What test voltage do I use for insulation resistance testing?

BS 7671 Table 64, applied by Regulation 643.3.2, sets the test voltages by circuit nominal voltage. For SELV and PELV circuits (extra-low voltage — not exceeding 50V AC or 120V ripple-free DC), the test voltage is 250V DC and the minimum acceptable IR value is 0.5 MΩ. For circuits up to and including 500V, with the exception of SELV and PELV (this covers the vast majority of UK domestic and commercial circuits at 230V single phase and 400V three phase), the test voltage is 500V DC and the minimum acceptable IR is 1 MΩ. For circuits above 500V, the test voltage is 1000V DC and the minimum IR is 1 MΩ — note that these are still low voltage circuits under the BS 7671 Part 2 definition, which runs to 1000V AC. In practice, most domestic and commercial IR testing uses 500V DC.

How do I carry out an insulation resistance test on a new circuit?

Ensure the circuit is de-energised and that all fuses or MCBs are removed or locked off. Link the line and neutral conductors together at the consumer unit or distribution board so the entire circuit (both conductors plus the CPC) is tested as a group. Disconnect the current-using equipment — luminaires, appliances, and anything containing electronics (LED drivers, motor capacitors, SPDs) that will be damaged by the 500V test voltage. Regulation 643.3.2 has the circuit wiring itself connected: leave switches in place and closed so the switch wiring is included in the test, and leave the accessories wired. Connect the IR tester between the linked L+N conductors and earth, apply the test voltage for at least 1 minute (some instruments recommend a shorter stabilisation period — follow the manufacturer guidance), and record the reading. Also test between L and N with the link in place (or remove the link and test L to N separately) to verify insulation between conductors.

What does a low insulation resistance reading indicate?

A low IR reading (below 1 MΩ, or suspiciously low for the installation age) indicates that the insulation of a conductor is degraded and allowing current to leak to earth or to another conductor. Common causes include: moisture ingress into a junction box, conduit, or light fitting; physical damage to cable insulation from rodents, staples, or mechanical impact; insulation degradation due to excessive heat (cables near downlights or in loft spaces where temperatures are high in summer); carbon tracking in accessories following arcing or overheating; and the presence of a connected electronic device that was not disconnected before testing. If a low reading is found, disconnect loads and accessories one by one to isolate the section with the problem. A clamp meter can also be used to detect leakage current on energised circuits.

Can I test insulation resistance on a live circuit?

No. IR testing requires the circuit to be de-energised (dead) and safely isolated before the test voltage is applied. The 500V or 1000V DC test voltage applied to a live conductor is extremely dangerous. The correct procedure for EICR work is: identify the circuit to be tested, carry out safe isolation in accordance with the HSE GS38 guidance and the Electricity at Work Regulations 1989, verify the circuit is dead with an approved voltage indicator, then proceed with IR testing. After IR testing is complete, verify the circuit is still safe to re-energise before restoring the supply.

What does Regulation 643.3 require?

BS 7671:2018+A4:2026 Regulation Group 643.3 governs insulation resistance testing. Regulation 643.3.1 requires the insulation resistance to be measured between live conductors, and between live conductors and the protective conductor connected to the earthing arrangement — line and neutral may be connected together for the second of these. Regulation 643.3.2 sets the pass criterion: the result is satisfactory where the main switchboard and each distribution circuit, tested separately with all its final circuits connected but with current-using equipment disconnected, is not less than the value in Table 64. Regulation 643.3.3 (redrafted by A4:2026) covers connected equipment that is likely to influence the result or be damaged: the Table 64 test is applied before that equipment is connected, and then, following connection, a 250V DC test is applied between live conductors and the protective conductor, which must read at least 1 MΩ. Note that manufacturer instructions may still recommend disconnecting some equipment for the 250V DC test. There is no threshold in 643.3 for investigating readings that are above the minimum — that guidance comes from Guidance Note 3, which sets 20 MΩ as the investigation point on a new installation.

When do I use IR testing to verify SELV or PELV circuit separation?

