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Thread: AFHA Update

  1. #1
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    AFHA Update

    Suppose you had a five year report done on a large commercial building, and later wanted to add a couple panels to the system and report. Does the study have to be completed for the whole building again, generally? Can the first report be used as a base and just calculate values for the additions?

    Does it make sense that adding two panels to a two-year-old report costs almost exactly the same as the initial report of dozens of panels?

    Asking for a friend

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    What does AFHA stand for?

    Google keeps sending me to West Virginia.
    "Electricity is really just organized lightning." George Carlin


    Derek

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    Arc flash hazard analysis.

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    Quote Originally Posted by George Stolz View Post
    Suppose you had a five year report done on a large commercial building, and later wanted to add a couple panels to the system and report. Does the study have to be completed for the whole building again, generally? Can the first report be used as a base and just calculate values for the additions?

    Does it make sense that adding two panels to a two-year-old report costs almost exactly the same as the initial report of dozens of panels?

    Asking for a friend
    No, yes, and maybe.

    AFHA are updated all of the time. I have one customer that sends me his 'additions' every quarter.
    I work off of the base model, and the new equipment simply gets added. A complete new report is not produced. The customer gets a revised table. Once a year, or so, a complete new report is generated.

    There are times I am asked to update a report produced by a different firm. In these cases I almost have to start from scratch by creating a new model, so it comes close to the original study especially if the original report input data is suspect and needs to be verified.

    It is also possible that the amount of 'overhead cost' is the same for 2 panels as it is for 24 panels.
    Just because you can, doesn't mean you should.

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    Ditto what Jim said

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    Adding a panel to an existing distribution system will not alter the results of the AFHA for any of the existing panels. You can, if you wish, assign the new panel the same arc flash category as its upstream panel - it certainly won't be worse. Two things would alter these statements:
    1. If the new panel powers large motors that would contribute fault current to the rest of the system.
    2. If the new panel is fed via a step-down transformer.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b;1937176[SIZE=4
    .. it certainly won't be worse. ..[/SIZE]
    Charlie, what about when the conductor length actually its impedance) is great enough to lower the arcing fault current and therefore slow down the clearing time of the protective device?
    Just because you can, doesn't mean you should.

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    Good question, Jim. My answer is that that extra impedance would impact the fault current (and arc flash energy) at the location of the new panel. So if you choose to simply assign the upstream panel's AFHA results to the new, downstream panel, you might be giving unnecessarily high numbers to the new panel. Your choice: revise the analysis or use the previous results.

    But that extra impedance won't alter the fault current (or arc flash energy) anywhere else. The analysis looks at a fault at one location at a time, and determines the fault current (and arc flash energy) at that location. The only impedances that enter into that analysis are those between the fault point and any energy sources in the system (i.e., utility and motor contributions). So if the new panel does not power any large motors, it will not have an influence on the arc flash energy felt at any other location.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    So if the new panel does not power any large motors, it will not have an influence on the arc flash energy felt at any other location.
    Absolutely.
    But other than that, the incident energy at the 'added' equipment is just an educated guess without doing the calculations.
    Just because you can, doesn't mean you should.

  10. #10
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    I agree, Jim. But if the upstream panel has a category 1 rating, you don't need a calculation to assign category 1 to the new panel. Also, if the upstream panel has a category 2 rating, and you are willing to live with a category 2 for the new panel, once again you don't need a calculation. On the other hand, if the upstream panel has a category 2 rating, but you would prefer to assign category 1 to the new panel, then you will need a calculation. That is all I am saying.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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