Checks a measured 1-minute insulation resistance reading against IEEE 43's traditional minimum-IR rule of thumb for rotating machinery: IR(1min, 40°C) ≥ kV_rated + 1 megohms — still a widely used quick field-acceptance check.
Standards followed
IEEE 43-2013 (recommended practice for testing insulation resistance of rotating machinery) — Table 3 minimum IR by winding type
What it can do
- Minimum required IR from the winding type (and rated voltage for pre-1970/field windings).
- Temperature correction of the measured reading back to the 40°C reference, using the common field rule of thumb that IR approximately halves for every 10°C rise (doubles for every 10°C fall).
- Pass/fail verdict.
Example problem
6.6kV form-wound motor built after 1970, measured 250 megohms at 40°C.
- Minimum required for a post-1970 form-wound winding = 100 megohms (IEEE 43 Table 3).
- Measured at the 40°C reference already, so no correction needed: 250 megohms.
- 250 ≥ 100 → pass. (The older kV + 1 = 7.6 megohm figure applies only to pre-1970 and field windings.)
Pass.
Want the full step-by-step version?
The kV + 1 MΩ value applies only to windings made before about 1970 and to field windings; modern form-wound windings need at least 100 MΩ and random-wound or sub-1 kV windings at least 5 MΩ. The temperature-correction rule of thumb (halving/doubling per 10°C) is a common field approximation, not a precise physical law — for critical decisions, take readings at or near 40°C directly rather than relying on the correction.