Electrical

Insulation Resistance Testing

Insulation resistance testing proves the insulation between conductors, and between conductors and earth, is sound before a circuit is energised — the right test voltage and the 1MΩ minimum are set by BS 7671, and a passing-but-low reading is still worth investigating.

Updated 20 Aug 20268 min read

Insulation resistance is a dead test, carried out during initial verification and periodic inspection, that checks the insulation separating your live conductors from each other and from earth hasn't broken down. A healthy installation reads in the hundreds of megohms; a failing one leaks current where it shouldn't, and that leakage is a shock and fire risk waiting to be energised.

This guide covers the test voltages BS 7671 specifies for different systems, the minimum acceptable values, what you must disconnect before testing to avoid damage and false readings, and how to interpret a result that passes the letter of the standard but looks wrong.

Key takeaways

Test voltage depends on the circuit: 250V DC for SELV/PELV, 500V DC for LV up to 500V, 1000V DC for circuits above 500V.
Minimum insulation resistance for a 230/400V circuit is 1MΩ, tested at 500V DC.
A healthy installation reads far higher — hundreds of megohms or off the top of the scale.
Test line-line and each live conductor to earth; disconnect or link out electronics, dimmers, and neon indicators first.
A reading that passes but is much lower than the rest of the installation warrants investigation, not a tick.

What the test proves and where it sits

Insulation resistance measures how well the insulation resists a DC test voltage — a high resistance means almost no current leaks across it, which is what you want. It is one of the dead tests in the BS 7671 initial verification sequence, carried out after continuity of protective conductors and ring final continuity, and before the installation is energised for live testing.

It sits where it does for a reason: a low insulation resistance reading tells you the installation is not fit to be made live, so you find that out with the power off, not by tripping a breaker or worse when you energise. On a periodic inspection it's equally central — degraded insulation from age, heat, damp or damage shows up here first.

Test voltages and minimum values

BS 7671 sets the test voltage by circuit nominal voltage. For SELV and PELV circuits, test at 250V DC with a minimum of 0.5MΩ. For low-voltage circuits up to and including 500V — which covers ordinary 230V and 400V installations — test at 500V DC with a minimum of 1MΩ. For circuits above 500V, test at 1000V DC with a minimum of 1MΩ.

The 1MΩ figure is the absolute floor for a pass, not a target. It exists as a hard limit below which a circuit is considered defective; a real, healthy installation should be nowhere near it. New wiring routinely reads hundreds of megohms or takes the meter off the top of its range, and anything that limps in just above 1MΩ is telling you something is wrong even though it technically passes.

Pass is not the same as good

1MΩ is the minimum, not the aim. A circuit reading 1.2MΩ passes but is degraded — compare it against the rest of the board, which should read far higher.

What to disconnect before you test

A 500V DC test voltage will damage electronic equipment and give false readings through anything that provides a deliberate path between conductors. Before testing, disconnect or unplug electronic devices, remove or bypass dimmer switches, disconnect surge protection devices and neon indicators, and remove electronic starters and controls. Anything left connected either gets damaged or drags the reading down and masks the true insulation.

Where devices cannot reasonably be removed, the recognised alternative is to link line and neutral together and test between that link and earth at a reduced risk of damage — you lose the line-to-neutral reading but still prove insulation to earth. Record what was disconnected or linked so the result can be understood later.

How to carry out the test

With the circuit isolated, proven dead and loads disconnected, test between live conductors and between live conductors and earth. On a single-phase circuit that means line to neutral, line to earth, and neutral to earth. On three-phase, test between each pair of lines, between each line and neutral, and between each line and neutral and earth.

Apply the test voltage for long enough to get a settled reading — insulation can charge like a capacitor, so the value may climb for a moment before stabilising. Record the lowest value obtained for the circuit. Where a whole installation is tested at once with all final circuits connected, a global reading well above 1MΩ is acceptable, but a low global reading means you split it down and test circuit by circuit to find the culprit.

Find the low one

A low global reading isn't a fail of the whole board — split the circuits and test them individually. One damp cable or one forgotten dimmer usually explains the lot.

Reading and acting on results

A reading comfortably in the hundreds of megohms across all combinations is a healthy pass. A reading below 1MΩ is a fail: the circuit must not be energised until the fault is found and corrected. A reading between the two — say 2MΩ on a circuit where everything else reads 200MΩ — passes but flags degraded or damp insulation, a nicked cable, or moisture ingress that will only get worse.

On a periodic inspection, a low or borderline insulation resistance typically drives a C2 where it represents a real shock or fire risk. The value is recorded on the schedule of test results either way; the number on the certificate is only meaningful alongside a note of what was connected and what the test conditions were.

Frequently asked questions

What is the minimum insulation resistance?

For a low-voltage circuit up to 500V tested at 500V DC, the minimum is 1MΩ. For SELV and PELV tested at 250V DC it's 0.5MΩ. These are absolute minimums — a healthy installation reads far higher, into the hundreds of megohms.

What test voltage do I use?

250V DC for SELV and PELV circuits, 500V DC for low-voltage circuits up to and including 500V (ordinary 230/400V work), and 1000V DC for circuits above 500V. The voltage is set by the circuit's nominal voltage, not by preference.

What do I disconnect before testing?

Anything electronic or with a deliberate path between conductors: electronic devices, dimmers, surge protection devices, neon indicators and electronic starters. The 500V test voltage damages them and they drag the reading down. Where they can't be removed, link line and neutral and test to earth.

Why is my reading low but still above 1MΩ?

A pass that's much lower than the rest of the installation points at degraded insulation, damp, a nicked cable or a device left connected. It passes the minimum but warrants investigation — split the circuits down and find which one is pulling the reading down.

From guidance to action

Electrical Installation CertificateRecord insulation resistance results on the schedule of test results.EICR certificate appLog insulation resistance on a periodic inspection and code any defects.