A complete guide to insulation resistance (IR) testing for UK electricians. Covers test voltages, the ≥1 MΩ minimum value, how to test new circuits, precautions on live circuits, and what Regulation 643.3 requires.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
For a standard 230V/400V circuit, BS 7671 requires a minimum insulation resistance of 1.0 MΩ, tested at 500V DC; SELV and PELV circuits are tested at 250V DC with a 0.5 MΩ minimum (Table 64). The test is required by Regulation 643.3 as a dead test, with the supply isolated and sensitive equipment disconnected. A low or steadily falling reading points to moisture, damaged insulation or a wiring fault.
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Insulation resistance (IR) testing is one of the fundamental verification tests required by BS 7671:2018+A4:2026 as part of both the initial inspection and testing of a new installation and the periodic inspection and testing of an existing installation (EICR). The purpose of the test is to verify that the insulation surrounding the conductors is in good condition and is providing adequate electrical separation between live conductors and between live conductors and earth.
Over time, electrical insulation degrades due to heat, moisture, mechanical damage, UV exposure, and chemical attack. As insulation degrades, its resistance decreases. At very low insulation resistance values, significant leakage current can flow through the insulation under normal operating conditions, causing energy loss, heating of the conductor insulation, and — if the leakage path is to earth — operation of residual current devices. In severe cases, low insulation resistance can lead to ignition of the insulation and electrical fire.
IR testing detects these problems before they become dangerous. A single IR test of a circuit takes only a few minutes and provides a quantitative measure of insulation health that can be compared against the minimum value required by BS 7671 Regulation 643.3 and tracked over successive EICR intervals to identify progressive deterioration.
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The test voltage for insulation resistance testing is specified in BS 7671 Table 64. Using the correct test voltage is essential — too low a voltage may not reveal insulation weaknesses, while too high a voltage can damage sensitive equipment.
Used for SELV and PELV circuits with a nominal voltage up to 50V AC. Minimum IR value: 0.5 MΩ. Applies to extra-low voltage circuits such as fire alarm wiring, security systems, and ELV lighting.
Used for circuits up to 500V nominal. This covers all standard UK domestic and commercial circuits (230V single phase, 400V three phase). Minimum IR value: 1 MΩ. This is the most commonly used test voltage for electricians.
Used for circuits above 500V nominal (high-voltage systems). Minimum IR value: 1 MΩ. Rarely used in routine domestic or commercial electrical work; more common in industrial high-voltage installations and some specialist EV charging infrastructure.
The test voltage is applied for a minimum period sufficient for the reading to stabilise. For most modern digital instruments, this stabilisation is indicated by a steady display. The traditional recommendation is to apply the test for 1 minute, but follow your instrument manufacturer's guidance.
BS 7671 Table 64 sets the minimum acceptable insulation resistance values. For circuits up to 500V (tested at 500V DC), the minimum is 1 MΩ. This is a regulatory minimum, not a design target.
Minimum insulation resistance and DC test voltage by circuit type
Per BS 7671:2018+A4:2026 Table 64, Regulation 643.3. A pass at the minimum is not a target — a sound new installation reads well above 100 MΩ.
Remember that IR values can be temporarily reduced by moisture and humidity — a wet morning can cause surface tracking on cable insulation in outdoor or poorly ventilated locations. Where IR is borderline due to suspected moisture, allow the installation to dry out and retest before condemning the wiring.
For a new circuit, IR testing is carried out before connection to the supply. Here is the correct procedure:
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Try the certificate tools freeWhen carrying out IR testing as part of an EICR on an occupied installation, additional precautions are required:
Residual Charge — Shock Hazard
Guidance Note 3 explicitly flags residual charge as a shock hazard, not merely an instrument feature. IR testing charges cable capacitance to the full test voltage (500V DC or higher). This charge can persist after the tester is disconnected and the circuit may remain live to touch until it is fully discharged. Always confirm that the instrument has completed its automatic discharge cycle — and verify with an approved voltage indicator — before touching conductors or reconnecting equipment.
BS 7671:2018+A4:2026 Regulation 643.3 is the primary regulation governing insulation resistance testing. It requires that insulation resistance tests are carried out on all new installations before they are energised and on existing installations during periodic inspection and testing.
The regulation specifies that the test must be carried out on each circuit with all equipment connected (or in a representative energised state), except for equipment that is liable to be damaged by the test voltage, which must be disconnected. This creates a practical challenge — modern installations have numerous electronic components that are damaged by IR test voltages, requiring them to be disconnected individually.
A significant procedural change introduced in the A4:2026 redraft addresses situations where sensitive equipment cannot be disconnected. Under the revised Regulation 643.3, the correct response is not to skip the test or simply record a limitation. Instead, the equipment shall be connected and a 250V DC insulation resistance test shall be performed with the equipment in circuit. The lower test voltage avoids damage to the connected electronics whilst still providing a valid verification of the wiring insulation. This replaces the previous informal practice of noting equipment as "not disconnected" and recording a limitation.
The minimum acceptable IR values from Table 64 are the legal minimum under BS 7671. Where the inspector considers that a reading — although above the minimum — represents a deteriorating condition warranting monitoring, a C3 (improvement recommended) observation should be recorded on the EICR.
Efficient insulation resistance testing requires a systematic approach. Here are practical tips from experienced inspection electricians:
Surge protective devices (SPDs) are required under BS 7671:2018+A4:2026 Regulation 443.4 where a risk assessment identifies transient overvoltage as a hazard (e.g. risk to life, public services, commercial activity, or a large number of occupants). They are increasingly common and easily missed during inspection. Before applying any IR test voltage, check the consumer unit, distribution board, and sub-boards for SPDs and disconnect them from the circuit. An SPD in circuit gives a reading of virtually zero ohms — which can falsely condemn a sound installation. Also check for arc fault detection devices (AFDDs), now recommended on AC final circuits under Reg 421.1.7. AFDDs contain electronics that are equally vulnerable to 500V DC test voltages and must be disconnected before testing.
On large commercial or industrial installations, test one section at a time and record the reading for each section. This makes it easier to identify which section has a low reading, rather than testing the entire installation as a single unit and getting a low combined result that is difficult to investigate.
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