How to test insulation resistance on a domestic circuit

Helpful video reference. We use GSH Electrical's tutorial "Insulation resistance testing" as the video reference for this guide. GSH Electrical is a UK specialist electrical training channel with more than 170,000 subscribers whose videos support City and Guilds and EAL apprenticeship programmes. The testing content here reflects current BS 7671 requirements.

1. Understand what the test checks

Every conductor in your installation is wrapped in plastic insulation. Over time, heat, moisture, physical damage or deterioration can reduce the quality of that insulation, meaning current can leak from one conductor to another or to earth. An insulation resistance test checks that enough insulation remains to prevent dangerous leakage.

The test applies a 500 V DC voltage between conductors and measures the resistance in megohms (MΩ). The higher the reading, the better the insulation. BS 7671 Table 64 sets the minimum acceptable value at 1 MΩ for circuits up to 500 V. A healthy, recently installed circuit typically reads several hundred MΩ or shows as infinite on the meter.

2. Gather the right equipment

You need a multifunction tester (MFT) capable of applying 500 V DC. Common UK makes include Megger, Metrel, Fluke and Kewtech. A basic continuity meter or multimeter is not suitable for this test: it does not apply the required voltage and the readings are meaningless for insulation assessment.

You also need an approved voltage indicator to confirm the circuit is dead before connecting the MFT, and insulated test leads rated for at least 1000 V. If you do not own test equipment, this test belongs with a qualified electrician who does.

3. Isolate the circuit and confirm dead

Switch off the MCB or remove the fuse for the circuit under test. Check the isolation using an approved voltage indicator at the furthest accessible point on the circuit. Do not rely on the consumer unit position alone: always verify at the circuit end as well.

If working in an occupied property, tape over the breaker and let anyone else in the building know the circuit is off. A second person restoring power mid-test creates a genuine hazard.

4. Disconnect all equipment and lamps

Remove every plug from every socket on the circuit. Switch off any fixed appliances at their local switches or FCUs. Remove all lamp bulbs, tubes and LED modules from the luminaires on the circuit.

Electronic control gear in modern LED drivers, dimmers, and smart switches can be permanently damaged by 500 V DC. Leaving any item connected also drags the reading down: a single portable appliance with 10 MΩ of internal resistance will pull a perfectly healthy circuit to a 10 MΩ reading, which could cause unnecessary alarm or a false fail.

5. Set up the MFT and connect the leads

Select the IR function on your MFT. Set the test voltage to 500 V DC. Most modern MFTs display this selection clearly on the front panel.

Connect the test leads to the consumer unit terminals for the circuit under test. For a standard line-to-earth test, the red lead goes to the line terminal and the black or green lead to the earthing bar. Make sure your MFT leads are plugged into the correct sockets on the meter body for the IR function.

6. Carry out the three standard tests

Line to Earth. Apply the test voltage and hold for the period specified by the meter manufacturer (typically until the reading stabilises, or one minute). Record the reading in MΩ.

Neutral to Earth. Move the red lead to the neutral terminal and repeat. Record the reading.

Line to Neutral (combined test). For whole-installation testing, link the line and neutral conductors together at the board and test between the combined conductors and earth. This is the standard method described in the On-Site Guide and 18th Edition guidance for a complete installation IR test. Record this combined reading separately.

7. Read and record the results

Any reading of 1 MΩ or above is a pass under BS 7671 Table 64. Readings below 1 MΩ mean the insulation has broken down and the circuit should not be put back into service until the fault is found and rectified.

In practice, a reading anywhere close to the 1 MΩ minimum on a domestic circuit warrants investigation even if it technically passes. Healthy domestic wiring typically reads well above 100 MΩ. A reading of, say, 2 MΩ passes the test but suggests a problem developing.

Record the date, circuit reference, test voltage, and reading on your EICR schedule or installation test certificate.

When to call us

An insulation resistance test is a standard part of an EICR (Electrical Installation Condition Report) and any initial verification after new work. If you are a landlord needing an EICR, or a homeowner who has had a shock from a circuit or found unexplained tripping, Richard can carry out a full periodic inspection and provide the test schedule and condition report. Most domestic EICRs in east Kent are completed in two to three hours.