Insulation Resistance
The resistance of the insulating material between a conductor and the body or the other conductors, measured with a dedicated test instrument by applying a test voltage above the normal operating voltage. Moisture, contamination and ageing lower this resistance; a fall is a sign of leakage current and coming failure.
An ordinary multimeter cannot make this measurement; the few volts applied by its ohm range are not enough to reveal leakage through micro cracks. An insulation tester applies a much higher DC voltage and calculates the resistance from the leakage current that flows.
- Precondition: the circuit must be completely de-energised and electronic boards, drivers and sensors must be disconnected. The test voltage can permanently damage semiconductors.
- Where it is used: motor windings, long supply cables, installations working outdoors. It is not a routine test on low-voltage vehicle-mounted electronics.
- Interpretation: the absolute limit varies with the application; the genuinely meaningful data is the trend of the same point over time. A continuously falling value shows that the insulation is degrading.
- Temperature: the measurement is temperature sensitive; measure under similar conditions when comparing.
Before carrying out this test, always plan which components will be disconnected; a driver board that has not been disconnected can be damaged in a single measurement.
The context in which this term is used: Installation, redundancy and field continuity