Direct and alternating current, the three-pin plug and the live, neutral and earth wires, the roles of fuses, circuit breakers and earthing, and the electrical power equation P = I V.

What this dot point is asking

CCEA Double Award wants you to tell direct current (d.c.) from alternating current (a.c.), describe the three-pin plug and the live, neutral and earth wires, explain the roles of fuses, circuit breakers and earthing, and use the power equation P = I V. The safety explanation and the power calculation are both common.

Direct and alternating current

The three-pin plug

The plug also contains a fuse in the live wire and is made with an insulating case and pins.

Safety features

Choose a fuse rated just above the appliance's normal working current.

Electrical power

Energy transferred by an appliance

The power of an appliance also tells you how much energy it transfers over time, using $E = P \times t$ (energy in joules, power in watts, time in seconds). A $2000\ \text{W}$ heater left on for $60\ \text{s}$ transfers $2000 \times 60 = 120000\ \text{J}$. Energy companies bill in kilowatt-hours rather than joules: one kilowatt-hour is the energy used by a $1\ \text{kW}$ appliance in one hour. A high-power appliance such as an electric shower therefore costs much more to run per minute than a low-power one such as a phone charger, which is why power ratings matter for both safety (fuse choice) and cost.

Examples in context

Example 1. A double-insulated hairdryer

Some appliances with plastic cases are double insulated and have no earth wire, because the user can never touch a live metal part.

Example 2. A trip switch (RCD)

A residual current device cuts the supply within milliseconds if it detects current leaking to earth, for example through a person, giving faster protection than a fuse.

Example 3. Why a frayed cable is dangerous

If the insulation wears away and the live wire is exposed, touching it gives a path for current through the body to earth, which can be fatal. The earth wire and an RCD reduce this risk by cutting the supply quickly if a fault current flows.

Try this

Q1. State the colour of the live wire and its job. [2 marks]

• Cue. Brown; it carries the alternating voltage from the supply.

Q2. An appliance uses $230\ \text{V}$ and draws $2.0\ \text{A}$. Find its power. [2 marks]

• Cue. $P = I V = 2.0 \times 230 = 460\ \text{W}$.

Q3. What is the job of the earth wire? [1 mark]

• Cue. To carry current safely to earth if a fault makes the case live, so the fuse blows.