How can aging insulation between turns be detected?

• A. Turn-to-turn insulation resistance decreases; use insulation testing (megger) and trending (polarization index).
• B. Thermal imaging shows hot spots.
• C. Vibration analysis detects mechanical loosening.
• D. Capacitance test acceptance.

Answer: (A)

Aging insulation between turns shows up as a decline in the winding’s insulation resistance, so the test that detects this most directly is insulation resistance testing with a megohmmeter followed by trending using the polarization index. A megger applies a high DC voltage and measures the resistance between turns; healthy, well‑insulated windings read as very high resistance. As paper insulation ages or moisture and degradation affect the impregnation, leakage paths develop and the measured resistance drops. Tracking the polarization index— the ratio of resistance at a longer test time to a shorter test time—helps reveal aging because aged insulation polarizes more slowly, causing the PI to fall over time. Lower IR and a decreasing PI are indications that insulation between turns is aging. Thermal imaging can show hot spots from other issues but doesn’t directly quantify aging insulation between turns, while vibration analysis targets mechanical problems, not electrical insulation aging. Capacitance tests can provide dielectric information but do not track aging between turns as effectively as IR testing with PI trending.