Insulation resistance tester
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An insulation resistance tester, known as a megohmmeter when it reads directly in megohms, is an instrument that applies a direct voltage to electrical insulation and indicates the resistance that the insulation presents. [1] [2] IEEE 43-2013 recognizes direct-reading megohmmeters and resistance bridges for the measurement and also describes calculating the resistance from separately measured voltage and current. [1] [3]
What it measures
The instrument measures insulation resistance, which IEEE 43-2013 expresses in megohms; one megohm equals 1,000,000 Ω. [4] [5] Readings taken at set times during a single application of voltage also yield the polarization index, and earlier readings in the same test yield the dielectric absorption ratio. [6] [7]
Principle
The instrument applies a direct voltage and determines the ratio of that voltage to the current through the insulation, a resistance in ohms because the ohm is the volt per ampere. [8] [2] Direct-reading megohmmeters may be hand-cranked, battery-powered or rectifier-powered. [1]
In the voltmeter and microammeter method, a regulated direct-voltage supply drives the current and the resistance is calculated from the two readings. [3] When a low resistance is measured, allowance is made for the voltage drop across the protective resistor in the test set. [9]
Used in
The instrument is used for the insulation resistance test and for the polarization index and dielectric absorption test. [10] [11] In acceptance testing, section 5.2 of ANSI/NETA ATS-2025 addresses the suitability of test equipment and section 5.3 the calibration of test instruments. [12]
Notes
- IEEE 43-2013, clause 9.1. IEEE 43-2013 recognizes direct-reading megohmmeters, whether hand-cranked, battery-powered or rectifier-powered, and resistance bridges as instruments for measuring insulation resistance. Datum instr.insulation-resistance-tester.direct-reading
- IEEE 43-2013, clause 9.2. IEEE 43-2013 treats insulation resistance as the quotient of a direct voltage applied across the insulation and the current that the voltage drives through it, read at a stated time after the voltage is applied. Datum tests.insulation-resistance.definition
- IEEE 43-2013, clause 9.2. IEEE 43-2013 also describes obtaining insulation resistance by calculation from a separately measured voltage and current, using a regulated direct-voltage supply, a voltmeter and a microammeter. Datum instr.insulation-resistance-tester.voltmeter-ammeter
- IEEE 43-2013, clause 9.2. IEEE 43-2013 expresses insulation resistance in megohms. Datum tests.insulation-resistance.megohm-unit
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 5. Applying the SI prefix mega, symbol M, which denotes the factor one million, to the ohm gives the megohm, equal to one million ohms.Value: 1,000,000 Ω. Datum qty.ohm.megohm
- 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
- 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
- NIST SP 811, web page updated 2025-08-18 edition, clause Table 3. In the NIST Guide to the SI, the ohm, symbol Ω, is the SI coherent derived unit of electric resistance and is expressed in other SI units as the volt per ampere.Value: V/A. Datum qty.ohm.definition
- IEEE 43-2013, clause 10. IEEE 43-2013 notes that when a low insulation resistance is measured, allowance is made for the voltage drop across the protective resistor of the test set. Datum instr.insulation-resistance-tester.protective-resistor
- Electrical Testing Reference editorial taxonomy, 2026-10 edition. The test method is named Insulation Resistance.Value: Insulation Resistance. Datum tests.insulation-resistance.title(provenance pending)
- Electrical Testing Reference editorial taxonomy, 2026-10 edition. The test method is named Polarization Index and Dielectric Absorption.Value: Polarization Index and Dielectric Absorption. Datum tests.polarization-index-and-dielectric-absorption.title(provenance pending)
- ANSI/NETA ATS-2025, clause 5.2, 5.3. Section 5.2 of ANSI/NETA ATS-2025 deals with the suitability of test equipment, and section 5.3 deals with the calibration of test instruments. Datum instr.insulation-resistance-tester.ats-suitability
Data records
- Section 5.2 of ANSI/NETA ATS-2025 deals with the suitability of test equipment, and section 5.3 deals with the calibration of test instruments.
