For electric motors, the insulation resistance is measured between the windings and the frame. For a good motor, you would expect to see 10 megohms or more. As the motor ages, the value of insulation can depreciate to lower values due to dust, dirt, moisture, loose armature, and other factors.
A general rule when measuring motor insulation resistance is as follows.
- 2 megohm or less (bad) 2 to 5 megohm (critical)5 to 10 megohm (suspect) 10 to 50 megohm (good) Above 50 megohm (excellent)
Transformer insulation resistance testing is carried out similarly but is more complex due to the nature and construction of transformers. The insulation resistance is carried out from each winding to earth or ground as well as between windings.
Often industrial transformers are manufactured to operate at very high voltages so high-voltage insulation resistance testing is often needed. Several thousand volts are applied to test for high voltage leakage and insulation breakdown. These systems can typically operate at 5000V and over.
We hope you have found our guide on PAT testers and insulation testers informative and useful.