Tech Tip #10: How Important is meg-ohm Testing?

To a technician evaluating a motor, a meg-ohm meter is like a Doctor’s blood pressure cuff.  It is a measurement you have to make.  It provides important information, and when it’s bad, it's bad.  But it is a one dimensional test, evaluating only the integrity of the insulation system to ground.  By itself, it does not provide enough information to diagnose overall motor health.  A high meg-ohm reading does not rule out motor electrical problems any more than a normal blood pressure reading rules out serious illness.

In addition, a meg-ohm test will miss inter-turn faults in the windings, it will miss poor connections, it can miss an open phase, and is totally unaffected by rotor problems.

So should you measure insulation resistance?  Of course, but recognize that much more information is needed to assess motor electrical health.  Combining insulation resistance with other AC based tests such as Motor Circuit Analysis can give you a complete picture of motor electrical health, whether for troubleshooting or condition monitoring.