Insulation tests

[JJME]

Hello all!

At my work we routinely do insulation tests on the feeders that are run to panelboards, large equipment such as chillers, etc.

We take as a baseline for our own quality control that a cable must have an insulation above 100 MOhm to by compliant. However, is there a problem when the results between two or more readings are different? I don't expect all the results to be exactly the same, but I also don't expect them to be too different.

For example, the insulation test results for a three phase cabling system (A, B and C) I'm looking at are these. The neutral is 'N' and ground is 'T'.

AB: 4,6 GigaOhm BC: 6,5 GigaOhm CA: 5,1 GigaOhm
AN: 4,3 GigaOhm BN: 9 GigaOhm CN: 5,2 GigaOhm
AT: 1,3 GigaOhm BT: 2,9 GigaOhm CT: 3,7 GigaOhm
NT: 3,4 GigaOhm

Look at, for example, BN... it is very different from AN and CN. Also, CT differs a lot from AT.

So the question is: is there any standard, code, whatever that says how much can these results differ from one another? If there is no standard, maybe a rule of thumb for good practice?

Thanks in advanced.

Regards!

 

[synchro]

For example, the insulation test results for a three phase cabling system (A, B and C) I'm looking at are these. The neutral is 'N' and ground is 'T'.

AB: 4,6 GigaOhm BC: 6,5 GigaOhm CA: 5,1 GigaOhm
AN: 4,3 GigaOhm BN: 9 GigaOhm CN: 5,2 GigaOhm
AT: 1,3 GigaOhm BT: 2,9 GigaOhm CT: 3,7 GigaOhm
NT: 3,4 GigaOhm

Look at, for example, BN... it is very different from AN and CN. Also, CT differs a lot from AT.

So the question is: is there any standard, code, whatever that says how much can these results differ from one another? If there is no standard, maybe a rule of thumb for good practice?

There should be no problem as long as all the measurements exceed your 100 Meg standard, especially by the more than 10X of your example. In fact I'd be suspicious if the measurements all matched each other very closely. For one thing, I suspect that the spacing between the pairs of conductors you cited are probably not all equal to each other. Also all materials have some imperfections, it's just that we have to make sure they are not significant for the application. In particular, the high value of the BN resistance compared to that of AN and CN is a a good thing not a bad thing. Of course if a measurement is at the high limit of the equipment then I'd do it over to make sure that proper connections were made.

 

[JJME]

There should be no problem as long as all the measurements exceed your 100 Meg standard, especially by the more than 10X of your example. In fact I'd be suspicious if the measurements all matched each other very closely. For one thing, I suspect that the spacing between the pairs of conductors you cited are probably not all equal to each other. Also all materials have some imperfections, it's just that we have to make sure they are not significant for the application. In particular, the high value of the BN resistance compared to that of AN and CN is a a good thing not a bad thing. Of course if a measurement is at the high limit of the equipment then I'd do it over to make sure that proper connections were made.

Thank you very much for the input!

 

[paulengr]

Biggest thing to watch out for is whether or not you are actually measuring what you expect. When the values are quite high make sure your ground reading is correct.