Domestic Uses and Safety (P2.3)
AC vs DC
- Direct current (dc): current flows in one direction only. Source: batteries, fuel cells.
- Alternating current (ac): current reverses direction periodically (sinusoidally). Source: mains supply, generators.
UK mains supply: 230 V, 50 Hz (50 cycles per second — current reverses 100 times/second).
Advantage of ac for transmission: easily stepped up/down with transformers → efficient long-distance transmission.
The three-core cable
UK mains appliances use a three-core cable:
| Wire | Colour | Function |
|---|---|---|
| Live | Brown | Carries the alternating current |
| Neutral | Blue | Completes the circuit; ~0 V |
| Earth | Green/yellow | Safety — connected to the metal casing |
Live wire is dangerous — 230 V relative to ground.
Neutral wire is at ~0 V (close to Earth potential).
Earth wire prevents electric shock: if the live wire touches the metal casing → current flows through earth wire → triggers fuse/circuit breaker.
Fuses and circuit breakers
Fuse: thin wire that melts if current exceeds the rated value → breaks the circuit. Placed in the live wire. Must be replaced after use.
Circuit breaker: electromagnetic switch that trips when current is too high; can be reset.
RCD (Residual Current Device): detects any difference in current between live and neutral → if there is a leakage (e.g. person touched live wire) → trips very quickly → much better protection against electrocution than a fuse.
Choosing the correct fuse
Rule: always choose the fuse rating just above the normal operating current.
Example: A 1,000 W kettle on 230 V: I = P/V = 1000/230 = 4.35 A → use a 5 A fuse (the next standard rating above 4.35 A). A 3 A fuse would blow immediately; a 13 A fuse would not blow before serious damage.
Earth wire and double insulation
Earth wire: required in appliances with metal casings (e.g. toasters, washing machines). If live wire contacts casing → current flows through earth → fuse blows → no electrocution.
Double insulation: some appliances have no metal parts that can become live — no earth wire needed (marked with □□ symbol, e.g. many power tools and hairdryers).
Common exam errors
- Connecting fuse to neutral wire — fuse must be in the LIVE wire.
- Confusing ac and dc on an oscilloscope — ac shows a sinusoidal wave; dc shows a flat line.
- Saying earth wire carries current normally — it only carries current in a fault condition.
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