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Picking a AFB Minimum?

Discussion in 'General Discussion' started by haze10, Nov 5, 2008.

  1. haze10 Well-Known Member

    If we want to simplify our labelling, and pick one value as minimum for the Arc Flash Boundary, and one value for the Restricted approach Boundary - what would you use.
    Originally, I would have gone with this:
    Flash boundaries:
    Up to 600 volts: 4.0 ft
    601 volts and over: 15.0ft

    Limited Approach Boundary:
    Up to 600 volts: 4.0 ft
    601 volts and over: 15.0ft

    But now, I am thinking this is more appropriate:

    Up to 600 volts: 10.0 ft
    601 volts and over: 20.0ft

    Limited Approach Boundary:
    Up to 600 volts: 10.0 ft
    601 volts and over: 20.0ft

    Obviously, if the calculated value comes up higher, then that is the value that goes on the label. But I think it helps in the learning process to have a standard minimum that covers 99% of the applications. There is a good probability that in many cases these values will cover 100%. I keep the Limited Approach Boundary the same as the AFB. I think there is little value in having a person dressed in AF PPE, able to cross the AFB but stop at the Limited Approach Boundary. But for teaching and implementation purposes, it helps to have them the same.

    I assume the 'over 601V' stops in typical petro/chem type installations of 13.8KV, with an upper limit of 23KV. If I had 369KV I'd be extending it.

    What do you think of this concept and what values would you choose.

    Remember, the purpose is to protect the worker, there is nothing wrong with the label stating a value higher than calculated - it just can't be lower than calculated.
    haze10, Nov 5, 2008
    #1

  2. blackdog New Member

    Our company has adopted a ten foot policy both the LAB and AFB. Seems a lot safer and simpiler for everyone involved.
    blackdog, Nov 6, 2008
    #2

  3. WDeanN Well-Known Member

    I don't like the idea of having a one-size-fits-all boundary. I've seen several calculated out to 30 or 40 feet.

    But yes, I set the minimum AFB equal to the greater of the calculated boundary (rounded up to the nearest foot) or the Limited Approach Boundary (as determined by the NFPA).
    WDeanN, Nov 6, 2008
    #3

  4. brainfiller Administrator

    I believe standardized is better similar to what Haze suggests. I have been pitching larger uniform boundaries for years, typically 8 to 10 feet for the arc flash protection boundary. I met a lot of resistance in the early years when I brought this up. A few people thought I should have my head examined and then slowly people seemed to realize it was a much better way. Slowly I began to see companies adopting the 8 or 10 foot boundary. I published this concept in the NEC Digest Article "How to Perform and Arc Flash Study in 12 Steps" a while ago. There is a link to the article somewhere on this forum.

    The concept is very simple. Instead of getting into a calculation contest and trying to figure out if the AFPB is 3.4 feet, 3.2 feet or 3.14159 feet, pick a larger standardized number. The AFPB is just the boundary from the prospective arc source where you need PPE. If you don't have on PPE, get out of the way - Period! 6 feet, 8 feet, 10 feet etc. just back up. Making this the Limited Approach Boundary also simplifies things. Unprotected and Unqualified people get out of the way. Of course there are exceptions if the larger boundary impacts pedestrian traffic lanes etc. but those issues tend to be exceptions.

    The best way to handle this is to perform the calculation study, evaluate the AFPB's at each voltage level, select the largest AFPB within reason and then round it up. The "within reason" part is because as many know, the IEEE 1584 formulas, can yield some pretty unrealistic boundaries if you have a very long clearing time. Like WDN mentioned, I have also seen boundaries of 30 feet and even 100 feet with a low short circuit current. You need to evaluate the clearing time and settings if you have one of these very large AFPB's. If you have a very large one and it is legitimate, keep it separete as an exception and keep the 8 or 10 ft boundary as the standard for the other locations.

    K-I-S-S really needs to be applied here for PPE, and the AFPB or the end users will become more confused, ignore it, or get it wrong.
    brainfiller, Nov 10, 2008
    #4

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