Any tips? Any good online resources for fault finding in particular? I’ve really struggled to find anything.
Regarding fault-finding:
In real life, key to this is a logical approach to fault finding – collecting as much information as possible (also known as “talking to people,” but including as well any paper-based documentation available like instruction manuals, schedules, plans etc), thinking about what the cause of the fault might be, then deciding on what appropriate tests would prove or disprove the theory. Break the problem down into smaller chunks – literally, in the case of circuits (divide the circuit into two, find out which half still has the fault), and eliminating certain areas. Although it is very unlikely for two different faults to simultaneously occur, it is quite feasible for one fault to go unnoticed for a long time (e.g. lack of main equipotential bonding) and then the symptoms only becoming apparent when another fault occurs (e.g. breakdown in insulation, causing a shock off pipework). The majority of fault diagnosis involves the use of appropriate dead tests – either continuity (to find open circuits) or insulation resistance (to find short circuits, or degraded insulation).
Regarding the 2365-03 practical fault-finding assessment:
It’s basically you assessing whether it’s a short (in which case, do an IR test) or an open circuit (in which case do a continuity test). There might be incorrect polarity, also (in other words, L-N swapped around) - you can determine this with continuity tests, i.e. test L at the board to L at the point of inspection - if open circuit, then try L at the board to N (or E) at the point of inspection - if low resistance, then it's probably polarity. Confirm by testing continuity of N (or E) at the board to L at the point of inspection.
If it’s an RCD that’s affected, then it’s an earth fault (either L-E or N-E). If an RCD-protected breaker trips, then in must be a L-N fault (as RCDs will not trip on these). If the scenario mentions “when it’s switched on” then it must be something to do with the switched L. So think things through, and make sure you have a logical approach to testing. Write down all your tests - whether it's continuity or insulation resistance, and what the results are.
For example:
"Symptoms of fault: RCD trips when light switch is turned on, on downstairs lighting circuit."
Logical thought:
(1) RCD trips, so must be L-E fault or N-E fault (would not trip on L-N fault)
(2) it only trips when light switch is turned on. If wired correctly, this will switch the L conductor only. So likely L-E fault (short).
Testing:
Insulation resistance test
L-E 0.00MΩ
N-E >500MΩ
L-N >500MΩ
Diagnosis: L-E short circuit
How would you rectify? Check connections from switch onwards, replace cable/accessory as appropriate.
Hope that helps, and good luck
