Insulation to Ground Resistance – What’s the Big Deal?

Electrical insulation to ground testing, also known as MegOhm testing, has been one of the primary methods used to for evaluating the condition of electric motor insulation for a century (or thereabout). The primary reason for its use has been to perform a safety check of the barrier between the current carrying conductors and the case of the electric motor.

A 1983 EPRI project determined that 37% of motor failures, across all size ranges, were detected as winding problems with 17% of that being insulation to ground. That relates to 6.3% of faults detected with insulation to ground readings FOLLOWING motor failure. In a 2000 study performed by PG&E (the Electric Motors Performance Analysis Tool project) on 480 Volt motors from 5 to 250 horsepower, 4% of the motors surveyed, that were determined to be in poor condition, had insulation readings below 100 MegOhms. In both cases, only 1 in 20 motors with problems were determined through insulation to ground testing, with 1 in 3 (or, 1 in 2 with the PG&E study) having winding shorts and/or insulation to ground faults. In effect, winding shorts are 6 times more likely to be the cause of a motor fault.
Now, when looking at the complete motor system, including the incoming power, controls, cables, motors, couplings and driven equipment, electric motors fail even less frequently than expected. In most cases, the cause of failure is the electric motor acting as a fuse. With motor faults being a conservative 15% of motor system faults (some surveys show higher, and others lower, numbers), with loose connections, poor contacts, cable faults, driven equipment failure, coupling faults, belt and sheave failure, being more prevalent, that leaves insulation to ground tests detecting a rate of failure of 1 for every 120+ motor system faults.

Conclusion:
With insulation resistance measurements detecting a conservative 0.8% of motor system faults, the following considerations should be made:
A motor system diagnostic program should be implemented that views the complete system and includes the ability to detect winding shorts, power quality, loose connections, cable faults and mechanical condition of the system. A combination of tools including MCA, MCSA, Vibration Analysis, Infrared Analysis and other technologies should be considered for the greatest possible return on your motor system program.
Insulation resistance is found to be lacking as a predictive maintenance tool, but should be included in any program for safetly issues. The values determined should be at least the IEEE 43-2000 recommended values.

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