I would not say they are quite worthless.
Every time I have had someone megger something and it was not what it should be it was a very low reading and pretty obvious there was a problem.
That's been my experience too.
But...
First place I would start is, were they working fine and now they no longer work, or is this a new installation (or new replacement equipment) and this is part of the commissioning process? The point is, I don't typically start off the troubleshooting process by megging the motors, that's for the end of the process when you have eliminated all the easier issues. It sucks to read a bad motor circuit and replace the motor, only to discover it never WAS the motor. The guy footing the bill will NEVER call you again...
For example, are the breaker instantaneous trip settings correct for those motors? If the motors were recently changed out with new Energy Efficient motors, nuisance SC tripping is a really common problem because the new motor designs have less resistance, so settings for older motors no longer work.
Other possible causes of SC tripping: moisture in the conduits, moisture in the motor windings, dust and moisture in the controllers. etc. etc.
As to the OLs tripping in 3 seconds, is there a change in the duct work or vane controls? Sometimes people fail to realize that a broken duct or a WIDE OPEN vane control system will OVERLOAD a blower motor, they are typically designed to start with low or NO FLOW, meaning the vanes are CLOSED and/or the ducts are intact. Some people mistakenly think that if the vanes are closed, the blower will overload, when it is EXACTLY THE OPPOSITE.
