Megger testing technique questions

[michaellabeit]

I listened in on a Megger webinar on insulation resistance testing by Jo Jewett, one of Megger's senior engineers. He said that the polarity doesn't matter for IR tests. Moreover, Megger's A Guide To Diagnostic Insulation Testing Above 1 kV states the following on page 6:


"With modern insulating materials there is little, if any, difference in the reading obtained, regardless of which way the terminals are connected. However, on older insulation, a little known phenomenon called electroendosmosis causes the lower reading to be obtained with the positive terminal connected to the grounded side of the insulation being tested."

[bob152637485]

When performing insulation resistance on equipment, does it matter which lead on a megger goes where? If I’m testing insulation resistance from A to B, does it matter if the Red lead goes on A or B?

I have personally used a Megger in both polarities just to test that, and from my experience I have found that there is no difference in reading. The way a resistance meter works(at least on something like a Fluke)is that it applies a small known voltage across the load, and then measures the current. Voltage times current gives you your resistance. A Megger is doing the same thing, only using a much larger known voltage.

Now, if you test in one direction, then try to test again in the other, it will take some time for you to get the correct reading, since it will now be magnetized in the original direction, and need to be overcome to magnetize in the new direction. I have also seen some insulation testers that have separate voltage and current leads, and on those, I am not sure what reading you would get if the voltage and current polarities were opposite.