical safety Standards. 2012.] Workplace Electrical Safety Standard are the most often utilized in the industry to perform arc flash hazard analysis. The formulas are based on incident energy testing performed and calculations conducted for selected range of prospective fault currents, system voltages, physical configurations etc.

Use of Incident Energy as a Measure of Burn Severity in Arc Flash Boundary Calculations


The IEEE P1584 Standard was developed by having incident energy testing performed based on methodology described in the ASTM F1959-99 standard. The incident energy to which the worker's face and chest could be exposed at working distance during an electrical arc event was selected as a measure for determining hazard risk category and calculating the arc flash boundary. The incident energy of 1.2 cal/cm2 ( 5.0 J/cm2 ) for bare skinwas selected in solving the equation for the arc flash boundary in IEEE P1584 [1584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society. September 2002. page 41]. Also, NFPA 70E [NFPA 70E Standard for Electrical Safety in the Workplace. 2012. page 10] states 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 Standard for Electrical Safety in the Workplace. 2012. page 26].The IEEE 1584 Guidestates that “the incident energy that will cause a just curable burn or a second degree burn is 1.2 cal/cm2 (5.0 J/cm2 )” [1584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society. September 2002. page 96]. To better understand these units, IEEE P1584 refers to an example of a butane lighter. Quote: "if a butane lighter is held 1 cm away from a person’s finger for one second and the finger is in the blue flame, a square centimeter area of the finger will be exposed to about 5.0 J/cm2 or 1.2 cal/cm2 “. However IEEE P1584 equations (5.8) and (5.9) for determining the arc flash boundary can also be solved with other incident energy levels as well such as the rating of proposed personal protective equipment (PPE). The important point to note here is that threshold incident energy level for a second degree burn or onset to second degree burn energy on a bare skin is considered constant value equal to 1.2 cal/cm2 (5.0 J/cm2) in IEEE P1584 Standard.

Flash Fire Burn Experimentations and Observations


Much of the research which led to equations to predict skin burns was started during or immediately after World War II. In order to protect people from fires, atomic bomb blasts and other thermal threats it was first necessary to understand the effects of thermal trauma on the skin. To name the few, are the works done by Alice M. Stoll, J.B.Perkins, H.E.Pease, H.D.Kingsley and Wordie H. Parr. Tests were performed on a large number of anaesthetized pigs and rats exposed directly to fire. Some tests were also performed on human volunteers on the fronts of the thorax and forearms. A variety of studies on thermal effects have been performed and thermal thresholds were identified for different degree burns. We will focus on second degree burn as this is the kind of burn used to determine the arc flash boundary in engineering arc flash analysis studies.

Alice Stoll pursued the basic concept that burn injury is ultimately related to skin tissue temperature elevation for a sufficient time. Stoll and associates performed experimental research to determine the time it takes for second degree burn damage to occur for a given heat flux exposure. Stoll showed that regardless of the mode of application of heat, the temperature rise and therefore the tolerance time is related to heat absorbed by the skin[Stoll, A.M., Chianta M.A, Heat Transfer Through Fabrics. Naval Air Development Center. Sept. 1970]. Results of this study are represented in Figure 1 line (A) along with other studies discussed below.Image
Figure 1. Stoll Criterion Time to Second Degree Burn for Various Incident Heat fluxes on Bare Human Skin


A. Stoll found that the results from her experiments could be predicted using Henrique’s burn integral [Torvi D.A., A Finite Model of Heat Transfer in Skin Subjected to a Flash Fire. University of Alberta. Spring 1992]. Henriques and Moritz were the first to describe skin damage as a chemical rate process and show that first order Arrhenius rate equation could be used to determine the rate of tissue damage.

In 1952, J.B.Perkins , H.E.Pease and H.D.Kingsley of the University of Rochester, investigated the relation of intensity of applied thermal energy to the severity of flash fire burns [J. B. Perkins, H. E. Pearse, and H. D. Kingsley, Studies on Flash Burns: The Relation of the Time and Intensity of Applied Thermal Energy to the Severity of Burns, University of Rochester Atomic Energy Project, Rochester, NY, UR-217, December 1952]. Comparing results of this study with those of Alice Stoll shows that a larger amount of energy is required to induce second degree burn. Results of this study are represented in Figure 1 line (B).

