Hi-pot testing CAN be destructive. A good operator can head off a lot of problems on a step test, though, if he does a one-minute megger test first. He then compares the megohm value calculated at each step during the step test. If at any time the calculated value of the hi-pot test approaches ore goes below the megger test, the hi-pot test should be terminated.
Sad fact is, though, I have had a 4160-volt motor fail a 2500-volt megger test one minute into the test, so even megger tests can be destructive. Of course, that motor was pretty much gone when we got to it, but it had been running prior to shutting down for "maintenance".
old field guy