# Insulation testing



## Paul S. (Sep 8, 2012)

I recently purchased a fluke insulation tester. (1507 is the model number, I think.) I tested some wiring today and I was getting about 550 megohms between the conductors using 500V test voltage. When I used 1000V test voltage I got 11.5 gigaohms. - I have very little experience using an insulation tester; in fact no experience. However, I have done some research and I am continuing to do so here. 
If I am not mistaken, 11.5 gigaohms is equal to 11,500 megohms. Why would I get more resistance with a greater test voltage? I would think it would be less... Am I backwards on this? Maybe I didn't read the tester correctly, I plan on trying it out again tomorrow if I have the time. (I have other work to do though.... I'm kind of swamped with work right now.) Any help on this will be appreciated. Thanks in advance.


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## Expediter (Mar 12, 2014)

With a megger you are testing for a possible high resistance fault. The higher voltage allows you to find out if you have a fault at a higher resistance than the lower setting could allow.

Read this thread for more information http://www.electriciantalk.com/f2/megger-testing-after-fire-72201/


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## Expediter (Mar 12, 2014)

If you use a too high voltage you can actually start to damage the insulation of the conductors you are testing. So careful with the higher voltages if you don't really need it.


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## Paul S. (Sep 8, 2012)

My understanding is that the typical test voltage should be twice the rating of the wire plus 1000 Volts. For standard NMB that would be 13,000V. I only used 1000V. - However, as I said, I am a novice at this and I do think it is wise to use the lower voltages unless I need, for some reason, to use the higher voltages. So I do appreciate your response and I will take that into consideration. Any thoughts on the readings I mentioned?


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## Meadow (Jan 14, 2011)

Paul S. said:


> My understanding is that the typical test voltage should be twice the rating of the wire plus 1000 Volts. For standard NMB that would be 13,000V. I only used 1000V. - However, as I said, I am a novice at this and I do think it is wise to use the lower voltages unless I need, for some reason, to use the higher voltages. So I do appreciate your response and I will take that into consideration. Any thoughts on the readings I mentioned?


I wouldn't go beyond 1000 volts on a 600 volt av rated conductor. No reason to either.


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## Paul S. (Sep 8, 2012)

I guess my main question here is why would I get a higher resistance reading using a higher voltage? It seems to me that at a higher voltage I would read less resistance not more.


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## chewy (May 9, 2010)

We have to test at a minimim of 500v for circuits operating up to 250v between phase and earth and 1000v between phase and earth for circuits above 250v. Thats required by the regulations for us. 

We also cant be less than 1 megohm for an installation and 20 megohms for a cable if that gives you a guideline...


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## bobelectric (Feb 24, 2007)

Check your math.


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## hardworkingstiff (Jan 22, 2007)

bobelectric said:


> Check your math.


What math? He read the meter at two different voltages and got a megaohm reading on one and a gigaohm reading on another. He is correct in that 11.5 gigaohms equals 11500 mega ohms so I'm confused on what math he should check.

Could you elaborate some please?


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## EBFD6 (Aug 17, 2008)

bobelectric said:


> Check your math.


I agree.



hardworkingstiff said:


> What math? He read the meter at two different voltages and got a megaohm reading on one and a gigaohm reading on another. He is correct in that 11.5 gigaohms equals 11500 mega ohms so I'm confused on what math he should check.
> 
> Could you elaborate some please?


The Range of the Fluke 1507 according to the Fluke website is:
*Insulation Test Range: 0.01MΩ to 10GΩ

*So getting a reading of 11.5G seems a little suspect. I think it may be possible that the op is misreading the meter display. Either way, something is not right. You don't get higher resistance with a higher voltage when using a megger. It will always be the other way around. I suspect operator error.


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## EBFD6 (Aug 17, 2008)

chewy said:


> We have to test at a minimim of 500v for circuits operating up to 250v between phase and earth and 1000v between phase and earth for circuits above 250v. Thats required by the regulations for us.
> 
> We also cant be less than 1 megohm for an installation and 20 megohms for a cable if that gives you a guideline...


You Meg from cable to earth (ground) and energize cables that are only 20 meg-ohms?

I would never energize a cable with that low of a resistance reading to ground. You should be looking at >400 meg-ohms minimum, IMO. Anything less, and there is a problem. Actually, if I get anything less than 1 Gig I'm a little nervous throwing the switch.


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## Bad Electrician (May 20, 2014)

I know the full scale on the 1507 is 11.0 Gigohm and I just laid an eyeball on the meter to verify and full scale at 500 VDC is 550 Megohm

You reading should go down as the voltage goes up assuming you have not changed anything in the circuit. But you are at full scale for the meter or as we use say infinity.


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## AllWIRES (Apr 10, 2014)

Paul S. said:


> I recently purchased a fluke insulation tester. (1507 is the model number, I think.) I tested some wiring today and I was getting about 550 megohms between the conductors using 500V test voltage. When I used 1000V test voltage I got 11.5 gigaohms. - I have very little experience using an insulation tester; in fact no experience. However, I have done some research and I am continuing to do so here. If I am not mistaken, 11.5 gigaohms is equal to 11,500 megohms. Why would I get more resistance with a greater test voltage? I would think it would be less... Am I backwards on this? Maybe I didn't read the tester correctly, I plan on trying it out again tomorrow if I have the time. (I have other work to do though.... I'm kind of swamped with work right now.) Any help on this will be appreciated. Thanks in advance.


