Megger Test
A Megger test (insulation resistance test) is a non-destructive electrical measurement that applies high DC voltage (typically 250V–5kV) to determine insulation integrity between conductors and ground by measuring resistance in megohms (MΩ) using the formula R = V/I.
On the shop floor, technicians disconnect and discharge equipment, isolate motor windings by removing terminal links, then apply the megohmmeter between each phase and ground (or phase-to-phase) to record leakage current. Readings >100 MΩ indicate excellent insulation, while <1 MΩ signal failure requiring repair. This test is crucial for preventive maintenance, as it detects deterioration trends like moisture ingress before catastrophic breakdown, ensuring equipment reliability and safety.
Why DC, not AC?
DC provides a stable output to accurately measure insulation resistance; AC causes capacitive charging and fluctuating readings.
What current components are measured?
The test analyzes Capacitive Charging Current (decays quickly), Dielectric Absorption Current (polarization), and Leakage Current (true indicator of insulation health).
Threshold for safety?
Electrical workers use >2 MΩ as a verification threshold to ensure a circuit is safe before energizing.
Is it predictive?
Yes; when performed annually with recorded ambient conditions, it detects deterioration trends (e.g., moisture ingress) before catastrophic breakdown.