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Discussion Starter · #1 ·
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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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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Discussion Starter · #4 ·
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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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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Discussion Starter · #6 ·
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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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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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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Check your math.
I agree.

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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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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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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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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Discussion Starter · #14 ·
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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Discussion Starter · #15 · (Edited)
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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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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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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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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