Q: Why do we measure insulation resistance on a transformer, and what do the results mean?
A: The insulation resistance (IR) test is a fundamental diagnostic tool used to assess the quality of the insulation system between windings and between windings and ground (core/tank).
Purpose:
• Detect moisture contamination, surface dirt, or carbonized paths in the insulation.
• Evaluate the effectiveness of drying treatment after repair or oil replacement.
• Provide a baseline for trending insulation condition over time.
Interpretation:
• A single IR reading is compared against manufacturer benchmarks or historical data.
• The Polarization Index (PI) - the ratio of the 10-minute to 1-minute IR reading - is a more reliable indicator:
|
PI Value |
Condition |
Interpretation |
|
≥ 2.0 |
Good |
Dry, healthy insulation |
|
1.0 – 2.0 |
Questionable |
May indicate moisture ingress |
|
< 1.0 |
Poor |
Insulation likely compromised |
Key Note: IR is a spot-check, not a conclusive pass/fail. Always combine with other tests (tanδ, DGA) for a full assessment. Temperature correction is also essential: IR values roughly halve for every 10°C rise.