Insulation Resistance Measurement — Method and Limit Values

The insulation resistance measurement checks whether the insulation between live conductors, and between live conductors and earth, is still sufficient to prevent leakage current and breakdown. This is a different measurement from the continuity test of protective conductors (§413/continuity) or the RCD function test (§531) — insulation resistance says something about the quality of the insulation itself, not about the protective conductor or the residual-current protection.

Limit values (table 61.1)

The test voltage is not arbitrary: a SELV/PELV circuit is tested with a lower voltage (250 V) than a standard 230/400 V circuit (500 V) — testing a SELV circuit at 500 V can damage the (lower-voltage-rated) insulation itself.

Test procedure

1. De-energize the installation — measuring insulation resistance under voltage is not possible and is dangerous.
2. Remove or disconnect lamps/sensitive electronics — semiconductors and electronic ballasts can be damaged by the test voltage or produce a false-low reading.
3. Leave fuses/circuit breakers in the final circuits in place, but close all switches in the final circuits — otherwise part of the circuit is not included in the measurement.
4. Measure between: phase-neutral, phase-earth, neutral-earth (or combined phase+neutral-earth for larger installations, depending on the tester).
5. Compare the value with the table value that corresponds to the nominal voltage of the measured circuit.

What a low value means

An insulation resistance that is (well) below the limit value usually indicates:

• Moisture in the installation (crawl spaces, outdoor connections, construction-damp new buildings).
• Aged or damaged cable insulation (particularly with older rubber/PVC insulation or rodent damage).
• A developing insulation fault that is not yet large enough to trip an RCD, but is already a warning signal for future failure.

Common mistakes

1. Attempting to measure under voltage — technically impossible with an insulation tester and dangerous; the installation must first be de-energized and secured against unintended re-energizing.
2. Using the wrong test voltage — applying 500 V to a SELV/PELV circuit instead of 250 V can damage the insulation of that circuit.
3. Not disconnecting electronics — produces false-low readings or damages equipment (frequency drives, LED drivers, dimmers).
4. Confusing insulation resistance measurement with the continuity test of the protective conductor — two different measurements with different purposes; one says nothing about the other.

Related

• Inspection
• §531 — Residual Current Devices (RCD)
• §413 — Automatic Power Off (AUV)
• Thermographic inspection