Fault Diagnosis and Rectification
Method beats guesswork
Fault diagnosis is marked on method, not luck. Start with information: what the client reports, when it began, what changed, and whether the fault is intermittent. Review existing certificates and schedules, then confirm the symptom yourself before touching anything. Isolate safely before working.
Work logically and sectionalise. Use the half-split technique - test at the midpoint of the circuit and decide which half contains the fault. Dead tests locate most faults: continuity for open circuits and high-resistance joints, insulation resistance for short circuits and earth faults. Live tests (Zs, RCD operation, voltage) confirm what the dead tests suggest.
Typical assessment faults: an open circuit from a loose terminal, a nail through a cable causing a line–earth fault, a reverse-polarity socket, a borrowed neutral causing RCD tripping, a faulty appliance causing nuisance tripping, and a ring final circuit with a high-resistance joint.
Worked example: an RCD trips whenever a particular lighting circuit is energised. Isolate, remove lamps and electronic gear, then measure insulation resistance line–earth and neutral–earth. A low reading points to damaged cable or water ingress in a fitting. If insulation resistance is healthy, look for a neutral–earth fault or a faulty luminaire before blaming the RCD.
After rectification, retest the circuit and record the results, issuing a Minor Electrical Installation Works Certificate for the repair. Never simply reset or uprate a protective device to stop it tripping - the device is reporting a fault, not causing one.