Which sequence correctly describes performing an insulation resistance test?

• A. Switch tester to 500 V; attach the clip to neutral then firmly press the other wire to line; press the yellow test button and wait until you get a reading; then test between line and earth and neutral and earth; the reading should be more than 1 MΩ (1000000 Ω)
• B. Switch tester to 250 V; attach the clip to neutral; press the test button; test between line and earth only; reading is not specified
• C. Switch tester to 500 V; clip to live; press the test button; test between line and earth; reading should be greater than 1 Ω
• D. Switch tester to 500 V; clip to earth; press test; test between line and earth and neutral and earth; reading should be less than 1 MΩ

Answer: (A)

Insulation resistance testing uses a megohmmeter to push a DC voltage through the insulation and measure leakage current; a high resistance means the insulation is good, while a low reading indicates leakage paths or faults. For a typical LV installation, you apply a test voltage of about 500 V and first ensure the circuit is isolated and all loads removed. Then you measure between each conductor and earth to check the insulation to ground. Testing both line-to-earth and neutral-to-earth captures leakage paths from all live conductors to earth, and you should expect readings in the megohm range. A minimum of around 1 MΩ per conductor to earth is a common benchmark, so readings greater than 1 MΩ show acceptable insulation. If you only test one path, or you expect readings in ohms, or you choose the wrong voltage, the test doesn’t reliably indicate insulation quality.