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1.2 versus 1.5 cal at Arc Flash Boundary

Discussion in 'NFPA 70E - Electrical Safety in the Workplace' started by haze10, Oct 6, 2008.

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  1. haze10 Sparks Level

    "...At voltage levels about 600V, the Flash Protection Boudary is the distance at which the incident energy equals 1.2 cal/cm2. For situations where fault clearing time is 0.1s or faster, the Flash Protection Boudary is the distance at which the incident energy level equals 1.5 cal/cm2..."

    First, let me verify. The second sentence, increasing to 1.5 cal/cm2, stands alone regardless of voltage. It is good for all voltages both above and below 600V?

    Second, if 1.2 cal/cm2 is the threshold for a 2nd degree burn, and since IE includes clearing time in the formula, why is it acceptable to increase the burn threshold to 1.5 cal/cm2.

    Third, do you think it is a good idea to add in this complexity to an already complicated guideline. How much difference does it really make to the boundary distance. I realize heat is proportional to the distance squared, so the reduction may be significant. But is it a good practice. I pound the 1.2 cal limit into my electricians, now I have to qualify it for clearing times less than 0.1 seconds. Are people actually do this and would it not be a better idea to just keep the 1.2 cal limit throughout and live with slightly wider AF boundaries?
  2. Zog Sparks Level

    I dont have a solid answer to that question but I would stick with the 1.2 cal for your guys. K.I.S.S. (Not calling you stupid)
  3. haze10 Sparks Level

    I answered one of my own questions. The 1.5 cal rule is ONLY for "Voltage Levels above 600V". I saw the 1.5 cal rule in the Annex, but in the body of the text 130.3.2 the headers is "Voltage Levels above 600V" so then anything that follows even if not specified would have to fall in that category.

    But can someone answer why the 1.5 cal rule came to be and how it performs for FR clothing built to a 1.2 cal world.
  4. haze10 Sparks Level

    Zog, no offense taken. "Stupid is as stupid does". I use the 1.2 cal value regardless of voltage because its not worth dealing with the complexity to gain a few inches in arc boundary. But I still would like to know why its there. For the 2014 release - I plan to submit a whole new guideline to totally replace Art 130 - and it will be 2 pages long.

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