Insulation resistance testing
The test
Insulation resistance is measured with a DC tester: 500 V DC for circuits rated up to and including 500 V, and 250 V DC for SELV and PELV. The minimum acceptable value is 1.0 MΩ for the 500 V test and 0.5 MΩ for the 250 V test. Test between live conductors and earth, and between live conductors. Test the installation as a whole at the origin first, with the main switch off, then investigate if the result is low.
Preparation
Everything that could be damaged or could give a false reading comes out or is disconnected: lamps, dimmers, electronic transformers, fluorescent and LED gear, timers, smoke detectors, IT equipment, surge protection devices and equipment with filters. Isolate and prove dead first. Test leads must be sound and the tester in calibration.
Recording and interpreting
Record the lowest value obtained, in MΩ. If the overall value is below 2 MΩ, test each circuit separately to find the culprit, then test sections of that circuit. A low reading is often moisture, a shared or borrowed neutral, a damaged cable, or equipment left connected.
Trap: leaving an SPD in circuit - it can conduct at 500 V and give a falsely low result.
Trap: assuming 1 MΩ is a good result. The regulation minimum is not a target; a healthy installation reads tens or hundreds of MΩ.
Rule of thumb: below 2 MΩ, split and retest; below 1 MΩ, it fails.