Figure 1 line (C) shows second degree burn threshold as reported by Wordie H. Parr [Wordie, H. Parr, Skill Lesion Threshold Values for Laser Radiation as Compared with Safety Standards. US Army Medical Research Laboratory. February 1969]. The results were obtained by exposing skin to laser radiation and determining dose-response relationship for producing different grades of burns. The Figure 1 shows that the Wordie H. Parr curve lies between those proposed by Alice Stoll and those proposed by the University of Rochester study. The explanation for these second degree burn threshold differences could be interpreted by the fact that thermal injury depends on energy absorbed per unit volume or mass to produce a critical temperature elevation. Skin reflectance and penetration greatly influence this absorption. Also, heat conduction in tissue is far more efficient for smaller than for larger irradiated areas and exposure to higher levels of irradiance would be possible before injury occurred. Indeed, with extensive irradiation, injury would occur at far lower level of irradiance [IPCS. Lasers and Optical Radiation. World Health Organization. Geneva, 1982].

After reviewing these three studies, it was concluded that the curve presented by Stoll is most suitable for evaluating the type of burn hazard expected with arc flash. Stoll's study is a good choice because it is more conservative than the other two studies and therefore minimizes cases where the burn severity for a specific thermal flux exceeds the associated degree of burn, and is less open to criticism.

Figure 1 also includes an arrangement of onset to corneal injury thresholds from CO2 laser radiation (see square markers on Figure 1) [IPCS. Lasers and Optical Radiation. World Health Organization. Geneva, 1982]. The data follows the trend similar to that observed by Stoll and others. The range of scatter in the data is thought to be mainly due to the use of different corneal image sizes.

Stoll’s results can be theoretically extended to include heat flux rates over 40cal/cm2/sec experimentally observed, and they are represented by line (D) on Figure 1. The observed and extrapolated data lines A and D can be expressed analytically as:

t = 1.3 * H^-1.43 ( Equation 1)


where t is time to second degree burn in seconds, H is heat flux in cal/cm2/sec.

As an example of using the Equation 1, the projected time to second degree burn at a heat flux rate of 2 cal/cm2/sec is approx 0.5 sec. During this time interval the skin would be exposed to a total of 1 calcm2incident energy (2 cal/cm2/sec x 0.5 sec = 1 cal/cm2 ) , whereas at 30 cal/cm2/sec flux the time to second degree burn is equal to 0.01 sec resulting in only 0.3 cal/cm2 incident energy exposure, yet inducing the same burn severity as the former less intense and more lasting exposure.

Discussion and Conclusion


Our understanding of the burn mechanism is not perfect or complete but it is sufficient for the practical purposes concerned here. The important point to notice from Figure 1 and Equation 1 is that the degree of burn injury depends not only and, in fact, not as much on the total dose of energy received by the skin but also on the rate at which the energy is received.

The concept of destructiveness of rapid liberation of heat is not new and is widely used in many industrial and military applications. Apart from total amount of heat released during an arc flash event, it is the high heat flux rate that causes the gaseous products of arc flash to expand and potentially generate high pressures similar to most explosive reactions. This rapid generation of high pressures of the released gas constitutes the explosion. The liberation of heat with insufficient rapidity will not cause an explosion. For example, although a kilogram of coal yields five times as much heat as a kilogram of nitroglycerin, the coal cannot be used as an explosive because the rate at which it yields this heat is much slower.

Figure 2 below shows onset to second degree burn energy threshold adjusted for heat flux rate as a function of exposure time. The onset to second degree burn energy threshold was calculated as a product of heat flux rate and time to second degree burn as per the Stoll’s data from Figure 1 lines A and D.
Image
Figure 2 - Threshold Incident Energy for a Second Degree Burn vs. Exposure Time


Figure 2 demonstrates that the threshold energy for a second degree burn injury is not a constant but rather a variable. Note that the 1.2 cal/cm2 onset to second degree burn energy for bare skin used in IEEE P1584, NFPA 70E and CSA Z462 ( dashed line on Figure 2 ) intersects with the curve produced using the Stoll’s data at one (1) second point on Figure 2. This observation supports the choice of Stoll’s curve that we made for evaluating the type of burn hazard expected with an arc flash.For exposures lasting less than 1 second the irradiance required for an injury would significantly increase as the duration of exposure decreased; however, the amount of incident energy required to cause a second degree burn would decrease. Equation 2 shown below is an analytical expression for the threshold line represented by Figure 2:

Eb = 1.2 * t^0.3 (Equation 2)


where t is exposure time in seconds. Eb is threshold incident energy in cal/cm2 that needs to be released during the exposure time t to cause second degree burn.

