Mains electricity

UK mains electricity is alternating current (ac) at 230 V and 50 Hz. This contrasts with the direct current (dc) from a battery, where current flows in one direction only.

ac vs dc

• Direct current (dc) — flows in one direction. Sources: cells, batteries.
• Alternating current (ac) — direction reverses periodically. Sources: mains, alternators.

On an oscilloscope:

• dc trace: a horizontal line (positive or negative).
• ac trace: a sine wave that oscillates above and below zero.

UK mains has frequency 50 Hz — the current reverses 100 times per second (each cycle has two reversals).

Why ac for transmission

ac can be stepped up to high voltage by a transformer, then stepped down for use. High voltage transmission means lower current → less power lost as heat (P = I²R). dc cannot easily be transformed this way.

The three-core cable

A standard UK mains cable has three insulated copper wires:

• Live (brown) — carries the alternating supply potential. Oscillates between roughly +325 V and −325 V (peak), around 0 V neutral.
• Neutral (blue) — completes the circuit at approximately 0 V, the return path.
• Earth (green/yellow) — only carries current if there is a fault. It safely conducts current to ground, tripping fuses or RCDs.

A live wire is dangerous even when a switch is "off" elsewhere, because it can be at +325 V relative to the body. Touching live to earth (e.g. via a damp body) gives a fatal current.

Why earth wires?

If the live wire shorts to a metal casing, the current rushes through the low-resistance earth wire. This surge blows the fuse, breaking the circuit before someone touches the casing.

Plug wiring summary

• Brown → Live (right of plug, viewed from back).
• Blue → Neutral (left).
• Green/Yellow → Earth (top).
• Plus: a fuse rated to blow before the cable melts (typically 3 A or 13 A in the UK).

RMS values

You'll often see "230 V" quoted — this is the rms (root-mean-square) voltage, the dc-equivalent that delivers the same average power. The peak voltage is about 325 V (= 230 × √2). At GCSE you typically just use 230 V.