Insulation Resistance Testing
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Premium section
The test applies a d.c. voltage 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.
Common questions
- Is there a free Insulation Resistance Testing quiz?
- Testing sits in the Testing & Inspection section, which is part of TradeTraining Premium — but the app is free to start, sample questions are open to everyone, and the free sections cover the fundamentals first.
- Where does Insulation Resistance Testing sit in the City & Guilds scheme?
- Testing maps to C&G 2365 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.