Megger Testing a Motor Before You Energise It: Insulation Resistance, PI Ratio and What the Numbers Actually Mean

A megger test of a motor applies a DC voltage between the winding and the frame, usually 500 V for a winding rated below 1,000 V, and reads the insulation resistance in megohms after one minute. Before a motor is energised, that reading must be corrected to 40 deg C and must clear the minimum. The WEG motor manual, citing IEEE 43, sets that minimum at 5 megohms for windings up to 1,000 V and 100 megohms above, and on windings above 1,000 V the polarisation index (the 10-minute reading divided by the 1-minute reading) should be at least 2.0. A reading that clears the minimum can still be telling you the winding is wet. For the motors New India Electricals Ltd (NIEL) supplies, the most useful reading is the first one taken on site, because every later reading is judged against it.

TL;DR

• A megger test measures insulation resistance (IR) between a motor winding and earth. The WEG installation manual sets 500 V DC for windings below 1,000 V, 1,000 to 2,500 V DC for 2,501 to 5,000 V windings, and 2,500 to 5,000 V DC for 5,001 to 12,000 V windings.

• IR falls sharply with temperature. The WEG correction table halves the value for every 10 deg C rise, so 400 megohms read at 20 deg C is 100 megohms at the 40 deg C reference.

• The published minimums are floors, not pass marks. The WEG M line manual, citing IEEE 43, lists 5 megohms up to 1,000 V and 100 megohms above, while ABB notes that a new machine usually reads in the thousands of megohms.

• The polarisation index (PI) is the 10-minute IR divided by the 1-minute IR. A PI of 2.0 is the minimum for Class B and Class F insulation, and the ratio largely cancels the temperature effect.

• A low reading on a new or stored motor usually means moisture, not a failed winding. WEG and ABB both prescribe oven drying before any decision to rewind.

• The most useful megger result is a trend. The US Bureau of Reclamation states that falling IR across periodic tests says more about deterioration than a single low value.

What does a megger test actually measure on a motor winding?

A megger test measures the resistance of a motor’s winding insulation to a steady DC voltage, expressed in megohms. “Megger” is the trade name of Megger Group, but site engineers use it for any insulation resistance tester, which the WEG manual calls a megohmmeter. It answers one question: is anything conductive, usually moisture or dirt, bridging the insulation between copper and iron?

That insulation is where motors fail. During the dismantling of a 75 kW, 100 HP induction motor filmed at the NIEL workshop, the presenter shows that the stator winding is insulated from the laminated core and between conductors, and notes that winding failure is one of the most common reasons an induction motor fails. The varnish protects that insulation against moisture and dust, and a megger test is the routine site check of it.

The reference document is IEEE 43. According to the IEEE Standards Association record for IEEE 43-2013, the 2013 edition covers stator and rotor windings of machines rated 750 W or greater, moved to inactive-reserved status on 21 March 2024, and is being revised as project P43. Motor manufacturers still quote its values in their manuals.

Which test voltage should you use for a 415 V, 3.3 kV or 11 kV motor?

The test voltage is set by the winding’s rated voltage, not by the highest voltage your tester can produce. Table 5.3 of the WEG Installation, Operation and Maintenance Manual of Electric Motors (document 50033244, revision 43, April 2026) gives the bands below, mapped to the voltages NIEL supplies up to the 11,000 V machines in our high voltage motor range.

The ABB low voltage motors manual (3GZF500730-85, revision H, June 2022) allows 500 or 1,000 V DC for LT motors. Pick one test voltage per motor and keep it for life, because readings at different voltages do not trend cleanly.

How do you megger a motor before energising it, step by step?

A pre-energisation megger test isolates the winding, tests each phase to earth for one minute, records the winding temperature and discharges the winding. The sequence follows section 5.4.1 of the WEG manual and section 3 of the ABB manual.

1. Isolate and earth. The motor must be stopped, cold and disconnected. WEG instructs that terminals be earthed before and after each measurement.

2. Disconnect everything outside the motor. ABB requires IR to be measured at the motor terminals with supply cables disconnected. WEG adds that switches, capacitors and other external devices considerably influence the reading and must be disconnected and earthed.

3. Open the star or delta links. WEG recommends testing each phase separately with the other two earthed. Testing all three together checks insulation to earth only and misses a phase-to-phase weakness.

4. Read at one minute. WEG specifies the reading one minute after voltage is applied. On windings above 1,000 V, hold the voltage for 10 minutes to capture the PI.

5. Record the winding temperature. Without it, the reading cannot be corrected or compared with any previous test.

6. Discharge. ABB requires the windings to be discharged against the earthed frame immediately after each measurement.

7. Restore and check the links. The test ends with the terminal box closed correctly. NIEL once flew an engineer to a site in Africa where a customer had wired a motor incorrectly and blamed the equipment; the fix took minutes, and the site team was then trained on correct installation practice.

Why must the reading be corrected to 40 deg C before you judge it?

Insulation resistance falls steeply as winding temperature rises, so an uncorrected reading is not comparable with anything. WEG corrects every reading to 40 deg C using Table 5.4 of its manual, which halves the resistance for every 10 deg C rise.

A motor looks healthier on a cool morning than it is. Two of the principals NIEL represents also disagree on the rule: ABB references IR to 25 deg C and halves it for each 20 deg C increase, while WEG references 40 deg C and halves it every 10 deg C. Apply the method of the manufacturer whose motor is on the plinth, and write that method on the test sheet.

What is a good insulation resistance value for a motor?

A good insulation resistance value is one far above the published minimum and stable against the motor’s own previous readings. The minimums mark the point below which a motor must not be energised. They do not describe a healthy winding.

