Faults, fuses, earthing and circuit breakers sim
The student works out each appliance's current and fuse, then makes earth faults, short circuits and shocks happen in a metal kettle and sees how the earth wire, fuse and circuit breaker keep the user safe.
Practical electricity · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Mains is 230 V a.c. Choose a fuse from the normal current, I = P ÷ V: the smallest standard fuse (3, 5 or 13 A) above it. For example, a 2300 W kettle takes 2300 ÷ 230 = 10 A, so it needs a 13 A fuse. If the live wire touches an earthed metal case, the earth wire gives a path of about 0.5 Ω (0.25 Ω live + 0.25 Ω earth). About 460 A then flows, far above 5 × 13 A, so the fuse melts at once (or the 16 A breaker trips) and the case is safe. For those few milliseconds the case sits at about 115 V. With no earth, the case sits at 230 V and nothing blows. A person (about 2 kΩ, dry, hand to feet) then carries I = 230 ÷ 2000 ≈ 115 mA, and above about 30 mA can kill. The fuse and switch go in the live wire. In the neutral, the appliance stays joined to 230 V after they open: once a neutral fuse has blown no current flows, but the case can still be live at 230 V. A real UK plug (BS 1363) has its fuse fixed to the live pin, so a fuse or switch in the neutral here stands for a plug, socket or appliance wired wrongly, with live and neutral swapped. With the fuse in the neutral and the earth connected, the 460 A earth-fault current goes from the case down the earth wire and never passes the fuse, so the fuse never blows and the current keeps flowing; pushed through the earth wire's 0.25 Ω, it holds the case at 460 × 0.25 = 115 V, so a person touching it gets about 58 mA. A double-insulated appliance (choose Hair dryer or TV) has no metal part that can become live, so it needs no earth. On the bench lie the plug's back cover and the screwdriver for its terminal screws. Model limits: the supply is treated as stiff (always 230 V at the appliance), with the wiring's resistance lumped into the fault path. Real fault-loop and body resistances vary widely, and wet skin conducts far more. The fuse holds at its rating and melts in t = 2.4 s ÷ ((I/rating)² − 1) above it. The fuse and the breaker are shown one at a time, though homes have both. RCDs, which would cut a 115 mA leak, are not modelled. The clamp-meter reading scatters by ±1% as the mains drifts. Not to scale. Time never runs on its own: each drag or button press is one step, and the fuse or breaker reacts within that step.
- 16 A circuit breaker (MCB) at the consumer unit
- Glow: fault current or current through a person
- Amber glow: normal current, live → element → neutral
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