Insulation resistance testing
You apply a d.c. test voltage between conductors and measure how much leaks through the insulation. Damaged, damp or crushed insulation shows up as a low reading.
Why it matters
Insulation that leaks current can cause electric shock, fires and nuisance RCD tripping. It’s also the test most likely to damage equipment if you don’t prepare properly.
Test voltages and minimum values
BS 7671 Table 64 (as amended):
| 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. FELV) | 500 V | 1 MΩ |
| Above 500 V | 1000 V | 1 MΩ |
A healthy domestic circuit usually reads hundreds of MΩ — many testers show >299 MΩ or >999 MΩ. A reading that passes but is much lower than similar circuits is worth investigating.
Before you test
- Safely isolate and prove dead.
- Disconnect or isolate anything that could be damaged or affect the reading: electronic equipment, dimmers, RCDs with electronics, surge protection, lamps, PIRs. Where something can’t be disconnected, test line and neutral connected together to earth.
- Tell everyone — a 500 V test can give a nasty shock.
Method
- Line to neutralTest between live conductors.
- Line to earthTest each live conductor to the cpc.
- Neutral to earthTest neutral to cpc. On three-phase circuits, test every combination.
- RecordRecord the lowest live–live and the lowest live–earth reading on the schedule, then reconnect everything you disconnected.
iZapp’s 3D Testing Bench lets you do this test on a virtual board with real-looking kit: place the probes, read the tester and record the result on a practice schedule. Some attempts have a fault planted for you to find.
Frequently asked questions
What is the minimum insulation resistance for a 230 V circuit?
1 MΩ, tested at 500 V d.c. (BS 7671 Table 64).
Why do I need to disconnect equipment first?
The test voltage can damage electronic equipment, and connected loads can give a falsely low reading.
How do I record an over-range reading?
Record it as the tester shows it, for example >299 MΩ or >999 MΩ.
Last updated 2026-10-14. Training summary of BS 7671 and IET Guidance Note 3 practice, written by NAPIT-registered electricians. Always check the current edition and amendment of BS 7671 and your scheme’s requirements.