Polarisation Index and DAR Calculator
PI and DAR from timed insulation resistance readings. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates DAR and PI from reading at 30 s, reading at 1 min and reading at 10 min. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Commonly quoted guide for PI: below 1 poor, 1 to 2 questionable, 2 to 4 acceptable, above 4 good. A very high resistance can give a low PI that is still healthy. Follow your test standard for pass limits.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60034-27-4, 2018 Ed 1.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Recommended test procedures for insulation resistance and polarisation index of rotating-machine windings are relevant. The calculator computes ratios only; test voltage, temperature correction, acceptance criteria and machine-specific limits remain outside scope. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Reading at 30 s | 1,000 MΩ |
| Reading at 1 min | 1,500 MΩ |
| Reading at 10 min | 4,500 MΩ |
- DAR = 1,500 / 1,000 = 1.5
- PI = 4,500 / 1,500 = 3
Assumptions
- Same test voltage and temperature for all readings
- Readings taken after the insulation is fully discharged
Commonly quoted guide for PI: below 1 poor, 1 to 2 questionable, 2 to 4 acceptable, above 4 good. A very high resistance can give a low PI that is still healthy. Follow your test standard for pass limits.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Reading at 30 s | MΩ | greater than 0 MΩ | |
| Reading at 1 min | MΩ | greater than 0 MΩ | |
| Reading at 10 min | MΩ | greater than 0 MΩ |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| DAR (60 s ÷ 30 s) | (number) | Shown to 2 decimal places | |
| PI (10 min ÷ 1 min) | (number) | Shown to 2 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with reading at 30 s
| Reading at 30 s (MΩ) | DAR | PI |
|---|---|---|
| 100 | 15 | 3 |
| 250 | 6 | 3 |
| 500 | 3 | 3 |
| 1,000 | 1.5 | 3 |
| 2,000 | 0.75 | 3 |
| 4,000 | 0.38 | 3 |
| 10,000 | 0.15 | 3 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60364-5-52 (selection and erection of wiring systems, current-carrying capacity)
- IEC 60364-4-43 (protection against overcurrent, short-circuit withstand k factors)
- IEC 60228 (conductor sizes)
- IEC 60287 (calculation of cable current rating)
Ampacity and grouping factors come from your cable data sheet or your licensed copy of the standard; see STANDARDS.md for the method constants Elecmetric does use. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter reading at 30 s, reading at 1 min and reading at 10 min. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Same test voltage and temperature for all readings. Readings taken after the insulation is fully discharged.
Which calculators should I use next?
Common next steps are Insulation Resistance Temperature Correction and Cable Resistance. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Insulation Resistance Formulas: PI and DAR: Insulation resistance formulas: polarisation index, dielectric absorption ratio, correction to 40 °C
More Cable Engineering calculatorsAll engineering calculatorsPolarisation Index and Dielectric Absorption Ratio in Elecmetric