Insulation Resistance Testing

[Mr. Spinosa]

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.

 

[meternerd]

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.

11.5 G is the full scale of the Fluke at 1000V. Notice that 550M is half of 11.5G. Full scale at 500V. You need to hold down the test button until the reading settles out at the lowest value. 1000V scale is the best test unless the equipment you are testing can't take that voltage. If you let go of the button before the reading stops dropping, you have garbage.

 

[Mr. Spinosa]

Thanks for your reply. I have done some reading and I contacted a Fluke tech support guy and had a real good talk with him. He helped me out a great deal. - I also looked more closely at the manual and found out that what you are saying is correct. I didn't understand the equipment I was using. At any rate, I understand it much better now than I did when I first started here. - I got infinite readings on all the wires I tested but knowing not to let go of the test button until the reading stops dropping is definitely a nugget I hope I can keep in my thick skull. Thanks again!