On second-opinion site visits — where the owner already had electrical work done and something is now wrong — the same handful of mistakes show up.
Mistake 1 — cable size below the protective device rating
A 2.5 mm² cable protected by a 32 A MCB is a fire pathway. Under fault, the cable will fail before the MCB trips. Cable size has to be matched to the protective device rating per the cable selection tables in SS 638. This is one of the first checks on an insurance audit and one of the first failures on amateur work.
Mistake 2 — daisy-chained sockets without spur control
Extending a ring main by chaining one socket off another, off another, off another, with no proper spur, breaks the ring and creates a long radial that is no longer protected the way it was designed to be. The DB still energises the circuit; it is just no longer the same circuit.
Mistake 3 — earth conductor missed or downsized
The CPC (circuit protective conductor / earth) has its own size requirement — typically the same size as the live conductor or larger. A green-yellow conductor that is too thin, or simply absent at a metallic accessory, defeats the RCD. Test before assuming.
Mistake 4 — mixing TT and TN-S earthing without engineering
In landed houses with separate outbuildings, the earthing arrangement sometimes drifts — TN-S in the main house, TT in the workshop, with a foreign earth path between them. This is engineering scope, not handyman scope.
Mistake 5 — loose terminations
- Under-torqued terminal screws on power circuits — heat up under load, then arc
- Stripped conductors that are too short for full terminal grip
- Strands cut off to fit a smaller terminal instead of using the right device
- Aluminium-to-copper terminations without the right bi-metallic terminal
Mistake 6 — no labelling
An unlabelled DB is the most predictable problem to fix and the most common one to find. Every isolation in the future — every appliance fault, every renovation, every emergency — costs ten times longer when no one knows which MCB feeds what.
The test reading is the proof
Insulation resistance, earth continuity and polarity readings recorded against each circuit are the receipt that the installation was done right. Without them, every claim about "the wiring is fine" is unverified.