BS 7671:2018+A4:2026 Regulations 643.4.1 to 643.4.3 require that the separation of live parts in SELV and PELV systems — and in circuits protected by electrical separation — is confirmed by a measurement of insulation resistance. The resistance values obtained must comply with Table 64. This applies to ELV circuits that are increasingly common in modern buildings: EV pilot/control circuits, fire detection wiring, Category 6 PoE circuits, and ELV lighting systems. The purpose of the test in this context is not just to verify cable insulation health but to confirm that the required degree of electrical separation from other circuits and from earth has been achieved. Read the acceptance value carefully: Regulation 643.4 states that the values obtained in the tests of Regulations 643.4.1 to 643.4.3 shall be at least that of the circuit with the highest voltage present, in accordance with Table 64. So the bare SELV/PELV figure of 0.5 MΩ at 250V DC only applies where no higher-voltage circuit is involved; where the ELV circuit is separated from a 230V circuit, the 1 MΩ value for the higher-voltage circuit applies. Guidance Note 3 adds that where the ELV conductors are separated from low voltage conductors by insulation alone — for example within a multicore cable shared with LV circuits — the test voltage is increased to 500V DC and the insulation resistance shall be not less than 1 MΩ. Record the result as a verification of the protection measure, not merely as a cable condition check.

TESTING GUIDE

Insulation Resistance Testing: BS 7671 Procedure and Minimum Values

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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10 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

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.

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The short answer

What is the minimum insulation resistance value?

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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Key takeaways

01Insulation resistance (IR) testing verifies that the insulation of conductors is intact and is not leaking current to earth or between conductors. BS 7671 Regulation 643.3 sets out the requirements.
02The test voltage applied depends on the circuit nominal voltage: 250V DC for SELV and PELV circuits, 500V DC for circuits up to and including 500V (most domestic and commercial circuits), and 1000V DC for circuits above 500V.
03The minimum acceptable insulation resistance value depends on circuit type. For SELV and PELV circuits (tested at 250V DC) the minimum is 0.5 MΩ. For circuits up to and including 500V — including all standard 230V domestic and 400V commercial circuits (tested at 500V DC) — the minimum is 1 MΩ. Guidance Note 3 goes further for new work: on a new installation, any value below 20 MΩ should be investigated.
04For new circuits, IR testing is carried out on the dead circuit before connection to the supply. All luminaires and appliances should be disconnected — many electronic devices are damaged by IR test voltages.
05On live circuit EICR work, IR testing requires safe isolation of the circuit under test and disconnection of sensitive electronic equipment before applying the test voltage.

01 · Testing Guide

Check Your Insulation Resistance Reading

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What your reading means — BS 7671 Table 64 plus what it says about the installation

Your reading

MΩ

The value shown on the tester

Unit

Circuit nominal voltage

Age of the installation

Previous reading, if you have one (MΩ)

MΩ

Optional — a sharp fall matters even when the current reading passes

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02 · Testing Guide

Why Test Insulation Resistance?

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.

03 · Testing Guide

Test Voltages — 250V, 500V, and 1000V DC

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.

250V DC

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.

500V DC

Used for circuits up to and including 500V nominal, with the exception of SELV and PELV. 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.

1000V DC

Used for circuits above 500V nominal. Minimum IR value: 1 MΩ. Rarely used in routine domestic or commercial electrical work; more common in industrial installations. Note these are still low voltage circuits — BS 7671 Part 2 defines low voltage as up to 1000V AC, so “above 500V” here does not mean high voltage.

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.

04 · Testing Guide

Minimum Acceptable Values — ≥1 MΩ

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.

BS 7671 Table 64 — at a glance

Minimum insulation resistance and DC test voltage by circuit type

Circuit

Test voltage

Min IR

SELV & PELV

250 V DC

0.5 MΩ

Up to & incl. 500 V

500 V DC

1.0 MΩ

Above 500 V

1000 V DC

1.0 MΩ

Per BS 7671:2018+A4:2026 Table 64, Regulation 643.3.2. A pass at the minimum is not a target — Guidance Note 3 says to investigate any new installation reading below 20 MΩ.

Sound new installation — Guidance Note 3 treats 20 MΩ as the investigation point on new work: below that, subdivide the installation and test circuits individually to find the cause, even though 1 MΩ would technically conform to the Regulations.
Older installation — judge in context — Guidance Note 3 is explicit that assessing a reading depends on the age and type of installation, the length of cable involved, and the environmental conditions. A value that would prompt investigation on new work can be acceptable in context on an existing installation, but a downward trend across successive inspections should be recorded and monitored.
Failure — below the Table 64 minimum — the circuit fails the test. Before coding it on an EICR, confirm the reading is a genuine insulation fault rather than a connected electronic device that was not disconnected.

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.

