Mains Electricity & the National Grid

A.C. vs D.C.

A.C. (mains, 230 V, 50 Hz)

Alternating current constantly changes direction. The p.d. alternates between positive and negative values.

D.C. (cell or battery)

Direct current flows in one direction only. The p.d. stays constant.

• A.C. is produced by an alternating p.d.
• D.C. is produced by a direct p.d.
• UK mains is an A.C. supply at about 230 V and a frequency of 50 Hz.

Three-Core Cable

Earth (green and yellow)

• At 0 V under normal conditions.
• Only carries current if there is a fault, to stop the casing becoming live.

Live (brown)

• Carries the alternating p.d. from the supply (≈ 230 V).

Neutral (blue)

• Completes the circuit and is at about 0 V.

Mains Safety

• Your body is at 0 V. Touching the live wire creates a large p.d. across you, driving a dangerous current.
• A live wire is dangerous even when the switch is open, because there is still a p.d.
• Fuses melt and break the circuit if too much current flows. Choose a fuse rating just above the normal operating current.

Example: An electric kettle is rated at 1150 W on a 230 V supply.

$I=\frac{P}{V}=\frac{1150}{230}=5\text{ A}$

Choose a 7 A fuse (must be higher than 5 A).

The National Grid

Power station → step-up transformer → high-voltage transmission cables → step-down transformer → homes and industry

• Electricity is transmitted at low current to reduce energy losses in the cables.
• To transfer the same power at lower current, the voltage must be increased.
• Step-up transformers increase voltage for transmission.
• Step-down transformers reduce it again before it reaches homes and businesses.

Transformer equation

$\frac{V_p}{V_s}=\frac{N_p}{N_s}$

• $V_p$ = primary p.d., $V_s$ = secondary p.d.
• $N_p$ = primary turns, $N_s$ = secondary turns