Regulation 643.3, Table 64, and the two-stage test under Amendment 2:2022. Pass criteria, procedure, and the EICR coding implications of the mistakes that cost electricians money.

Insulation resistance testing verifies that the insulation between live conductors, and between live conductors and earth, is intact. It is one of the dead tests prescribed by Chapter 64 of BS 7671:2018+A2:2022 and it is mandatory on every new installation, addition, alteration, and periodic inspection.

The test works by applying a DC voltage between conductors (or between conductors and the protective conductor connected to the earthing arrangement) using an insulation resistance tester. Any leakage current flowing through the insulation is measured, and the instrument displays the resistance in megohms. A low reading indicates that the insulation has broken down, that moisture or conductive contamination is present, or that conductors are short-circuited.

Left uncorrected, degraded insulation causes nuisance RCD tripping, localised heating, fire, and the risk of electric shock from exposed-conductive-parts that should not be live. It is the single most valuable piece of information an inspector can record about the condition of cable insulation.

The 250 V DC retest provision in Regulation 643.3.3 was formalised under Amendment 2:2022. It gives installers a practical route for re-proving a circuit after commissioning without risking damage to electronic loads that may not tolerate 500 V DC. It is not a substitute for the 500 V DC cable test before connection of equipment. Both stages are required.

Table 64 is the regulatory floor. Values below it are non-compliant. Values above it can still be a concern in context, particularly on new work.

Circuit nominal voltage (V)	Test voltage DC (V)	Minimum insulation resistance (MΩ)
SELV and PELV	250	0.5
Up to and including 500 V, with the exception of the above systems	500	1.0
Above 500 V	1000	1.0

Source: BS 7671:2018+A2:2022, Table 64. Table 64 is also applied when verifying insulation resistance between non-earthed protective conductors and Earth. FELV circuits are tested at the same voltage as that applied to the primary side of the source and must meet the test requirements for low voltage circuits.

1. Isolate the circuit, lock off, and prove dead with an approved voltage indicator.
2. Disconnect the incoming neutral at the main earthing terminal or main neutral link. Any parallel path through another installation will produce a false reading.
3. Confirm the protective conductor of the switchgear or enclosure is connected to the main earthing terminal.
4. Disconnect pilot and indicator lamps, capacitors, surge protective devices (SPDs), and any voltage-sensitive electronic equipment.
5. Close all circuit breakers, main switches, and two-way/intermediate switches so the full run of cable is included in the test. Test two-way and intermediate switches in all combinations.
6. Remove lamps and any current-using equipment that cannot withstand 500 V DC. This includes LED drivers, electronic fluorescent starters, dimmers, touch switches, delay timers, PIR sensors, emergency lighting, RCBO functional earth leads, AFDDs.
7. Select the correct test voltage: 500 V DC for standard LV, 250 V DC for SELV/PELV, 1000 V DC above 500 V.
8. Connect the test leads per Regulation 643.3.1 and record the lowest reading for each group.

Regulation 643.3.2 allows the test to be applied to the main switchboard and each distribution circuit separately. On a simple dwelling with no sensitive equipment, a whole-installation test with the line conductors linked together and the neutral connected to earth at the MET gives a valid single reading. The instrument will normally read in excess of 200 MΩ and most display out of range, typically shown as >999 MΩ.

On larger or more complex installations a whole-installation test is not useful. A single faulty circuit dominates the measurement and hides the state of every other healthy circuit. Subdivide to distribution-circuit level as a minimum, and down to final-circuit level where a problem is suspected. This is also the only way to identify a specific failure when it occurs.

For a four-core SWA, use the five-test sequence: L1-L2, L1-L3, L2-L3, then L1+L2+L3 linked to N, then L1+L2+L3+N linked to E. Record the lowest reading in the first group as the between-live-conductors value, and the lowest of the last two as the live-conductors-to-earth value. A four-step sequence (L1-L2, L1+L2 to L3, L1+L2+L3 to N, L1+L2+L3 to E) is acceptable for experienced testers, but the five-step sequence is required when you need to identify which conductors are affected.

Where equipment cannot be disconnected – a commissioned smart home with hard-wired dimmers and touch panels, a fire alarm loop with electronic detectors, an installation with bonded SPDs and fixed LED drivers – Regulation 643.3.3 provides a two-stage route.

The 250 V DC step is not a shortcut. It is a commissioning check designed to prove the installation after equipment that could not tolerate 500 V DC has been fitted. The cable verification at the Table 64 voltage must still have been carried out before equipment was connected. Where a Stage 1 result is missing, the tester is relying on the Stage 2 reading alone and the certificate cannot be signed in full confidence.

