480 volt motors I test from T lead to ground, then between T leads. Testing at 1000v with my Fluke 1507. I shoot for at least 50 megs from T-Gr and 0 megs between T leads. I have found one motor that showed open(infinite resistance) between T leads, that's why I check.
My method isn't scientific at all, I don't do any timed tests or any DAR/PI functions. I simply start testing, if the resistance starts climbing I'll watch it for 10-15 secs or so. If it starts dropping, I'll usually watch it until it stops.
Usually when I'm called out it's because a breaker/overload is tripping. Almost all of the motors I've tested have tested pretty good to complete junk. There hasn't been a whole lot of in between, so it's usually been pretty straight forward to troubleshoot without having to make any real judgement calls.
On the other hand I've tested a lot of wire that's read 0.1-0.3 megs, etc and yet the motor is still running. Moisture in conduits seems to play a big part in your megger readings. I've pulled some wire out that's megged low and couldn't ever find a nick or scrape in it. I think it must develop very small cracks in the insulation you just can't see with the naked eye.
I've had no formal training and haven't read that Biddle megger book some guys on the forum recommend, so you can take my advice for what it's worth. Nothing.:grin: