A megger test on transformer windings is typically used to assess what?

• A. Low insulation resistance
• B. Insulation resistance is independent of voltage rating
• C. High insulation resistance appropriate for voltage rating
• D. No insulation resistance is required

Answer: (C)

A megger test checks how well the insulation of transformer windings resists leakage current when put under high DC voltage. The test applies a known voltage and measures the insulation resistance, with the goal that this resistance be very high. That high value should be appropriate for the winding’s voltage rating, meaning the insulation can reliably withstand service voltages with a margin. If the resistance is too low, it indicates leakage paths through moisture, contamination, or aging insulation, signaling degraded insulation that could fail under normal operation. The point isn’t that insulation resistance should be zero or that it’s independent of voltage rating—the protection a winding needs depends on its rated voltage, and the megger helps verify that the insulation can handle that voltage.