As an example of using Equation 2 above, consider 1, 10 and 100kA faults in 600 Volt grounded switchgear with one (1) inch gap between conductors. The table below summarizes Arcing Current, Incident Energy and the Arc Flash Boundary (AFB) predicted using IEEE P1584 Empirical Model. We deliberately assigned arc duration to 1, 0.1, and 0.01 seconds for the 1, 10 and 100kA faults respectively which is consistent with inverse nature of typical protective device time-current characteristics. Column F lists AFB values calculated using 1.2 cal/cm2 onset to second degree burn Incident Energy recommended by IEEE P1584 Guide. Column I lists AFB values calculated using onset to second degree burn energy evaluated from Equation 2 and published in column H.

Image

Note that the amount of incident energy the person would be exposed to remains the same and equal to 2.1 cal/cm2 in all three instances (Column D). The arc flash boundary also remains the same when Incident Energy at AFB is assigned 1.2 cal/cm2 value onset to second degree burn energy as recommended in IEEE P1584. Therefore applying same onset to second degree burn energy for the above fault scenarios would make them appear to be of same severity. However, the arc flash boundary drastically changes when incident energy at AFB is being evaluated using Equation 2. AFB will now increase with an increase of the available fault current, predicted arcing current and heat flux released by an arc.

Therefore, using onset to second degree burn energy for bare skin exposure fixed to 1.2 cal/cm2 in calculating the arc flash boundary for arc durations other than one (1) second is, as far as we are concerned, open to dispute and, in our strong opinion, heat flux rate should be factored-in when estimating skin damage imposed by an arc flash. Using the 1.2 cal/cm2 energy for exposure times less than one second will result in undervalued arc flash boundaries while resulting in conservative but save arc flash boundaries for exposure times more than one (1) seconds. As the IEEE 1584 Guide states, the Guide’s equations (5.8) and (5.9) [1584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society. September 2002. page 13] can be used to calculate the arc flash boundaries with boundary energy other than 1.2 cal/cm2, and we believe the equations should be in fact solved for boundary energy computed using the Equation 2 especially for cased when arc duration is less than one (1) second.

by Michael Furtak,Application Engineer and Lew Silecky, Manager Technical Sales & Marketing of Mersen Canada Toronto, Inc.

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PostPosted: Tue Apr 24, 2012 4:38 pm 
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sorry, the equation 2 should actually read Eb = 1.2 * t^0.3

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PostPosted: Thu Apr 26, 2012 4:20 pm 
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It may take some searching but I've seen a data set that suggests that the incident energy requirement INCREASES below one second. I have never seen data from Alicia Stoll's experiments below 1 second but I have also never actually seen the original work. If the incident energy rate increases below 1 second then 1.2 cal/cm^2 is a conservative minimum.


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PostPosted: Fri Apr 27, 2012 3:50 am 
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Found it. Your analysis is mostly correct above 1 second. Some additional data circulating out there suggests at very long time intervals (>10 seconds) there are additional problems but since most folks use a cutoff below that point it suffices to say that the "very" extended range case does not matter.

Below 1 second, data from Privette shows what I described...the Stoll curve increases rather than continues to decrease. Although it is an indirect reference, see "Protective Clothing Guidelines for Electric Arc Exposure", IEEE Trans. on Ind. Applications, V 33, No. 4 (1997), pp. 1041-1054). Authors are Neal, Bingham, and Doughty.

The reasons for this and references are given as well. Above 1 second, I agree pretty much with what you are describing...that the 1.2 cal/cm^2 "cutoff" is not a constant. I would caution you though that this is all assuming that the incident energy is purely thermal which testing done by Mersenne has shown that this is not always the case.


