R-Pro › Field cases › Air conditioning
Megger Insulation Resistance Test — Compressor / Motor / Breaker Safety
Likely causes
Pre-measurement safety + correct test voltage
Check: **Power MUST be off** for measurement. Breaker OFF + locked out (LOTO). DC bus capacitor residual voltage = wait 10–15 min natural discharge or short with insulated screwdriver. **Megger voltage**: 220V single-phase = 250V or 500V megger / 380V 3-phase = 500V or 1000V / **Compressor winding = 500V megger mandatory** (residential and industrial). Measure between compressor discharge/suction pipe (or terminal) and ground. Ohm meter ×1K scale = supportive (not precise).
What to do: **Pass/fail criteria** (compressor/motor): **≥1 MΩ = OK**, continue use. **0.5–1 MΩ = caution**, plan replacement soon. **<0.5 MΩ = replace immediately** (insulation failure, fire/shock risk). **0 or negative = short, stop and replace immediately**. **After measurement**: discharge residual voltage stored in insulation again (megger applies voltage, charges capacitance). **Log** = record date, values, action in operations log — for warranty claims and legal evidence.
**Field trick**: user says 'breaker keeps tripping' = 80% chance of zero insulation. Megger mandatory. **R-22 old systems** = gradual insulation degradation common (mineral oil aging). **R-32 new** = insulation stays stable (POE synthetic oil). Megger itself = annual calibration mandatory. Korean KS C IEC 60092-509 standard. Residential megger KRW 50–150k, industrial 300–800k.
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