Motor Feeder Insulation Resistance and Polarization Index at Commissioning
A 200 kW, 400 V three-phase cage induction motor is being commissioned on an industrial site. The feeder is a 4 × 185 mm² copper XLPE/SWA cable, 120 m long, run on a tray with two other loaded cables. The motor has been in storage in an unheated warehouse for seven months. Insulation resistance is measured at 500 V DC with the cable and motor connected and the drive isolated: L1–PE 0.9 MΩ, L2–PE 1.1 MΩ, L3–PE 0.8 MΩ, all at a conductor temperature of 22 °C. A polarization index test gives 1.3. The client's specification requires a minimum of 1 MΩ for a 400 V machine, and the site electrician argues that 0.8 MΩ is close enough and that the motor should simply be run to dry it out. Production wants the line started the next morning.
1. Interpret the insulation resistance and polarization index results, and decide whether the motor may be energized for the production start.
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What the numbers say
- A healthy 120 m XLPE feeder plus a dry winding should read in the tens of MΩ or more. Readings of 0.8–1.1 MΩ are one to two orders of magnitude low.
- The three phases sit within a narrow band and there is no single phase-to-earth outlier. That pattern points to a distributed moisture or contamination path, not a discrete insulation failure in one winding.
- The polarization index of 1.3 is the decisive result. Below about 2.0 the ten-minute value barely rises above the one-minute value, which is the classic signature of conduction-dominated leakage through moisture rather than healthy absorption behaviour.
What the 1 MΩ specification value means
- It is a floor derived from the kV + 1 rule (about 1.4 MΩ for a 0.4 kV machine at 40 °C), not a target for a 200 kW machine expected to run for years. Two of the three phases are below even that floor, and the readings were taken at a favourable 22 °C.
Decision Do not energize. Applying 400 V to a damp winding risks flashover at the overhangs, an earth fault trip, and damage to the drive. The electrician's 'run it to dry it' proposal is only defensible for marginal readings on a small machine with the drive disconnected and with continuous monitoring - not here. Dry the winding, retest, and explain the delay to production on the basis of the measured trend, not opinion.
2. How would you separate the contribution of the feeder cable from that of the motor, and which additional tests would you perform?
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Separate the two items first - the combined test cannot tell you which side is wet, and the remedy differs completely.
- Isolate, lock off, prove dead, and allow the drive DC link to discharge before touching terminals.
- Disconnect the motor at its terminal box, spread and tape the cores, then test the cable alone at 500 V DC for one minute. A 120 m XLPE cable in good order should read well above 100 MΩ, normally into the GΩ range. A low cable reading points to moisture in the glands, a damaged sheath during the tray pull, or water tracking in the SWA bedding.
- Test the motor alone: each winding to frame and between windings after removing the star or delta links, at 500 V DC.
Additional tests
- Guard terminal: if surface contamination on the terminal block makes readings unstable, use the guard connection to bypass surface leakage so the true through-insulation value is seen.
- Winding resistance with a micro-ohmmeter at ambient, corrected to 20 °C, checked for balance between phases against factory data.
- Space heater circuit: confirm it exists, is rated correctly and was actually energized during storage - this is usually the root cause of the whole finding.
- Visual inspection of the terminal box for condensation, rust and contamination, and of the winding overhangs for tracking.
- Never insulation-test through the drive; it must be fully disconnected and isolated during any test on the motor or cable.
3. State the acceptance criteria and the records you would require before signing the machine over.
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Drying and acceptance
- Dry with space heaters or warm forced air and record insulation resistance daily so the trend, not a single reading, is the evidence.
- Accept the motor when the winding-to-earth value is stable and well clear of the floor - in practice at least 50 MΩ at the machine temperature with the polarization index at or above 2.0 - and the cable alone reads above 100 MΩ. Where the manufacturer publishes factory test values, compare against those rather than against the specification minimum alone.
- Accept only when both the isolated motor and the isolated cable pass; a combined passing value can hide a failing item.
Records for the handover file
- Instrument make, model, serial number and calibration date; test voltage; test duration; conductor temperature and ambient humidity at the time of each test.
- Per-phase readings for the combined, cable-only and motor-only tests, plus the polarization index and the daily drying trend.
- Winding resistance values corrected to 20 °C compared with factory data.
- Corrective actions taken and the justification for accepting the machine.
Then complete the commissioning set: protection settings verified against the calculated fault current, drive parameters and earth-fault detection configured, rotation and no-load run checked, loaded run with thermal imaging of terminals, and only then the machine handover sheet signed against named evidence.