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PostPosted: Fri Apr 27, 2012 10:02 am 
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Hi PaulEngr

I wish you used direct references instead of the indirect ones. I have found the mention of Privette's empirical findings in the indirect reference you are citing but I don't have and I couldn't find the original Privette's article to find out how they come up with their results and, therefore, I am not in the position to discuss Privette's work.

However, while trying to find the Privette's article on the web, I've come up with couple more references supporting the material presented in the article:

The figure below has being copied from "Thermal Output of Pyrorechnic Compositions and Evaluation of Skin Burns" Dr. B Lawton. Royal Military Collefi of Science, Cranfield University, UK. Notice please the match between 2nd Degree Burns Pyrotechnic Fires line ( highlighted in yellow color ) and Figure 2 from our article:

Image

Next, is the work called "A Finite Element Model of Heat Transfer in Skin Subjected to A Flash Fire" by David Andrew Torvi, University of Alberta, 1992. Check please values highlighted in tables below and compare them to Figure 1 from our article:

Image

Image
I've also come across couple more references supporting the material presented in the article but I won't cite them because they are indirect references and I prefer to stay away from citing such references.

Note that NFPA 70E old edition from year 2004 used to assume 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 (1.2 cal/cm^2)." This reference is GONE and DOES NOT appear in the newest NFPA 70E year 2012 edition. I believe the article answers the question WHY.

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PostPosted: Fri Apr 27, 2012 12:14 pm 
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I'm no fan of indirect references as well but sometimes the original references are so obscure that the indirect ones are all you've got. The references in the indirect reference go back to ASTM 1959 data, which is likely to itself be an indirect reference (or cite something really obscure). I looked at this issue years ago and stopped when I found the article I referenced since this plus 70E-2004 seemed to suggest the existence of a minima at or near the 1 second range.

If I have my unit conversions correct, the conversion from kJ to cal is multiply by 239. The conversion from m^2 to cm^2 is multiply by 10,000, so the conversion from kJ/m^2 to cal/cm^2 is to multiply by 0.0239.

The resolution of the chart as posted simply isn't good enough to pick some data points and run the conversion from it.

Using the above numbers and the conversion (it's late on a Friday and I'm not sure of what I'm doing), 166.4 kJ/m^2 is 3.98 cal/cm^2, which causes a second degree burn at 0.2 seconds. 83.2 kJ/m^2 is 1.99 cal/cm^2, which causes a second degree burn at 0.5 seconds. Based on this data unless there's something wrong with my unit conversion, it seems to support the idea that as the time interval decreases, the required incident energy to achieve a second degree burn increases. I suspect there may be a math error here since part of the information referenced says "Stoll" but I don't have anything else to go on. Is my math correct in converting kJ/m^2 over to cal/cm^2?


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PostPosted: Fri Apr 27, 2012 12:48 pm 
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Tables 3.2 and 3.12 from "A Finite Element Model of Heat Transfer in Skin Subjected to A Flash Fire" by David Andrew Torvi, University of Alberta, 1992 list Heat Flux in kW/m^2, not in kJ/m^2. Therefore, your conversion is not exactly right as you missed per second notation. In other words, kW/m^2 is same as kJ/m^2/sec. Now, take the values obtained from converting kW/m^2 to cal/cm^2/sec and compare them with Line A from Figure 1 to see the match. Indeed, 166.4 kW/m^2 (or 3.98 cal/cm^2 /sec ) AND 83.2 kW/m^2 (or 1.99 cal/cm^2/sec ) irradiance is required to cause a 2nd degree burn within 0.2 and 0.5 sec interval respectively, resulting in 3.98 cal/cm^2 /sec x 0.2 sec = 0.8 cal/cm^2 and 1.99 cal/cm^2/sec x 0.5 sec = 1 cal/cm^2 incident energy exposure respectively. What you say is pretty much exactly what I am trying to prove. A quote from the article: "the irradiance required for an injury would significantly increase as the duration of exposure decreased; however, the amount of incident energy required to cause a second degree burn would decrease."

I thank you very much for your input. I've spent more time today researching the topic and I've come up with even more solid evidence supporting points presented in the article.

