Wiring, Cables, Connectors, Fuses and Circuit Protection
What the exam tests
Cable sizing, terminals, fuses, relays, grounds and circuit protection.
Core
- Cable must carry the load current with acceptable voltage drop. Long runs need larger cross-section; 1.0 mm² is not automatically suitable for a 30 A circuit.
- Crimp terminals with the correct tool, then pull-test. Solder joints crack under vibration unless strain-relieved.
- Fuses protect the wiring, not the device. Select the fuse rating to protect the smallest cable in the circuit, and never fit a larger fuse to stop it blowing.
- Blade fuses have time-current curves; a 10 A fuse carries 10 A indefinitely but opens quickly at 20 A. Fusible links protect main feeds. Circuit breakers may be cycling or manual reset.
- Relay ISO terminals: 30 = battery feed, 87 = output to load, 85/86 = coil, 87a = normally closed output. A suppression diode across the coil prevents back-EMF spikes.
- Grounds must be clean bare metal, tight, and protected from corrosion. A poor ground can make several unrelated circuits misbehave.
Worked example
A 55 W lamp at 13.8 V draws 4 A. If the circuit uses 1.0 mm² cable rated at 10 A, a 10 A fuse protects the cable and tolerates lamp inrush. If someone fits a 20 A fuse, the cable can overheat before the fuse opens. Calculate current first, then check both the fuse and the cable ratings. A fuse that keeps blowing is a symptom to diagnose, not a rating to increase.
Traps
Fitting a bigger fuse; using the body as a random return; confusing relay terminals 87 and 87a; overlooking a shared ground; replacing a fusible link with ordinary wire.