Insulation resistance: correction to 40 °C
Insulation resistance roughly halves for every 10 °C rise in winding temperature, so readings taken at different temperatures are compared after correction to 40 °C. A falling trend matters more than one reading.
Result
Corrected to 40 °C
Recommended minimum (IEEE 43)
Test voltage
Above the minimum
How it is calculated
Correction factor Kt = 0.5^((40 − t) / 10), R₄₀ = Kt × Rt. IEEE 43 minimum after 1 minute: 5 MΩ for random-wound and low-voltage windings, 100 MΩ for form-wound windings, kV + 1 MΩ for windings made before 1970. Test voltage: 500 V DC up to 1000 V rated, 500–1000 V up to 2500 V, 1000–2500 V up to 5000 V.
Reference only. The maker's manual and class rules take priority.
Reference table
| °C | Kt |
|---|---|
| 0 | 0.062 |
| 5 | 0.088 |
| 10 | 0.125 |
| 15 | 0.177 |
| 20 | 0.250 |
| 25 | 0.354 |
| 30 | 0.500 |
| 35 | 0.707 |
| 40 | 1.000 |
| 45 | 1.414 |
| 50 | 2.000 |
| 55 | 2.828 |
| 60 | 4.000 |
| 65 | 5.657 |
| 70 | 8.000 |
| 75 | 11.314 |
| 80 | 16.000 |