5KV Megger Shows Continuous Variation In The Resistance Value.

Humidity?

Are you breathing anywhere near the test set up, or, is it completely isolated in a controlled environment?

It appears the PI value of the test is .90804?

Possible causes:

Loose test connection?

Poor quality or unstable power supply to test set?

Incorrect test voltage value?

Incorrect test procedure or test equipment for application?

Inductance? (Adjacent to HV and/or high current carrying conductors or cables?)

Capacitance? (Long length of cable(s)? Filter or PF capacitors connected?))

Surge arrestors and/or other protective devices still connected in the system when test voltage is applied?

Moisture in cable/equipment?

High resistance carbon fault(s)?

Leaking/failing conductor insulation?

Leaking/failing cable/equipment conductor termination?

Faulty/Failing test equipment/set?

Hope this gives you an idea where to look.

Poor quality or unstable power supply to test set? MAY be

If the power supply to the test set is not stable the readings will not be consistent and will not be dependable. You must stabilize the test set power supply by using a constant voltage source such as a UPS but make sure the ground circuit is intact and functional whatever system is used.

Inductance? (Adjacent to HV and/or high current carrying conductors or cables?) YES

You may have to de-energize the other conductors in the area long enough to conduct the testing.

Leaking/failing conductor insulation? if its the case, dun u think the resistance value should be on the lower side instead of varying continuously.

If you examine your test results the numbers are cycling from low to high then back to low which is indicative of arcing occurring through weak insulation, water, or carbon left over from a fault. Improper terminations can also display this same pattern.

Leaking/failing cable/equipment conductor termination? May be

Is this new cable(s) or recently repaired cables?

Are the readings the same on all three phase conductors? If so; more than likely the cable terminations are either the wrong type for the cable, or they have been improperly installed.

It is very important that you determine what type of cable you are dealing with, get all pertinent information on the type of terminations and the correct procedure for installing the terminations onto the cable then inspect each and every termination to verify if everything is in order.

I have observed the same test pattern on cables that have semi-conductor layers which were not cut back far enough resulting in break-over arcing during testing and/or operation.

Knowing and following the cable manufacturer's specific instructions is critical to the operation and longevity of the cable(s) as well as the equipment it is attached to.

Is it old cable that has been in service for a long period of time and suddenly failed?

If so; You more than likely have water impingement (wet cable interior) and/or have suffered a fault which has left enough carbon at the fault site(s) to allow arcing to occur during testing.

Thanks for providing the valuable inputs. i will definitely take these into consideration. one more thing i want to confirm is the PI test is meant for motors and generators and not for cables. here we are testing MV (11KV) cable before going for Hi-Pot test to ensure everything is okay.

It was not specified, but if the device under test is a stator, it started out wet and is drying, you will get fluctuations as it dries.

Another question: Does the time represent a momentary test or the duration of each consecutive test?

With resistance this high, a very tiny bit of leakage results in a large numerical variation. (10 GΩ in parallel with 35 GΩ gives 8 GΩ). So I think anything in the environment could give you this much variation. Is there any historical data you can look at? What is your requirement?

I would say off hand a bad connection, high moisture, or varying temperature, but I would go by the lowest reading...I would review the testing procedure and check for proper methods first though....

https://www.instrumart.com/assets/Megger-insulationtester.pdf

5KV Megger measurement is sensitive even to small environmental variations and usable ONLY on equipment rated for this testing. Now assuming that this is your case, is your Megger instrument AND cabling verified to be in good working order, not to mention clean and dry as it should, and its batts in good shape? Because some 8:1 resistance ratio is a little odd, and no matter how bad some connection is or someone is breathing on it, it cannot produce that result, at this voltage, without you hearing the produced... sparks that is. S.M.

My first thought, as someone else here, is that you are breathing too close to the equipment......or someone is!!

My second thought is why worry? that is spectacularly high insulation values....!!!

In my day, seeing even 100 Meg Ohms would have been considered wonderful!!

I have to admit that I have never had a device (Megger) that could indicate even a single "GΩ" acurately, let alone the values you are looking at!!!

....don't times change!!!

I have never, even see a 5KV Megger by the way......

They've been available (in hand-held models) for many years now, from Megger Corp. and other good test instrument manufacturers (AEMC, etc.). 10kVDC and 15kVDC meggers are not unusual these days. Many of them run off a handful of AA cells, and will give you several years of test usage from that set of batteries. Here are some example links:

http://megger.com/us/products/ProductDetailsBySubGroup.php?PSGC=I105&BS=I

http://www.aemc.com/

http://www.fluke.com/Fluke/tten/Insulation-Testers/Fluke-1555-1550C.htm?PID=69212

5kV is not a big problem but G-Ohms sure is of problem in measurement. Measurement cable insulation may be much lower.

The thing that is causing the phenomenon is something called "worry". So stop doing that immediately.

Artefact in measurement is often like a discovery only to collapse one day and end up in worry.