7.1 Insulation Resistance Testing

7.1 Insulation Resistance Testing

Quick Answer: Insulation resistance testing (BS 7671 Regulation 643.3, with the values in Table 64) verifies that the insulation between live conductors and between live conductors and earth is sufficient to prevent dangerous leakage or short circuits before the installation is energised. Table 64 tiers the test voltages and minimum values by nominal circuit voltage: 250 V DC / 0.5 MΩ for SELV and PELV, 500 V DC / 1 MΩ for circuits up to and including 500 V, and 1000 V DC / 1 MΩ for circuits above 500 V.

Purpose

Insulation resistance testing is the third dead test in the standard sequence of initial verification and periodic inspection. Its purpose is to confirm that the insulation between live conductors (line–neutral, line–line) and between each live conductor and earth is adequate to prevent leakage currents and short circuits when the installation is energised. A low or absent reading indicates a fault — damaged cable insulation, moisture ingress, or connected equipment not disconnected — that could cause danger, fire, or nuisance RCD operation on energisation. The test is performed with the installation isolated (dead).

The Instrument

An insulation resistance tester (sometimes called a "megger") is used. It must:

• Measure in megohms (MΩ) — not ohms
• Provide a selectable DC test voltage of 250 V, 500 V, and 1000 V
• Apply the voltage long enough to charge the insulation capacitance and return a stable reading
• Meet the accuracy and current-delivery requirements of BS 7671 and GN3 (10th Edition, 2026)

A standard multimeter cannot perform this test — it lacks both the voltage output and the MΩ measurement range.

Test Voltages and Minimum Values

BS 7671:2018+A4:2026 Regulation 643.3.2 sets the requirement and Table 64 gives the test voltage and minimum insulation resistance for each band of nominal circuit voltage:

SELV = separated extra-low voltage; PELV = protective extra-low voltage. The 230/400 V single-phase and three-phase installations that dominate UK domestic and commercial work fall in the middle band: 500 V DC test, minimum 1 MΩ.

Test Procedure

1. Isolate the installation and confirm dead using an approved voltage indicator (prove-test-prove).
2. Disconnect or link out electronic equipment that could be damaged by the DC test voltage — dimmer switches, electronic controls, LED drivers, surge protective devices. Where the manufacturer's data allows, terminals may be linked together to isolate the electronic components from the test voltage while still testing the fixed wiring. RCDs may need consideration depending on the instrument and test configuration.
3. Remove lamps and disconnect sensitive loads where required.
4. Test between live conductors: L–N for single-phase; for three-phase, test L1–L2, L1–L3, L2–L3, and each phase to neutral.
5. Test between each live conductor and earth: L–E (and L1–E, L2–E, L3–E for three-phase). Either link all live conductors together and test the bundle to earth, or test each conductor individually.
6. Apply the test voltage for a stable period — the reading rises as the insulation capacitance charges, then stabilises.
7. Record the steady reading on the schedule of test results.

Three-Phase Installations

For a three-phase installation, every phase-to-phase and phase-to-earth combination is tested:

• L1–L2, L1–L3, L2–L3 (phase-to-phase)
• L1–N, L2–N, L3–N (phase-to-neutral)
• L1–E, L2–E, L3–E (phase-to-earth)

Alternatively, link L1, L2, L3 and N together and test the bundle to earth in a single operation for the L–E measurement.

Why DC, Not AC?

DC stresses the insulation consistently and reveals weaknesses that an AC test would mask. The DC test charges the insulation capacitance, so the reading rises then stabilises — a healthy installation shows a climbing megohm value that plateaus. A falling or unstable reading suggests moisture, contamination, or a fault.

Whole-Installation Minimum and Sectioning

When the installation is tested as a whole, the minimum is 1 MΩ for a 230/400 V installation tested at 500 V DC. For large installations, parallel paths through many circuits can pull the overall value below 1 MΩ even when every individual circuit is healthy. GN3 (10th Edition) allows the installation to be tested in sections if the overall value is below the minimum; each section must still meet the applicable minimum.

Worked Example

A 230/400 V three-phase installation is tested at 500 V DC. The overall L–E reading is 0.7 MΩ — below the 1 MΩ minimum. The tester splits the installation into three distribution circuits and re-tests each: 12 MΩ, 8 MΩ, and 0.4 MΩ. The third section is below minimum and is investigated — a damaged cable run is found and repaired. After the repair, each section exceeds 1 MΩ and the installation is certified.

Common Causes of Low Readings

• Moisture in conduit, trunking, or accessories
• Damaged insulation — cable crushed at a fixing or nicked during stripping
• Connected equipment not disconnected (a common error — electronic filters present a low resistance to earth by design)
• Cable damage from drilling or fixings

Procedure Checklist

• Isolate and prove dead
• Disconnect/link out electronic equipment per manufacturer's data
• Remove lamps and sensitive loads
• Select correct test voltage for the circuit nominal voltage
• Test L–N (and all phase combinations for 3-phase)
• Test L–E (each phase to earth)
• Apply voltage until reading stabilises
• Record steady reading on schedule of test results