Insulation Leakage Test NM Cable

[growler]

Got a call from an electrical engineer and he wants the circuits in a home addition megged. It appears that a homeowner built an addition to his house without a permit and now the electrical inspector wants a EE to sign of on the project before a CO issued ( walls finished ). The discrepancy in square footage was caught by the loan company ( they require the CO before the loan goes through). My question is this. I have never megged any romex and I'm not sure what the proper reading would be. I visited the Southwire site and they give you some numbers to play with and then say in a foot note that field test are not reliable. Have any of you done this sort of thing and if so what were the results. I learned to use a megger in the military about 30 years ago but they provided the specs.

 

[charlie tuna]

Re: Insulation Leakage Test NM Cable

growler,
i see no problem with it --- i would meg it at 500 volts between the hot to ground after disconnecting "everything" from receptacles.

 

[charlie tuna]

Re: Insulation Leakage Test NM Cable

note: you will have to be extreamily careful not to meg any hard wired appliances or equipment because you could damage the electronic control with the high voltage created by the meggar. and if you didn't isolate such equipment, not only could you damage it, but it would also read as a ground!!!! i seriously doubt this test is worth doing ---- but i'm not an engineer!!!!!!!

 

[apauling]

Re: Insulation Leakage Test NM Cable

FEMA has created the appearance that a megger test is a pass on the wiring safety by requiring as a pass in flooded houses. I thought it was a fail test, where only the fail was truely known.

There may be many hazards that a megger will not uncover.

paul

 

[charlie b]

Re: Insulation Leakage Test NM Cable

I hope that the engineer is planning to do more than get a megger reading, before ?signing off? on the project. I don?t know if I would even take on such a project. But if I did, I would want to do a load calculation (to prove that the addition did not overload the service). I would open up and look at all of the outlets (to check for adequate wire size and type, and for proper grounding). I would determine which outlets were powered from which breaker (to verify that no breaker is overloaded). I would check for GFCI and AFCI devices, if the nature of the addition would have required them. Finally, I would write a report that described my review, and that clearly stated the limitations of my review (i.e., could not see the conduit, since the walls were completed). My signature and seal would be on the report, not on any design document. For example, I might provide ?record drawings,? if the owner wanted them (and was willing to pay for them), but I would not seal those drawings.

 

[friebel]

Re: Insulation Leakage Test NM Cable

I am going to address checking the NM wiring with a Megger. The other issues of proper size wire etc., the AHJ can, and will address that.
If I was doing the testing I would do like one thread ahead of this one suggested testing at the 500 volt level. I would attach one lead to ground and the other lead to each one of the current carrying conductors.
Now the next question is, "what reading will be acceptable".
Naturally, I would like to see "infinity", and you should see close to that on a new installation. The electrical Rule of Thumb is a reading of One-Megohm would be the minimum reading that you should accept. NOTE: I would not accept a reading of ONE-MEGOHM on a new installation. I would want it to be up to Infinity, or very close to it on a new wiring job.
If you would want to discuss this further, you can send me a private message, and I will give you my phone number and email address if you would like to discuss this further.I could also give you the name of the engineer at the AVO Company that sells Meggers and he would send you a couple of booklets on the subject of Meggers, and how to use them. The one comment that was made on a previous thread stated that he did not see any advantage or purpose for doing a Megger check.
That is how we do Preventative Maintenance in the electrical industry, and I have done it for many years.

 

[oliver100]

Re: Insulation Leakage Test NM Cable

1000 ohms/volt is an old practice rule of thumb.

 

[mdshunk]

Re: Insulation Leakage Test NM Cable

I've made it a habit of megging every new install, and I'm pretty fast at it. I would add that a megger will blow up the GFCI receptacles, so watch out for that. Obviously, you will need to have all the light switches in the "on" position (megg three ways in BOTH positions), and you will need all the light bulbs removed.

 

[growler]

Re: Insulation Leakage Test NM Cable

Thanks for the advice Charlie b. I have not herd from the homeowner today, I think the cost estimate may have put him off. I suggested he open the dry wall for inspection. It's my guess that the homeowner thought that the PE would show up and bill for a couple hrs. and sign off on the work. I know that the engineer had made one trip to do a visual inspection and was a little uneasy that he had bothered to show up at all ( I think we all know that feeling). I think that the megger idea was to show due diligence and maybe put the homeowner off. This guy seemed very cautious and I don't blame him. I think that after accepting the job he felt honor bound to continue but any avenue of escape would be welcomed. I wouldn't want to take responsibility for someone else's work when it's impossible to give it a proper inspection. It's a
shame that for a couple hundred bucks in permit fees all this hassle could have been avoided.

 

[steved]

Re: Insulation Leakage Test NM Cable

Hipot testing is required for motorhomes and manufactured/modular homes. The Slaughter Company (a manufacturer of hipot testers) has some information about this procedure.

Spell-checked version of the above:
Hipbone testing is required for metronomes and manufactured/modular homes. The Slaughter Company (a manufacturer of hippo testers) has some information about this procedure.

 

[friebel]

Re: Insulation Leakage Test NM Cable

I would like to make a comment on the subject of the HyPot Tester versus the Megger testing. It has been my experience that the HyPot tester, commonly known, possibly as a self-destructive test. In other words what I mean by that statement is, that you can destroy the equipment that you are testing by imposing a voltage level that the equipment cannot take. Whereas the Megger tester, generally will not destroy the equipment that you are testing.
It has been my experience, that only once in my lifetime that I could not find the problem with a Megger check, and the HyPot tester found the tracking problem on a slate board where a fuse assembly was mounted. By imposing a very high voltage from the HyPot tester, we found the tracking problem to ground that had been giving us the problem.We only experienced this problem when there was a high level of moisture in the air. Then the fuse would blow because of the tracking problem on the slate to ground.
I am not against HyPot testing, I am only suggesting that we need to be careful when we are using a HyPot tester versus a Megger Check. I certainly would welcome any comments on this matter.

 

[growler]

Re: Insulation Leakage Test NM Cable

Thanks Steved. I didn't end up doing the test on the none permitted addition anyway. I think the homeowner was still trying to get around the rules. I wished the engineer luck. I hate dealing with situations like that, neither the HO or the EE spoke English very well and there are real communications problems. I did find the Slaughter site very interesting. If these test are good enough for the manufactured home industry ( the most strictly controlled of the housing industry). I would think that it would be a good idea to test some of the older homes in such a manner. It would be interesting to see how old the insulation needs to get before it really starts to break down. Unfortunately such test are not required. The real estate companies don't even want a real electrical inspection, they prefer the home inspector version ( broken cover plate & and a couple of double taps ).