Insulation resistance test: what it is and what reading is OK

An insulation resistance test checks whether cable insulation is leaking current. The tester applies a DC voltage (usually 500 V DC for 230/400 V circuits) and measures resistance between conductors and between each conductor and PE. The minimum for a domestic installation is 1 MΩ; a healthy, dry installation typically reads tens or hundreds of MΩ.

Insulation resistance test: 500 V, 1 MΩ minimum and good readings

For 230/400 V installations to IEC standards (Europe, UK and most of the world). North America uses different voltages, colours and rules: see the “Around the world” notes.

Why it is done

Insulation ages. It can be scuffed on a sharp edge, scorched or damp. Damaged insulation means leakage current, a shock risk and a fire risk. This test finds those faults before they cause harm, even where you cannot see them: inside the wall.

How the test is done

The circuit is isolated from the supply, and sensitive equipment (electronics, LED drivers, power supplies) and surge protectors are disconnected. Otherwise they lower the reading or get damaged.
The insulation tester (often called a "megger") applies a test voltage: 500 V DC for ordinary 230/400 V circuits.
Measurements are taken L-N, L-PE and N-PE (on three-phase, between all conductors).

Test voltage and minimum reading

Values from IEC 60364-6 (adopted in Europe as HD 60364-6):

Circuit nominal voltage	Test voltage (DC)	Minimum insulation resistance
up to 50 V (SELV and PELV)	250 V	0.5 MΩ
above 50 V up to 500 V (normal 230/400 V circuits)	500 V	1 MΩ
above 500 V	1000 V	1 MΩ

If surge protectors cannot be disconnected, the test voltage may be reduced to 250 V, but the result must still be at least 1 MΩ.

What good looks like in practice

Reading	Verdict
below 1 MΩ	too low: find the fault
1 to 10 MΩ	acceptable, but keep an eye on it
tens to hundreds of MΩ	healthy, dry installation

When a whole installation reads too low, it is split into groups of circuits and each is tested separately to find the weak one. A low reading on one circuit usually means a damp box, a damaged cable or an appliance someone forgot to disconnect.

When it is done

On initial verification of a new installation, after major alterations, and during periodic inspections. The result goes into the test certificate.

Around the world:
• UK: BS 7671 uses the same values (500 V test, 1 MΩ minimum for 230/400 V circuits) and records them on the Electrical Installation Certificate or EICR.
• USA: the NEC does not set a general minimum for building wiring; insulation testing is mainly specified by equipment standards and maintenance practice.

Important: insulation resistance testing is done by a qualified person with a calibrated instrument. 500 V DC can damage connected equipment and give a nasty shock. This page explains the test; it does not replace it.