05 · Testing Guide

Testing New Circuits — Step by Step

For a new circuit, IR testing is carried out before connection to the supply. Here is the correct procedure:

1Disconnect all equipment — remove all luminaires, disconnect all appliances, and remove or disconnect electronic equipment from the circuit. Leave switches in the closed (on) position so the switch wiring is included in the test.
2Link L and N at the board — at the consumer unit, link the line and neutral terminals of the circuit together so both conductors are tested simultaneously. The CPC (earth) is left separate.
3Test L+N to E — connect the IR tester between the linked L+N conductors and the earth (CPC). Apply 500V DC and record the reading.
4Test L to N — remove the link and test between L and N to check insulation between the two live conductors.
5Record results — note the measured IR value, the test voltage, and the circuit designation on the schedule of test results.
6Discharge before touching — as noted in Guidance Note 3, the cable capacitance holds the test voltage as residual charge that persists after the tester is removed and can cause a shock. Confirm the instrument has completed its discharge cycle before touching any conductors or reconnecting equipment.

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06 · Testing Guide

Live Circuit Precautions for EICR Work

When carrying out IR testing as part of an EICR on an occupied installation, additional precautions are required:

Safe isolation first — always carry out safe isolation of the circuit before applying the IR test voltage. This is a legal requirement under the Electricity at Work Regulations 1989. Use an approved voltage indicator (GS38 compliant) before and after isolation to confirm the circuit is dead.
Warn occupants — inform the building occupants that circuits will be isolated and equipment may be disconnected during testing. Agree on a programme of isolation to minimise disruption.
Identify and protect sensitive equipment — before isolating each circuit, identify all connected equipment. Do not apply IR test voltages to circuits with connected surge protective devices, LED drivers, motor drives, or any equipment with electronic components unless those components are physically disconnected.
Identify and disconnect AFDDs — under the A4:2026 redraft of Reg 421.1.7, arc fault detection devices are now required on single-phase AC final circuits supplying socket-outlets rated up to 32 A in high rise residential buildings, houses in multiple occupation, purpose-built student accommodation and care homes, and recommended for the same circuits in all other premises. Expect to meet them far more often on A4:2026 boards. Their internal electronics are vulnerable to 500V DC test voltages. Check for AFDDs alongside SPDs before applying any test voltage.

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.

07 · Testing Guide

Regulation 643.3 Explained

BS 7671:2018+A4:2026 Regulation Group 643.3 is the primary requirement governing insulation resistance testing, and it splits into three parts.

643.3.1 says what to measure: the insulation resistance shall be measured between live conductors, and between live conductors and the protective conductor connected to the earthing arrangement. During that second measurement, line and neutral may be connected together — which is why the standard site method is to link L and N and test the pair to earth.

643.3.2 sets the pass criterion, and its wording is precise: the result is satisfactory where the main switchboard and each distribution circuit, tested separately with all its final circuits connected but with current-using equipment disconnected, is not less than the appropriate value in Table 64. The circuit wiring stays connected; the current-using equipment comes off. Note also that BS 5839-1 sets more specific requirements for fire detection and fire alarm system wiring.

643.3.3 is the part redrafted by A4:2026, and it deals with connected equipment that is likely to influence the measurement or be damaged by the test. It sets a two-stage sequence, not an either/or. First, the Table 64 test is applied prior to the connection of that equipment. Then, following connection of the equipment, a 250V DC test is applied between live conductors and the protective conductor connected to the earthing arrangement — and that reading must be at least 1 MΩ, not the 0.5 MΩ some assume from the 250V row of Table 64. A note to the regulation warns that manufacturer instructions may still recommend disconnecting particular equipment even for the 250V DC test.

The Table 64 values are the minimum for compliance with BS 7671. Where the inspector considers that a reading — although above the minimum — represents a deteriorating condition warranting monitoring, that observation should be recorded on the EICR.

08 · Testing Guide

For Electricians: Practical IR Testing Tips

Efficient insulation resistance testing requires a systematic approach. Here are practical tips from experienced inspection electricians:

Always Check for SPDs Before Testing

Surge protective devices (SPDs) are required under the A4:2026 redraft of Regulation 443.4.1 where the consequence of a transient overvoltage could be serious injury to or loss of human life, failure of a safety service as defined in Part 2, or significant financial or data loss. For all other cases, protection shall be provided unless the owner of the installation declares it is not required. A4:2026 deleted the old risk assessment method in Regulation 443.5 and Annex A443 — so on new work the default is that an SPD is fitted, and you will meet them far more often. 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 left in circuit drags the reading down and can falsely condemn a sound installation. Also check for arc fault detection devices (AFDDs), which Reg 421.1.7 now requires on socket-outlet final circuits up to 32 A in high rise residential buildings, HMOs, purpose-built student accommodation and care homes, and recommends elsewhere. AFDDs contain electronics that are equally vulnerable to 500V DC test voltages and must be disconnected before testing.

Test by Section on Large Installations

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.

Frequently Asked Questions About Insulation Resistance Testing

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