Insulation resistance testing applies a d.c. voltage between conductors that should be insulated from each other and measures the leakage in megohms, proving the insulation has not broken down, been damaged or got wet. Table 64 in Chapter 64 of BS 7671:2018+A4:2026 sets the test voltage and minimum result: 250 V d.c. and 0.5 MΩ for SELV and PELV, 500 V d.c. and 1.0 MΩ for circuits up to and including 500 V (all normal 230 V and 400 V work), and 1000 V d.c. and 1.0 MΩ above 500 V. It is a dead test, carried out after continuity and before any live testing, and is required on every EIC and, within agreed limitations, every EICR.
- Test at 500 V d.c. for 230/400 V circuits and accept nothing below 1.0 MΩ; SELV and PELV are tested at 250 V d.c. with a 0.5 MΩ minimum.
- Isolate, prove dead and disconnect vulnerable equipment before testing. The test voltage is high enough to damage electronics.
- On an EICR with equipment connected, link line and neutral and test to earth. Record the live-to-live test as a limitation.
- Record the reading as the tester shows it: >200 MΩ or >999 MΩ, not a rounded number.
- A low but passing reading (1 to 2 MΩ) is a fault in progress and should be investigated and noted.
What does an insulation resistance test actually measure?
The tester puts a steady d.c. voltage across two conductors and measures the tiny current that flows through the insulation between them. Resistance = voltage ÷ current, displayed in MΩ. Good PVC insulation on a short domestic circuit leaks so little that the instrument reads over range. Insulation that is cracked, crushed, overheated, damp or contaminated leaks more and the reading drops. The test is done at a higher voltage than the circuit's normal voltage so that weaknesses that would only show under stress are found now rather than in service.
The test is carried out between:
- each live conductor and every other live conductor (line to neutral on single-phase; L1-L2, L2-L3, L1-L3 and each line to neutral on three-phase), and
- each live conductor and the protective conductor (earth), with the protective conductor connected to the main earthing terminal.
What test voltage do I use and what is the minimum reading?
| 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), i.e. 230 V single-phase and 400 V three-phase | 500 V | 1.0 MΩ |
| Above 500 V up to 1000 V | 1000 V | 1.0 MΩ |
These are the figures in Table 64 of BS 7671. They are minimum values below which the circuit fails. A healthy new circuit will be hundreds of megohms or over range; a healthy older installation is typically tens of megohms or more. IET Guidance Note 3 advises that a reading under 2 MΩ, although a pass against the table, should be investigated because it indicates a latent defect.
Where surge protective devices, RCDs with electronics or other equipment likely to influence the result cannot be disconnected, BS 7671 permits the test voltage to be reduced to 250 V d.c. for that measurement, but the 1.0 MΩ minimum still applies. Note the reduced voltage in the remarks column.
What should I disconnect before testing?
The 500 V test voltage will damage or give false readings from anything electronic left on the circuit. Before testing:
- Isolate the circuit at the board, lock off and prove dead with a proving unit and approved voltage indicator.
- Remove or isolate vulnerable equipment: dimmer switches, electronic timers and PIRs, USB socket-outlets, smoke and heat alarms (unplug from their bases), boiler and heating controls, LED drivers and fluorescent ballasts, shaver sockets, extractor fans with electronic timers, EV chargers, PV inverters and any surge protective device on the board.
- Deal with lighting: either switch the lights off at the switch (so line and neutral are separated at the load) or remove lamps. For two-way and intermediate circuits, test with the switches in each position if you are testing line to neutral, so every strapper is included.
- Open any neutral-to-earth links that are deliberate, for example at a transformer or a UPS, and note them.
- Open RCBOs and RCDs where practicable, or test on the load side of them, as their electronics can sit between neutral and earth.
How do I test line plus neutral to earth?
On an EICR the installation is in service and full disconnection of every item of equipment is often not practicable. The accepted approach, described in IET Guidance Note 3, is to connect line and neutral together (at the outgoing terminals of the device, or using the tester's leads with a link) and test between the combined live conductors and earth. Because line and neutral are at the same potential during the test, nothing connected between them sees any voltage, so electronic equipment is safe. Anything connected between a live conductor and earth (a filter capacitor, a neon indicator, a faulty heating element) still shows up.
Record the result in the live-to-earth column of the schedule of test results and enter LIM in the live-to-live column, with the limitation stated on the report ("Insulation resistance tested L+N to E with equipment connected; L-N not tested"). On an EIC for new work this shortcut is not acceptable: both tests must be done, because nothing is connected yet and the L-N test is the one that finds a neutral-to-line short before the circuit is first switched on.
How do I interpret a low reading?
| Reading (500 V test) | Interpretation | Action |
|---|---|---|
| Over range (>200 MΩ, >999 MΩ) | Insulation in good condition | Record as displayed |
| 20 MΩ to over range | Normal for an installation in service | Record |
| 2 to 20 MΩ | Acceptable; may indicate age, moisture or a long circuit | Record; consider noting if well below other circuits |
| 1 to 2 MΩ | Pass, but GN3 recommends investigation | Locate the cause; observation likely |
| Below 1 MΩ | Fail | C2 in most cases; find and rectify or report |
| Near zero | Short circuit or connected load | Check disconnection first, then look for the fault |
A reading that drops to zero on a single circuit is very often something you forgot to disconnect rather than a cable fault: a lamp left in, a boiler still connected, an old immersion heater element. Check that before pulling cables about. Genuine low readings come from a nail or screw through a cable, water in an outside socket or junction box, a crushed cable behind a cupboard, a rodent-damaged loft cable, degraded rubber insulation on very old wiring, or a neutral touching the earth terminal inside an accessory.
To find which part of a circuit is at fault, split it: disconnect at a junction or accessory halfway along and test each half. On a ring final, break the ring at the board and test each leg. Testing line to earth and neutral to earth separately (rather than L+N linked) tells you which conductor is affected, which narrows the search further.
Low readings across every circuit at once, on a damp day in an old property, usually point to moisture in the board rather than a fault on each circuit, and often improve once the enclosure is opened and dried. Re-test and note it.
When do I record ">200 MΩ" or ">999 MΩ"?
Most multifunction testers top out at either 200 MΩ or 999 MΩ (some at 1000 MΩ or 2 GΩ). When the reading exceeds the instrument's range the display shows the limit with a ">" symbol. Copy that onto the schedule exactly, including the symbol. The two conventions are simply a product of which tester you own; both are far above the 1 MΩ minimum.
Do not write "999" or "200" on its own: without the symbol it reads as a measured value, and an assessor comparing your figures with your instrument's specification will spot it. Equally, do not round a real reading of 85 MΩ up to over range. A circuit that was 85 MΩ five years ago and is 4 MΩ today is deteriorating, and that comparison only works if both figures are honest.
Where does the insulation resistance test sit in the test sequence?
BS 7671 Chapter 64 and GN3 give the order for initial verification: continuity of protective conductors and bonding, continuity of ring final circuit conductors, insulation resistance, SELV/PELV and electrical separation, dead polarity, earth electrode resistance, then the live tests: polarity confirmation, earth fault loop impedance, prospective fault current, RCD operation and functional testing. Insulation resistance comes after continuity because a broken cpc or open ring needs fixing before the circuit is stressed, and before the live tests because you do not want to energise a circuit with a line-to-earth short.
For the continuity tests that precede it, see Ring Final Circuit Testing: r1, rn, r2 and R1+R2 Explained, and for the live tests that follow, Earth Fault Loop Impedance Testing: Ze, Zs and Ipf Explained and RCD Testing Explained: Types, Test Currents and Trip Times.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £5 a month.