Generator Test

This brings to mind a question I had about meggering a few years back. I had been told (on a previous job) that the most important thing about meggering was to compare current results with previous ones but then was put on a job where we were to check several motors monthly but different people did it and were given no access to previous readings. Do you think this was a reasonable test? Also I had been told to megger with positive to ground but others used negative ground. I found that I got different values + than -.

One reason you may get different values for (+) to ground and (-) to ground may be due to some contaminants behaving like a semiconductor relative to the windings. Variances I have seen using insulation resistance (IR) testers have been upwards of 10% difference in readings.

For that matter, just taking the same measurement on a piece of equipment several times in the exact same manner can result in fairly significant differences, especially if the equipment has been in service for a good long time. Just the nature of the beast.

A long history showing trends is much more useful than a single snapshot.