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 Post subject: arc flash calculation basicPosted: Fri Apr 06, 2018 5:52 am

Joined: Wed Nov 01, 2017 6:09 am
Posts: 27
Results from arc flash calculation gives me heat flux( cal/s/cm2) and heat energy(cal/cm2) at various distance. As I understand it is the energy that it’s used the choose correct ATPV on clothing. What is the practical use of the heat flux?

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 Post subject: Re: arc flash calculation basicPosted: Sun Apr 08, 2018 10:15 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
The relationship between a second degree burn and incident energy is not linear. The onset of a second degree burn at 1 second is about 1.2 cal/cm2. It increases to about 2.0 cal/cm2 at 2 seconds. Using heat flux it's essentially a constant. But heat flux never caught on and PPE is rated using incident energy.

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 Post subject: Re: arc flash calculation basicPosted: Mon Apr 09, 2018 6:11 pm
 Sparks Level

Joined: Wed Jun 10, 2009 5:00 pm
Posts: 267
Location: Toronto
Jensaugust12 wrote:
Results from arc flash calculation gives me heat flux( cal/s/cm2) and heat energy(cal/cm2) at various distance. As I understand it is the energy that it’s used the choose correct ATPV on clothing. What is the practical use of the heat flux?

Heat flux should be used to determine onset to second degree burn energy on a bare skin. Incident energy value alone without knowing how fast the energy was delivered (expressed by heat flux) is pretty useless. A quote from A.Stoll "Heat Transfer in Biotechnology" below summarizes the issue of using a critical thermal load approach in determining arc flash boundary:

"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."

ASTM F1959/F1959M Standard Test Method for Determining the Arc Rating of Materials for Clothing provides an equation for determining the onset to 2nd degree burn energy:

"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.

What software program are you using to calculate heat flux and does it have an option for determining onset to 2nd degree burn energy as a function of heat flux or time?

_________________
Michael Furtak, C.E.T.

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