Insulation Resistance — What Does That Number on Your Report Mean?
If you've ever seen an electrical installation test report, it usually includes an insulation resistance value in megohms (MΩ) among other readings. This page helps you understand what that number means — it's not a guide to measuring it yourself.
How the measurement is done (and why it's not a DIY task)
Safe de-energization
Before testing, the electrician disconnects the circuit from supply and confirms it's safe — testing a live circuit incorrectly can damage equipment or be dangerous.
Testing with a megohmmeter
A megohmmeter (part of a multifunction installation tester) applies a test voltage of 250V, 500V or 1000V DC, depending on the circuit's voltage class — for example SELV/PELV or standard 230/400V circuits.
Comparing the result to the standard
In practice a minimum of roughly 1 MΩ is often required, but the exact acceptable threshold depends on the specific circuit and applicable standard (LVS HD 60364-6 / IEC 60364-6) — there's no single universal number.
Recording it in the report
The measured value, method used, and result (pass/fail) are recorded in a test report — a document often required for handover, insurance, or regulatory purposes.
What to know about insulation resistance
The minimum often required in practice
This figure comes up often as a reference point, but the actual acceptable threshold always depends on the specific circuit and standard — check your own report rather than assuming one universal number.
What the number shows
Insulation resistance shows how well the wiring's insulation prevents current leakage between conductors or to earth — a higher value means lower risk of electric shock or fire from damaged insulation.
Test voltage depends on the circuit
Megohmmeters test at 250V, 500V, or 1000V DC — lower voltages for low-voltage circuits (like SELV/PELV), higher voltages for standard 230/400V circuits.
There's no single universal threshold
The exact acceptable value depends on the circuit type and applicable standard — it's shown on your test report, not a generic table online.
The measurement itself needs a calibrated instrument
The test must be done with a calibrated megohmmeter and the circuit correctly de-energized beforehand — this part isn't safe to try without proper training.
The result gets documented
Every test ends with a report recording the values, method, and conclusion — a document often needed for handover or insurance.
How we can help
If you've seen a number on your report you're not sure about, we can explain it and suggest next steps.
- Plain-language explanation of your own report
- Help understanding if the value is within range
- Guidance if the result is borderline
- Advice on whether re-testing is needed
- An honest opinion, no sales pressure
Frequently asked questions
What is a megohmmeter and how is it different from a regular multimeter?
A megohmmeter measures very high resistance (in megohms) using a test voltage of 250-1000V DC — a regular multimeter isn't built for this and won't give a reliable result.
Why does the test voltage vary — 250, 500, or 1000V DC?
The test voltage is chosen based on the circuit's nominal voltage class — low-voltage circuits (like SELV/PELV) use a lower test voltage, standard 230/400V circuits use a higher one.
Does a reading below 1 MΩ always mean a problem?
Not necessarily — a minimum of around 1 MΩ is often required in practice, but the exact acceptable threshold depends on the specific circuit and standard. It's best to leave the interpretation to a certified electrician who knows what value is expected for your circuit.
Can I measure insulation resistance myself at home?
The measurement itself requires a calibrated instrument and safe de-energization of the circuit beforehand — done incorrectly it can damage equipment or be dangerous, so it's a job for a certified professional.
What happens if the measurement doesn't meet the report's requirements?
The electrician assesses which part of the circuit or connection could be the cause and recommends the necessary repair or a re-test afterward.
Unsure what a number on your report means?
Book a consultation — we'll help you understand what your test report actually means, no sales pressure.