Mains electricity and the National Grid

ac vs dc

• Direct current (dc) — current flows in one direction (cells, batteries).
• Alternating current (ac) — current changes direction many times per second. UK mains is 230 V at 50 Hz (50 cycles per second).

ac is used for mains because it can be transformed up or down, dc cannot.

The three-pin plug

Other features:

• The fuse is in the live wire — melts if too much current flows.
• The cable grip clamps the cable so wires can't be pulled out.

Earthing and fuses

If a fault makes the metal casing live:

1. Earth wire provides a low-resistance path to ground.
2. Large current flows.
3. Fuse melts (or RCD trips), disconnecting the appliance.

This protects the user from electrocution.

Power equation

P = V x I

For a 3 kW kettle on UK mains: I = P/V = 3000/230 = ~13 A.

So a 13 A fuse is appropriate. The fuse rating must be just above the normal operating current.

The National Grid

Power stations -> step-up transformer -> high-voltage transmission lines (up to 400 kV) -> step-down transformers near towns -> 230 V to consumers.

Why high voltage for transmission?

• Power loss in cables = I^2 R.
• For a fixed power, higher V means lower I (because P = VI).
• Lower I means much less heat loss in the resistance of the cables.

Transformers

A transformer changes the voltage of an ac supply.

• Step-up transformer increases V (and decreases I) — used at the start of transmission.
• Step-down transformer decreases V — used near homes.

V_p / V_s = N_p / N_s, where N is the number of turns.

WJEC exam tip

For "why is mains transmitted at high voltage?" you must include the formula reasoning: P = VI -> for fixed P, higher V gives lower I, and power loss in cables is I^2 R. Just saying "less energy lost" without the I^2 R link usually scores partial marks only.