Fluctuating Insulation Resistance

[stilllearning]

Hi folks

I was doing and Insulation Resistance check on a ring circuit at home today and got the following result: P-N >299MOhm
N-E > 299MOhm

All was fine until I tested between Phase and Earth. The initial reading was 185MOhm this then increased up to about 275MOhms.

I split the ring at one of the sockets and got a reading of >299MOhms on one of the legs but the other leg still gave the same result i.e started to settle at 185MOhms and then increased to about 280MOhms. I then tested at the sockets on this part of the leg; the one closest to the CU gave a similar reading but as I tested the sockets that were progressively farther away I got a lower reading. The final socket on this part of the leg gave an initial reading of about 135Mohm and finally settled on about 160Mohms. The results between Phase and Neutral and between Neutral and Earth were not affected.

I’m retraining to be a sparky at present and I’m not sure about interpreting the results of this test. Even though the measurements are in the acceptable limits I don’t understand why the results should fluctuate on the same ring and why the reading increases if I leave the test running. Any ideas??

Thanks

 

[Lectrician]

Good old capacitance

I wouldn't blink an eye with those values.

 

[stilllearning]

Thanks a lot for the quick reply Lectrician.

As I said, I'm new to this so I just wanted to be sure.

Cheers

 

[Adam_151]

Just be glad most modern IR testers discharge it automatically

Otherwise you could test a long run of cabling and then touch it and let go sharpish

 

[securespark]

Either way, both lower readings are well above.

 

[stilllearning]

Thanks for the replies everyone.

 

[jimbob100_99]

Above 2MOhm your ok Above 100MOhm your in heaven!!!!