You ran the megger test. You have a number on the screen. Now what?
This is where most guides leave you hanging. They tell you “higher is better” and move on. But in the real world, you need to know whether that reading means the equipment is safe to run, needs monitoring, or should be taken out of service immediately.
This guide gives you the interpretation tables, the decision framework, and the practical know-how to make that call — backed by the actual values from IEEE 43, IEC 60204-1, and IEC 60364-6.
Table of Contents
The Three Numbers That Matter
Every insulation test gives you at least one of these numbers. Understanding which one you have — and what it tells you — is the first step.
1. Spot reading (IR value in MΩ or GΩ) — The resistance at a single point in time, usually at 60 seconds. It’s the most basic measurement. Every megger gives you this. It’s heavily dependent on temperature.
2. Polarization Index (PI) — PI = IR at 10 minutes ÷ IR at 1 minute. It tells you how well the insulation absorbs charge over time. Independent of temperature and equipment size.
3. Dielectric Absorption Ratio (DAR) — DAR = IR at 60 seconds ÷ IR at 30 seconds. A quick version of the PI. Same principle, shorter test.
Spot Reading Interpretation
General guidelines
| IR Reading | Condition | Action |
|---|---|---|
| Above 100 MΩ | Excellent | No concerns. Continue normal testing program. |
| 10 MΩ – 100 MΩ | Good | Acceptable for most equipment. Monitor normally. |
| 1 MΩ – 10 MΩ | Marginal | Investigate. May be acceptable for older or large equipment. |
| Below 1 MΩ | Poor | Do not operate. Investigate cause immediately. |
What affects your reading
Temperature — Insulation resistance roughly halves for every 10°C rise. A motor reading 50 MΩ at 30°C would read about 200 MΩ at 10°C. Always record the temperature.
Humidity — High humidity increases surface leakage. Test above the dew point whenever possible.
Equipment size — Larger equipment has more insulation surface area, which means more leakage paths. A 5,000 kW motor will naturally read lower than a 5 kW motor in identical condition.
Minimum Values by Equipment Type
These are the specific minimum values from the standards. Use these — not the generic “1 megohm per kilovolt” rule.
Motors (per IEEE 43-2013)
| Motor Type | Minimum IR at 40°C | Clause |
|---|---|---|
| Form-wound coils (made after 1970) | 100 MΩ | 12.1 |
| Random-wound coils and form-wound <1 kV | 5 MΩ | 12.1 |
| Windings made before 1970 | (kV + 1) MΩ | 12.1 |
Machinery (per IEC 60204-1)
| Circuit Type | Minimum IR | Test Voltage |
|---|---|---|
| Power and control circuits | ≥ 1 MΩ | 500V DC |
| Circuits with heating elements | ≥ 0.5 MΩ | 500V DC |
LV installations (per IEC 60364-6)
| Circuit Voltage | Test Voltage | Minimum IR |
|---|---|---|
| SELV/PELV (up to 50V) | 250V DC | ≥ 0.5 MΩ |
| Up to 500V | 500V DC | ≥ 1.0 MΩ |
| 500V – 1,000V | 1,000V DC | ≥ 1.0 MΩ |
Protection relays (per IEC 60255-5)
| Parameter | Requirement |
|---|---|
| Test voltage | 500V DC ± 10% |
| Minimum IR (new) | ≥ 100 MΩ |
Polarization Index (PI) Interpretation
For motors (IEEE 43-2013, Table 3)
| PI Value | Condition | Action |
|---|---|---|
| Below 1.0 | Dangerous | Do not operate. Severe moisture or contamination. |
| 1.0 – 1.5 | Poor | Investigate. Failing unless Class A insulation. |
| 1.5 – 2.0 | Questionable | Acceptable only for Class A. For Class F/H, investigate. |
| 2.0 – 4.0 | Good | Healthy insulation. |
| 4.0 – 8.0 | Excellent | Clean, dry insulation. |
| Above 8.0 | Investigate | May indicate brittle insulation on older motor types. |
Minimum PI: 1.5 for Class A, 2.0 for Class B, F, and H.
For transformers (IEEE C57.152-2013, Clause 7.2.13.4)
| PI Value | Condition |
|---|---|
| Below 1.0 | Dangerous |
| 1.0 – 1.1 | Poor |
| 1.1 – 1.25 | Questionable |
| 1.25 – 2.0 | Fair |
| Above 2.0 | Good |
Don’t apply motor PI criteria to transformers. Transformer oil has a PI close to 1.0 naturally.
When PI is not valid
Per IEEE 43-2013 Clause 12.2.2: if IR at 1 minute exceeds 5 GΩ, the PI may not be meaningful. The insulation is clearly excellent — trust the absolute value.
Dielectric Absorption Ratio (DAR) Interpretation
| DAR Value | Condition |
|---|---|
| Below 1.0 | Dangerous — resistance is dropping |
| 1.0 – 1.25 | Poor |
| 1.25 – 1.6 | Questionable |
| 1.6 – 2.0 | Good |
| Above 2.0 | Excellent |
Temperature Correction
For motors (IEEE 43-2013)
Reference temperature: 40°C
Formula: R₄₀ = R_measured × 2^((40 − T) / 10)
Example: Measured 200 MΩ at 20°C. Correction to 40°C: 200 ÷ 2² = 200 ÷ 4 = 50 MΩ. That 200 MΩ is only 50 MΩ at the reference temperature.
