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megger reading variation

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jclough

jclough

Industrial
May 25, 2001
83
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?
 
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I have no idea what any of those things are, but the limited info suggests ome sort of diode is being created.
 
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jclough, maybe the megger is responding to ground potential(I'm assuming that the endcap with the bearing is for a motor and it is grounded). Maybe the megger has some reference to ground. If possible, try the test with the endcap isolated from ground.
 
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Hi, most likely explaination is corrosion forming a battery.
 
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I'm with MacGyverS2000 on this one. I don't think cbarn24050's "battery theory" is sound because the electrochemical potential is going to be of the order of a volt and the Megger will be testing at hundreds of volts.

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.
 
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