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Insulation temperature correction
Bring insulation resistance to 40 °C for rotating machines or to 20 °C for transformers, check the polarization index and DAR, correct the power factor of transformers and bushings to 20 °C and compare with your past measurements.
IEEE 43correction to 40 °C, PI and minimum resistanceANSI/NETAtable 100.14, factors to 20 °CIEEE C57.12.90PF correction table withdrawn in 2010IEEE C57.19.01PF of new OIP bushings
Results
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— %
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Charts
- Corrected to the reference
- As measured
- Minimum or limit
- Method factor
- K = 1, reference temperature
- Measured temperature
Calculation and details
Step-by-step calculation
| Year | Measured | Temp. or K | Corrected |
|---|
What it means for the equipment
What it is for
Insulation resistance halves, or worse, with every 10 °C rise, and power factor rises with temperature. A 2000 MΩ reading on a cold motor and a 600 MΩ reading on the same motor just stopped can describe the same insulation. To decide whether a winding is taking on moisture or aging, each measurement has to be brought to the same reference temperature and compared with the earlier ones. This tool makes that correction with the method you choose, evaluates the polarization index and DAR, and plots the trend of up to six past measurements.
How it is calculated
IEEE 4310 °C rule, 40 °C reference
R40 = KT · RT · KT = 0.5(40 − T)/10It is the traditional IEEE 43 rule: resistance halves for every 10 °C rise. The 2013 edition keeps it for thermoplastic insulation (asphalt-mica and other older systems). Megger gives it as a general rule of thumb: 2 MΩ at 20 °C is 0.5 MΩ at 40 °C.
IEEE 43-2013thermoset insulation
KT = exp[−1245 · (1/(T + 273) − 1/313)] from 10 to 40 °C · KT = exp[−4230 · (1/(T + 273) − 1/313)] from 40 to 85 °CFor epoxy-mica and polyester, the 2013 edition splits two ranges. The constants were taken from the EASA technical summary of the standard; check them against your copy of IEEE 43 before using them in a formal report. Below 40 °C this insulation changes much less than with the 10 °C rule: at 20 °C, KT = 0.76 against 0.25.
ANSI/NETAtable 100.14, oil insulation column
R20 = K · RTFor oil-filled transformers the tool uses the factors of table II of Megger's "A Stitch in Time" guide, oil-filled transformer column, which NETA reproduces in its table 100.14: 0.25 at 0 °C, 0.50 at 10 °C, 1.00 at 20 °C, 1.98 at 30 °C, 3.95 at 40 °C, 7.85 at 50 °C, 15.85 at 60 °C and 44.70 at 75 °C. Between rows the tool interpolates on a logarithmic scale.
IEEE 43polarization index and minimum resistance
PI = R10 min / R1 min · DAR = R60 s / R30 s- Rotating machines, IEEE 43: minimum PI of 1.5 for class A and 2.0 for classes B, F and H. If the 1-minute resistance corrected to 40 °C exceeds 5000 MΩ, PI is not meaningful. Minimum 1-minute resistance at 40 °C: kV + 1 MΩ for windings built before 1970 and field windings, 100 MΩ for AC form-wound windings built after 1970 and 5 MΩ for random-wound and form-wound windings below 1 kV.
- Transformers and DAR: Megger table I, which the manufacturer itself calls tentative. PI below 1, dangerous; 1 to 2, questionable; 2 to 4, good; above 4, excellent. DAR from 1.0 to 1.25, questionable; 1.4 to 1.6, good; above 1.6, excellent. The tool flags the 1.25 to 1.4 range, which the table does not classify, as an alert.
- DAR is calculated as 60 s / 30 s, the Megger and AEMC definition. Some instruments offer other time pairs; if you use another one, do not compare it with these thresholds.
- Neither PI nor DAR is corrected for temperature: they are ratios of two readings at the same temperature.
IEEE C57.12.902010 editionIEEE C57.19.01bushings
FP20 = K · FPTIEEE C57.12.90 withdrew its general power factor correction table in 2010, noting that the variation with temperature "is large and erratic, and no single curve fits all cases". That is why the tool does not make up a curve: use the factor from the transformer or bushing manufacturer's table, or the one from your test set. Each bushing manufacturer publishes its own by type.
- Typical reference: 0.5 % at 20 °C for oil-filled power transformers (Bureau of Reclamation, FIST 3-31, 8.1) and for new OIP bushings (IEEE C57.19.01, table 6). You can change the limit.
- Status (Tiza convention): pass up to the limit, alert up to twice the limit and fail above that. For a bushing with a 0.5 % limit, twice the limit matches the 1 % above which IEEE Std 62 suggests seeking expert help.
- Current practice prefers to test with the insulation close to 20 °C or to use dielectric frequency response (DFR), which corrects each unit with its own curve.
Trend
Δ = Xcurrent / Xprevious − 1Each past reading is corrected with the same method, with its own temperature or its own K factor, and the current reading is compared with the one just before it. Tiza convention: alert if the corrected resistance drops 50 % or more, or if the corrected power factor rises 50 % or more.
Limits of the calculation
- It is an estimate, not a diagnosis. The factors are averages of many units; yours may behave differently, especially if it is wet or contaminated.
- The farther from the reference you measure, the larger the correction error. If you can, test with the insulation close to 40 °C (machines) or 20 °C (transformers).
- The thermoset constants of IEEE 43-2013 come from a technical summary, not from the text of the standard. The Megger and NETA tables apply to oil-filled transformers, not to dry-type units or units with natural ester.
- The temperature that matters is the insulation temperature: winding temperature by resistance or detector, or top oil after a rest period, not ambient.
- Insulation resistance and power factor do not detect every defect. A decision about the equipment should rest on the full set of tests and on the factory history.
Frequently asked questions
How is insulation resistance corrected to 40 °C?
IEEE 43 brings the reading to 40 °C with R40 = KT · RT. The traditional rule assumes that resistance halves for every 10 °C rise: KT = 0.5^((40 − T)/10), so 2000 MΩ measured at 20 °C equal 500 MΩ at 40 °C. The 2013 edition keeps that rule for thermoplastic insulation and gives another expression for thermoset insulation (epoxy-mica), which changes much less below 40 °C. Any factor is an average: the best comparison is between readings taken at similar temperatures.
What is the minimum polarization index of a motor?
IEEE 43 recommends a polarization index (R at 10 min / R at 1 min) of at least 1.5 for class A insulation and 2.0 for classes B, F and H. If the 1-minute resistance corrected to 40 °C exceeds 5000 MΩ, the index is no longer meaningful. It also sets a minimum 1-minute resistance at 40 °C: 100 MΩ for most AC form-wound windings built after 1970, 5 MΩ for random-wound and form-wound windings below 1 kV, and kV + 1 MΩ for older windings and field windings.
How is the power factor of a transformer corrected to 20 °C?
With the factor from the manufacturer's table or from the test set: PF20 = PFT · K. IEEE C57.12.90 withdrew its general table in 2010 because the variation of power factor with temperature is large and erratic, and no single curve fits every case. That is why current practice prefers to test with the insulation at a known temperature close to 20 °C, or to use dielectric frequency response (DFR), which allows an individual correction for each unit. As a typical reference, the power factor of an oil-filled transformer should not exceed 0.5 % at 20 °C.
We test the insulation of your equipment on site
We measure insulation resistance, polarization index, power factor and dielectric response on transformers, bushings and motors, in Colombia, or elsewhere on request, with the temperature recorded and the trend against your past tests.