Insulation Resistance Testing
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Full course section in Plus
The test applies a d.c. between conductors and measures the leakage. For a normal 230/400 V circuit that is 500 V d.c. with a minimum acceptable value of 1 MΩ; and circuits are tested at 250 V with a 0.5 MΩ minimum. A circuit that scrapes past 1 MΩ has passed and still deserves investigation — healthy new wiring reads far higher.
Most of the skill is in what you disconnect first. Electronic equipment, dimmer switches, , and anything with capacitors will either be damaged by 500 V d.c. or will drag the reading down and be blamed for a fault it does not have. Lamps come out, appliances come off, and on a live installation-in-progress the line and neutral are linked together to test as one against earth.
A low reading is not automatically a rewire. Damp is the usual culprit on an older installation, long circuits read lower than short ones because leakage adds up along the run, and a single wet junction box can hold down a whole circuit. Split the circuit and test the halves — the number tells you where to look next.
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- Can I try Insulation Resistance Testing free on this page?
- Yes. The public sample below accepts answers and gives feedback and retries without an account, payment or energy. It is a short revision sample, not the complete course or a full mock paper. The full course section requires Plus.
- Where does Insulation Resistance Testing sit in the City & Guilds scheme?
- Testing maps to Unit 304 / 2391. On TradeTraining it lives in the Testing & Inspection section as bite-size lessons with practice questions.
- What test voltage is used for insulation resistance?
- 500 V d.c. for circuits up to 500 V, with 250 V d.c. used for and circuits and 1000 V d.c. for circuits above 500 V. The instrument must be able to hold that voltage while delivering a small test current.
- What is the minimum acceptable insulation resistance?
- 1 MΩ for a 230/400 V circuit and 0.5 MΩ for or . Those are floors rather than targets — a reading close to the floor on new work usually means a fault you have not found yet.
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Insulation resistance
The resistance between conductors that should be completely separated from each other. It is measured in megohms, line to neutral and each to earth, with a test instrument applying an elevated DC voltage. Healthy insulation reads very high; damaged cables, moisture or pinched conductors show up as a low reading long before they show up as a shock or a fire.
Read the full Insulation resistance glossary entrySELVSeparated Extra-Low Voltage
An extra-low-voltage system supplied from a safety source such as an isolating transformer, kept electrically separate from earth and from every higher-voltage system. Because the voltage is low and there is no path back to earth, touching one conductor cannot drive a dangerous current through you — the classic protective measure for shaver sockets and garden lighting.
Read the full SELV glossary entryPELVProtective Extra-Low Voltage
An extra-low-voltage system fed from a safety source like SELV, but with one point deliberately connected to earth. The earthed reference suits control circuits and electronics that need it, at the cost of losing SELV's complete separation — the difference between the two is a reliable exam question.
Read the full PELV glossary entryBasic protection
Protection against touching live parts during normal, fault-free service. It is delivered by insulation around conductors, barriers and enclosures around terminals, and placing live parts out of reach. It is the first layer of shock protection; fault protection is the second, and an installation needs both.
Read the full Basic protection glossary entry