What Is an Insulation Resistance (IR / Megger) Test?
Quick answer — An insulation resistance (IR) test, run with a megger (insulation tester) or the winding, measures the DC insulation resistance between windings and between windings and earth, and its ratio to detect moisture, contamination and a weakened insulation. The values are compared over time and against the absorption ratio (PI or DAR) to judge the condition.
What is an insulation resistance test on a transformer?
What it does
A DC voltage is applied and the leakage current measures the insulation resistance. Low resistance indicates moisture, contamination or a weak path; a rising value with time is a good sign.
Testing winding to earth and winding to winding
Both are measured, and the results (and the polarisation index) help distinguish a dry, sound insulation from a moist or contaminated one.
Interpretation
Compare against the manufacturer or a baseline and use the trend. A sudden drop, or a low polarisation index, points to moisture or a developing defect.
References & standards
- IEC 60076-1
- IEEE C57.12.90
Related
Related manufacturers
Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp
Related components
Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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