- IEEE 43-2013 recognizes direct-reading megohmmeters, whether hand-cranked, battery-powered or rectifier-powered, and resistance bridges as instruments for measuring insulation resistance.
- IEEE 43-2013 notes that when a low insulation resistance is measured, allowance is made for the voltage drop across the protective resistor of the test set.
- IEEE 43-2013 also describes obtaining insulation resistance by calculation from a separately measured voltage and current, using a regulated direct-voltage supply, a voltmeter and a microammeter.
Sources
- ANSI/NETA ATS-2025, Acceptance Testing Specifications for Electrical Power Equipment and Systems, 2025 edition, NETA. Cited: clause 5.2, 5.3. 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 9.1; clause 9.2; clause 5.4; clause A.1; clause 10. Held as: Purchased single-user copy. 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 5; clause Table 3. Held as: Government work. Publisher page
Obtaining the documents
- ANSI/NETA ATS-2025: publisher catalog or sales page
- IEEE 43-2013: publisher catalog or sales page
Changelog (24 entries)
| Date | Revision | Change | Reason | By |
|---|---|---|---|---|
| 2026-10-13 | created (instr.insulation-resistance-tester.ats-suitability) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | provenance noted (instr.insulation-resistance-tester.ats-suitability) | locator from the NETA index table of contents (section and table numbers, titles and tables_referenced only); no body text used | agent | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.ats-suitability) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (instr.insulation-resistance-tester.direct-reading) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.direct-reading) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (instr.insulation-resistance-tester.protective-resistor) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.protective-resistor) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (instr.insulation-resistance-tester.voltmeter-ammeter) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.voltmeter-ammeter) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | 1 | created | hand-written exemplar for the article generator (Task 10) | agent |
| 2026-10-13 | created (qty.ohm.definition) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.definition) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (qty.ohm.megohm) | read from the government page (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (qty.ohm.megohm) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (tests.insulation-resistance.definition) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (tests.insulation-resistance.definition) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (tests.insulation-resistance.megohm-unit) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (tests.insulation-resistance.megohm-unit) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (tests.insulation-resistance.title) | seeded from the test taxonomy | agent | |
| 2026-10-13 | created (tests.polarization-index-and-dielectric-absorption.dar-thirty-second) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (tests.polarization-index-and-dielectric-absorption.dar-thirty-second) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (tests.polarization-index-and-dielectric-absorption.pi-definition) | method fact restated from the licensed-copy digest (Task 10 exemplars) | agent | |
| 2026-10-13 | reviewed (tests.polarization-index-and-dielectric-absorption.pi-definition) | fresh-context review task-10-exemplars-review.md + fix round 1 | agent | |
| 2026-10-13 | created (tests.polarization-index-and-dielectric-absorption.title) | seeded from the test taxonomy | agent |
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Permanent revision URL: https://www.electricaltestingreference.com/instruments/insulation-resistance-tester/rev/1/. Current version: https://www.electricaltestingreference.com/instruments/insulation-resistance-tester/.
Plain
"Insulation resistance tester." Electrical Testing Reference. Revision 1, 2026-10-13. https://www.electricaltestingreference.com/instruments/insulation-resistance-tester/rev/1/.
Legal (Bluebook, internet source)
Electrical Testing Reference, Insulation resistance tester (rev. 1, Oct. 13, 2026), https://www.electricaltestingreference.com/instruments/insulation-resistance-tester/rev/1/.
BibTeX
@misc{etr-instruments-insulation-resistance-tester-rev1,
title = {Insulation resistance tester},
howpublished = {\url{https://www.electricaltestingreference.com/instruments/insulation-resistance-tester/rev/1/}},
organization = {Electrical Testing Reference},
publisher = {Electrical Testing Reference},
year = {2026},
note = {Revision 1; last substantive change 2026-10-13}
} Wikipedia
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