The max value changes for the voltage applied on that meter. Look for the > sign. Both those numbers should have it on the screen. It just means you've gone past the max reading.


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## Paul S. (Sep 8, 2012)

I plan on using the tester again today using some different scenarios. I think I'm just going to do some experimenting; maybe testing a piece of romex with a staple through it, some things like that. Thank you for your replies. I will check the display again.


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## Paul S. (Sep 8, 2012)

I plan on using the tester again today using some different scenarios. I think I'm just going to do some experimenting; maybe testing a piece of romex with a staple through it, some things like that. Thank you for your replies! I will check the display again. - Also when using your insulation tester would you disconnect the grounds to make sure you do not send any voltage back to the panel? When I used the insulation tester yesterday I disconnected everything and then measured between each of the conductors.


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## AllWIRES (Apr 10, 2014)

Find a motor to play with. Even a ceiling fan would work, then you can flip through the three switch positions too. I was always told to test at twice the operating voltage.

You should always isolate before using a megger.


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## Cow (Jan 16, 2008)

Leave the grounds connected, it's fine.

I also have Fluke 1507, it's served me well that last few years.


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## Expediter (Mar 12, 2014)

Paul S. said:


> My understanding is that the typical test voltage should be twice the rating of the wire plus 1000 Volts. For standard NMB that would be 13,000V. I only used 1000V. - However, as I said, I am a novice at this and I do think it is wise to use the lower voltages unless I need, for some reason, to use the higher voltages. So I do appreciate your response and I will take that into consideration. Any thoughts on the readings I mentioned?


No, it is not twice the voltage PLUS 1000v. It is only up to twice the voltage. I didn't have time last night to elaborate and there are many threads here about megging.
You are correct to use the lowest setting for your situation. It sounds as if there is no problem whatsoever with the piece of cable you tested. 

However, I have made the mistake in the past of leaving the red lead in the lower resistance jack on the meter the one that only tests to 22k ohms. So when I tested for a higher resistance, It read open, even when I knew it should have some type of lower resistance reading. 

I don't know the exact meter you have, but mine has a jack for lower resistance using only the 9v battery power not the 50v+ the other jack has available. I think I have a 1587.


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## Bad Electrician (May 20, 2014)

Start low work to a higher voltage by starting at the lowest voltage you will avoid frying any equipment that may be connected to the conductors under test.

1000 VDC 

Get a soft pencil and some paper and make 1,000,001 scratches and Megger that in various places.

For the record you have an Insulation Resistance Tester I do not believe Fluke ever calls it a Megger a Trademarked name of one of their competitors.


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## Bad Electrician (May 20, 2014)

Tried to post the video but it isn't working.
http://i159.photobucket.com/albums/t127/brianjohn2580/00022_zps5591fc21.mp4


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## Wirenuting (Sep 12, 2010)

Bad Electrician said:


> Tried to post the video but it isn't working. http://i159.photobucket.com/albums/t127/brianjohn2580/00022_zps5591fc21.mp4


Post it on you tube and link it here.


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## Zog (Apr 15, 2009)

Bad Electrician said:


> Tried to post the video but it isn't working.


We have seen it before :whistling2:


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## Paul S. (Sep 8, 2012)

Thanks for all the posts! I just contacted Fluke tech support too. - After reading the posts here, tech support, and checking the manual I have a much better understanding on how to use this tester. Again, I appreciate all the posts!


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## Big John (May 23, 2010)

For 600V class power equipment, the standard test voltage is 1kVDC, and that includes basically all building wire. That said, _do not_ assume that this applies to everything connected to 600V class equipment. 

We recently saw that mistake where a guy hooked up to a switchboard full of digital metering. Yeah, 480V electronics ain't a fan of 1000V.


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## Jlarson (Jun 28, 2009)

Big John said:


> For 600V class power equipment, the standard test voltage is 1kVDC, and that includes basically all building wire. That said, _do not_ assume that this applies to everything connected to 600V class equipment.
> 
> We recently saw that mistake where a guy hooked up to a switchboard full of digital metering. Yeah, 480V electronics ain't a fan of 1000V.


I asked a guy that was meggering all the newly installed I/O into a PLC cabinet for commissioning tests, "did you reconnect everything to the PLC when you were done?" all I got was this look :blink:, I knew it was gonna be bad :laughing:


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## Big John (May 23, 2010)

Jlarson said:


> ...All I got was this look :blink:....


 Did you know that look makes a noise? It really does, I've heard it. Every time someone makes that face at me on a job I just hear this loud "GOD DAMMIT!" Everywhere I go, it's the same, and I've never figured out where that noise is coming from.


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## Jhellwig (Jun 18, 2014)

Paul S. said:


> I guess my main question here is why would I get a higher resistance reading using a higher voltage? It seems to me that at a higher voltage I would read less resistance not more.


You aren't getting a higher resistance. That it just the top of the scale for those two voltages. It is just how the meter is. There is nothing wrong with the wire or the meter.

If you look next to the number you will see >. It just means that the reading it greater than whatever the number is instead if saying infinity.


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## Jlarson (Jun 28, 2009)

Big John said:


> Did you know that look makes a noise? It really does, I've heard it. Every time someone makes that face at me on a job I just hear this loud "GOD DAMMIT!" Everywhere I go, it's the same, and I've never figured out where that noise is coming from.


:laughing:

I've also hear "you dumb mother ****er" a few times its really weird, must be the wind.


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