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Insulation Resistance Degraded: How Low Matters, Threshold Decision Tree

Troubleshooting Equipment Updated Jun 19, 2026 ยท Electrical

Why this matters

A megohmmeter gives a number, and the number alone tells you almost nothing without a threshold to compare it against. A motor reading 50 megohms might be fine or might be flagged for service depending on its voltage class and its trend over prior tests. A feeder reading 2 megohms is a problem; the same value on a soaked outdoor run after rain might be moisture that bakes out. Knowing the action thresholds, and reading the trend rather than a single snapshot, is what separates a defensible "return to service" from a callback when the windings ground out a month later. This article fixes the insulation-resistance bands and the corrections (temperature, dielectric absorption) that make the reading meaningful.

Symptom presentation

The trigger is often a tripped ground fault, a motor that grounds under load, a megger reading taken during PM that came in low, or a cable that ohms low to ground. The equipment may still run. Insulation degradation is progressive: it shows as falling megohm values across successive tests, a poor dielectric absorption ratio, or a polarization index that has dropped from prior years. Surface contamination and moisture mimic real insulation failure and must be ruled out.

Quick checks

  • Use the correct test voltage for the equipment. Low-voltage (under 600 V) systems are typically tested at 500 V or 1000 V DC; do not over-stress.
  • Record temperature. Insulation resistance roughly halves for each ~10 C rise; correct every reading to 20 C (or 40 C for some motor standards) before comparing.
  • Test line-to-ground and line-to-line as the equipment allows; isolate the section under test from connected loads and the supply.
  • Dry and clean visibly wet or dirty surfaces, then retest; surface leakage is not insulation failure.

Decision thresholds

The classic one-megohm-per-kilovolt-plus-one rule of thumb sets a floor, but modern standards favor trend and ratio over a single number.

  • Motors/cables, corrected reading comfortably above the IEEE 43 minimum (roughly 100 megohms for older windings, or 5 megohms minimum for newer form-wound machines per IEEE 43, scaled by voltage): acceptable. Record, trend, return to service.
  • Reading at or near the minimum, or a polarization index (PI, 10-minute over 1-minute) below about 2.0 on a machine that should read higher: marginal. The insulation is aging or damp. Investigate; consider drying, cleaning, and a repeat test before condemning.
  • Reading well below the voltage-based minimum (for example a 600 V feeder under ~1 megohm corrected), or a dielectric absorption ratio (60s/30s) below ~1.25: failed or failing insulation. Do not energize on a hard ground; locate the fault.
  • A trend showing a sharp drop from prior tests (for example halving year over year after correction): flag for action even if the absolute number still clears the floor. The trajectory is the warning.
  • Near-zero or dead-short readings: a solid ground fault. Find and clear it before any energization.

Always state the correction: a 30 C winding reading 10 megohms corrects to a different (higher) value at 20 C; comparing an uncorrected hot reading to a cold spec produces false failures.

Confirming diagnosis

  • Repeat after drying: if a low reading climbs substantially after heat or time, the fault was moisture/surface contamination, not the insulation body.
  • PI / DAR ratios: a low spot reading with a healthy ratio (PI above 2, DAR above 1.4) suggests sound but contaminated insulation; a low reading with a poor ratio confirms degraded insulation.
  • Isolation test: disconnect the suspected section. If the low reading clears when a downstream device or cable is isolated, that downstream item is the ground, not the conductor.
  • Step-voltage / repeat at higher test voltage (within rating): insulation that drops sharply as test voltage rises has a weak spot.

Remediation

  • Moisture/contamination: clean and dry (space heaters, low-temp bake for motors per the machine's procedure), retest, and return to service when corrected values recover.
  • Degraded cable insulation: replace the affected run; do not patch insulation that has failed the threshold.
  • Degraded motor windings: send for rewind or replace, depending on machine value and the standard's guidance.
  • Single grounded device downstream: isolate and replace the faulted item; retest the conductor clean.
  • Document corrected values, ratios, and the trend for the next PM cycle so the next technician reads against your baseline.

A megohmmeter applies hundreds to over a thousand volts DC and stores charge in the cable/winding capacitance. Discharge and ground the tested conductor after every test before touching it. Never megger an energized circuit or a circuit connected to sensitive electronics, surge devices, or unrated equipment; the test voltage will damage them. De-energize, isolate, and lock out first.

References

  • IEEE Std 43, Recommended Practice for Testing Insulation Resistance of Electric Machinery, minimum values, PI, and temperature correction.
  • IEEE Std 95, Recommended Practice for Insulation Testing of AC Electric Machinery with High Direct Voltage.
  • NETA MTS (Maintenance Testing Specifications), insulation-resistance acceptance values for cables and equipment.
  • NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, insulation testing intervals and trending.
  • NFPA 70E, Standard for Electrical Safety in the Workplace, for de-energization, isolation, and discharge procedures.

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