megger reading variation
megger reading variation
(OP)
Copied this from another forum. Someone suggested that a different forum might help....
Ran across something I don't understand today. Had to perform a megger test on an endcap per customer specs. What is being checked is the resistance between a hardcoated bearing bore liner and the endcap. The idea is to limit current leakage through the motor bearings. One megohm or greater is considered good.
Here's the odd part...the test polarity seems to be important. Parts which fail with one polarity pass with the other. Insulation resistance reads much higher when the positive probe is connected to the endcap and the negative is connected to the bearing liner.
How can this be?
Ran across something I don't understand today. Had to perform a megger test on an endcap per customer specs. What is being checked is the resistance between a hardcoated bearing bore liner and the endcap. The idea is to limit current leakage through the motor bearings. One megohm or greater is considered good.
Here's the odd part...the test polarity seems to be important. Parts which fail with one polarity pass with the other. Insulation resistance reads much higher when the positive probe is connected to the endcap and the negative is connected to the bearing liner.
How can this be?





RE: megger reading variation
RE: megger reading variation
RE: megger reading variation
RE: megger reading variation
DanDel's additional current path theory would be good for an AC test at some kilohertz frequency but shouldn't happen at DC. I am assuming this is a simple DC test with a modern plastic cased instrument.
Corrosion forming a rectifying contact, however, is a well known phenomenon. Ever hear of copper oxide rectifiers? Even rusty bolts are also known to rectify.