Electrical Testing Reference

Polarization Index and Dielectric Absorption

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The polarization index and dielectric absorption test extends the insulation resistance test by taking readings at two different times during a single application of direct voltage and expressing them as a ratio. [1] [2] The polarization index divides the reading at 10 min by the reading at 1 min, while the dielectric absorption ratio uses a pair of readings taken earlier in the test. [3] [4] [5] Both appear in ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023, and IEEE 43-2013 describes the polarization index for the windings of rotating machines. [6] [7] [8]

Definition

IEEE 43-2013 defines the polarization index as the insulation resistance measured after the voltage has been applied for 10 min, divided by the insulation resistance measured after 1 min. [1] [3] [4] Annex A of the recommended practice describes variants built on shorter reading times; one of them divides the one-minute reading by a reading taken at 30 s, a ratio of the kind known as the dielectric absorption ratio. [9] [10] [5]

Being ratios of two resistances, both quantities are dimensionless, and the NIST Guide to the SI notes that the unit one is generally left unwritten when such values are stated. [11] [1]

Purpose

The index adds a time dimension to the insulation resistance test. IEEE 43-2013 notes that readings at intervals through the test are commonly recorded so that resistance can be plotted against time, and it reads a sharp fall in the index between tests as a sign of contamination, moisture or cracked insulation. [12] [13] A film of moisture on a winding at or below the dew point lowers both the resistance and the index. [14]

During maintenance the index also serves as a guide to when the drying of a winding can end. [15] IEEE 43-2013 applies no temperature correction to it. [16]

Principle

The index rests on the absorption current, the part of the measured current that dies away while the test voltage is held. [17] Because that component decays, a reading taken late in the test differs from the one-minute reading, and the ratio of the two summarizes how the insulation behaves over the first minutes of voltage application. [1] [17] In small random-wound machines the absorption current decays within a few seconds, which is why IEEE 43-2013 regards the index as not meaningful for them. [18]

Two further conditions limit its use. Where the one-minute resistance at the reference temperature is very high, the measured current becomes so small that the recommended practice allows the index to be disregarded. [19] A continuous stress-control coating over the whole end-winding region can pull the index toward unity. [20]

General procedure

The measurement uses the arrangement of the insulation resistance test: the winding is deenergized, external equipment is disconnected and grounded, and the conditions of the test are recorded. [21] [22] [23] The test voltage is applied and held constant, the one-minute reading and the later reading are taken without interrupting it, and intermediate readings are commonly noted for plotting. [24] [3] [12]

When a hot winding cools during the test and the index comes out high, IEEE 43-2013 calls for the test to be repeated at a lower winding temperature. [16]

Evaluation

Clause 12.2 of IEEE 43-2013 recommends minimum polarization index values that depend on the thermal class of the insulation, and the recommended practice expects a machine to meet both these and its one-minute insulation resistance recommendations before it returns to service or undergoes an overvoltage test. [25] [26] The index is not applied to non-insulated field windings, such as squirrel-cage rotor windings, or to direct-current armatures with exposed commutator copper. [27] Annex A states that no agreed pass or fail criteria exist for the shorter variant ratios. [9]

ANSI/NETA ATS-2025 evaluates polarization index results for rotating machinery in its own text, beginning with section 7.15.1 for AC induction motors and generators, and it also uses a dielectric absorption ratio, which IEEE 43-2013 treats only as an informative variant. [28] [29] The two documents do not set identical criteria in every case, and the numerical criteria are set out in the cited clauses. [28] [25]

Acceptance and maintenance testing

ANSI/NETA ATS-2025 calls for the test at acceptance and ANSI/NETA MTS-2023 in maintenance testing. [6] [7] Among the equipment classes, the acceptance specification lists it for liquid-filled shunt and current-limiting reactors, and the maintenance specification lists it for the same reactors and for induction voltage regulators. [30] [31] [32]

