We have an issue regarding a rectifier 750DC for 3rd rail traction power for passenger trains. With our new megger unit, we tried to test our rectifier unit. Right away, I got zero Mega omhs reading, and after a minute will slowly have a low value, and after 3 or 5 minutes it continues to increase. But when I use my analog type, it gets a low value of 0.5Mega omh.
We have some capacitors inside for the DC output filtering, but is it OK to energize this rectifier even though it had zero insulation? We physically investigate this matter before energization and found out nothing for this zero meg…
My opinion, formed without a lot of details which would be nice to have, is that the rectifier will be fine to energise. There are very likely other components drawing the output resistance down, perhaps a voltmeter, perhaps leakage through semiconductors, perhaps any of a multitude of other things.
As a more general comment: I would generally try to avoid using a megger on the output of a semiconductor rectifier or a drive because the readings are usually misleading and it is fairly easy to damage semiconductors.
Above is a snippet.