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PostPosted: Fri Apr 27, 2012 1:24 pm 
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OK, now based on this data it again seems to correspond to what you are suggesting...that we have to model the second degree burn threshold via the Arrhenius equation. One implication of this that jumps to mind immediately is the question about the 1.2 cal/cm^2 threshold as you right stated. But extrapolating away from this, we do not have to consider any cases other than "PPE 0" (1.2 cal/cm^2). The reason is that the ASTM test uses the Stoll curve directly and passes/fails arc resistant cloth samples based on passing above or below the Stoll curve at any point over the time interval of the test. It is simply going to make the threshold a curve based on opening time. The PPE manufacturers will love this!


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PostPosted: Fri Apr 27, 2012 1:55 pm 
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I agree that PPE manufacturers would love to see changes to the existing ASTM standard they currently using to test the PPE, or see a new test standard reflecting, in particular, the issue brought up in the paper. I kept the article limited to the evaluation of threshold incident energy for a second degree burn only and I've kept the issue of testing the PPE for my next article.

I also agree that the issue brought up the in the paper affects a lot of the "PPE 0" cases escalating them in many cases up to PPE 1.

I am not so sure if everyone who has already done arc flash analysis and calculated arc flash boundaries based on the 1.2 cal/cm^2 threshold recommended by IEEE P1584 and NFPA 70E will be happy to realize they have to redo the analysis to accommodate for the variable threshold incident energy for a second degree burn, re-print the labels, re-label the equipment, buy new PPE etc. Sincerely, I wish I was wrong.

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PostPosted: Sun Apr 29, 2012 5:40 am 
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arcad wrote:
I am not so sure if everyone who has already done arc flash analysis and calculated arc flash boundaries based on the 1.2 cal/cm^2 threshold recommended by IEEE P1584 and NFPA 70E will be happy to realize they have to redo the analysis to accommodate for the variable threshold incident energy for a second degree burn, re-print the labels, re-label the equipment, buy new PPE etc. Sincerely, I wish I was wrong.


Just in terms of modeling, there are now many "holes". Among them:
-Stoll curve below 1 second. The previous study that I referenced seemed to suggest that this was a non-issue. Your information says just the opposite.
-Type of energy (radiative heat vs. plasma for instance). This seems to be a "good/bad" situation. Some PPE is much more resistant to plasma, others are less than expected.
-Equipment design. IEEE 1584 test data for "arc in a box" is effectively metal enclosed gear without phase barriers. This is atypical design.
-Understanding of arc blast. Recent data that I've seen suggests that arc blast is only relevant at a certain point in time...too short a time interval, and it does not develop fully. Too long and it becomes a constant.
-Whether development of an arcing fault is instantaneous as currently assumed. This is commonly seen in British literature but some of the statements (at least 100 ms fault time required before arcing faults are a safety concern) are flat out incorrect.
-Consideration for both the average and distribution (look at probability, not just 50th percentile)
-Whether an arc can be sustained, especially with long arc gaps or long time periods.


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PostPosted: Mon May 07, 2012 10:04 am 
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Lots of good info above. There is MUCH dispute on this and it should be discussed. I suggest joining IEEE 1584 and ASTM F18.

The papers above are useful and Alan Privette's paper on skin burn in arc flash did indicate that in less than 1 second the second degree burn to human skin was closer to 2-3 cal. This is why NESC uses 2 cal for the cut off. I personally think this is more reasonable and has data to support it. I have read Alan's paper but have written him for the full citation to purchase a copy from IEEE.


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PostPosted: Mon May 07, 2012 10:15 am 
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Here is a quote about the paper from Tom Neal.

"Privette [12], [13] empirically extended the work of Stoll to exposure times down to 0.01 s and found for very short exposures of from 0.1 to 0.01 s that 1.5–2.0 cal/cm is required to cause a white burn on a depilated rat. For short exposure times of 1 s or less, the total absorbed energy required to cause a second-degree burn in human tissue, i.e., destruction of the epidermis, ranges between 1.2–3.2 cal/cm . The more recent data in [12] and [13] converges on the range of 1.2–1.6 cal/cm ."
“Protective Clothing Guidelines for Electric Arc Exposure” Thomas E. Neal, Allen H. Bingham, and Richard L. Doughty IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 33, NO. 4, JULY/AUGUST 1997 1041
It cites Alan’s work as
A. Privette, “Electric arc test method development,” American Society for Testing and Materials, West Conshohocken, PA, Rep. for ASTM F-18 Committee Task Force F18.10.07, June 10, 1992
I’m relatively sure he published this in IEEE.