Notes

  1. IEEE 43-2013, clause 5.4. IEEE 43-2013 defines the polarization index as the insulation resistance read after ten minutes of applied voltage divided by the insulation resistance read after one minute. Datum tests.polarization-index-and-dielectric-absorption.pi-definition
  2. Electrical Testing Reference editorial taxonomy, 2026-10 edition. The polarization index is the ratio of two insulation resistance readings taken at two times after a direct voltage is applied, the later reading divided by the earlier one. Datum gloss.polarization-index.definition
  3. IEEE 43-2013, clause 5.4. The later of the two readings that make up the polarization index under IEEE 43-2013 is taken ten minutes after the test voltage is applied.Value: 10 min. Datum tests.polarization-index-and-dielectric-absorption.ten-minute
  4. IEEE 43-2013, clause 5.3. Under IEEE 43-2013 the reference insulation resistance reading is the one taken when the test voltage has been applied for one minute.Value: 1 min. Datum tests.insulation-resistance.one-minute-reading
  5. Electrical Testing Reference editorial taxonomy, 2026-10 edition. The dielectric absorption ratio is the ratio of two insulation resistance readings taken a short time apart early in a direct-voltage test, the later reading divided by the earlier one. Datum gloss.dielectric-absorption-ratio.definition
  6. ANSI/NETA ATS-2025, clause 7.20.3.2.B.2. Polarization Index and Dielectric Absorption is addressed in ANSI/NETA ATS-2025. Datum tests.polarization-index-and-dielectric-absorption.governed-by.ansi-neta-ats(provenance pending)
  7. ANSI/NETA MTS-2023, clause 7.20.3.2.B. Polarization Index and Dielectric Absorption is addressed in ANSI/NETA MTS-2023. Datum tests.polarization-index-and-dielectric-absorption.governed-by.ansi-neta-mts(provenance pending)
  8. IEEE 43-2013. The recommended practice describes direct-voltage procedures for measuring the insulation resistance and polarization index of stator and rotor windings and explains how to interpret the results. Datum std.ieee-43.scope
  9. IEEE 43-2013, clause Annex A. Annex A of IEEE 43-2013 describes variants of the polarization index built on shorter reading times and states that no agreed pass or fail criteria exist for them. Datum tests.polarization-index-and-dielectric-absorption.annex-variants
  10. IEEE 43-2013, clause A.1. One variant in Annex A of IEEE 43-2013 divides the one-minute insulation resistance by a reading taken thirty seconds after the voltage is applied.Value: 30 s. Datum tests.polarization-index-and-dielectric-absorption.dar-thirty-second
  11. NIST SP 811, web page updated 2025-08-18 edition, clause Table 3, note b. The NIST Guide to the SI notes that the symbol for the derived unit one is generally omitted when values of dimensionless quantities are stated. Datum qty.dissipation-factor.unit-one
  12. IEEE 43-2013, clause 5.4. IEEE 43-2013 notes that readings at several intervals during the test are commonly recorded so that resistance can be plotted against time. Datum tests.polarization-index-and-dielectric-absorption.intermediate-readings
  13. IEEE 43-2013, clause 11.1. IEEE 43-2013 reads a sharp fall in the polarization index between tests, like a sharp fall in the one-minute insulation resistance, as a sign of contamination, moisture or cracked insulation. Datum tests.polarization-index-and-dielectric-absorption.trend-fall
  14. IEEE 43-2013, clause 6.2, 6.3.1. IEEE 43-2013 notes that when a winding is at or below the dew point a film of moisture can form on it and lower both the insulation resistance and the polarization index. Datum tests.insulation-resistance.dew-point
  15. IEEE 43-2013, clause 11.2. IEEE 43-2013 notes that the polarization index can indicate when the drying of a winding may be stopped. Datum tests.polarization-index-and-dielectric-absorption.drying
  16. IEEE 43-2013, clause 6.3.4, 11.1. IEEE 43-2013 applies no temperature correction to the polarization index, and it calls for a repeat test at a lower temperature when a hot winding that cooled during the test gives a high result. Datum tests.polarization-index-and-dielectric-absorption.no-temperature-correction
  17. IEEE 43-2013, clause 5.4. IEEE 43-2013 links the usefulness of the polarization index to the absorption current, the part of the measured current that dies away while the voltage is held, so the index loses its meaning where that current disappears within seconds. Datum tests.polarization-index-and-dielectric-absorption.absorption-current
  18. IEEE 43-2013, clause 5.4. IEEE 43-2013 regards the polarization index as not meaningful for small random-wound machines, because their absorption current dies away within a few seconds. Datum tests.polarization-index-and-dielectric-absorption.small-machines
  19. IEEE 43-2013, clause 12.2.2. When the one-minute insulation resistance at the reference temperature is very high, the measured current is so small that IEEE 43-2013 allows the polarization index to be disregarded. Datum tests.polarization-index-and-dielectric-absorption.high-ir-disregard
  20. IEEE 43-2013, clause 12.2.3. IEEE 43-2013 notes that a continuous stress-control coating over the whole end-winding region can pull the polarization index toward unity. Datum tests.polarization-index-and-dielectric-absorption.stress-control
  21. 29 CFR 1910.333, current edition, clause 1910.333(a)(1). Under 29 CFR 1910.333(a)(1), live parts to which an employee may be exposed are to be deenergized before the employee works on or near them, unless the employer shows that deenergizing creates added hazards or is infeasible. Datum tests.insulation-resistance.osha-deenergize
  22. IEEE 43-2013, clause 8. IEEE 43-2013 has cables, surge arresters, capacitors, voltage transformers and similar external equipment disconnected from the winding and grounded before the test, with any item left connected noted in the record. Datum tests.insulation-resistance.external-equipment
  23. IEEE 43-2013, clause 7. IEEE 43-2013 lists what is recorded with each test, among them the winding temperature, the temperature, humidity and dew point of the surrounding air, how long the machine has been out of service, the voltage applied and how the winding was connected. Datum tests.insulation-resistance.records
  24. IEEE 43-2013, clause 10. IEEE 43-2013 brings the test voltage to its full value as quickly as practical and keeps it constant for the whole measurement. Datum tests.insulation-resistance.voltage-application
  25. IEEE 43-2013, clause 12.2, Table 3. Clause 12.2 of IEEE 43-2013 recommends minimum polarization index values that depend on the thermal class of the insulation. Datum tests.polarization-index-and-dielectric-absorption.minimum-shape
  26. IEEE 43-2013, clause 11.2. IEEE 43-2013 expects a machine to clear both its polarization index and its one-minute insulation resistance recommendations before it is returned to operation or given an overvoltage test. Datum tests.polarization-index-and-dielectric-absorption.both-minimums
  27. IEEE 43-2013, clause 12.2.1. IEEE 43-2013 does not apply the polarization index to non-insulated field windings, such as squirrel-cage rotor windings, or to direct-current armatures with exposed commutator copper. Datum tests.polarization-index-and-dielectric-absorption.not-applicable
  28. ANSI/NETA ATS-2025, clause 7.15.1.D.2. ANSI/NETA ATS-2025 section 7.15.1 evaluates polarization index results for AC induction motors and generators in its own text, and its criteria do not coincide in every case with the minimums recommended in IEEE 43-2013. Datum tests.polarization-index-and-dielectric-absorption.ats-rotating
  29. ANSI/NETA ATS-2025, clause 7.15.1.B.2. ANSI/NETA ATS-2025 uses a dielectric absorption ratio for rotating machinery, a measure that IEEE 43-2013 describes only as an informative variant. Datum tests.polarization-index-and-dielectric-absorption.ats-dar
  30. ANSI/NETA ATS-2025, clause 7.20.3.2.B.2. Polarization Index and Dielectric Absorption is specified for liquid-filled shunt and current-limiting reactors in acceptance testing at clause 7.20.3.2.B.2 of ANSI/NETA ATS-2025.Value: liquid-filled shunt and current-limiting reactors. Datum tests.polarization-index-and-dielectric-absorption.equipment.reactor.acceptance(provenance pending)
  31. ANSI/NETA MTS-2023, clause 7.20.3.2.B. Polarization Index and Dielectric Absorption is specified for liquid-filled shunt and current-limiting reactors in maintenance testing at clause 7.20.3.2.B of ANSI/NETA MTS-2023.Value: liquid-filled shunt and current-limiting reactors. Datum tests.polarization-index-and-dielectric-absorption.equipment.reactor.maintenance(provenance pending)
  32. ANSI/NETA MTS-2023, clause 7.12.1.2.B.2. Polarization Index and Dielectric Absorption is specified for induction voltage regulators in maintenance testing at clause 7.12.1.2.B.2 of ANSI/NETA MTS-2023.Value: induction voltage regulators. Datum tests.polarization-index-and-dielectric-absorption.equipment.regulator.maintenance(provenance pending)

