Inspection, Testing and Commissioning
Core concepts
Know the sequence of initial verification: visual inspection before testing, then continuity of protective conductors, ring final circuit continuity, insulation resistance, polarity, earth electrode resistance (where applicable), earth fault loop impedance, RCD operation, and functional testing. Understand instrument use, test voltages, acceptable results and the importance of safe isolation before testing. Be familiar with the purpose of each test and how results are recorded and compared with design values.
Traps in questions
Common traps include testing before a thorough visual inspection, using the wrong test voltage for insulation resistance, or misinterpreting a low insulation reading caused by connected equipment. Watch for questions that reverse the test order, or that confuse prospective fault current with earth fault loop impedance. Another trap is forgetting to verify RCD trip times at the correct test currents.
Worked example / rule of thumb
For insulation resistance on a 230 V circuit, test at 500 V DC and expect a result of at least 1 MΩ (higher is better). If a low reading appears, disconnect sensitive equipment and retest before condemning the cable. Rule of thumb: always isolate, prove dead, and re-prove the tester before and after work.