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PostPosted: Mon May 07, 2012 10:22 am 
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ASTM F18 is considering using the Henriques model but or a burn model from DuPont which uses Henriques with some adjustments. This will change some arc ratings but the subtle tweaking of ratings will not make the calcualtions better. Only more data and a better model from IEEE 1584 will do that but the nature of an arc is such that if the average in a particular arc is 1.2. It is likely some part of the arc is MUCH higher than than and most of the arc is much LOWER than that. Just something to consider. I feel the 1.2 is too conservative and the use of cotton is not conservative enough in some cases.

Lots of room for improvement.


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PostPosted: Mon May 07, 2012 11:14 am 
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As I've already said, I have seen the mention of Privette's empirical findings in the indirect reference you are citing but I don't have and I couldn't find the original Privette's article to find out how they come up with their results and, therefore, I am not in the position to discuss Privette's work.

At least one other heavily cited paper reports that more (!) than 1.2 cal/cm2 incident energy is required to 2nd degree burn during 0.1 second exposure to the arc. The article is based on assumption that all arcing energy is translated into the arc flash energy, which is very questionable for short arc durations.

As far as I am concerned, IEEE 1584 Empirical Model is the way to go, other that the variable threshold energy for a second degree burn injury evaluated using Equation 2 should be used in determining arc flash boundaries instead of the 1.2 cal/cm2 constant value widely (though not exclusively) used at present

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PostPosted: Thu May 17, 2012 6:17 am 
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The article has now been published in the Spring 2012 issue of Electrical Line magazine
http://digital.olivesoftware.com/Olive/ODE/ElectricalLine/default.aspx?href=ELM/2012/03/01
I've also uploaded the pdf file for your convenience.

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PostPosted: Fri Nov 09, 2012 2:44 pm 
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I've finally obtained a copy of Alan Privette report that involved arc flash testing on rats. The extension to Stoll's curve proposed by Privette paper (see Figure 10 below) indeed suggests that the amount of incident energy to 2nd degree burn remains almost constant and does not exceed 2 cal/cm2 in the extended interval from one (1) second and less.

Image

Also, observe proposed deviation from the original Stoll’s curve on Figure 9 (copied below) in time interval above 10sec suggesting more heat is required to cause 2nd degree burn with lower intensity heat fluxes.

Image
The key point is that Alan Privette work is based upon tests where the test animals were shielded with flame retardant fabrics. Of course, burn injury through this fabric is not equivalent to the burn injury induced in exposed skin. Therefore, even though the incident energy levels were measured, A. Privette's finding are not applicable to bare skin injury predictions and shouldn't be exploited to justify NFPA 70E 1.2 cal/cm2 (5Joules/cm2) minimum threshold energy for a 2nd degree burn, as they currently are.

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PostPosted: Fri Nov 09, 2012 6:14 pm 
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Please don't use the term "exploit" (unless you are watching too much conspiracy TV). This is unkind and manipulative to attempt to paint those on the committee as uncaring, unfeeling or trying to serve some subversive agenda. I can assure you this is NOT the case. If that were true your ideas would be accepted very quickly because they would make EVERYONE wear AF PPE and equipment would have to be switched out overnight to keep from killing the population (or sunburning the population, which is what a second degree burn is).

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This is a technically interesting thread.

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An NFPA 70E taskforce is looking into this subject, I believe at your request. However this is a quite complicated subject and I know of NO ONE burned with our use of these numbers on bare skin but from what I can see and know from my 18 year’s experience, you may be technically correct.

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However, here are some things to consider:

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1. Arc flash calculations are not nearly as far along as skin burn modeling.

2. Idealistically you are asking that AF boundaries prevent all second degree burn possibilities (if we do what you want AF boundaries would be MUCH greater). But arcs are focused events while the boundaries are not so MANY boundaries will become unmanageable in a real work setting forcing PPE to be worn by all workers in a plant in many cases up to hundreds of feet away.

3. One of the tests of a scientific theory is real life correlation. No real life evidence suggests AF boundaries need to be greater. Actually the opposite is true.

