Testing, Verification and Fault Finding
Core concepts
Understand the sequence of tests required for verification of an installation: visual inspection, continuity of protective earthing, insulation resistance, polarity, circuit separation, and earth fault loop impedance. Know the correct instruments and their safe use, including insulation resistance testers, earth loop impedance testers, RCD testers and multimeters. Understand how to interpret results against the relevant limits and how to record them.
Traps in questions
Candidates often perform tests in the wrong order or forget to isolate sensitive equipment before insulation resistance testing. Questions frequently test whether a low insulation resistance reading is a fail or a measurement error, and whether the tester's voltage is appropriate. Another trap is confusing continuity testing with insulation resistance testing. Be careful with the polarity test, which confirms active and neutral are correctly connected, and with the difference between fault finding and verification.
Worked example
An insulation resistance test between active-neutral and earth returns a reading below the required minimum. First confirm the tester is set correctly and the installation is isolated. Then disconnect loads and repeat. If the reading remains low, divide the installation into sections to locate the faulty circuit. Record the final reading and confirm it meets the required value before energising.
Rule of thumb
Always inspect before testing, isolate before insulation resistance testing, and prove the tester on a known source before and after use.