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

[mmccullough]

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.

 

[petersonra]

I think the issue is going to be that very few professionals are going to be willing to put their professional license on the line to save a few bucks for the insurance company.

I don't think there is any way to reliably tell whether the wiring is degraded from age or from the fire until after you rip it out. Even then it may be hard to tell.

I agree with the engineer that it would be possible to have hidden damage in internal spaces that would not be readily apparent.

You will need to set some criteria in advance for what you would consider an acceptable hi-pot test for each chunk of cable. What is your pass-fail criteria going to be?

 

[Strathead]

Is Hipot test the best way to test the resistance of of the insulation of NM cable?
... 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. ...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.

Curiosity. I thought that hipot testing was actually just a higer voltage version of "megger" testing. This would be more, not less detrimental to the broken down insulation wouldn't it?

 

[mmccullough]

The hipot test sends double the current through the line and finds weak points in the wire. With the age of the wire we are dealing with this would highly detrimental. Our thought is that no testing is going to differentiate between damage caused by heat from a fire as to age. We visually inspected all wiring possible and saw no melted or broken down insulation.

 

[rbalex]

Curiosity. I thought that hipot testing was actually just a higer voltage version of "megger" testing. This would be more, not less detrimental to the broken down insulation wouldn't it?

Not necessarily, unless it is an AC Hi-Pot, in which case you will practically guarantee the conductor’s destruction. Unless intentionally destructive, DC testing, even at relatively high levels, only reveals actual shorts/ground-faults. AC testing, even at relatively low levels above the conductor rating, will often destroy conductors that are still serviceable.

Unless you're willing to replace without testing, I recommend getting a qualified testing service, to conduct the tests.

 

[Strathead]

Not necessarily, unless it is an AC Hi-Pot, in which case you will practically guarantee the conductor?s destruction. Unless intentionally destructive, DC testing, even at relatively high levels, only reveals actual shorts/ground-faults. AC testing, even at relatively low levels above the conductor rating, will often destroy conductors that are still serviceable.

Unless you're willing to replace without testing, I recommend getting a qualified testing service, to conduct the tests.

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?

 

[rbalex]

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?

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.

 

[brian john]

1. You would no Hi-Pot NM cable.
2. If you megger to industry standards, per the manufacture, NEMA and NETA standards, you should be able to adequately ascertain the condition of the cable.

If you are unsure of how, what and why, you might be better off hiring someone to perform the testing.

 

[brian john]

I think the issue is going to be that very few professionals are going to be willing to put their professional license on the line to save a few bucks for the insurance company.

I don't think there is any way to reliably tell whether the wiring is degraded from age or from the fire until after you rip it out. Even then it may be hard to tell.

I agree with the engineer that it would be possible to have hidden damage in internal spaces that would not be readily apparent.

You will need to set some criteria in advance for what you would consider an acceptable hi-pot test for each chunk of cable. What is your pass-fail criteria going to be?

Actually, that what electrical testing firms do, test and ascertain the condition of cables, conductors, busway, circuit breakers...........

 

[brian john]

The hipot test sends double the current through the line and finds weak points in the wire. With the age of the wire we are dealing with this would highly detrimental. Our thought is that no testing is going to differentiate between damage caused by heat from a fire as to age. We visually inspected all wiring possible and saw no melted or broken down insulation.

It does not send double the current, a high current test set would perform that test, but current testing is typically not performed on distribution conductors.

A Insulation resistance tester, megger sends a higher than normal DC voltage based on predetermined valuses as stated in different standards.

 

[brian john]

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]

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.

 

[zbang]

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.

 

[brian john]

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]

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.

 

[the blur]

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.

 

[mivey]

...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?

 

[tkb]

Why argue with the architect?
If you are going to be paid to replace the cables, take the money and be happy.

 

[Smart $]

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.

 

[brian john]

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.