Glossary term
Insulation Resistance
A measure of how strongly electrical insulation resists leakage current between conductors or between a conductor and ground.
Definition
metricInsulation resistance is the resistance offered by insulating material to leakage current between conductive parts.
Electrical insulation is intended to prevent current from flowing where it should not. Insulation resistance testing applies a DC test voltage and measures the resulting leakage current to estimate resistance. Low or declining insulation resistance can indicate moisture, contamination, ageing, thermal damage, mechanical damage, partial tracking, cable defects, winding degradation, or installation faults.
Insulation resistance indicates how effectively insulation prevents unwanted current flow. A high value means very little leakage current flows for a given test voltage. A low value suggests that the insulation path may be contaminated, wet, damaged, aged, overheated, cracked, carbonized, or otherwise compromised.
The basic relation is Ohm’s law:
where V_{test} is the applied DC test voltage and I_{leak} is measured leakage current. Values are commonly reported in megaohms or gigaohms.
Testing
Insulation resistance testing is used on motors, generators, transformers, switchgear, cables, heaters, busbars, and electrical installations. A test instrument applies a specified DC voltage, often much higher than operating control voltage, and measures the resulting leakage. Test voltage must be chosen for the equipment rating; too low a voltage may miss defects, while too high a voltage can stress insulation unnecessarily.
The reading is time dependent because insulation behaves partly like a capacitor and because absorption currents decay after voltage is applied. For this reason, one-minute readings, polarization index, dielectric absorption ratio, and trend data are often more informative than a single instantaneous value.
Interpretation
Insulation resistance is strongly affected by temperature, humidity, surface contamination, cable length, winding size, and equipment geometry. Large machines and long cables naturally have more leakage paths and capacitance than small devices. Temperature correction may be required when comparing readings over time.
The most useful information is often the trend. A single value above a minimum threshold may not prove health if the value has been falling steadily. Conversely, a low reading after washing, storage, or humid exposure may recover after drying, cleaning, or heating.
Common mistakes
A common mistake is treating insulation resistance as a complete dielectric test. It is a useful screening and maintenance metric, but it does not replace high-potential testing, partial discharge testing, tan-delta testing, visual inspection, or functional protection checks where those are required. Another mistake is failing to discharge the equipment after testing. Cables, motors, and windings can retain dangerous stored charge after a DC insulation test.