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).

Electrical power is the rate of energy transfer. P = IV and E = Pt. Household electrical energy is often billed in kilowatt-hours: 1 kWh is the energy transferred by a 1 kW device operating for 1 hour, equal to 3.6 MJ.

A fuse contains a wire designed to melt when current becomes too large. Circuit breakers disconnect a faulty circuit automatically and can be reset. Earthing a metal case provides a low-resistance path so a fault current is large enough to operate the protective device, helping prevent the case remaining live. Electrical work on mains circuits should be carried out only with appropriate training and isolation procedures.

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

The live conductor carries an alternating potential relative to earth and is dangerous to touch. The neutral conductor completes the circuit and is maintained close to earth potential under normal operation. The earth conductor is a safety connection to exposed metal cases; it normally carries no current. If a fault connects live to the case, the earth path allows a large fault current so a fuse or circuit breaker can disconnect the supply.

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

Fuses, breakers and electrical safety

A fuse contains a thin wire designed to melt if current exceeds a safe value for long enough. Circuit breakers disconnect the circuit when a fault is detected and can usually be reset. Protective devices must be chosen so they allow normal operation but disconnect dangerous fault currents.

Do not say a fuse 'reduces the current'. In normal operation it has little effect; under excessive current it breaks the circuit.

Power, energy and cost

Electrical power is P = IV. Energy transferred is E = Pt. A kilowatt-hour is the energy transferred by a 1 kW device operating for 1 hour. Electricity cost is calculated from energy used in kWh multiplied by the tariff per kWh.

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.

Exam technique

In safety questions, identify the fault, the protective path and the device that disconnects the supply. In energy-cost questions, convert watts to kilowatts and time to hours before calculating kWh.

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.