IEEE 43 · Insulation resistance (megger) testing

Insulation resistance testing per IEEE 43

Choosing the test voltage, correcting to 40 °C, polarization index and DAR, and minimum IR concepts for rotating machines — with a worked 460 V motor example.

Need the standard? IEEE Std 43 is available from IEEE Standards →

How to run an IEEE 43-style IR test, step by step

Step	What you do	Watch out for
1	Lock out, verify zero energy, and disconnect surge caps, VFD/soft-starter leads, and anything electronic	Megging through a drive can destroy it — and the drive reads as a fault
2	Choose the DC test voltage from the winding's rated voltage	IEEE 43 steps test voltage up by rating; lower-voltage machines get a lower test voltage
3	Record winding temperature (or ambient for a machine that has sat idle) and humidity	Without a temperature, the reading cannot be trended
4	Apply voltage and record readings at set times — commonly 30 s, 1 min, and 10 min	Keep the voltage steady; do not stop early if you want a PI
5	Correct the 1-minute reading to the 40 °C reference	Use the correction the standard gives for the insulation type, or your test set's built-in correction
6	Compute PI (10 min ÷ 1 min) and/or DAR (short-time ratio)	Very high readings make PI unreliable — the standard addresses this
7	Compare with the minimum IR and PI recommendations, and with prior tests on the same machine	The trend over years tells you more than one reading
8	Discharge the winding to ground for at least as long as the test ran	Windings store charge — a charged winding can hurt someone

Our own summary, not the standard text. IEEE Std 43 is published by IEEE. Values cited here are the long-standing 2000/2013-edition guidance; IEEE inactivated the 2013 edition in 2024 pending revision, so confirm against the current edition, the machine manufacturer's instructions, and your project test spec (often NETA ATS/MTS), which may set different acceptance values. Not engineering advice.

The four numbers, in plain terms

Measurement	What it is	What it tells you
IR (1-minute)	Megohms from winding to ground after 1 minute at the DC test voltage	Overall insulation condition, dominated by surface moisture and contamination
IR corrected to 40 °C	The 1-minute reading adjusted to a common reference temperature	Lets you compare this year's test with last year's taken at a different temperature
Polarization index (PI)	Ratio of the 10-minute reading to the 1-minute reading	Healthy insulation keeps absorbing charge, so resistance keeps rising; wet or dirty insulation flattens out
Dielectric absorption ratio (DAR)	Ratio of two short readings (commonly 60 s ÷ 30 s)	A quick screen when a 10-minute test is not practical; not a substitute for PI on critical machines

Worked example — 460 V, random-wound induction motor after a flood

Input / step	Result	Source
Winding rating	460 V, random-wound (below 1 kV)	Nameplate
DC test voltage	500 V	IEEE 43 guidance for windings below 1 kV
1-minute reading at 20 °C winding temperature	12 MΩ	Measured
Correct to 40 °C (rule-of-thumb: resistance roughly halves per 10 °C rise)	12 × ½ × ½ ≈ 3 MΩ	Illustrative correction — use the standard's method
Compare to the recommended minimum for this winding type	Below the 5 MΩ recommendation	IEEE 43 minimum-IR guidance
10-minute reading / PI	15 MΩ ÷ 12 MΩ = 1.25 — below the 2.0 generally recommended for modern (Class B and higher) insulation	IEEE 43 PI guidance
Decision	Do not energize. Clean and dry the winding, then retest and trend	Field practice

Worked example values only. The halving-per-10 °C rule is a simplification; IEEE 43-2013 ties the correction to insulation type.

Common field mistakes

Mistake	What to do instead
Using the highest voltage on the tester for every machine	Pick the test voltage from the winding rating — overstressing old insulation can fail it
Recording a reading without a temperature	Always log winding temperature so the reading can be corrected and trended
Passing a machine on one "good enough" reading	Compare to past tests; a steady drop matters even above the minimum
Megging with the drive, surge caps, or RTD leads still connected	Isolate the winding and ground RTDs and CTs not under test
Forgetting to discharge after the test	Ground the winding for at least as long as the test ran (longer after a PI)

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Reading a megger test like a test tech

Where to get the standard. IEEE Std 43, the recommended practice for testing insulation resistance of electric machinery, is available from IEEE Standards. The 2013 edition — the one most test specs still cite — was inactivated by IEEE in 2024 while a revision moves forward, so check which edition your spec calls for. This page explains the concepts in our own words; it does not reproduce the standard’s tables.

Test voltage follows the winding rating. IEEE 43 recommends a DC test voltage that steps up with the machine’s rated voltage. Low-voltage motors get a modest test voltage; medium-voltage machines get more. Using the biggest button on the tester for everything overstresses old insulation and can fail a winding that would have run for years.

Temperature correction. Insulation resistance falls steeply as temperature rises, so IEEE 43 corrects the 1-minute reading to a 40 °C reference before comparing it with minimums or past tests. Older practice used a simple rule — resistance roughly halves for every 10 °C rise — and the 2013 edition refines the correction by insulation type. Many modern test sets do the correction for you if you enter the winding temperature.

PI and DAR. Good insulation keeps soaking up charge for minutes, so its resistance keeps climbing. The polarization index divides the 10-minute reading by the 1-minute reading; the dielectric absorption ratio does the same with two shorter readings. A flat ratio points to moisture or contamination even when the absolute number looks acceptable.

Worked example. A flooded 460 V random-wound motor reads 12 MΩ at 1 minute with the winding at 20 °C, tested at 500 V DC. Corrected to 40 °C with the rule-of-thumb, that is about 3 MΩ — below the 5 MΩ recommendation for this winding type. The 10-minute reading of 15 MΩ gives a PI of 1.25, short of the 2.0 generally expected. Dry it out, retest, and trend. Record results on the equipment inspection checklist or in your project closeout checklist test records. Field PM keeps test reports, photos, and QA/QC sign-offs with the job — see Field PM for electrical contractors.

FAQ

What voltage should I megger a 480 V motor at?+

IEEE 43 recommends the DC test voltage based on winding rating; for windings rated below 1000 V, the guidance is 500 V DC. Higher-rated windings get higher test voltages. Always check the manufacturer and your project test spec.

What is a good megger reading for a motor?+

IEEE 43 gives recommended minimums for the 1-minute reading corrected to 40 °C, which depend on the winding type and age. As one example, random-wound and form-wound windings rated below 1 kV have a recommended minimum of 5 MΩ. In practice most healthy machines read far higher, and the trend over time matters more than one number.

What is a good polarization index?+

PI is the 10-minute reading divided by the 1-minute reading. For modern Class B and higher insulation, IEEE 43 generally recommends a PI of at least 2.0. When the 1-minute reading is very high, PI loses meaning, and the standard addresses that case.

Why correct insulation resistance to 40 °C?+

Insulation resistance drops sharply as temperature rises, so the same motor reads very differently on a cold morning and after running. Correcting to a common 40 °C reference makes readings comparable from test to test.

What is the difference between PI and DAR?+

Both are ratios that show whether resistance keeps rising as the insulation absorbs charge. PI uses the 10-minute and 1-minute readings; DAR uses two shorter readings (commonly 60 s and 30 s) and is a quicker screen. PI is the IEEE 43 benchmark.

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