For transformers (IEEE C57.152-2013)
Reference temperature: 20°C. Same formula, different reference.
Trending: Why One Reading Is Never Enough
A single reading is a snapshot. It tells you where the insulation stands right now, not where it’s going.
Healthy trend: Readings stay stable or fluctuate within a narrow band over multiple tests.
Warning signs:
- Steady decline over consecutive tests, even if each reading is above minimum
- Sudden 50%+ drop from previous reading
- One phase significantly lower than the other two
- PI declining while absolute IR remains acceptable
What to record every time: Equipment ID, date, ambient temperature, equipment temperature, humidity, test voltage, IR at 30s/1min/10min, PI, DAR, and any notes.
Five Real-World Scenarios
Scenario 1: New motor reads 500 MΩ at 25°C — Normal. This is your baseline. Record it.
Scenario 2: Motor reads 3 MΩ, PI is 2.5 — The PI is good (insulation absorbs charge normally), but 3 MΩ is below the 5 MΩ minimum for random-wound motors. Likely surface contamination, not bulk failure. Clean the windings and retest.
Scenario 3: Motor reads 80 MΩ, PI is 1.1 — The IR looks fine, but PI of 1.1 is below the 2.0 minimum for Class F. Insulation is saturated with moisture or contamination. Investigate before operating.
Scenario 4: Transformer reads 150 MΩ, PI is 1.3 — Per IEEE C57.152, PI of 1.3 is “fair.” The 150 MΩ is reasonable for an in-service oil-filled transformer. Compare to previous readings.
Scenario 5: Motor reads 8 GΩ, PI is 0.9 — Disregard the PI. Per IEEE 43, if IR exceeds 5 GΩ, the PI may not be meaningful. The insulation is in excellent condition.
Red Flags That Mean “Stop Right Now”
- IR below 1 MΩ — Universal minimum for almost all equipment
- PI below 1.0 — Resistance is decreasing over time. Dangerous.
- Sudden 50%+ drop from previous reading
- One phase dramatically lower than the other two on a three-phase motor
- Completely flat reading from 30 seconds through 10 minutes — insulation is saturated
Common Mistakes
Comparing readings at different temperatures without correction. A motor at 20°C reading 100 MΩ has better insulation than a motor at 40°C reading 80 MΩ — but only if you correct both to the same reference.
Using motor PI criteria on transformers. PI of 1.5 fails a Class F motor but is “fair” for a transformer.
Ignoring the trend. A reading of 50 MΩ is fine if stable. The same reading is alarming if previous readings were 200, 180, 150, 120, 80 MΩ.
Not discharging before reading. Residual charge affects the initial reading. Discharge for at least 1 minute before testing. After a PI test, discharge for 40 minutes minimum.
Applying the old “(kV + 1) MΩ” rule to modern equipment. IEEE 43-2013 replaced this with 100 MΩ for form-wound coils made after 1970.
FAQ
What is a good insulation resistance reading?
It depends on the equipment. For a 400V random-wound motor: ≥5 MΩ per IEEE 43. For industrial machinery: ≥1 MΩ per IEC 60204-1. For protection relays: ≥100 MΩ per IEC 60255-5. New equipment should read much higher than the minimums.
My reading is high but the PI is low. What does that mean?
If IR exceeds 5 GΩ, IEEE 43 says the PI may not be meaningful at those tiny current levels. Your insulation is excellent — trust the absolute value.
How often should I test?
Critical motors: every 6 months. Standard motors above 100 kW: annually. Building installations: every 3–5 years. Transformers: IR every 2–3 years, DGA annually.
Key Takeaways
- Three numbers matter: spot reading (IR), polarization index (PI), and dielectric absorption ratio (DAR).
- Use the correct standard for your equipment type — not a one-size-fits-all rule.
- Temperature correction is essential. IR halves for every 10°C rise.
- Motor PI minimum: 2.0 for Class F/H. Transformer PI: 1.25 is “fair.” Different equipment, different criteria.
- Trending beats single readings. Record conditions and plot over time.
- IR below 1 MΩ or PI below 1.0 = do not energize.
Standards Referenced
| Standard | Key Content |
|---|---|
| IEEE 43-2013 | Motor IR minimums, PI minimums, PI invalid above 5 GΩ, temp correction to 40°C |
| IEEE C57.152-2013 | Transformer PI interpretation, temp correction to 20°C |
| IEC 60204-1 | Machinery: 500V DC, ≥1 MΩ |
| IEC 60204-11 | HV machinery: rated V or 5 kV, ≥1 MΩ |
| IEC 60364-6 | LV installations: 250/500/1000V DC, ≥0.5–1.0 MΩ |
| IEC 60255-5 | Protection relays: 500V DC, ≥100 MΩ |