Free Insulation Resistance Test Report

The insulation resistance test, done right

A megger test is the cheapest insurance on an electrical job. Ten minutes with an insulation tester before energizing finds the nicked conductor at a fitting, the wet splice in a handhole, and the motor that sat in a flooded pit — before they become a fault, a tripped main, or an arc-flash event. The test only protects you if it's recorded properly, though. A reading scrawled on a cable tag won't satisfy a commissioning agent, and it can't be trended next year.

The free workbook above gives you a professional IR Test Report for cables and equipment, a PI & DAR sheet for motors, transformers and critical cable, and instructions for doing the test safely.

Before you connect the tester

1. De-energize, lock out, and verify absence of voltage per NFPA 70E. An IR test is not a substitute for a voltage test. Our NFPA 70E approach boundaries chart and electrical JSA template help with the planning.
2. Isolate what shouldn't see the test voltage: surge arresters and capacitors, VFDs and soft starters, electronic trip units, meters, control transformers, PT and CT secondaries. Megging through a drive can destroy it.
3. Lift both ends of cable when you can. Testing a feeder with the panel landed tests the panel too — which is fine if you want that, but know which one you're reading.
4. Clean and dry the terminations. Surface leakage across a dirty lug reads like bad insulation. Use the guard terminal to separate surface leakage from volume leakage on critical tests.
5. Check the instrument. It must be in calibration — the report flags an expired calibration date.

Choosing the test voltage

Pick the DC test voltage from the equipment's rating and the project specification. Typical practice:

Higher is not better. Hitting old insulation with more voltage than it needs can fail it. Record the voltage you actually used.

Recording readings: phase to ground and phase to phase

For three-phase cable and equipment, the report takes six 1-minute readings: A–G, B–G, C–G (each phase to ground with the other phases grounded) and A–B, B–C, C–A. The sheet finds the lowest reading, applies the temperature correction, and compares it to the minimum you enter. Look at the phases against each other, too: in the example on the template, a motor branch reads 850 and 900 MΩ on A and B but 45 MΩ on C — C fails, and the note says where to look (the pull, and the fittings where the insulation can be nicked).

If the tester shows over-range (">5 GΩ"), enter its full-scale value as a number and note it. Text entries are ignored by the formulas.

Temperature correction

Insulation resistance drops as temperature goes up — for many insulation systems it roughly halves for every 10 °C rise. A cable that reads 2,000 MΩ on a cold morning might read 1,000 MΩ on a hot afternoon with nothing wrong. To compare tests, every reading is corrected to a reference temperature:

• NETA ATS tables commonly correct to 20 °C for cables and electrical apparatus.
• IEEE 43 corrects rotating machine windings to 40 °C.

You enter the correction factor (a multiplier) for the recorded temperature from your standard's table or your test set's built-in correction. If you leave it blank, the sheet uses 1.0 and marks the result "(no temp corr.)" so nobody mistakes it for a corrected value. See our IEEE 43 insulation resistance testing guide for a worked example.

Polarization index and dielectric absorption ratio

A single 1-minute number tells you about surface moisture and contamination. Timed readings tell you about the insulation itself. When DC voltage is applied, good insulation keeps absorbing charge and the resistance keeps climbing for minutes; wet or contaminated insulation flattens out almost immediately.

• Polarization index (PI) = 10-minute reading ÷ 1-minute reading. IEEE 43 recommends a minimum around 2.0 for modern Class B and higher windings.
• Dielectric absorption ratio (DAR) = 60-second reading ÷ 30-second reading. A quick screen when a 10-minute test isn't practical; minimums are a rule of thumb, not a standard.

One catch: when the 1-minute reading is very high (on the order of 5,000 MΩ), IEEE 43 notes the PI may not mean much — the current is so small the ratio is noise. The PI & DAR sheet switches to "PASS on IR" above the threshold you set. The example flooded motor reads 10, 12 and 15 MΩ — PI 1.25 — and fails: clean it, dry it out, and retest.

What makes a test report hold up

• Instrument, serial number, calibration date and due date on every page.
• Test voltage and duration for every reading.
• Temperature and humidity — without them, nobody can trend the result.
• What was connected during the test (both ends lifted? motor leads disconnected?).
• The minimum you judged against, and where it came from (spec section, manufacturer, standard).
• Signatures — the technician, the witness (owner or commissioning agent), and the reviewer.

After the test

Ground and discharge every item for at least as long as the test voltage was applied — longer after a 10-minute PI. Windings and long cables hold a charge that can hurt someone. Re-land the conductors, torque the terminations, and update the status on your cable schedule. Failed items get tagged out and stay off until they're repaired and retested. The completed reports go in the closeout package — see the project closeout checklist.

Keep test records with the job, not in a folder

Field PM's QA/QC module ties test reports, inspections and hold-point sign-offs to the project, so the turnover package is ready when the work is. Built by a licensed electrical contractor. Start a free 30-day trial — no credit card required.

This template is a field test record, not engineering advice. Acceptance criteria come from the project specification, the equipment manufacturer and the standards you test to (IEEE 43, ANSI/NETA ATS and MTS), which are referenced here in our own words — use the current editions. All testing must follow NFPA 70E and your company's electrical safety program.