Insulation resistance is a measure of how well the windings of a motor are isolated from the earth and from each other. A megger applies a high DC voltage and reads the leakage current, converting it to a resistance value expressed in megaβohms (MΞ©). High resistance indicates good insulation, while low values suggest moisture, contamination, or ageing.
The Polarisation Index (PI) is the ratio of the resistance measured after 10 minutes to that measured after 1 minute. Because the insulation absorbs charge over time, a healthy system shows an increase in resistance, giving a PI greater than 1. The PI provides a timeβdependent view of insulation quality, making it more reliable than a single reading.
Interpretation guidelines are commonly used: PI < 1.0 indicates poor insulation, 1.0β2.0 suggests marginal condition, and PI > 2.0 is considered good. Additionally, many standards require a minimum resistance of 1β―MΞ© per kilovolt of applied test voltage. Falling below this threshold flags the motor for further inspection or corrective action.
What is insulation resistance?
How does a megger work?
What does the Polarisation Index indicate?
Why is high insulation resistance important?
How often should I check insulation resistance?
What factors can affect insulation resistance readings?
Can I use a megger on any electrical device?
Results are for informational purposes only and do not constitute professional advice.