Data records

Related entries

Sources

  • 29 CFR 1910.333, 1910.333 - Selection and use of work practices, current edition, OSHA. Cited: clause 1910.333(a)(1). Held as: Government work. Publisher page
  • ANSI/NETA ATS-2025, Acceptance Testing Specifications for Electrical Power Equipment and Systems, 2025 edition, NETA. Cited: clause 7.20.3.2.B.2; clause 7.15.1.D.2; clause 7.15.1.B.2. Held as: Purchased single-user copy. Publisher page
  • ANSI/NETA MTS-2023, Maintenance Testing Specifications for Electrical Power Equipment and Systems, 2023 edition, NETA. Cited: clause 7.20.3.2.B; clause 7.12.1.2.B.2. Held as: Purchased single-user copy. Publisher page
  • Electrical Testing Reference editorial taxonomy, Electrical Testing Reference editorial taxonomy of test methods, 2026-10 edition, Electrical Testing Reference. Held as: Own observation.
  • IEEE 43-2013, Recommended Practice for Testing Insulation Resistance of Electric Machinery, 2013 edition, IEEE. Cited: clause 5.4; clause 5.3; clause A.1; clause Annex A; clause 11.1; clause 6.2, 6.3.1; clause 11.2; clause 6.3.4, 11.1; clause 12.2.2; clause 12.2.3; clause 8; clause 7; clause 10; clause 12.2, Table 3; clause 12.2.1. Held as: Purchased single-user copy. Publisher page
  • IEEE 43-2013, IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery, 2013 edition, IEEE. Held as: Public page. Publisher page
  • NIST SP 811, NIST Guide to the SI, Chapter 4: The Two Classes of SI Units and the SI Prefixes, web page updated 2025-08-18 edition, NIST. Cited: clause Table 3, note b. Held as: Government work. Publisher page

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@misc{etr-tests-polarization-index-and-dielectric-absorption-rev1,
  title = {Polarization Index and Dielectric Absorption},
  howpublished = {\url{https://www.electricaltestingreference.com/tests/polarization-index-and-dielectric-absorption/rev/1/}},
  organization = {Electrical Testing Reference},
  publisher = {Electrical Testing Reference},
  year = {2026},
  note = {Revision 1; last substantive change 2026-10-13}
}

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{{cite web |title=Polarization Index and Dielectric Absorption |website=Electrical Testing Reference |publisher=Electrical Testing Reference |url=https://www.electricaltestingreference.com/tests/polarization-index-and-dielectric-absorption/rev/1/ |date=2026-10-13 |access-date= |version=1}}