Mains electricity, plug wiring and the National Grid

UK mains supply is alternating current (ac) at 230 V and 50 Hz. Cells and batteries supply direct current (dc) — the current flows in one direction only. On an oscilloscope, dc is a flat horizontal line; ac is a sine wave that crosses the time axis 100 times per second.

The three-pin plug

Pin	Wire colour	Job
Live	brown	Carries the alternating pd from the mains (≈ 230 V).
Neutral	blue	Completes the circuit; sits near 0 V.
Earth	green/yellow	Safety wire — only carries current if a fault sends current to the casing.

A correctly wired plug also has the cable grip clamping the outer sheath (not the individual wires) and a fuse rated just above the device's working current.

How a fuse + earth wire protect the user

If a live wire touches a metal casing, current flows live → casing → earth wire → ground. The surge melts the fuse, breaking the circuit before the user can be electrocuted. Plastic-cased (double-insulated) appliances do not need an earth wire — they have the symbol of a square inside a square.

The National Grid

Power stations generate at ~25 kV. Step-up transformers raise this to 275 kV or 400 kV for transmission. High voltage means low current (since P = VI), so wires waste less energy as heat (P_loss = I²R). Step-down transformers drop the pd to 230 V for homes.

CCEA tip

When asked "why is the grid run at high voltage?", you must mention BOTH "lower current" AND "less energy lost as heat in the cables (I²R)". One half scores B1 of B2.