Megging romex. What values am I looking for?

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bradyman1

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I recently purchased a new fluke 1507. I have a job for a residence that had a small fire. I have been requested to megger the wiring to verify its integrity. Can anyone give me any direction as to what voltage I should run my tests at and what values I am looking for? Do the different types of home wiring (I.e. Bx, cloth covered romex, modern romex, etc.) have different pass/fail values?
 
At 1000 volts, I think 11 megohms is the top end of the scale. If you hit that (which is easy to do), you're golden.

I meg new installs starting at 100 volts. If things are good, the 1507 should red-line fairly fast unless it's a long run. If it passes at 100 volts, I move up to 250, 500 then 1000 volts.

Existing wiring I start at 50 volts. This way, if there's anything still attached to the circuit (lamp, radio, doorbell transformer etc.) it will make its presence obvious by the circuit failing. At 50 volts you won't burn it out.
 
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480sparky said:
At 1000 volts, I think 11 megaohms is the top end of the scale. If you hit that (which is easy to do), you're golden.

I meg new installs starting at 100 volts. If things are good, the 1507 should red-line fairly fast unless it's a long run. If it passes at 100 volts, I move up to 250, 500 then 1000 volts.

Existing wiring I start at 50 volts. This way, if there's anything still attached to the circuit (lamp, radio, doorbell transformer etc.) it will make its presence obvious by the circuit failing. At 50 volts you won't burn it out.
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Gigaohms.

I do it the same way as 480 does. Start at 50v and crank up the voltage in steps if it tests out okay. If you get a 0.0 megaohms right off the bat you most likely have something still connected, light bulb, appliance, etc. Look for things to unplug or disconnect. I'd remove GFCI's and smokes from circuits too just to be cautious and wirenut the wires together to keep the circuit passing through.

I'm told testing H-G or N-G won't ever fry anything but as you soon as you go H-N with your megger cranked up, you'll smoke whatever was still connected. I have the 1507 also and being able to test at low voltages first is a HUGE help when doing this kind of stuff where you don't know what still might be hooked up.
 
480sparky said:
At 1000 volts, I think 11 megohms is the top end of the scale. If you hit that (which is easy to do), you're golden.

I meg new installs starting at 100 volts. If things are good, the 1507 should red-line fairly fast unless it's a long run. If it passes at 100 volts, I move up to 250, 500 then 1000 volts.

Existing wiring I start at 50 volts. This way, if there's anything still attached to the circuit (lamp, radio, doorbell transformer etc.) it will make its presence obvious by the circuit failing. At 50 volts you won't burn it out.
Click to expand...


In my opinion, you shouldn't megger wire rated at 600V with a 1000 volt megger. Since you are talking about romex, and the maximum voltage is 240 the any scale over 250 is unnecessary. Many meggers only have a 50 and a 500V setting, then use the 500, but never the 1,000.

I used to work on nuclear submarines. The Spec for meggering wires for 120 volt, 240 volt and 480 volt systems was, using the 500 volt scale, 100,000 ohms or more between conductors and/or to ground. I never saw one less than 1 megohm that wasn't just plain failed.

I am pretty sure that guideline would be safe for a house since it is good enough for a 688 class submarine.
 
Since mergers can use DC how would that affect say a dimmer still connected to the circuit and hitting it with 500 vdc?
 
infinity said:
Since mergers can use DC how would that affect say a dimmer still connected to the circuit and hitting it with 500 vdc?
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I would add to my statement that you never megger wires that are hooked up to anything. I guess it would be OK to megger through a breaker, but I wouldn't. Never a load.
 
Strathead said:
I would add to my statement that you never megger wires that are hooked up to anything. I guess it would be OK to megger through a breaker, but I wouldn't. Never a load.
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As long as the applied voltage is equal to or less than the loads' nominal voltage, what damage can be done?

I routinely meg out existing circuits starting at 50 volts to root out loads that are still attached. Hacks and homeowners are notorious for hiding things
 
GoldDigger said:
Certainly not an AFCI or GFCI breaker. Not sure about a simple shunt trip, but any electronic trip would be a no-no.
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why is that? the electronics associated with these breakers are completely isolated from the power contacts. the worst i would expect would be that they might trip, unless the voltage used exceeded the contact rating in the GFCI/AFCI. Stick to 100V for testing such circuits and it should not matter one bit.
 
petersonra said:
why is that? the electronics associated with these breakers are completely isolated from the power contacts. the worst i would expect would be that they might trip, unless the voltage used exceeded the contact rating in the GFCI/AFCI. Stick to 100V for testing such circuits and it should not matter one bit.
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The GFCI only measures current via non contact current transformer. But the electronics that process that signal get their power from the AC line (via resistor/capacitor/diode network, not through a transformer.) The relay trip of the contacts is isolated from those contacts, but once again the control power comes from the line.
Show me a breaker with a separate control power input that you can disconnect and I will agree with you on that one.
Now as long as your Megger voltage is no higher than the line voltage, no problem.
 
