Polarization index (PI) is a motor insulation test that compares insulation resistance after 10 minutes of applied DC voltage to the resistance after 1 minute, per IEEE Std 43. A spot resistance reading tells you whether insulation clears a threshold right now; PI tells you whether it still absorbs charge the way healthy insulation should, which often separates a winding that survives the next hot summer from one that grounds out on the next start.
DC current through a winding mixes capacitive charging current that decays almost instantly, absorption current that decays slowly over minutes as the insulation polarizes, and steady leakage current. Insulation resistance (IR) is voltage over current at a given instant, so IR keeps climbing for several minutes as absorption current fades. PI is that climb as a ratio.
A megohmmeter applies a constant DC test voltage (other windings and the frame grounded) and holds it for 10 minutes: roughly 500 V DC for windings below 1000 V, 500 to 1000 V DC for 1000 to 2500 V windings, and higher for larger machines. The technician logs IR at exactly 1 minute and again at 10 minutes, then divides. The related dielectric absorption ratio (DAR) uses 60-second over 30-second readings from the same test, a faster screen, but PI's longer window reads more clearly on machines with more winding capacitance.
IEEE 43 ties the minimum acceptable PI to the winding's thermal insulation class rather than one number for every motor.
| PI value | Interpretation |
|---|---|
| Below 1.0 | Poor. Little absorption; consistent with moisture, contamination, or degraded insulation. |
| 1.0 to 2.0 | Questionable. Below the recommended minimum for Class B and higher systems. |
| 2.0 to 4.0 | Good. Meets or exceeds the typical minimum for Class B, F, and H systems. |
| Above 4.0 | Excellent on modern insulation systems. |
IEEE 43 sets 1.5 as the recommended minimum PI for older Class A insulation and 2.0 for Class B, F, and H, which cover most motors in service today. A PI at 1.0 means the two readings are essentially equal and no meaningful absorption is happening, pointing to contamination, moisture, or degraded insulation. One exception runs the other way: on older varnished-cambric, shellac mica-folium, or asphaltic windings, a PI above roughly 8 can mean the insulation has aged and turned brittle, not that it is unusually healthy.
A spot insulation resistance test gives an absolute megohm value checked against a minimum (commonly rated kV plus 1 MΩ for older or field windings, 100 MΩ for modern form-wound AC coils, or 5 MΩ for random-wound and low-voltage stators, at 40°C). That number trends well but is sensitive to temperature, humidity, and lead length, so one low reading on a humid day can look worse than the insulation really is. PI cancels most of that because both readings come from the same test under the same conditions. PI has a limit of its own: once IR at 1 minute climbs above roughly 5000 MΩ at 40°C, both readings are dominated by noise and PI stops being meaningful. IEEE 43 calls for both a minimum spot IR and, where applicable, a minimum PI: spot IR anchors absolute condition and catches wet windings, while PI anchors the trend and catches gradual aging a single reading would miss.
A single PI reading is a snapshot; a PI history on one motor is a maintenance signal. A winding holding PI of 3.5 for years that suddenly reads 1.8 has lost absorption capability, even if spot IR still clears the minimum, often well before failure. It is the same logic behind preventive maintenance built on vibration and thermal trending. Log PI at every planned electrical PM and compare it against the machine's own history first, IEEE 43 minimums second. Common errors that produce a misleading PI: stopping early and estimating the 10-minute value, letting test voltage sag under load, testing a still-wet winding, and comparing readings taken at very different winding temperatures. Always discharge the winding fully to ground after testing before touching leads.
PI and spot IR are periodic, manual electrical tests, usually run during shutdowns or rewind acceptance. They pair naturally with mechanical checks already common on rotating equipment, such as vibration severity screening, to build a full picture of motor health. Fabrico reads machine condition and OEE straight from the line, using computer vision to catch faults that vibration or current sensors alone can miss, and auto-routes a work order the moment a real loss is detected, built and hosted in the EU with EU data residency and backed by ISO 27001, ISO 20000-1, and ISO 9001 certification. Book a Fabrico demo to see how it fits around the tests your team already runs.
For Class B, F, or H insulation, IEEE 43 recommends a minimum PI of 2.0. A PI of 2.0 to 4.0 is generally good, above 4.0 is excellent. Older Class A systems have a lower minimum of 1.5.
Spot IR is a single reading, so its value shifts with winding temperature, which is why IEEE 43 references spot IR minimums to 40°C. PI is a ratio of two readings minutes apart on the same winding at nearly the same temperature, so temperature effects mostly cancel out.
On older varnished-cambric, shellac mica-folium, or asphaltic windings, a PI above roughly 8 can mean thermally aged, brittle insulation rather than unusually healthy insulation, so very high readings on legacy systems deserve a second look.
No. IEEE 43 calls for both where applicable. Spot IR checks absolute condition against a minimum megohm threshold and catches contamination or moisture, while PI checks absorption behavior and catches gradual aging a single reading would miss.
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