Fault Diagnosis and Rectification
Fault Diagnosis and Rectification
This domain tests systematic diagnosis: gathering information, isolating safely, using instruments to divide the circuit, confirming the root cause, then rectifying and re-verifying. Random part-swapping earns nothing.
Method
- Interview the user and review earlier certificates and test results for a baseline.
- Isolate and prove dead, then inspect for damage, heat discolouration, water and rodent ingress.
- Dead tests: continuity, insulation resistance, polarity. Live tests: Zs, RCD ramp and trip times, prospective short-circuit current. Use a leakage clamp to separate N-E from L-E paths and thermography for hot terminations.
- Half-split long circuits, disconnecting sections and retesting rather than guessing.
- Rectify to the original design, retest, record the outcome, and amend the certificate or issue a minor works certificate.
Worked example
A 30 mA RCBO on a socket circuit trips at random, more often in wet weather. Whole-circuit insulation resistance L-E is 0.6 Mohm, so the circuit is split at a midpoint: one leg reads greater than 299 Mohm, the other 0.4 Mohm. Further splitting locates an outdoor socket with water ingress and a degraded membrane. The reasoning: random tripping plus low insulation resistance points to an earth-leakage path, not a faulty device, so the RCBO is not replaced.
Traps
Replacing the protective device without finding the cause; missing an N-E fault because only L-E was tested; leaving a CPC disconnected after testing; failing to retest and update certification after repair; reading a 22 mA ramp result as proof of a healthy 30 mA device.