4. I believe you are right on the studies. The committee (actually MANY committees including ISO, ASTM F23, ASTM F18 and EU committees) decided to be conservative on this subject by using a defensible value which we could all agree on to simplify this complex subject since IR will usually ONLY cause second degree burns (which are curable) in an arc event. The plasma or clothing ignition can cause third degree (scarring burns) and it doesn't characteristically reach out even close to the AF boundary with our limited science.

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This is an engineering problem with many movable parts and we had to set something as a boundary. We chose not to use the most conservative info because it would have made the final answer too conservative to those of us who had experience causing the standard to fail the “smell test”.

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I'm a pragmatist and want workers protected but I think there are more important things to focus on in the standards. We need to eliminate the fatalities and permanent disabilities, not make electrical equipment so it can't even be worked around without PPE to prevent possible “sunburns” in a one in a million event, in what I believe would still be VERY few cases. I have about 200 accident investigations under my belt in AF and don't see this as an issue. Arc Flash IS an issue but broader boundaries are not going to save any lives or really any burns.

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I can also assure you there is no conspiracy to "exploit" workers to get anyone's jollies on seeing sunburns from arc flashes.



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PostPosted: Fri Nov 09, 2012 8:50 pm 
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Dear Hugh,

In your earlier comments you indicated "that in less than 1 second the second degree burn to human skin was closer to 2-3 cal. This is why NESC uses 2 cal for the cut off". I am glad you've just pointed that I "may be technically corrent".

Also, I appreciate you finally acknowledging that I am "right on the studies". It means that the recent literature search I've done and the support material (including experimental evidence) I shared with the NFPA group produced the result. Indeed, I am gratefull to member of the NFPA Commitee who recently approached me questioning the article claim that heat dose for a 2nd degree burn could be a fraction only of the industry accepted 1.2 cal/cm2 threshold incident energy (as exemplified by Figure 2 from the article) and inspired me to perform more research. I wish I had all this recently revealed material available before the article was published in IAEI magazine in July 2012. I believe that having more proof available prior to taking the material to print would help me to write even more comprehensive article.

What you are saying however implies that NFPA 70E commitee was aware of the problem but "chose not to use the most conservative info because it would have made the final answer too conservative to those of us who had experience causing the standard to fail the “smell test”." I wish you were wrong and NFPA chose a different path. Your observation above the NFPA course of action really concerns me.

Now, once the NFPA was publicly called for action, it is up to the NFPA 70E taskforce to address the issue of using 1.2cal/cm2 as a minimum incident energy for a 2nd degree burn, implement much needed amendments, and abandon defending existing arc flash boundaries and the available numeric value to cause the damage.

Indeed, the arc flash article above was conceived only as an education tool to raise public awareness of the potential hazard. I assure you that the article was not planned to undermine anyones credibility including NFPA. Reasonable effort was done to approach standard organization and get answers and the organisation interpretation before taking the material to print.

You may consider the article material as an alternate view to NFPA interpretation. You may disagree on it. But please, don't shoot the messenger.

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http://arcadvisor.com


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PostPosted: Sun Nov 11, 2012 6:43 pm 
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Privette's work was done on bare skin to my knowledge.

You could come to ASTM F18 and get involved in the committees who raised this issue to save lives 18 years ago and fatalities in the US dropped 57% during that time and changed the world of electrical work. Lets make it better!

Love to meet you there.


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PostPosted: Sun Nov 11, 2012 8:52 pm 
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elihuiv wrote:
Privette's work was done on bare skin to my knowledge.


I am afraid your knowledge is based on indirect references like the one from T. Neal's "Protective Clothing Guidelines for Electric Arc Exposure” cited in your earlier comments. To the contrary, my most recent observation that the test animals were shielded with flame retardant fabrics is based on the original Privette's report [A. Privette, "Progress report for ASTM Burn Stufy", Duke Power Company, 1992]. Ultimately, the results of Privette's study were used to predict bare skin injury with all consecutive outcomes, including the minimum heat dose value to cause the damage from NFPA 70E, and other safety standarts.

A quote from A.Stoll [A.Stoll, "Heat Transfer in Biotechnology", Advances in Heat Transfer, v.4. Academic Press. 1967] summarizes the issue of using 1.2 cal/cm2 as a threshold incident energy to 2nd degree burn. 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."

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