Insulation resistance tester
Updated 2026-10-13
This is revision 1 of the entry. It is the current revision. The current version is at /instruments/insulation-resistance-tester/.
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] [Electrical Testing Reference] 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] [2]
What it measures
The instrument measures insulation resistance, which IEEE 43-2013 expresses in megohms; one megohm equals recorded on Electrical Testing Reference. [Electrical Testing Reference] [Electrical Testing Reference] 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. [Electrical Testing Reference] [Electrical Testing Reference]
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. [Electrical Testing Reference] [Electrical Testing Reference] 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. [2] When a low resistance is measured, allowance is made for the voltage drop across the protective resistor in the test set. [3]
Used in
The instrument is used for the insulation resistance test and for the polarization index and dielectric absorption test. [Electrical Testing Reference] [Electrical Testing Reference] 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. [4]
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 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 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
- 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
- IEEE 43-2013, Recommended Practice for Testing Insulation Resistance of Electric Machinery, 2013 edition, IEEE. Cited: clause 9.1; clause 9.2; clause 10. Held as: Purchased single-user copy. Publisher page
Obtaining the documents
- ANSI/NETA ATS-2025: publisher catalog or sales page
- IEEE 43-2013: publisher catalog or sales page
Changelog (10 entries)
| Date | Revision | Change |
|---|---|---|
| 2026-10-13 | created (instr.insulation-resistance-tester.ats-suitability) | |
| 2026-10-13 | provenance noted (instr.insulation-resistance-tester.ats-suitability) | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.ats-suitability) | |
| 2026-10-13 | created (instr.insulation-resistance-tester.direct-reading) | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.direct-reading) | |
| 2026-10-13 | created (instr.insulation-resistance-tester.protective-resistor) | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.protective-resistor) | |
| 2026-10-13 | created (instr.insulation-resistance-tester.voltmeter-ammeter) | |
| 2026-10-13 | reviewed (instr.insulation-resistance-tester.voltmeter-ammeter) | |
| 2026-10-13 | 1 | created |
Cite this page
Cite the governing standard for the fact itself; cite this page as the finding aid.
Permanent revision URL: https://www.testinstrumentreference.com/instruments/insulation-resistance-tester/rev/1/. Current version: https://www.testinstrumentreference.com/instruments/insulation-resistance-tester/.
"Insulation resistance tester." Test Instrument Reference. Revision 1, 2026-10-13. https://www.testinstrumentreference.com/instruments/insulation-resistance-tester/rev/1/.
Test Instrument Reference, Insulation resistance tester (rev. 1, Oct. 13, 2026), https://www.testinstrumentreference.com/instruments/insulation-resistance-tester/rev/1/.
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organization = {Test Instrument Reference},
publisher = {Test Instrument Reference},
year = {2026},
note = {Revision 1; last substantive change 2026-10-13}
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