Insulation Resistance Minimum Values: BS 7671 Guide

Insulation resistance (IR) is a measurement of how effectively the insulation material around electrical conductors prevents current leakage between them. In a perfect cable, the insulation would have infinite resistance — no current would leak between conductors. In practice, all insulation materials allow a tiny amount of leakage current, and this leakage increases as the insulation ages, gets damaged, or is exposed to moisture and contaminants.

The insulation resistance test applies a DC voltage between the conductors and measures the resulting leakage current. From Ohm's law (R = V/I), the instrument calculates the insulation resistance in megohms (MΩ). A high reading indicates good insulation; a low reading indicates that the insulation is compromised and current is leaking.

Insulation resistance testing is test number three in the GN3 testing sequence, carried out after continuity testing and before live tests. It is performed with the circuit de-energised and all loads disconnected.

BS 7671 Table 64 (Regulation 643.3.2, Chapter 64) sets out the minimum insulation resistance values. In earlier editions this table appeared as Table 61 or Table 61.1 in Chapter 61 — A4:2026 renumbered Chapter 61 to Chapter 64. The values themselves are unchanged:

Note that the general minimum for SELV/PELV circuits is 0.5 MΩ, but for all standard circuits (230V and 400V), the minimum is 1.0 MΩ. In everyday domestic and commercial testing, you will almost always be testing at 500V DC with a minimum of 1.0 MΩ.

Selecting the correct test voltage is critical. Using too high a test voltage on a circuit rated for lower voltages can damage the insulation. Using too low a test voltage will not adequately stress-test the insulation and may miss defects.

For standard domestic and commercial work in the UK, the vast majority of circuits operate at 230V single-phase or 400V three-phase. These all fall in the "up to and including 500V" category and are tested at 500V DC.

SELV (Separated Extra-Low Voltage) and PELV (Protective Extra-Low Voltage) circuits are tested at 250V DC. These include doorbell transformers, LED strip driver outputs, garden lighting transformers, and bathroom shaver socket SELV supplies. Always check the circuit voltage before selecting the test voltage on your MFT.

Auto-validates every IR reading on site

Enter your insulation resistance reading into Elec-Mate and the app instantly validates it against the BS 7671 minimum of 1 MΩ.

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The 1 megohm minimum is the absolute floor for insulation resistance on standard circuits. Any reading of 0.99 MΩ or below is a fail. However, treating 1 megohm as a "target" would be a mistake — it is the minimum acceptable value, not a good value.

For context, new PVC-insulated cable typically produces insulation resistance readings of 200 MΩ or higher per circuit. A brand-new installation that produces readings of only 2 or 3 megohms would technically pass but should raise serious questions about the quality of the installation. Similarly, an existing installation that has dropped from 50 MΩ at the last EICR to 2 MΩ at this one shows a significant trend of deterioration that warrants investigation even though the absolute value still passes.

IET guidance recommends investigating any reading below 2 MΩ, even though the minimum is 1 MΩ. A reading between 1 and 2 MΩ passes but indicates that the insulation is close to failure and may fail at the next periodic inspection. Recording this as a C3 (improvement recommended) on an EICR is common practice.

Several factors affect insulation resistance readings beyond the actual condition of the cable insulation. Understanding these factors helps you interpret your results correctly and avoid false failures.

When you get a low IR reading (below 2 MΩ), systematic troubleshooting is needed to identify the cause. The process involves progressively isolating sections of the circuit to narrow down where the fault is.

Common causes of low IR include: water ingress into buried or concealed cables (especially after heavy rain or plumbing leaks), nails or screws through cables behind plasterboard, rodent damage to cable insulation, thermal degradation in overheated cable runs (behind radiators, in insulated ceiling voids), and deteriorated rubber insulation on pre-1970 wiring systems.

Three-phase circuits require additional conductor combinations to be tested. As well as the standard all-live-to-earth and live-to-live tests, you must test between each phase combination.

The full set of tests for a three-phase, four-wire circuit is: L1+L2+L3+N to E (all live conductors to earth), L1 to L2, L1 to L3, L2 to L3, L1 to N, L2 to N, L3 to N. All readings must be at least 1.0 MΩ. In practice, many electricians use the "all connected together" method — connect all live conductors together and test to earth in one go, then separate them and test between each pair. The first test confirms the cable-to-earth insulation; the individual tests confirm the inter-conductor insulation.

For three-phase motors, the motor windings themselves have their own insulation resistance that is tested separately from the cable. Motor winding IR testing is beyond the scope of standard BS 7671 testing and is covered by BS EN 60034-1. However, if a three-phase circuit feeding a motor produces a low IR reading, disconnecting the motor and re-testing will clarify whether the problem is in the cable or the motor.

For more on testing three-phase installations, see our dedicated three-phase testing guide.

Insulation resistance results are recorded on the schedule of test results on the EICR or EIC. For each circuit, record the IR value in megohms, the test voltage used (250V, 500V, or 1000V), and the conductor combination tested.

Where the instrument displays a reading greater than its maximum range (e.g., "> 200 MΩ" or "> 999 MΩ"), record the value as shown — for example, "> 200" in the IR column. This indicates excellent insulation. Do not record it as "pass" or "OK" — the actual value (or the maximum displayed value) must be recorded.

If any circuit produces a reading below 1.0 MΩ, the circuit fails and the fault must be investigated and rectified. On an EICR, a failing IR reading is recorded as an observation with the appropriate observation code. On an EIC, the certificate should not be issued until the insulation fault is rectified.

Elec-Mate auto-validates every insulation resistance reading as you enter it. Any reading below 1.0 MΩ is flagged immediately in the schedule of tests, and the app prompts you to record an observation with the appropriate code. Readings between 1 and 2 MΩ trigger a warning suggesting further investigation.

Instant IR validation on site

Enter your insulation resistance readings and Elec-Mate validates each one against the BS 7671 minimum of 1 MΩ. Failures are flagged in red.

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The app records the test voltage alongside each IR value and includes the complete schedule of tests with auto-validated IR, R1+R2, Zs, and RCD results — all cross-referenced against BS 7671 limits for the protective device on each circuit.