Several Megaohm resistance between Conductors in Underground Conduit

[Kornfeld]

I recently installed undergrgound IMC conduit and ran several gauge 14 THHN wires through the conduit. Using my multimeter, I measure some conductance between the wires that varies from 3 to 9 Megaohm. Is this something I should worry about? I can't imagine what could be causing this problem. Do I need to pull the wires and start over again?

 

[augie47]

Per ECM article:
Anything reading between 2 megohms and 1000 megohms is usually considered a good reading, unless other problems have been noted.

 

[NewtonLaw]

What meter did you use? What applied voltage was placed on the cables during the test ? How long was the cable run? Was the conduit RGS, EMT, PVC? Where the cables opposite the testing end, in free air? Terminated on a device?

 

[paulengr]

Per ECM article:
Anything reading between 2 megohms and 1000 megohms is usually considered a good reading, unless other problems have been noted.

ECM is wrong. This is not what any standard calls for. 2 megaohms temperature corrected would only meet the minimum for a rotor or very old (pre 1970) mica and asphalt motor insulation, something I’m not sure still exists anywhere.

Plus you need to know the temperature. NETA for instance calls for a minimum 100 megohms, temperature corrected, measured at 1000 V for 1 minute. The wiring fails if the test was done correctly. That’s also a pretty common cable spec. On motors the test voltage and passing criteria is different and frequently we test the cable and loads together. Same with transformers.

I hardly ever see anything that low on good cable. Either the test was not done correctly or the cable is bad.

Plus gigaohms or higher is fine IF it’s not an open connection giving a false reading, something you need to check for when you see gigaohms or higher. On ordinary fresh, dry, isolated #14 THHN-2 or XHHW I’d expect almost a gigaohm or more.

 

[Kornfeld]

It's a Fluke multimeter; What the test voltage is, I couldn't say. There's roughly a 70 feet run of THHN/THWN gauge 14 cable in IMC conduit. At the open ends (of which there are two) the conductors are in air. Sometimes when I check with the multimeter, I get no conductance at all. Other times, in the megaOhms. I can't detect any particular pattern. Should I be using a different sort of meter?

 

[ptonsparky]

ECM is wrong. ...

Thank you.

 

[tom baker]

You need to use a megger or megohmeter. A DVM has high impedance, and is not the correct meter. A megger, as they are called applies a 500 or 1000 vdc, measures the current, and using ohms law, displays resistance. The wires can not be connected at either end

 

[SSDriver]

You must use a Megohmeter and not a standard DVM. I use a Fluke 1587 and it works great. For quick and easy checks I usually do a 1min test(on longer runs I will wait until the readings stabilize and then wait 1 min). I generally like my readings over 100M ohms(almost all circuits are over 1G ohms on new wires), Anything under 20M Ohms and I see issues. Between 20M and 80M I see random issues with GFCI, AFCI, or VFD's depending on the situation. I've seen motors hooked up with no VFD's or GFI's run below 20M ohms with no issues for years. I don't get overly complicated with the math and I have not had any issues. Now if you are trying to do periodic measurements and comparing them to past measurements to see breakdown then you have to compensate for temperature change.

 

[Kornfeld]

Thank you all for this valuable information. I've ordered a megohmeter and will take it from there.

 

[tom baker]

It used to be a megger (trademark, hand crank) were very expensive, most shops did not have one in every service truck. Fluke has a combo dvm-insulation tester that is very affordable. I am not sure of the fluke number it may be the 1587

 

[hillbilly1]

I have a Megger MIT 400/2, not exactly cheap, but way less than what they previously cost. I also have a Bidel hand crank model I bought used about 20 something years ago. I think it was as much as what I gave for the new megger!