2 · IGCSE Physics
Mains electricity and fuses
Pick an appliance power and supply voltage to find the current and the right plug fuse, then create a fault and see how the earth wire and fuse protect you.
I = P ÷ V
Fuse: next rating above I
Fault: I = V ÷ R(fault path)
R(earth path) ≈ 0.6 Ω, R(body) ≈ 2000 Ω
Wire colours: live brown, neutral blue, earth green/yellow
Fuse: next rating above I
Fault: I = V ÷ R(fault path)
R(earth path) ≈ 0.6 Ω, R(body) ≈ 2000 Ω
Wire colours: live brown, neutral blue, earth green/yellow
Try this
- At 230 V set P = 2000 W. Work out I, then fit a 3 A fuse and watch what happens; which fuse is correct?
- Choose “Live wire touches metal casing” with the earth connected and watch the fuse. Press Reset, untick the earth wire, tick “Person touches casing” and compare the current through the person with the fuse rating.
- Switch to 120 V and find the largest power a 13 A fuse can supply. Why do lower-voltage countries need thicker cables for the same appliance?
Exam tip: when explaining earthing, give the full chain — live wire touches casing, a large current flows through the low-resistance earth wire, the fuse melts and breaks the live connection, so the casing is no longer live.