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Flash boundary distance

Discussion in 'General Discussion' started by amohammed, Jun 16, 2009.

  1. amohammed New Member

    Jim,

    In your article published in NEC digest in august 2007 "How to perform an arc-flash study in 12 step", you've recommended standardizing the flash boundary distance to six feet for live work. I can't seem to understand how can one work on a piece of equipment 6 feet away? (when I say work I mean racking in breakers or any other maintenance operation)
    I know remote controlling of breaker racking is feasible at much larger distances. But I am sure this method of operation is pointless at just 6 feet.

    I've learned quite a bit from this article and I appreciate your contribution. For those oblivious of the article, I've attached the links.

    Part 1

    Part2

    Part 3

    I welcome comments from other members as well.

    Thank you,
    Aleen
    amohammed, Jun 16, 2009
    #1

  2. jghrist Well-Known Member

    My interpretation is that if the calculated arc flash boundaries are all less than 6 feet, simplify by using 6 feet everywhere. This doesn't mean you can't work at a distance less than 6 feet, only that you are required to use appropriate PPE when you do.

    For instance, at one piece of equipment, the IE at the working distance may be 7 cal/cm² and the arc flash boundary 4 feet. At another piece of equipment, the IE may be 23 cal/cm² and the arc flash boundary 5'-4". You would simplify by requiring PPE whenever you are interacting with either piece of equipment within 6' of it. On the first, you would use Cat 2 PPE (8 cal/cm²) and on the second you would use Cat 3 PPE (25 cal/cm²).
    jghrist, Jun 16, 2009
    #2

  3. amohammed New Member

    Now that makes sense. Thanks for the interpretation.

    Aleen
    amohammed, Jun 16, 2009
    #3

  4. McFlash New Member

    The question begs a definition.......What exactly is the Arc-Flash Protection Boundary? When an arc flash hazard exists it is the approach boundary within which a person could receive a second degree burn if an arc flash were to occur. Right now this boundary is a calculated value and is different for every piece of equipment in a facility. By standardizing the boundary you are taking confusion out of when to put on PPE since it is the same distance at every piece of equipment.

    Be careful though I would do an either or case....the boundary should be set to 6ft like suggested or farther if calculations lead to farther distances.
    McFlash, Jun 16, 2009
    #4

  5. MIEngineer Member

    Not to hijack the thread.....but here goes.

    Does anyone actually use the 4 different approach distances indicated in 70E. From my limited involvement with AF studies(<=480V) the arc flash boundary is much farther away from the equipment than the shock protection distances. Do people actually enter the arc flash boundary wearing the required AF PPE without shock protection as required by limited approach distance? If so, what does this gain?
    MIEngineer, Jun 30, 2009
    #5

  6. brainfiller Administrator

    Wow, this thread slid right on by me :eek: - sorry. Here is what I meant by standardizng.

    The Arc Flash Protection Boundary defines the minimum distance from a prospective arc flash where you need to wear PPE when an arc flash hazard exists. Over the years I have heard many people get excited about having different boundaries for each location based on the results of the arc flash calculation study. The concern is confusion.

    Since the boundary applies more to those that are not properly protected, my take is if you are not performing the work and wearing appropriate PPE then just get out of the way. Splitting hairs over where the unprotected person must be i.e. 3 ft, 4ft, 5 ft, etc. makes this more complicated than it needs to be - just have the non protected people stay out of the way.

    Of course there could be exceptions to this such as needing a helper close by but not standing within the AFPB. But.... if something goes wrong and the unprotected helper needs to rescue you during an arc flash, it could be quite bad.

    What many people have been doing is adopting a standardized AFPB based on the largest (within reason) one from the study. This could mean 6 or 8 feet everywher. Combine this with using a 2 level PPE approach and you have a much simpler program. Again, the large AFPB should not affect the protected person doing work, it only affects the unprotected person by keeping them out of the way. Also keep in mind, we don't know everything (not even close) about how the arc flash acts. Last week I threw a bus bar (it was placed on the two stabs of a pad mounted transformer) about 20 feet with only 15 kA of arcing current at 480/277V. That's well beyond the AFPB.

    Hope it helps!
    brainfiller, Jul 1, 2009
    #6

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