Insulation Resistance Measurements

PP91:

 

 

Yes, read about "dielectric absorption". So we waited to measure the time taken to reach a high value. But the issue is that the time it is taking to reach a high value is in the range of 10-20mins approx for each channel. This is producing a huge overhead. 

There is far more I don’t know about this subject than I do, so I have more questions than answers.


If the insulation test is run for twenty minutes, how long should be allowed for the discharge?


Andy Betteridge 

mapj1:

At 1000V you will be seeing all the effects insulation can throw at you in everything, the PCBs,  the relays, perhaps the flux used to solder the PCBs, and any finger prints on the exposed ends of wires -  everything will matter at this level of resistance - to get the best part of 10 gig-ohms, you are asking for  something like 100nA or less to flow, now that  that is a mere 10^12 electrons per second. - what area of insulation are you stressing -  each atom is ~ 250pico metres radius, or if you prefer to think of it as atoms, then each square mm of insulation to live metal interface exposes about  16 million million atoms, it is not reasonable over many hundreds of square mm of contact area between insulation and  live metal not to expect a detectable percentage of the atoms to have either a free electron (steady leakage current ) or a slightly wobbly one (slow polarisation and dielectric recovery)


If you really do need to measure giga-ohms on the test specimens, then you need to be gapping the insulation to give longest possible creepage paths and large clearances to keep the E field down, cleaning everything till it gleams, baking it to drive of moisture and then encapsulating as much as possible in HV laquer, or alternatively after cleaning and bake off, doing the whole test in vacuum. 

Can you test at lower voltage ? - if the resistance looks higher , you may also be suffering the onset of corona discharge to the air. This can be tracked down, either using a MW radio, or a UV detector. (or just hopefully polishing off all surfaces that are not smooth - cut wire ends make quite good needle electrodes.)


But before all that, as others have said, check if you really do need to measure 10 gigohms, or if a lower pass limit can be set - a few million ohms is normally OK for a  a wiring installation, and much easier to handle.

Thanks for the Reply.


This gave me some perspective. Thanks for the explanation. I understand some of the things to be taken care of written here and already have been implemented. Since there are many things which affect these high IR values, wanted to get some info which can be improved. Lower voltages(100V) has also been tested, which gives rise to similar issues, the ramp up time is comparatively less here.

Sparkingchip:

PP91:

 

 

Yes, read about "dielectric absorption". So we waited to measure the time taken to reach a high value. But the issue is that the time it is taking to reach a high value is in the range of 10-20mins approx for each channel. This is producing a huge overhead. 

There is far more I don’t know about this subject than I do, so I have more questions than answers.


If the insulation test is run for twenty minutes, how long should be allowed for the discharge?


Andy Betteridge 

Thanks for the reply.


For the discharge time, we are following the one mentioned in the megger pdf i.e 4 times the charge time.

mapj1:

The reverse effect depends on the mechanism - there may be no stored charge - if it was mostly caused by current flowing to drive off damp then although you put charges through the system to do that, you do not get them back however long you wait, as that is essentially a resistive effect. However, the low resistance will be back in a few hours of days if you do not control the environmental conditions.

If it is dielectric absorption and recovery (loose but not free charges) then the recovery time is similar to the the charge up time.

I;d also question how the parts are going to be used after testing - if 100nA really matters, they will pass on the day, but may well fail after a few days in storage.