Rectifier Megger Value 1

Possibly differences in how the instruments behave when presented with a low resistance. Sounds like you either have an output capacitor or a long cable, or both.

thanks scottyUK, yah we have some capacitors inside for the DC output filterring, but is it ok to energize this rectifier even though it had a zero insulation,we phically investigate this matter before energization and found out nothing for this zero meg. Thanks alot scottyUK

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 rectfier or a drive because the readings are usually misleading and it is fairly easy to damage semiconductors.

A Megger creates more confusion than it solves problem as soon as you leave simple installations and apparatus. If you suspect a failure in a rectifier or its connected load, then divide and conquer. Have you separated that third rail and checked it separately? That is most likely where most of the bad insulation is. If you get zig Megoms on the third rail, then check the rectifier. Use a DMM first, that won't kill anything.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.