ABB states that a new machine usually reads in the thousands of megohms. A new 415 V motor reading 20 megohms has passed the IEEE 43 minimum, yet sits in WEG’s “Regular” band and reads a small fraction of what a new winding usually shows. That points to moisture picked up in transit or storage, not to a sound motor.

What does the PI ratio tell you that a one-minute reading cannot?

The polarisation index (PI) is the ratio of the insulation resistance at 10 minutes to the resistance at 1 minute, measured at a relatively constant temperature, as defined in the WEG M line manual (document 11171349, revision 17, January 2025). In clean, dry insulation the resistance keeps climbing as the dielectric absorbs charge, so the 10-minute reading sits well above the 1-minute reading. In wet or contaminated insulation, leakage dominates and the two stay close.

PI is useful because it is a ratio. Megger’s technical note on the polarisation index test explains that a large machine’s temperature barely changes during the 10-minute test, so PI can be treated as independent of temperature.

Three cautions apply. Megger warns that a PI above 5 can indicate brittle or cracked insulation, and that a sudden PI rise of more than 20 percent without maintenance is a warning sign. PI cannot be calculated if the tester runs out of range. And the WEG M line manual lists PI as “not applicable” for windings up to 1,000 V, so on most LT motors the corrected one-minute IR and its trend carry the decision.

For NIEL’s range, the Class B and Class F rows are the ones that matter. The Spectrum series LT motors in NIEL’s motor catalogue carry Class F insulation with Class B temperature rise, and the slipring motors in our rerolling mill duty range use Class B or Class F insulation. A PI of 2.0 is the floor for both.

Why does a low reading on a new motor usually mean moisture, not a failed winding?

A low insulation resistance on a new or stored motor is usually condensation, and the WEG manual says so directly: if insulation resistance is low, moisture may be present in the stator windings.

The fix is heat, not a rewind:

• WEG: remove the end shields, rotor, fan, fan cover and terminal box, then oven dry the wound stator at a maximum of 120 deg C for two hours, longer for larger motors. Cool, re-measure and repeat. WEG considers a rewind only if repeated drying fails.

• ABB: oven dry at 90 deg C for 12 to 16 hours, then 105 deg C for 6 to 8 hours, with drain plugs removed during heating and refitted afterwards.

Prevention is cheaper. WEG instructs that space heaters stay switched on throughout storage and whenever an installed motor is out of service. The general arrangement drawings for the heavy duty slipring mill motors in NIEL’s catalogue show a separate auxiliary terminal box for the space heater, and that range’s stators and rotors are flood impregnated with polyester resin varnish against chemicals and moisture. On humid duties such as cooling tower motors, the heater circuit belongs on the commissioning checklist.

Moisture is a running failure mode too. When NIEL’s engineers rank what kills motors in rerolling mills, water and moisture ingress near the cooling beds comes second only to scale and dust, and the corrective advice is correctly positioned drain plugs and moisture-resistant winding treatment.

How should IR and PI results change what you do next?

The megger result decides one of three things: energise, investigate, or dry and retest.

The last row is the one plants miss. The US Bureau of Reclamation’s FIST Volume 3-1 states that decreasing insulation resistance across periodic tests is more indicative of deterioration than a low value, and adds that machines above 10,000 kVA should pass on both IR and PI.

Weak supplies raise the stakes. NIEL’s guidance on derating equipment for African installations is that weaker supply quality and wider voltage fluctuation argue for greater insulation margin. On drive-fed motors at 550 V or 690 V, a DC megger test does not prove the winding can withstand drive voltage peaks; that needs inverter-duty insulation specified at purchase, as covered in our guide to VFD panels and what they do to your supply.

What should a pre-energisation insulation test record contain?

A pre-energisation test record should let a different engineer repeat the test in five years and get a comparable number. Capture these ten items for every motor:

1. Serial number, rated voltage, kW and insulation class, with a nameplate photograph.

2. Tester make and model, and the DC test voltage, which must not change in future tests.

3. Winding temperature, ambient temperature and relative humidity.

4. Confirmation that cables, capacitors and surge devices were disconnected at the terminals.

5. One-minute IR for each phase to earth, other phases earthed.

6. Correction method (WEG to 40 deg C or ABB to 25 deg C) and corrected values.

7. For windings above 1,000 V, the 10-minute IR and PI for each phase.

8. On slipring motors, a separate rotor winding reading.

9. Space heater status in storage, and any drying cycle with temperature and duration.

10. Decision taken, engineer’s name and date.

NIEL asks maintenance heads to keep each motor’s datasheets and nameplate photographs ready before calling any supplier, and the insulation history belongs in the same file. It is the evidence in a rewind-or-replace decision, which our analysis of why an ageing motor costs more than a new one covers from the cost side.

Need a motor that clears its first megger test on site?

NIEL has supplied industrial AC and DC motors since 1969, to customers in more than 45 countries, with offices in Bengaluru, Zambia, Malawi and Nigeria. Send us the motor’s nameplate photo and your first IR and PI readings, and our engineers will tell you whether to energise it, dry it and retest, or plan a replacement.

Find the right motor for your duty:

• High voltage motors: 3.3 kV to 11 kV machines, where the 10-minute PI test applies.

• Rerolling mill duty motors: built for scale, dust and moisture near the cooling beds.

• Crane duty and slipring motors: the rotor winding needs its own reading.

• Cooling tower motors: humid duty, so the space heater belongs on the commissioning checklist.

• Stock inventory: replacement motors ready to ship when a winding cannot be saved.

Call +91 80 42434343, email support@newindiaelectricals.com, or contact our team with your test record.

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