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In your beach episode, your skin absorbed a total of 29 cal/cm^2 over a 20-minute period. That's only 0.02 cal/cm^2 per second
The incident energy in cal/cm² takes time of exposure into account, and it is the total energy, not the energy per unit of time, that determines the arc flash hazard.It isn't just the amount of energy you absorb, it's the amount per unit of time that relates to the burn.
The incident energy in cal/cm² takes time of exposure into account.
and it is the total energy, not the energy per unit of time, that determines the arc flash hazard.
The beach analogy doesn't work because injury is not linearly related with time for low radiant intensity levels. You probably won't get a second degree burn by sitting 4 feet away from a 100 W lamp for 2.6 hours.
I'd say assume linearity for real arc flash calculations. Usually you limit the exposure time to 2 seconds anyway on the assumption that if there is an arc flash the worker won't stand there in front of the arc longer than 2 seconds unless his movements are restricted.Do you mean at some point of a time injury would then become linearly related with time? What would be the threshold time for such transition?
could you please show analytical expression / formula describing the linearity between injury and time you would recommend for "real arc flash calculations"?I'd say assume linearity for real arc flash calculations.
The 1.2 ca/cm2 is based on that value causing a 2nd degree burn in 1/10th of a second.
Thank you, I'll think about it. The main reason I started this discussion was to see what do you guys think about using 1.2 cal/cm2 as a minimum incident energy for a second degree burn, and if anybody else can see the problem with it as well ( looks like me only is having the problem with it so far ).You are free to submit a proposal to the IEEE 1584 and NFPA 70E committees if there is something you feel should be changed.
After reading the Ralph Lees article, I noticed that the author assumes all arcing power is being converted into arc flash power, with the arc flash boundary equation expressed as function of arcing power. I have a problem with such an assumption ( arc flash power equals to arcing power ) especially for short time durations in the order of 100msec. That would be the same as assume that, when turning incandescent light bulb on, all the energy supplied to the light bulb would instantly be converted into ligh energy emitted by the bulb filament ( indeed it looks instantly to human eye but not that instanly through oscilloscope connected to light sensor viewing the bulb). My calculations show that only at most 1/3 of the arcing energy is being converted into arc flash energy ( incident energy ) within 100msec after the arc was initiated. The calculations include light energy emitted during the 100msec "heating" cycle, and "cooling" cycle thereafter. Taking that 1/3 correction factor into account, 1.7 cal/cm2 arcing power for 1/10th of a second reported in the Ralph Lee paper translates into approx 0.6 cal/cm2 incident energy exposure for 1/10th of a second, which is roughly half of the 1.2cal/cm2 threshold used by IEEE 1584 and NFPA 70E."The" Ralph Lee paper, the one that started it all. It is required reading for anyone doing arc flash studies (Or should be).
Can anybody explain why 1.2 cal/cm^2 incident energy was selected in solving the equation for the arc flash boundary in IEEE P1584 and NFPA 70E? Does it imply that an exposure to less than the 1.2 cal/cm^2 is not enough to cause a second degree burn?
follow the [URL unfurl="true"]http://arcflashforum.com/threads/2221/[/url] link to find the answer to your question