What is an insulation resistance test?

Glossary · Testing, inspection and certificates

How an insulation resistance test works

Every cable in a home has insulation around the conductors. Over time heat, damage, moisture or poor workmanship can wear that insulation down. An insulation resistance test checks that it is still doing its job.

The IET describes it plainly. Insulation resistance testing, or megger testing as it is more commonly known, tests the resistance of the insulation of all live parts in electrical circuits. The measured value is compared with the minimum values in the standard, to verify that the insulation has not broken down through damage to cables or other parts of a circuit.^{[1]Institution of Engineering and Technology (IET), History of insulation resistance testing (Wiring Matters, July 2020)} NICEIC lists it as one of the circuit tests in an EICR. Its purpose is to make sure all wires are properly insulated, with no risk of the insulation breaking down and causing faults or an electric shock.^{[2]NICEIC, Electrical Installation Condition Reports (EICRs)}

The minimum value

The minimum value that BS 7671 sets for insulation resistance in a normal installation is 1 megohm. The IET traces this value back to the 17th Edition and says it is still used today.^{[1]Institution of Engineering and Technology (IET), History of insulation resistance testing (Wiring Matters, July 2020)} It also notes that there is no documented reason for choosing 1 megohm, but that it has proved a useful starting point for judging the condition of cables and equipment.^{[1]Institution of Engineering and Technology (IET), History of insulation resistance testing (Wiring Matters, July 2020)}

A reading under 1 megohm does not meet the requirements of the current edition. The IET says it should be given a classification code. Finding out why the value is so low takes further investigation, which the IET treats as fault-finding rather than part of the inspection itself.^{[3]Institution of Engineering and Technology (IET), Implications of BS 7671:2018+A4:2026 on an EICRs (Wiring Matters, July 2026)} Our page on EICR observation codes explains the codes.

The IET’s list of live tests for a typical home with an RCBO on each circuit names only those three, and puts insulation resistance among the others.^{[4]Institution of Engineering and Technology (IET), Minimizing unnecessary live testing for initial verification (Wiring Matters, May 2025)}

Example

Hannah books an EICR for her Horsforth house. The electrician tests each circuit and writes the insulation resistance for each on the schedule of test results. The lighting circuit upstairs gives a reading below the minimum. That does not meet BS 7671, so the electrician gives it a code on the report. Working out whether the cause is a damaged cable, a damp junction or a faulty fitting is a separate job that follows.^{[3]Institution of Engineering and Technology (IET), Implications of BS 7671:2018+A4:2026 on an EICRs (Wiring Matters, July 2026)}

What people get wrong

A low insulation resistance reading is not the same as a full rewire. It tells the electrician the insulation on that circuit is not good enough, and they then need to find the reason. The cause could be one damaged part, and finding it is separate from the inspection.

People also assume the test is something an owner can run with an instrument bought online. It is not. NICEIC is clear that an EICR involves specialised tests and equipment and should never be a DIY job.^{[2]NICEIC, Electrical Installation Condition Reports (EICRs)}

The bottom line

An insulation resistance test checks that the insulation around your wiring is intact. The minimum value is 1 megohm, and anything lower is recorded on the report with a code. It is one of the standard tests in an EICR and in the certification of new work, and it needs a competent electrician. You can see how we test on our EICR testing page.