testing
testing
2 tests need to be performed. First is the resistance test to be done to ensure the ground is OK. Then the second is the test of the insulation. Depending on where you are performing the test will depend on what you need to test first. If you are at the motor I check the resistance of the individual phases compared to each other and then the resistance to ground. Phase to phase should be less than 1 ohm. Phase to ground should be infinity. Either of these not in that range is cause for concern.
Other resistance check is to verify my grounding point is good. So I pick a point for the grounding and then verify this by checking several other locations on the motor housing. Reading less than 1 ohm is what you want.
Then with the ground good and the resistance between phases less than 1 ohm you can pick a lead and megger the insulation.
For this there are several factors. Size of motor and age. If you dont have a reference then contact the manufacturer for full details. But as a general rull you need to be in the meg ohms for a reading. Some say more than 500 megg ohms. But I have 30 motors I check weekly and as long as I am over 500Kohms the manufacturer says I am OK. Mine vary from 200 meg to 1 gig ohm. But its the reading over time that gives you a betterpicture.
Check out the megger web page. They build meggers for a living and have a great book you can look at called a STiTCH IN TIME. It has geat info on testing with a megger.
Hope this helps. Jonathon
