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Insulation Resistance -old requirement

Ismael Ochoa
I have just come across a insulation resistance (IR) test requirement stating that the IR  R2/R1 > 0.7 where R1 is the IR before a Dielectric test ( 2000VAc for 1 min ) and R2 is the IR after the Dielectric test. Has anybody seen this requirement before?, if so what are the technical basis for this? Nowadays, standard only requests that IR should greater than certain value which depends on the equipment rated voltage.
Comments will be appreciated

I
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    Do you have more information about the type of equipment this test was for ? With modern solid plastic insulations the results are so near open circuit that the results are dominated by contamination or surface tracking. I am aware that certain types of capacitor insulation, such as oily paper, actually change during a hi pot test, as the application of the test voltages tends to pull the conductors together and oil gets squeezed a bit,  but then this  ' self heals' later. I have seen specs with different hot and cold limits, and if the hi pot test was DC, I;d be wondering about dielectric recovery, but this is not it.
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    Sounds like it's checking that the HiPot (2000V) test hasn't damaged the insulation (too  much). HiPot tests usually aren't done during routine testing (because they can stress otherwise sound insulation) so it sounds like a one-off manufacturing test, or possibly even a type test on a sacrificial unit.

      - Andy.
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    Are you sure that you're not getting confused with the Polarisation Index?  PI is the ratio of IR after 1 minute with IR after 10 minutes.  So R1/R10.  It is not an old requirement, it is still widely carried out on motors and generators.
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    John Russell:

    Are you sure that you're not getting confused with the Polarisation Index?  PI is the ratio of IR after 1 minute with IR after 10 minutes.  So R1/R10.  It is not an old requirement, it is still widely carried out on motors and generators.




    John, that is interesting and helpful. (I have Googled PI so I now know what it is.) My question is, however, can the PI test be done with yer ordinary Megger, or does it require different equipment?

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    Chris Pearson:



     My question is, however, can the PI test be done with yer ordinary Megger, or does it require different equipment?



    Ordinary Megger, though I would suggest not one of the (now antique) hand wound ones that I remember from my youth........

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    Here's an interesting read I found on LinkedIn.....seemed to explain the process of of comparing insulation values for contiued use in rotating machines.

    https://www.linkedin.com/pulse/power-generator-motor-ir-pi-test-importance-mehdi-arab/


    Legh
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    Alasdair Anderson:




    Chris Pearson:



     My question is, however, can the PI test be done with yer ordinary Megger, or does it require different equipment?



    Ordinary Megger, though I would suggest not one of the (now antique) hand wound ones that I remember from my youth.......



    Saves going to the gym!

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    I tested a motor today. IR > 999 MΩ pretty quickly and maintained over 10 minutes, so it seems to be a pass.


    If I have understood this correctly, the meter must apply 500 V (or whatever) for the 10 minute period. On the first attempt, my MFT switched itself off, so one has to ensure manually that test voltage is maintained every three or four minutes in order to apply a steady potential.


    I started with fully charged batteries, but they needed to be recharged again afterwards. Where did the energy go?


    Above all, did I perform the test correctly?

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