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Thread: Proper way to determine if NM cable has been damaged by heat from a fire

  1. #11
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    Quote Originally Posted by rbalex View Post
    The old "crank-type" were DC. Some newer units MAY be AC; don't know, don't recommend them. But, as I said, they practically guarantee unnecessarily destroying conductors. I believe most conductor manufacturers recommend against AC insulation resistance tests.

    A 500Vdc setting is already below the conductor rating (600V) anyway.
    1000 VDC is the recommended voltage for 600 AVC conductors.
    Brian John
    Leesburg, VA

  2. #12
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    Quote Originally Posted by Strathead View Post
    Never really thought about it before, and just did a Google search for megger. HMMMM! The OP, or his advisers have obvious misconceptions, since a Megger (trademark R) is a Hi pot tester. Just like a Bandaid is a bandage. That said, I never paid much attention to AC or DC, just set the switch to 50 or 500 as required and spun the handle. In most cases, are the old Meggers AC or DC output?
    Meggers are DC and measure resistance, in megohms and higher, Hi-Pots are AC and measure leakage current.
    Brian John
    Leesburg, VA

  3. #13
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    Quote Originally Posted by brian john View Post
    Actually, that what electrical testing firms do, test and ascertain the condition of cables, conductors, busway, circuit breakers...........
    The question is will the testing cost more, or even close to, the replacement. Doesn't make sense to spend $100 to test $10 of cable + $40 of installation.

  4. #14
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    Quote Originally Posted by zbang View Post
    The question is will the testing cost more, or even close to, the replacement. Doesn't make sense to spend $100 to test $10 of cable + $40 of installation.
    Depending on the amount of cables our minimum would be $1,280.00
    Brian John
    Leesburg, VA

  5. #15
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    Quote Originally Posted by mmccullough View Post
    Is Hipot test the best way to test the resistance of of the insulation of NM cable?

    We inspected a turn of the century building that had a fire that was contained to the basement area and now the Architect is claiming that all of the wiring in the building needs to be replaced due to the insulation being exposed to heat. We disagreed due to the lack of heat damage to any of the finishes and fixtures located outside of the fire area. We had thought about using a Megger to test the resistance but we are dealing with older wiring that has already had break down of the insulation due to age. Now they have hired their own EE that said the copper wiring is more conductive than the surrounding materials and high temperatures could travel along the without damaging the walls & finishes but minutely damaging the wiring and/or insulation. So they have now recommended to Hipot Test all of the wiring.

    We need to know if Hipot testing would be helpful or if there is another test method that would be more suitable for our situation.
    This EE, seems to be book smart and should stay in the library, away from working people.

    Performing an insulation resistance test per NETA standards is the way to go, depending on what a visual inspection shows.
    Brian John
    Leesburg, VA

  6. #16
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    very interesting subject, and many factors to consider. What are the insurance co requirements ? they certainly don't want another fire after the rebuild process.

  7. #17
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    Quote Originally Posted by brian john View Post
    ...depending on what a visual inspection shows.
    I think that would be key in a fire as there should be some evidence of scorching or heat presence of some sort.

    What has always bugged me is a meg of wires after lightning but what else do you do other than open the walls? In truth, the cable could be busted wide open and insulation missing but the air gap (or remaining insulation) might give an acceptable insulation reading. Might a TDR show anything valuable in that case (maybe you would have to have some violent physical results present from the lightning)? How do you test for possible lightning damage other than a meg test?
    BB+/BB=?

  8. #18
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    Why argue with the architect?
    If you are going to be paid to replace the cables, take the money and be happy.
    Tim
    Master Electrician
    New England
    Yesterday's Technology at Tomorrow's Prices

    Answers based on 2011 NEC

  9. #19
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    Quote Originally Posted by tkb View Post
    Why argue with the architect?
    If you are going to be paid to replace the cables, take the money and be happy.
    Sounds like the OP'er represents the paying party and wanting to keep repair costs at a minimum.
    I'll never get there. No matter where I go, I'm always here.

  10. #20
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    Quote Originally Posted by tkb View Post
    Why argue with the architect?
    If you are going to be paid to replace the cables, take the money and be happy.

    We have had customers say replace after we complete testing and everything is 100% acceptable. They have the cash and want to part with it. by all means go with it.

    We had a job where the customer had us do just that, when the insurance company saw our test reports they refused to pay the customer. What finally happened I have no idea as we explained to the customer, he told us to replace based on our data, he was liable for paying, case closed as far as I am concerned.
    Brian John
    Leesburg, VA

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