Inspection, Testing, and Fault Diagnosis
What the candidate must know
This domain joins visual inspection, dead testing, live testing, and systematic fault finding. Candidates must know the sequence: inspect before testing, test dead before live, and record results against limits. Core tests include continuity of protective and bonding conductors, insulation resistance, polarity, earth loop impedance, RCD trip time and current, and functional testing of detection and lighting circuits. Fault diagnosis requires a logical method: confirm the symptom, divide the circuit, test at boundaries, and change one variable at a time.
Traps that appear in questions
- Testing insulation resistance with electronics or surge devices still connected, giving a false fail or damaging equipment.
- Recording a pass without comparing to the required minimum, or ignoring temperature correction.
- Confusing a high loop impedance reading caused by a poor joint with a genuinely long run.
- Replacing a device before proving whether the fault is upstream or downstream.
- Skipping functional tests of alarms and emergency lighting because the electrical tests passed.
Worked example / rule of thumb
An RCD trips instantly at random. Insulation resistance reads 0.4 MΩ on one final circuit, below the 1 MΩ threshold, pointing to moisture ingress rather than a faulty device. Rule of thumb: test in the order continuity, insulation, polarity, loop, RCD, function, and diagnose by halving the circuit rather than by substitution.