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Megger tests: breakdown voltage puzzle
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SG-1:
When testing between two wires or one wire & earth, the wire has some small amount of capacitance.  Displacement current through the test sample will be continuous with AC. It will stop with the DC when the voltage becomes stable, after ramping up.  The AC is always transferring some amount of power through the test sample. If enough power is transferred carbonization occurs & the sample fails. 
taydin:

--- Quote from: SG-1 on November 02, 2019, 09:01:14 pm ---What are you seeing that makes you think the resistance is changing with voltage ?

--- End quote ---

I did lots of measurements on various items. The resistance increases along with 500V, 1 kV, 2.5 kV test voltages. But at 5 kV seems to break this trend and the resistance is lower.

I remember reading about this somewhere in an application note, but can't find it now  :(
floobydust:
A look at IEC standard for insulation 60664,

6.1.3.6 DC voltage test
The d.c. voltage test with a test voltage equal to the peak value of the a.c. voltage is not fully equivalent to the a.c. voltage test due to the different withstand characteristics of solid insulation for these types of voltages. However in case of a pure d.c. voltage stress, the d.c. voltage test is appropriate.

"... With regard to creepage distances, the time under voltage stress influences the number of occasions when drying out can result in surface scintillations with energy high enough to entail tracking.
"... With regard to tracking, an insulating material can be roughly characterized according to the damage it suffers from the concentrated release of energy during scintillations when a surface leakage current is interrupted due to the drying-out of the contaminated surface."

I guess you'd have to do more research on scintillation to understand what is going on.
SG-1:

--- Quote from: taydin on November 02, 2019, 10:49:59 pm ---
--- Quote from: SG-1 on November 02, 2019, 09:01:14 pm ---What are you seeing that makes you think the resistance is changing with voltage ?

--- End quote ---

I did lots of measurements on various items. The resistance increases along with 500V, 1 kV, 2.5 kV test voltages. But at 5 kV seems to break this trend and the resistance is lower.

I remember reading about this somewhere in an application note, but can't find it now  :(

--- End quote ---

I see a similar trend with the Fluke 1507, but it is just that the lower voltage tests have a lower end scale or max reading.
SG-1:
Scintillation seems to be partial discharge or corona.
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