Domestic electricity

Revision notes

Domestic mains supplies use alternating potential difference. In a typical three-core cable the live conductor alternates at a high potential relative to earth, the neutral conductor completes the circuit and is close to earth potential, and the earth conductor is a safety connection to exposed metal cases. Wire colours depend on the current wiring standard; in modern UK wiring they are brown (live), blue (neutral) and green/yellow (earth).

Mains supply and alternating current

The UK mains supply is alternating current. The potential difference repeatedly changes direction, with a nominal frequency of 50 Hz and a nominal voltage around 230 V. Direct current flows in one direction, as supplied by cells and batteries. When comparing a.c. and d.c., describe current direction rather than saying one is 'stronger'.

Live, neutral and earth

Double-insulated appliances use insulating cases or construction that prevents accessible metal becoming live, so they do not require an earth conductor.

Power, energy and cost

A high-power appliance can still use less total energy than a lower-power appliance if it operates for much less time. Always separate power from energy.

The National Grid

Electricity is transmitted at high potential difference so the same power can be carried with a smaller current. Since cable heating depends strongly on current, lower current reduces energy wasted as thermal energy in transmission lines. Step-up transformers increase the p.d. for transmission and step-down transformers reduce it to about 230 V for homes; they need a.c.

Three-core cable

Most appliances connect to the mains with three-core cable. The live wire is brown and carries the alternating potential difference of about 230 V. The neutral wire is blue and completes the circuit; it is at or close to 0 V. The earth wire has green and yellow stripes and is a safety wire that only carries current if there is a fault. UK mains electricity is ac at 50 Hz.

The National Grid connects power stations to consumers. Step-up transformers increase the potential difference to very high values for transmission. For the same power, a higher potential difference means a lower current, so less energy is wasted heating the cables. Step-down transformers reduce the potential difference to a safe level for homes.