Testing, Fault Finding and Maintenance
Core Concepts
Understand testing instruments: multimeter, insulation resistance tester (megger), earth loop impedance tester, RCD tester, and clamp meter. Know tests: continuity, insulation resistance (≥1 MΩ for LV), polarity, earth electrode resistance, and RCD trip time. Understand fault finding techniques: half-split, substitution, voltage/current measurement, and visual inspection. Know maintenance types: preventive, predictive, and corrective. Understand the importance of documentation and safe isolation before testing.
Traps in Questions
- Insulation resistance test must be done with power off and equipment disconnected.
- A megger applies high voltage (500 V or 1000 V); never use on sensitive electronics.
- Continuity test should be done with a low-reading ohmmeter; ensure leads are zeroed.
- RCD test: 30 mA RCD should trip within 300 ms at rated current.
- Earth loop impedance must be low enough to ensure disconnection within required time.
Worked Example / Rule of Thumb
Example: To test insulation resistance of a circuit, isolate, disconnect sensitive equipment, set megger to 500 V, connect between live and earth, and measure. Reading should be ≥1 MΩ. If reading is low, divide circuit to locate faulty section. Rule of thumb: For a 30 mA RCD, test at ½× (15 mA) should not trip, at 1× (30 mA) should trip within 300 ms, and at 5× (150 mA) should trip within 40 ms.