Insulation Resistance Guide: Megger Testing & IEEE 43
What Is Insulation Resistance?
Insulation resistance (IR) measures how well the insulation between conductors, or between conductors and ground, resists current flow. Low IR indicates degraded insulation, moisture, contamination, or damage — all of which can cause short circuits, ground faults, or electrocution.
IEEE 43 Minimum Resistance Values
| Equipment Voltage | Minimum IR (MΩ) | Test Voltage (DC) | Good IR |
|---|---|---|---|
| 250V | 0.25 | 500V | >50 MΩ |
| 600V | 1.60 | 500–1,000V | >100 MΩ |
| 1,000V | 2.00 | 1,000V | >200 MΩ |
| 2,400V | 3.40 | 2,500V | >500 MΩ |
| 4,160V | 5.16 | 5,000V | >1,000 MΩ |
| 5,000V | 6.00 | 5,000V | >1,000 MΩ |
Formula: Rmin = kV + 1 MΩ (for motors per IEEE 43-2000)
Temperature Correction
Insulation resistance drops approximately 50% per 10°C increase in temperature. Correct readings to 40°C:
R40°C = Rmeasured × KT
| Temperature | Correction Factor (KT) |
|---|---|
| 0°C | 0.125 |
| 10°C | 0.25 |
| 20°C | 0.50 |
| 30°C | 0.71 |
| 40°C | 1.00 |
| 50°C | 1.41 |
| 60°C | 2.00 |
| 70°C | 2.83 |
| 80°C | 4.00 |
Test Procedure
- De-energize: Lock out / tag out the equipment. Verify zero voltage.
- Disconnect: Disconnect the equipment from all loads and controls.
- Discharge: Discharge all capacitors and windings to ground.
- Connect: Connect megger between the conductor and ground (or between phases).
- Apply voltage: Apply test voltage for 1 minute (standard per IEEE 43).
- Read: Read resistance at the end of 1 minute.
- Record: Record the value, temperature, and test voltage.
PI (Polarization Index) Test
PI = R10min / R1min. A good PI indicates healthy insulation:
| PI Ratio | Condition |
|---|---|
| > 4.0 | Excellent |
| 2.0 – 4.0 | Good |
| 1.0 – 2.0 | Questionable |
| < 1.0 | Bad — do not energize |
Common Causes of Low IR
- Moisture: #1 cause. Water reduces resistance dramatically. Bake or dry the equipment.
- Contamination: Dirt, oil, chemicals on insulation surfaces create leakage paths.
- Age: Insulation deteriorates over time from heat cycling and oxidation.
- Overheating: Excessive temperature degrades insulation permanently.
- Mechanical damage: Vibration, abrasion, or physical impact damages insulation.
Common Mistakes
- Testing energized equipment: Always verify zero voltage before connecting a megger.
- Not correcting for temperature: A reading of 5 MΩ at 20°C is equivalent to 2.5 MΩ at 40°C — potentially below minimum.
- Testing with equipment connected: Disconnect all loads, VFDs, and controls. They give false low readings.
Standards Reference
- IEEE 43-2000 — Recommended Practice for Testing Insulation Resistance of Rotating Machinery
- NFPA 70B — Recommended Practice for Electrical Equipment Maintenance
- NEMA MG-1 — Motors and Generators
- IEC 60364-6 — Verification
Frequently Asked Questions
What is a good insulation resistance reading?
For motors and cables below 600V: >100 MΩ is good, >1 GΩ is excellent. The IEEE 43 minimum is kV + 1 MΩ (e.g., 0.48 kV + 1 = 1.48 MΩ). Readings below 5 MΩ are suspect; below 1 MΩ is dangerous. Always correct for temperature.
What voltage should I use for insulation testing?
For equipment rated ≤600V: use 500V or 1,000V DC test voltage. For 600V–5kV equipment: use 2,500V or 5,000V DC. Never exceed the insulation rating. For sensitive electronics, use 250V DC test voltage to avoid damage.
What is the polarization index?
PI = R(10 min) / R(1 min). A ratio above 2.0 indicates healthy insulation that improves under sustained voltage (absorption effect). A ratio below 1.0 means insulation is degrading under voltage — do not energize. PI testing reveals moisture and contamination that a single IR reading might miss.
How often should I test insulation resistance?
For critical motors: annually. For general equipment: during scheduled maintenance (every 1–3 years). For new installations: before energizing. For suspect equipment: immediately. Trending IR values over time is more valuable than a single measurement.
Can I test insulation resistance on live equipment?
Absolutely not. Insulation resistance testing (megger) applies high DC voltage (500–5,000V) to the equipment. Testing energized equipment is extremely dangerous and will damage the test instrument. Always lock out/tag out and verify zero voltage before testing.