Table 64 is a minimum. A new installation should comfortably exceed 200 MΩ per circuit, and most final circuits on a dwelling show the instrument’s out-of-range symbol. IET Guidance Note 3 recommends that any new installation giving a result below 20 MΩ should be investigated, even though a value as low as 1 MΩ technically complies. It is a sensible engineering threshold, not a regulatory one.

On a periodic inspection, a circuit reading just above 1 MΩ is not a regulatory failure, but it is an indicator of moisture ingress, damaged cable sheathing, or degraded termination insulation. Any circuit below 2 MΩ should be investigated before the EICR is signed off as satisfactory.

Record the exact reading the instrument displayed. Where the value exceeds the instrument range, record it as shown (for example, >999 MΩ) and note the instrument’s upper limit on the certificate. A blank or ambiguous result against a required test line is a documentary failing under Regulation 653.2.

These are the errors that repeatedly show up in EICRs, initial verification certificates, and field audits. The coding shown reflects Electrical Safety First Best Practice Guide 4 and the IET Guidance Note 3 interpretation of Regulation 651.

1. Testing with the incoming neutral still connected to the network

The installation shares a neutral with neighbouring installations via the DNO network. A test with the neutral still connected gives artificially low readings and can damage the instrument or a neighbouring installation. Coding: FI on an EICR because the result is unreliable and the test must be repeated.

2. Applying 500 V DC to a SELV or PELV circuit

Regulation 643.3.2 and Table 64 specify 250 V DC for SELV and PELV. Using 500 V DC will destroy transformers, LED drivers, and controllers. Coding: any consequent unsafe condition is C2, and the replacement cost is on the tester.

3. Failing to disconnect electronic loads before a 500 V test

Dimmers, RCDs, RCBOs, AFDDs, SPDs, emergency lighting, and LED drivers either mask a cable fault or are damaged by the test. Use Regulation 643.3.3 two-stage testing when equipment cannot be removed. Coding: a final-circuit reading below 1 MΩ is C2 (potentially dangerous) because of the increased fault current and fire risk.

4. Omitting the live-conductors test (L-N or L-L)

Regulation 643.3.1(i) requires measurement between live conductors. Skipping L-N, or on three-phase skipping L1-L2, L1-L3, L2-L3, misses an entire class of insulation fault. Coding: C3 (improvement recommended) where the inspector cannot repeat the test; on a new EIC it is an invalid certificate.

5. Testing only at the consumer unit on a larger installation

Regulation 643.3.2 requires each distribution circuit to be tested separately. A single reading at the origin reports the lowest of every parallel path and hides a failing circuit among healthy ones. Coding: documentary non-compliance with Regulation 653.2, and where the inspector cannot identify the affected circuit, FI.

6. Leaving RCBO functional earth leads connected

Modern RCBOs have a functional earth lead that must be disconnected for the insulation resistance test. Leaving it connected produces a false low reading through the internal electronics. Coding: invalid test; where it cannot be disconnected safely, apply Regulation 643.3.3 two-stage testing and record the limitation.

7. Recording “>999 MΩ” without stating the instrument’s upper range

The certificate must record what the instrument displayed. An out-of-range reading is not a numeric value and the upper limit of the instrument range must be identified on the certificate or in the notes. Coding: administrative non-compliance; a new EIC with an unrecorded range is invalid.

8. Accepting a reading just above 1 MΩ as satisfactory on an EICR

A circuit reading of 1.2 MΩ passes Table 64 but almost certainly indicates water ingress, damaged sheathing, or a failing joint. The duty to apply professional judgement under Regulation 651 means a result near the floor is an investigation trigger. Coding: C3 (improvement recommended) at minimum, C2 (potentially dangerous) where the cause is evident.

Chapter 65 and Regulation 653.2 require insulation resistance testing on periodic inspection to the extent practicable without causing danger or damage. In practice that means: disconnect what can safely be disconnected, leave in place what would cause danger or damage to disconnect, and record every limitation in the Extent and Limitations section of the EICR.

Where a circuit cannot be tested at the full Table 64 voltage because of fixed electronic equipment, apply the 250 V DC commissioning test under Regulation 643.3.3, record the reason, and note that the cable insulation has not been verified at Table 64 voltage since the original installation. A repeated “did not test” limitation across successive EICRs is not acceptable indefinitely under the Electricity at Work Regulations 1989. Plan a shutdown to permit a full test.

For further reference on coding decisions, see our guide to EICR coding (C1, C2, C3 and FI) explained, and for whole-installation record keeping see iCertifi’s BS 7671 certificate app.

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