| Time (min) | Winding 1 → Frame | Winding 2 → Frame | Winding 3 → Frame | |||
|---|---|---|---|---|---|---|
| Reading | Corrected @40°C | Reading | Corrected @40°C | Reading | Corrected @40°C | |
💡 Taking the readings
Apply the test voltage continuously and read without interrupting it — each reading is a snapshot of the same energization, not a separate test. Record the winding temperature at the time of test, not the ambient, because the correction to 40 °C is applied to the winding. If the 1-minute corrected reading is already very high (above 5000 MΩ), IEEE 43 cautions that PI loses meaning and trending the IR value itself is the better indicator.
Disabled by default. Each winding should be tested individually. Only enable this if all windings are confirmed identical.
| Winding | 1-min IR @40°C (MΩ) | Min IR (MΩ) | Min IR | DAR | PI | Test Voltage | Overall |
|---|
💡 Why trending beats a single pass/fail
A single insulation-resistance measurement tells you whether the winding is above a floor. A trend tells you where it is heading. A machine that reads 900 MΩ but has fallen from 4000 MΩ over three test cycles is degrading, even though it passes every acceptance criterion. Consistent conditions matter: compare readings corrected to the same 40 °C reference, taken at the same test voltage and the same winding configuration.
What This Record Captures
This is a field test record for motor insulation resistance, dielectric absorption ratio, and polarization index — the three results that come out of one continuous 10-minute application of DC test voltage. IR tells you the absolute condition of the insulation against a minimum. DAR and PI tell you how the insulation behaves over time, which is what exposes moisture and contamination.
Recommended DC Test Voltage — IEEE 43 Table 1
| Winding rated voltage (V) | Insulation resistance test direct voltage (V) |
|---|---|
| < 1000 | 500 |
| 1000 – 2500 | 500 – 1000 |
| 2501 – 5000 | 1000 – 2500 |
| 5001 – 12000 | 2500 – 5000 |
| > 12000 | 5000 – 10000 |
The worksheet checks the voltage you enter in the sidebar against this table using the nameplate voltage and flags anything outside the range as a warning, not a failure — a project specification may legitimately call for a different value.
Minimum Insulation Resistance — IEEE 43 Table 3
| Minimum IR at 40 °C | Applies to |
|---|---|
| kV + 1 MΩ | Most windings built before about 1970, all field windings, and others not described below (kV = rated machine voltage in kilovolts) |
| 100 MΩ | Most DC armature and AC windings built after about 1970 (form-wound coils) |
| 5 MΩ | Most machines with random-wound stator coils and form-wound coils rated below 1 kV |
Temperature Correction
Insulation resistance roughly halves for every 10 °C rise in winding temperature, so readings taken at different temperatures cannot be compared until they are normalized. IEEE 43 references all values to 40 °C using the doubling rule:
At 20 °C, Kt = 0.25 — a 200 MΩ reading corrects to 50 MΩ at the 40 °C reference.
Because DAR and PI are ratios of two readings taken at the same temperature, the correction factor cancels out entirely. Correcting the individual readings still matters for the minimum-IR check and for trending.
Temperature correction assumes the winding is at a uniform, known temperature. If the reading came from an infrared spot check on a frame that has not thermally soaked, the correction inherits that error — which is why the record captures the temperature source.
Interpreting DAR and PI
| Index | Formula | Common acceptance |
|---|---|---|
| DAR | IR at 1 min ÷ IR at 30 s | ≥ 1.4 (NETA motor default in this worksheet; instrument-maker guidance often cites 1.25 as the lower acceptable bound) |
| PI | IR at 10 min ÷ IR at 1 min | ≥ 2.0 for thermal class B, F and H; ≥ 1.5 for class A per IEEE 43 |
Both thresholds are editable in the sidebar because the governing value is whatever the project specification and the machine manufacturer say it is. On dry, clean insulation the resistance climbs steadily as absorption and polarization currents decay, giving a high ratio. On wet or contaminated insulation the leakage current dominates and the curve flattens — the ratio collapses toward 1.0 even when the absolute megohm value still looks acceptable.
Which Index Is Required
- NETA ATS / MTS motor modes: machines rated 200 HP / 150 kW and larger are evaluated on polarization index; smaller machines are evaluated on DAR. Enter horsepower or kW on the Record tab and the worksheet selects the governing index. At exactly 200 HP / 150 kW the worksheet requires PI, the more complete of the two.
- IEEE 43 reference mode: PI is the primary index; DAR is reported as informational.
- Worksheet mode: nothing is enforced by machine size — whatever data is present is evaluated against your thresholds.
The index that is not governing is still calculated and shown, marked informational. Recording both costs nothing once the readings are taken.
Safety and Test Discipline
- Absence of voltage. De-energize, lock out, and verify absence of voltage per NFPA 70E before connecting anything. The worksheet flags readings entered before that box is checked.
- Discharge. A tested winding is a charged capacitor. IEEE 43 guidance is to ground the winding for at least four times the duration the test voltage was applied — a 10-minute test means grounding for 40 minutes or more. The worksheet computes and displays this from the longest reading interval entered.
- Guard terminal. Surface leakage across dirty or damp end turns is measured as if it were insulation failure. The guard terminal routes that current away from the measuring circuit so the reading reflects the insulation itself.
- Condensation. If the winding is at or below the dew point, moisture on the surface makes the reading meaningless. The worksheet computes the dew point from ambient temperature and humidity and raises a warning.
- Surge and winding damage. Insulation resistance testing is a DC test at modest voltage. It will not find turn-to-turn faults — that requires a surge comparison test.
Common Mistakes
- Comparing this year's uncorrected reading to last year's corrected one
- Correcting to 40 °C using ambient temperature instead of winding temperature
- Interrupting the test voltage between the 1-minute and 10-minute readings, which restarts the absorption current and invalidates PI
- Treating a passing minimum IR as a passing test when the PI has collapsed
- Applying a test voltage well above the IEEE 43 table range because “higher finds more”
- Skipping the discharge period on a large machine
Standards Status
#selftest.