Quick answer: Insulation resistance (IR) in electrical panels often falls after humid weather or condensation, because moisture and contamination on insulating surfaces create leakage paths, and some insulation materials absorb water. But a low reading doesn't by itself tell you the cause, and a rebound after drying doesn't prove the insulation is sound. Compare readings only when they are taken on the same circuit, at the same test voltage and reading time, and corrected for temperature. Record local humidity, dew point and surface temperature with each reading. A qualified electrical person decides whether equipment can be energised. Moisture control reduces the conditions that cause humidity-related declines.
Quick answer
- Moisture and contamination lower surface IR; temperature also changes readings.
- Compare like with like: same circuit, voltage, time, temperature correction.
- Pair IR records with dew point and surface temperature; electrical decisions stay with qualified staff.
What an IR reading represents
An IR test applies a DC voltage and measures current. The current includes capacitive charging, absorption into the insulation, surface leakage and conduction through the insulation, and the mix changes over the test period. Surface leakage rises sharply when surfaces are damp or dirty. Readings also fall as temperature rises; for rotating machines, IEEE Std 43 corrects readings to a reference temperature.
Make readings comparable
| Item | Record |
|---|---|
| Asset and circuit | Same test object and configuration (connected loads isolated the same way) |
| Test voltage | Same voltage |
| Reading time | Same, e.g. 1 minute; or polarisation index (10 min ÷ 1 min) |
| Temperature | Insulation or ambient temperature; correction applied |
| Instrument | Model, calibration date |
| Environment | Local air temperature, RH, dew point; surface temperature |
| Weather | Recent rain, fog, temperature swings |
Read the pattern
| Pattern | Possible meaning | Next step |
|---|---|---|
| Falls in humid weather, recovers when dry | Surface moisture or contamination | Inspect surfaces; moisture control; cleaning |
| Falls and doesn't recover | Damage, absorbed moisture, contamination | Electrical investigation |
| Low on one circuit only | Local wetting or a fault on that circuit | Inspect that compartment and its cable entry |
| Gradual decline over months | Ageing, contamination build-up | Maintenance plan |
None of these is a diagnosis on its own.
Align moisture and electrical records
- Put IR readings on the same timeline as dew point and surface temperature.
- Note condensation events (surfaces at or below dew point).
- Note door openings, shutdowns and heater or dehumidifier status.
See diagnosing enclosure condensation and cable trench moisture.
Keep electrical acceptance separate from moisture control
- Whether to energise, repair or return to service follows the equipment's procedures and a qualified electrical person's decision.
- Moisture control is designed separately: find the moisture path, then size heating or dehumidification for what remains; see heater vs ventilation vs dehumidifier.
Our electrical cabinet dehumidifiers keep the enclosure dew point below its surfaces.
FAQ
Frequently asked questions
Does humidity affect insulation resistance?
Yes; moisture on surfaces and in some insulation creates leakage paths and lowers readings.
Why does insulation resistance recover after drying?
Surface moisture evaporates; but recovery doesn't prove the insulation is undamaged.
Can a dehumidifier improve insulation resistance?
It reduces humidity-related surface leakage by preventing condensation; it doesn't repair damaged insulation.
Sources
- IEEE Std 43, Recommended practice for testing insulation resistance of electric machinery.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