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Cow said:
Gigaohms.

I do it the same way as 480 does. Start at 50v and crank up the voltage in steps if it tests out okay. If you get a 0.0 megaohms right off the bat you most likely have something still connected, light bulb, appliance, etc. Look for things to unplug or disconnect. I'd remove GFCI's and smokes from circuits too just to be cautious and wirenut the wires together to keep the circuit passing through.

I'm told testing H-G or N-G won't ever fry anything but as you soon as you go H-N with your megger cranked up, you'll smoke whatever was still connected. I have the 1507 also and being able to test at low voltages first is a HUGE help when doing this kind of stuff where you don't know what still might be hooked up.
Click to expand...


My little brain says, not true. If you actually have a short to ground on the neutral for example and it is after the load, the the megger will travel through the load back to ground hand has every chance to fry it.
 
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Strathead said:
In my opinion, you shouldn't megger wire rated at 600V with a 1000 volt megger. Since you are talking about romex, and the maximum voltage is 240 the any scale over 250 is unnecessary. Many meggers only have a 50 and a 500V setting, then use the 500, but never the 1,000.

I used to work on nuclear submarines. The Spec for meggering wires for 120 volt, 240 volt and 480 volt systems was, using the 500 volt scale, 100,000 ohms or more between conductors and/or to ground. I never saw one less than 1 megohm that wasn't just plain failed.

I am pretty sure that guideline would be safe for a house since it is good enough for a 688 class submarine.
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Please point me to an article stating I should only megger Romex at 500vdc. I regularly megger 480v motors and thhn at 1000vdc. With Romex rated at 600v just like thhn why would I test it any differently just because it only carries 120v or 240v in a dwelling? Obviously you can find problems at 1000vdc that don't seem like problems at 500vdc. I would rather err on the side of caution and test at high voltages than simply test at 500vdc because in your opinion that's "good enough". But, to each their own...
 
Strathead said:
In my opinion, you shouldn't megger wire rated at 600V with a 1000 volt megger. Since you are talking about romex, and the maximum voltage is 240 the any scale over 250 is unnecessary. Many meggers only have a 50 and a 500V setting, then use the 500, but never the 1,000.

I used to work on nuclear submarines. The Spec for meggering wires for 120 volt, 240 volt and 480 volt systems was, using the 500 volt scale, 100,000 ohms or more between conductors and/or to ground. I never saw one less than 1 megohm that wasn't just plain failed.

I am pretty sure that guideline would be safe for a house since it is good enough for a 688 class submarine.
Click to expand...

I guess that's where "good enough for the government work" saying came from :)
My hat off to you, bud. Thank you for serving our country.
 
Cow said:
Please point me to an article stating I should only megger Romex at 500vdc. I regularly megger 480v motors and thhn at 1000vdc. With Romex rated at 600v just like thhn why would I test it any differently just because it only carries 120v or 240v in a dwelling? Obviously you can find problems at 1000vdc that don't seem like problems at 500vdc. I would rather err on the side of caution and test at high voltages than simply test at 500vdc because in your opinion that's "good enough". But, to each their own...
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I would love to see a Mike Holt video on megging, the kind that he made when he drove a 50-ft ground rod in and measured the resistance with each 10 foot increment.
I too would be very curious to see what latent issues you can find at, say, 1000 vs 500 volts.
 
Cow said:
Please point me to an article stating I should only megger Romex at 500vdc. I regularly megger 480v motors and thhn at 1000vdc. With Romex rated at 600v just like thhn why would I test it any differently just because it only carries 120v or 240v in a dwelling? Obviously you can find problems at 1000vdc that don't seem like problems at 500vdc. I would rather err on the side of caution and test at high voltages than simply test at 500vdc because in your opinion that's "good enough". But, to each their own...
Click to expand...

I suggest you use common sense. Romex insulation is rated at 600V A 600 volt insulation that shows a problem at 1000 volts doesn't necessarily have a problem. Well that is until you broke down the insulation by injecting 1000 volts in to it. You shouldn't regularly test a 480 volt motor with a 1000 volt megger either.
 
Strathead said:
In my opinion, you shouldn't megger wire rated at 600V with a 1000 volt megger. Since you are talking about romex, and the maximum voltage is 240 the any scale over 250 is unnecessary. Many meggers only have a 50 and a 500V setting, then use the 500, but never the 1,000.

I used to work on nuclear submarines. The Spec for meggering wires for 120 volt, 240 volt and 480 volt systems was, using the 500 volt scale, 100,000 ohms or more between conductors and/or to ground. I never saw one less than 1 megohm that wasn't just plain failed.

I am pretty sure that guideline would be safe for a house since it is good enough for a 688 class submarine.
Click to expand...


But isn't the peak RMS of 600 volts actually closer to 1000?
 
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