Insulation Resistance Testing

An insulation resistance tester (often called a "Megger") applies a high DC voltage — 500 V, 1 kV, 2.5 kV or 5 kV depending on the equipment rating — and measures resistance in MΩ or GΩ.

Common checks:

• Spot test — a 1-minute reading.
• Polarisation index $PI = \dfrac{R_{10\,min}}{R_{1\,min}}$ — for motors and transformers, a PI above about 2 indicates good insulation; below 1 is poor.

Always isolate and discharge the circuit before and after testing, and disconnect sensitive electronics that the test voltage could damage.