BS 7671 Table 64 sets the minimum insulation resistance for a 230 V circuit at 1.0 MΩ, tested at 500 V DC, and IET Guidance Note 3 says any reading under 2 MΩ should be investigated even though it passes. A low reading on a domestic circuit is far more often a connected load, a neon, or damp than a genuinely damaged cable, so the job is to split the circuit and disconnect things methodically until the reading recovers, not to start pulling cables. This guide gives a working order of checks, the readings that point to each cause, and how to record and code the result on an EIC or EICR.
- Minimum IR is 1.0 MΩ at 500 V DC for low-voltage circuits; investigate anything under 2 MΩ.
- Test L-N, L-E and N-E separately if you can. The pattern of which pair reads low tells you what kind of fault you have.
- Disconnect loads first: neons, dimmers, smoke alarms, boiler wiring centres, LED drivers and USB sockets all pull the reading down.
- Then split the circuit at accessible points and test each leg to localise the fault.
- Record the actual reading, the test voltage, what was disconnected and, on an EICR, code it honestly.
What does the test voltage and the pattern of readings tell you?
Insulation resistance is measured with the circuit dead and isolated, at 500 V DC for circuits up to 500 V nominal. The reading depends on which conductors the fault sits between:
| Reading pattern | Likely cause |
|---|---|
| L-N low, L-E and N-E fine | A load or device still connected between line and neutral: lamp, neon, electronic control, contactor coil |
| L-E low, N-E fine | Line conductor insulation damaged or a line-to-earth leakage path: damp in a fitting, a screw through the line core, a burnt terminal |
| N-E low, L-E fine | Neutral-to-earth fault: a classic RCD-tripper. Neutral touching the CPC in a back box, damp in an outside light, a borrowed neutral |
| All three low and similar | Damp, condensation or aged insulation affecting the whole cable |
| Fine at 250 V, low at 500 V | Marginal insulation breaking down under stress; often heat-damaged PVC or old rubber |
That last row is why the test is done at 500 V and not 250 V. Insulation that only fails under stress will fail in service eventually.
Why disconnect loads before anything else?
The commonest low reading on a domestic EICR is not a fault at all. Anything with electronics connected across line and neutral will drag the L-N figure down and can also give odd readings to earth through its filtering components. The usual suspects:
- Neon indicators in switched fused spurs, cooker switches and shower isolators.
- Dimmer switches and electronic time delay switches.
- Mains smoke and heat alarms with battery backup.
- Boiler wiring centres, programmers, room thermostats and zone valves.
- LED drivers and lamps, especially integrated LED downlights.
- USB sockets and smart sockets.
- Extractor fans with timers or humidistats.
- Immersion heater and storage heater controls.
GN3 recognises that on an existing installation disconnecting every item may not be practical, and permits testing with L and N linked together to earth, which protects electronic equipment from the 500 V and removes the L-N reading from the equation. If you do that, record it in the test results and the limitations. A low L+N to earth reading with everything else disconnected is a real fault.
Lamps are a special case. A filament lamp in circuit reads as a low L-N resistance, obviously, but so does a neon lamp in a switch, and so does a fluorescent fitting's ballast. Remove lamps, or switch the lighting circuit off at the switches and test both sides, before recording a fail on a lighting circuit.
How do you split the circuit to localise the fault?
Once loads are out and the reading is still low, split the circuit into sections and test each. Start at the consumer unit and work outwards:
- Disconnect the circuit at the board (line, neutral and CPC) and test the whole circuit. This is your baseline.
- Ring finals: disconnect both legs and test each leg separately to earth. One leg reading low halves the search area immediately. Then open the ring at a socket roughly halfway round and test again.
- Radials: disconnect at an accessible junction, spur or switch and test the board side and the far side separately.
- Lighting: separate the switch drops. A low reading that disappears when the switch wire is disconnected points at the drop or the switch box, not the ceiling rose loop.
- Keep going until the low section is a single cable run between two accessible points. That is the cable to inspect or replace.
Note the reading at each stage. On a fault-finding visit charged by the hour, the client is paying for the method, and the notes are what justify the invoice and, later, the certificate.
What are the physical causes to look for?
Once the low section is identified, look for the fault before you start replacing cable:
Moisture. Outdoor lights, garden sockets, shower pull cords, isolators in bathrooms, junction boxes in lofts under a leak, and cables entering from outside through unsealed holes. Damp faults often read a few hundred kΩ, vary with the weather, and give a reading that creeps up while the test voltage is applied as the tester dries the leakage path. A damp fault will trip an RCD when it rains and hold when it is dry.
Mechanical damage. A screw or nail through a cable behind plasterboard, a cable trapped under a floorboard nail, a cable rubbed through on a joist edge or a metal back box with no grommet. Ask the client about recent work: shelving, kitchen fitting, a new floor. The fault will be near it.
Heat. Cables run over or through insulation and loaded to their limit, cables bunched in a ceiling void above downlights, cables behind an oven housing. PVC that has been cooked reads lower under 500 V and may look discoloured or brittle when you find it.
Age. Rubber-insulated cable from before about 1960 will have perished and reads low across the whole circuit. Early PVC in some installations has degraded. There is no repair; these circuits need rewiring, and the reading, together with the visible condition, is the evidence.
Rodents. Chewed insulation in lofts and under floors, usually in several places.
Borrowed neutrals. A neutral-to-earth reading that is low only when another circuit is connected points to a shared neutral, common on old two-way lighting between landing and hall. See our guide to Insulation Resistance Testing Guide: Voltages, Minimums, Method for the test sequence.
What if the reading is between 1 and 2 MΩ?
It passes. BS 7671 requires at least 1 MΩ and a 1.5 MΩ reading meets that. But GN3 advises further investigation because a reading that low on a domestic circuit means something is degrading, and it will not go up on its own. Record the actual figure, not ">1". On an EICR a borderline reading can be a C3 with an observation such as "insulation resistance of circuit 4 measured 1.4 MΩ; recommend investigation and remedial work at the next opportunity". On an EIC for new work, a 1.5 MΩ reading on a brand-new circuit is not acceptable in practice: new cable reads in the hundreds of MΩ, and something is wrong.
How do you code a low insulation resistance reading on an EICR?
The code depends on what the reading implies about danger, not the number alone. Electrical Safety First's Best Practice Guide 4 and NICEIC guidance point the same way:
| Situation | Suggested code |
|---|---|
| IR below 1 MΩ, cause found, live conductor exposed or damaged, no RCD | C1 |
| IR below 1 MΩ, cause not visible, RCD protection present | C2 |
| IR below 1 MΩ, cause not found within agreed limitations | FI, with C2 where the risk justifies it |
| IR between 1 and 2 MΩ, no other defects | C3 or note in the observations |
| Low reading caused by connected equipment, confirmed by disconnection | No code; record the method |
Wording matters. "Insulation resistance low" tells the client nothing. "Circuit 7 (kitchen sockets): insulation resistance L-E 0.3 MΩ measured with all loads disconnected; fault localised to cable run between socket adjacent to sink and socket by back door; moisture evident in back box. Circuit isolated pending repair" is an observation the next electrician can act on. How to Write EICR Observations That Stand Up has more examples.
Recording the result on the schedule of test results
On the schedule, enter the actual figure in MΩ for each test carried out (L-L, L-E, or L+N to E as appropriate), the test voltage, and in the remarks what was disconnected or linked. Where your tester's top reading is >200 MΩ or >999 MΩ, record that; do not write "OK". If the circuit was left isolated, say so. The schedule is the record that protects you if the installation fails later. How to lay it out is at How to fill in a schedule of test results, column by column.
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.