Insulation Resistance vs Hipot: What Each Test Actually Proves

IR and Hipot Measure Different Things

Two tests are routinely confused. The hipot (dielectric withstand) test answers “does the insulation survive a defined overvoltage for a defined time without breaking down?” The insulation resistance (IR) test answers “how well does the insulation resist a small DC voltage, and therefore how leaky is it?” A part can pass one and fail the other, and a buyer who only sees the hipot result is missing half the picture.

Both are destructive in the sense that a failure damages the part, and both are useful as production screens — but only if the test conditions are specified precisely enough that two suppliers’ results can be compared.

Test Comparison

Hipot vs insulation resistance testing
Item	Hipot (dielectric withstand)	Insulation resistance
Voltage	AC (often 1,500–3,000 V RMS) or DC equivalent	DC, typically 500 V or 1,000 V
What is measured	Pass/fail breakdown, sometimes leakage current threshold	Resistance in MΩ, at a defined electrification time
Duration	1 min type test; 1–5 s in production	60 s electrification (or as specified)
Typical limit	No breakdown; leakage below a stated threshold	≥ 100 MΩ at 500 V DC, or ≥ 1,000 MΩ for high-grade parts
Detects	Gross barrier defects, voids, contamination across the surface	Moisture, ionic contamination, incomplete cure, marginal barrier thickness
Risk to part	High — a failed part is damaged	Low, but repeated tests can polarize and mislead

Why IR Catches What Hipot Misses

A hipot test stresses the barrier for seconds. A moisture-contaminated or partially cured part can survive that pulse and still fail in the field, because its leakage current under continuous DC stress is high enough to heat the insulation over time. IR measured after a defined electrification period exposes exactly that condition. This is why IR is the better screen after impregnation and after humidity conditioning, and why a curing problem shows up as low IR rather than as a hipot failure.

IR also reveals ionic contamination from flux residues or handling. If a supplier’s IR distribution drifts downward across lots, the process has changed — usually a varnish batch or a washing step.

Test Conditions That Must Be Stated

Test voltage (DC for IR; AC RMS or DC for hipot) and the ramp rate.
Electrification time before the reading is taken — IR changes for tens of seconds as the insulation polarizes.
Temperature and humidity at test, since IR falls sharply with moisture.
Which windings are tested against which, and whether the core is included.
Pass criteria, including the leakage-current limit for hipot at production speed.
Whether the test is 100 % or sampled, and the AQL for the sampled case.

Production Practice

A common and workable sequence is: IR on 100 % of units at low voltage as a fast screen, then hipot on 100 % at reduced duration (1 s) with a leakage limit, plus a full 60 s type test on the first article and on a sampling basis. Repeating a full hipot on every unit is not necessary and adds stress; repeating IR after a humidity chamber is essential for parts destined to damp environments.

Also record the test station’s calibration and the fixture’s own leakage. A fixture with dirty contacts adds tens of megohms of apparent leakage and produces false failures that cost more in rework than the test saves.