Fault Diagnosis, Testing and Repair Practice
Fault Diagnosis, Testing and Repair Practice
Fault Diagnosis, Testing and Repair Practice tests a safe, logical method for finding open circuits, short circuits, earth faults and loose connections, and for proving a repair.
Core rules and concepts
- Safe isolation: inform the user, switch off, isolate, lock off, prove the voltage tester on a known source, test the circuit, then work. Treat capacitors as live until discharged.
- Tests: continuity on a dead circuit, insulation resistance (usually at least 1 MΩ at 500 V for a domestic circuit; manufacturers may specify higher), polarity, earth continuity, RCD trip time, and load current with a clamp meter.
- Symptoms: open circuit - no current and full voltage at the break; short circuit - excessive current and a tripped protective device; earth fault - leakage current and a tripped RCCB; loose connection - intermittent operation, heat and arcing.
- Repair cycle: gather the complaint, inspect, test, locate, replace, retest and record.
Worked example
A socket is dead, but the MCB is on. Test for voltage at the MCB output (present), then at the socket (absent). With the circuit isolated, continuity-test the phase and neutral from the board to the socket. If phase is continuous but neutral is open, inspect junction boxes and socket terminals; a loose neutral is the likely fault. Reasoning: the MCB being on proves nothing about the downstream conductor.
Traps that cost marks
- Testing live when a dead test is possible.
- Assuming the MCB is faulty without testing downstream.
- Using a megger on equipment containing electronics.
- Failing to prove the tester before and after use.