Mains alternating current, the live, neutral and earth wires in a plug, the dangers of electricity, and how fuses, circuit breakers and earthing provide safety, with the power equation.

What this dot point is asking

WJEC Double Award Unit 3 wants you to describe mains alternating current, the three wires in a plug, the dangers of electricity, and how fuses, circuit breakers and earthing keep us safe, and to use the power equation.

Mains alternating current

The mains supply is a.c. because it is easy to generate and to change its voltage with transformers for transmission.

The three wires in a plug

Appliances with a metal case must be earthed; many with plastic cases are double insulated and do not need an earth wire.

The dangers of electricity

Electricity can cause electric shock (current passing through the body) and fires (from overheating cables or overloaded sockets). Damaged insulation, frayed cables and overloaded sockets are all hazards. Safety devices reduce these dangers by cutting off the supply when something goes wrong.

Fuses, circuit breakers and earthing

• Fuse: a thin wire that melts and breaks the circuit if the current gets too high, stopping the flow. A fuse is chosen just above the appliance's normal current.
• Circuit breaker: a switch that trips (turns off) automatically if the current is too high; it can be reset, unlike a fuse.
• Earthing: if a fault makes the metal case live, the earth wire gives a low-resistance path, so a large current flows, melting the fuse and cutting off the supply, making the case safe.

The power equation

Energy and power at home

Appliances are labelled with their power in watts (W) or kilowatts (kW), which tells you how fast they transfer energy. A higher-power appliance uses more energy each second, so it costs more to run for the same time. The energy transferred can be worked out from $E = Pt$ (energy = power x time), and electricity meters measure energy in kilowatt-hours. Knowing the power equation $P = IV$ and that energy = power x time lets you compare the running costs of different appliances and explain why high-power devices need a thicker cable and a larger fuse.

Double insulation

Some appliances have a plastic case and are double insulated, meaning there is no way for the user to touch a live metal part even if a wire comes loose. These appliances do not need an earth wire, because the plastic case cannot become live. They carry a special double-insulation symbol (a square inside a square). This contrasts with metal-cased appliances, which must be earthed, and is a common point in safety questions: the type of casing decides whether an earth wire is needed.

Try this

Q1. State the colour of the live wire. [1 mark]

• Cue. Brown.

Q2. An appliance uses 230 V and 2 A. Calculate its power. [2 marks]

• Cue. $P = IV = 2 \times 230 = 460\,\text{W}$.