Help with megger results of NM-B

[crtemp]

I have an inspector that is requiring a megger test on a house that got sheetrocked before it got inspected.
I just need to know if I'm doing this correctly. Here is the tester I'm using

I am disconnecting the hot and neutral from the service panel and connecting the neutral to the black lead and the hot wire to the red lead. The entire circuit is tied through using wirenuts and no devices are installed. When I turn the megger to 500v and push the test button I get a result of >2000 with no change in value no matter how long I hold down the test button. Is this an expected result? Even if I don't have any wires hooked up to the tester and push the test button I get the exact same results. I don't have the instructions for this tester but from what I can gather from Google searches this is the only way I can find to do the test. Is this correct or am I way off on this one?

 

[480sparky]

You can contact Fluke for a copy of the instructions.

Sound like 2000mohms is the maximum it can measure. If so, hold the test for 30-60 seconds. If you still have 2000mohms, you're good.

 

[luckylerado]

2000 is the limit at 500 volts for that tester.

Here is the manual for the 1520.

 

[crtemp]

You can contact Fluke for a copy of the instructions.

Sound like 2000mohms is the maximum it can measure. If so, hold the test for 30-60 seconds. If you still have 2000mohms, you're good.

If I test under 1000v it can measure up to 4000 mohoms. Would this be a better test?

 

[GoldDigger]

If I test under 1000v it can measure up to 4000 mohoms. Would this be a better test?

A reading of 2000 megohms is just fine for your NM-B and should satisfy the inspector that there are no nails driven through the insulation. It will not catch nicks in the insulation or uninsulated connections that are not touching anything at the moment, so I am not sure the inspector is really getting all that much information about what might have been done wrong withing the walls. (Such as splices outside a box, crossed neutrals, bootleg grounds, and more.)
Going to 1000V test voltage on wire with insulation rated for only 600V is asking for trouble.

 

[luckylerado]

If I test under 1000v it can measure up to 4000 mohoms. Would this be a better test?

No. Not necessary. Your wire is 600 volt and your service is Max 240 no doubt.

The inspector is just twerked off. at 500V you are getting a passing reading.

 

[ADub]

Going to 1000V test voltage on wire with insulation rated for only 600V is asking for trouble.

This is news to me?


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[don_resqcapt19]

... The inspector is just twerked off. ....

Not so sure about that....around here if he was twerked off, the drywall would be coming off.

 

[Cow]

I would think you'd need to megger all three ways to do a proper test. H-G, N-G, H-N. If you keep getting 2000MOhms on all tests, you're good to go.

 

[mkgrady]

At what test result would it fail? Is 100 megohms ok? Is 50 megohms ok? How about 5 megohms?

 

[kwired]

A reading of 2000 megohms is just fine for your NM-B and should satisfy the inspector that there are no nails driven through the insulation. It will not catch nicks in the insulation or uninsulated connections that are not touching anything at the moment, so I am not sure the inspector is really getting all that much information about what might have been done wrong withing the walls. (Such as splices outside a box, crossed neutrals, bootleg grounds, and more.)
Going to 1000V test voltage on wire with insulation rated for only 600V is asking for trouble.

I fully agree that there is still a lot that could be wrong that will not cause a fail of the test prescribed.
I also can't believe the inspector wouldn't at least want to witness such test if he is going to prescribe it, otherwise why not feed him with results he will want to see:roll:

I have no problem with testing 600 volt conductor insulation with a 1000 volt test voltage. If you were applying 600 volts RMS to that conductor the peak voltage is going to be about 850 volts. If you had 180 degree displacement (basically 600 volts single phase) between two conductors the difference between the two conductors when voltage is at opposing peaks would be about 1700 volts.

 

[GoldDigger]

@kwired,
The voltage of 1700 between two conductors will have to cross two thicknesses of 600 (850 peak) insulation. No problem.
1000V DC from a Megger from wire to wire is also no problem. But if the insulated ungrounded conductor is touching grounded metal (including a bare EGC) anywhere in the system you will be applying 1000V across only one layer of insulation when you test from ungrounded conductor or neutral to ground.

 

[Strathead]

At what test result would it fail? Is 100 megohms ok? Is 50 megohms ok? How about 5 megohms?

That depends on the standard, however, I used to work for the navy on nuclear submarines and the minimum resistance of 600 volt wiring with a 500 volt megger was only 100,000 ohms. If that is good enough for a nuclear submarine, I suspect it is good enough for what most of us do. I have seen wiring in the area of 3-4 megohms when it was in an underground conduit full of water.

 

[kwired]

@kwired,
The voltage of 1700 between two conductors will have to cross two thicknesses of 600 (850 peak) insulation. No problem.
1000V DC from a Megger from wire to wire is also no problem. But if the insulated ungrounded conductor is touching grounded metal (including a bare EGC) anywhere in the system you will be applying 1000V across only one layer of insulation when you test from ungrounded conductor or neutral to ground.

Understood. I still think 1000V test is not going to hurt it. A 600 volt nominal circuit will likely see 1000 volt or more transients at times.

 

[luckylerado]

I still think 1000V test is not going to hurt it....

Absolutely true. The 1000V test will not harm the 600V wire and in fact is the NETA suggested voltage for testing the wire if the wire is listed as 600V. Testing it as a system IAW NETA standards would allow for a test voltage of 500V for systems of 250V or less.


NETA ATS-09

7.3.2.2.2
Perform insulation-resistance test on each conductor with respect to ground and adjacent
conductors. Applied potential shall be 500 volts dc for 300-volt rated cable and 1000 volts dc
for 600-volt rated cable. Test duration shall be one minute.

T 100.1

 

[ptonsparky]

Megged from workbench drywall screw to footing rebar.
Add a little water.
Add jumper from EG to work bench.
Zoom in on meter.

What is a safe reading for megging romex in new construction? Old?
Did the Inspectors request prove anything?

 

[macmikeman]

What code section is your inspector using to demand such testing from you?

 

[Strathead]

What code section is your inspector using to demand such testing from you?

Why would you even want to go there? Someone put him in the position of getting an inspection on wiring that is covered up. In my area, the inspector would be fully within his rights to refuse the inspection and require the drywall to be torn off. Shoot they won't even allow one-siding in the City limits. When all the inspector asks for is meggering, you had best say, "Yes sir!" and thank your lucky stars! Maybe even buy him a cup of coffee.

 

[iwire]

What code section is your inspector using to demand such testing from you?

The 'do this or you can pull all the drywall back down' code

 

[luckylerado]

Why would you even want to go there? Someone put him in the position of getting an inspection on wiring that is covered up. In my area, the inspector would be fully within his rights to refuse the inspection and require the drywall to be torn off. Shoot they won't even allow one-siding in the City limits. When all the inspector asks for is meggering, you had best say, "Yes sir!" and thank your lucky stars! Maybe even buy him a cup of coffee.

Agreed. Count your blessings, This could have been a whole lot more expensive.