Insulation Resistance Testing (IR Test)
How to perform and read a 250 V / 500 V insulation resistance test, and what a low reading is telling you.
BS 7671:2018+A2:2022 Reg 643.3GS38
Insulation resistance testing applies a high d.c. voltage between conductors that should be electrically separate, and measures how much current leaks across the insulation. It's a dead test — everything is isolated — and it catches damaged cables, moisture ingress and wiring errors before the installation is energised.
Test voltage and minimum values
| Circuit nominal voltage | Test voltage (d.c.) | Minimum insulation resistance |
|---|---|---|
| SELV and PELV | 250 V | 0.5 MΩ |
| Up to and including 500 V (incl. 230/400 V) | 500 V | 1.0 MΩ |
| Above 500 V | 1000 V | 1.0 MΩ |
BS 7671 Table 64 values. 1 MΩ is the pass floor — a healthy new installation usually reads far higher (tens or hundreds of MΩ).
1 MΩ passes, but treat low as suspect
A reading between 1 and 2 MΩ is a pass on paper but a warning in practice. It usually means moisture or slightly degraded insulation. Investigate rather than sign it off and forget it.
Doing the test
1
Isolate and prove dead
Safe-isolate the installation or circuit. IR testing is a dead test and the 500 V output will damage connected electronics.
2
Disconnect vulnerable equipment
Remove or disconnect items that can't take 500 V d.c. — dimmers, electronic PIRs, RCDs' electronics, surge protection devices (SPDs), and connected appliances. Or test to a point that excludes them.
3
Link line and neutral (initial verification)
For new work, link L and N together and test between the linked conductors and earth. This protects any connected equipment and speeds the test. Where required, also test between line and neutral.
4
Apply the test
Select 500 V, connect the leads, and press test. Hold until the reading stabilises.
5
Record the lowest value
Log the result per circuit on the schedule of test results. The lowest reading is the one that matters.
SPDs and RCDs will skew or fail the test
Modern consumer units contain surge protection and electronics that present a low resistance to 500 V d.c. Disconnect SPDs before testing, or you'll chase a 'fault' that is really the protection device doing its job.
Reading a low result
- Near-zero across L–N: a dead short, a connected load still in circuit, or a crushed cable.
- Low L–E or N–E but healthy L–N: an earth fault — nail/screw through a cable, water in an accessory, or a trapped conductor in a back box.
- Reading that climbs slowly then holds: capacitance charging on a long run — normal; wait for it to settle.
- Reading that drifts down under test: moisture. Common in outdoor circuits and new plaster; often recovers as things dry.
Split the run to find it
If a circuit fails, disconnect it at a mid-point (a junction or an accessory) and test each half. The half that fails contains the fault. Repeat to close in on it.
Reference material for qualified electrical work. Figures follow BS 7671 and the IET On-Site Guide unless stated; always confirm against the current edition and the standard that applies to your installation.