Insulation Resistance
Insulation Resistance (IR) is the measure of an insulating material’s ability to resist electric current flow between conductive parts or from a conductor to ground, calculated as the ratio of applied DC test voltage to leakage current (R = V/I), expressed in megaohms (MΩ) or gigohms (GΩ). It is a key indicator of insulation health.
On the shop floor, IR is tested using a megohmmeter that applies a known DC voltage (typically 500V–10kV) across insulation and measures leakage current to assess integrity. In CMMS/ServiceGrid, IR test results are logged as predictive maintenance data to track insulation degradation trends, trigger maintenance alerts, and schedule preventive actions before failures occur. Common use cases include testing motor windings, cable assemblies, transformers, and switchgear to detect moisture, contamination, or aging.
- Moisture ingress: Absorbed water drastically lowers IR, leading to short circuits or ground faults if not detected early.
- Contamination: Dust, oil, or chemical residues on insulation surfaces create leakage paths, reducing IR and causing premature failure.
- Aging/deterioration: Long-term thermal/electrical stress causes insulation to crack or degrade, resulting in slowly tapering IR values that indicate impending breakdown.
What is the minimum acceptable IR value?
Rule of thumb: ≥1 MΩ per 1,000V operating voltage, with a minimum of 1 MΩ; industrial installations typically require 10–100 MΩ+, high-voltage systems >1 GΩ.
Why use DC instead of AC for IR testing?
DC avoids capacitive charging currents, isolating true leakage current for accurate resistance calculation.
How does IR differ from dielectric strength (HIPOT)?
IR assesses bulk insulation quality and leakage; HIPOT tests dielectric strength to find localized defects by applying high AC voltage until failure.
What does a trending IR decline indicate?
Normal, expected deterioration; sudden drops signal contamination, moisture, or damage requiring immediate intervention.
Can IR testing be non-destructive?
Yes—IR testing uses voltages below dielectric failure thresholds, making it non-destructive and suitable for routine maintenance.