Insulation resistance testing: voltages, limits, records
A single reading tells you almost nothing. A trend, taken the same way each time, predicts failures months ahead.
Isolate, lock out, verify dead and discharge before connecting. Insulation testers apply high DC voltage and windings and cables store charge afterwards — always discharge through the instrument before touching terminals. Disconnect drives, soft starters and surge protection first.
Choosing the test voltage
Too low and the test tells you nothing about stress at working voltage; too high and you damage the insulation you are trying to assess. The usual practice:
| Equipment rated voltage | Test voltage | Typical instrument |
|---|---|---|
| Up to 240 V control circuits | 250 V DC | Handheld 250/500/1000 V tester |
| 415 V motors, LT cables | 500 V DC | Handheld 500 V / 1 kV tester |
| Up to 1100 V systems | 1000 V DC | 1 kV digital tester |
| 3.3 kV / 6.6 kV machines | 2.5–5 kV DC | 5 kV tester with PI / DAR |
| 11 kV cables, HT motors | 5 kV DC | 5 kV tester, guard terminal |
A guard terminal matters on dirty or damp installations: it diverts surface leakage so you measure the insulation, not the muck on the terminal box.
What counts as acceptable
The old rule of thumb — 1 MΩ per kV of rating plus 1 MΩ at 40 °C — is a floor, not a target. A healthy modern LT winding usually reads hundreds of megohms. If a motor that read 800 MΩ last quarter reads 90 MΩ today, it is failing even though both numbers "pass".
Two ratios turn a single number into a diagnosis:
Polarisation index (PI)
10-minute reading ÷ 1-minute reading. Above 2 is good; below 1.5 warrants investigation. Very high-resistance windings can show a low PI while being healthy — read it with the absolute value.
Dielectric absorption (DAR)
60-second reading ÷ 30-second reading. The quick version when a ten-minute test is not practical during a short shutdown window. Above 1.4 is generally acceptable.
Insulation resistance roughly halves for every 10 °C rise in winding temperature. Comparing a hot machine against a cold baseline creates false alarms and hides real degradation. Record winding temperature every time and correct to a common reference before trending.
A record format that survives an audit
Keep one row per test, per machine, in a single sheet — not scattered across shift diaries. The columns that matter:
- Date, equipment tag, rating and location
- Test voltage applied and instrument serial number
- R–E, Y–E, B–E and phase-to-phase readings at 1 minute
- 10-minute reading and PI where taken
- Winding temperature and ambient humidity
- Tested by, and any deviation from the previous reading
Test the same machines at the same point in the maintenance cycle, with the same instrument where possible. Consistency is what makes the trend trustworthy — and a trend is what lets you plan a rewind at a convenient shutdown instead of at 2 a.m. on a Sunday.
Need the right tester for your plant?
Tell us the highest voltage class you maintain and whether you need PI, DAR and downloadable records. We will recommend a Metravi, Kyoritsu or Fluke model that fits the budget and the job.
Quick answers
What is the minimum acceptable value?+
1 MΩ per kV of rating plus 1 MΩ at 40 °C is the classic floor, with about 1 MΩ as an absolute practical minimum for LT machines. Treat it as a floor — the trend is the real test.
What is a good PI?+
Above 2 is good, below 1.5 is questionable. Very high-resistance windings can read a low PI while being healthy, so never judge on PI alone.
Can I test with the VFD connected?+
No. Disconnect the drive output cables first — test voltage on drive terminals destroys the power electronics. See our drives page for related commissioning notes.