Mains Electricity and the Domestic Supply

The electricity that comes out of the sockets in your home is very different from the steady supply of a battery: it is alternating, much higher in voltage, and potentially lethal. Understanding how the mains supply is delivered safely — through the three wires of a plug, the earth wire and the fuse — is essential, both for the exam and for staying safe. This lesson, part of the "Powering Earth" section of Topic P6 (Global challenges) of OCR Gateway Combined Science A, contrasts alternating and direct current, gives the UK mains values, explains the wiring of a three-pin plug, and sets out the safety roles of the earth wire, fuse and double insulation.

By the end of this lesson you should be able to distinguish alternating current (a.c.) from direct current (d.c.), state the UK mains voltage and frequency, describe the three-pin plug and the colour and role of each wire, and explain how the earth wire, fuse and double insulation keep users safe.

This lesson is mainly AO1 (recalling the UK mains values, the wiring colours and each wire's job), with AO2 when you explain how the earth wire, fuse and double insulation work together to protect the user.

Alternating Current and Direct Current

There are two kinds of electric current, distinguished by the way the current flows:

Direct current (d.c.) flows one way only, in a single, constant direction. It is what you get from a cell or battery: the current always flows from the + terminal round to the − terminal.
Alternating current (a.c.) constantly changes direction, flowing first one way and then the other, back and forth, many times each second. The mains supply is a.c.

The number of complete back-and-forth cycles per second is the frequency, measured in hertz ( $\text{Hz}$ ). An a.c. supply is produced by a generator and is used for the mains because its voltage is easy to change using transformers (which only work with a.c.), making it efficient to transmit over long distances.

graph TB
  Source["Electricity supply"] --> DC["Direct current d.c.<br/>one direction only<br/>from cells and batteries"]
  Source --> AC["Alternating current a.c.<br/>changes direction repeatedly<br/>the mains supply"]

Exam Tip: d.c. flows in one direction (batteries/cells); a.c. repeatedly changes direction (the mains). Frequency, in hertz, is the number of cycles per second. The mains is a.c. because its voltage can be changed by transformers for efficient transmission.

The UK Mains Supply

In the UK, the mains electricity supply has:

a potential difference of about $230 \text{ V}$ (sometimes quoted as 230 volts), and
a frequency of $50 \text{ Hz}$ — meaning the current changes direction and back $50$ times every second.

This $230 \text{ V}$ is far higher than the few volts of a battery, which is why the mains can deliver large powers to appliances such as kettles and heaters — but it is also high enough to be dangerous, capable of giving a fatal electric shock. The rest of this lesson is about how the supply is wired and protected so it can be used safely.

Exam Tip: Learn the two mains values exactly: UK mains is about $230 \text{ V}$ and $50 \text{ Hz}$ . These figures appear regularly in recall and calculation questions.

Power and Energy of Mains Appliances

The reason the mains supply is delivered at a fairly high voltage is that it lets appliances transfer energy quickly — that is, at a high power. The power of an appliance connected to the mains is given by:

$P = VI$

where $P$ is the power in watts ( $\text{W}$ ), $V$ is the mains potential difference (about $230 \text{ V}$ ) and $I$ is the current the appliance draws in amperes ( $\text{A}$ ). A kettle rated at over two kilowatts draws a current of around ten amperes, whereas a phone charger rated at only a few watts draws a tiny fraction of an ampere. The power rating printed on an appliance therefore tells you both how fast it transfers energy and, using the equation above, how large a current it draws — which is exactly what decides the fuse it needs.

The total energy an appliance transfers depends on both its power and how long it is switched on:

$E = Pt$

where $E$ is the energy transferred in joules ( $\text{J}$ ), $P$ is the power in watts and $t$ is the time in seconds. This is why leaving a high-power appliance such as an electric heater running for a long time uses a great deal of energy — and costs a lot of money — while a low-power device left on for the same time uses far less.

Worked Example — Current, power and energy

A microwave oven is rated at $1150 \text{ W}$ and runs from the $230 \text{ V}$ mains. Calculate the current it draws, and the energy it transfers if it is used for $300 \text{ s}$ (five minutes).

Step 1 — rearrange $P = VI$ to find the current: $I = \dfrac{P}{V} = \dfrac{1150}{230} = 5 \text{ A}$ .

Step 2 — use $E = Pt$ to find the energy: $E = 1150 \times 300$ .

Step 3 — calculate: $E = 345\,000 \text{ J}$ (that is $345 \text{ kJ}$ ).

Answer: the microwave draws a current of $5 \text{ A}$ and transfers $345\,000 \text{ J}$ of energy in five minutes. Because its normal current is $5 \text{ A}$ , it would be fitted with a fuse rated just above this — a $13 \text{ A}$ fuse, since the next standard size, $5 \text{ A}$ , would sit right on the operating current.

Exam Tip: Two equations tie this section together: $P = VI$ (power from voltage and current) and $E = Pt$ (energy from power and time). A higher-power appliance draws a larger current, which is why it needs a higher-rated fuse and a thicker cable that will not overheat.

The Three-Pin Plug

A UK mains appliance connects to the supply through a three-pin plug, containing three wires — the live, the neutral and the earth — each with a fixed colour:

the live wire is brown, and carries the alternating potential difference (about $230 \text{ V}$ ) from the supply;
the neutral wire is blue, and completes the circuit, staying at about $0 \text{ V}$ ;
the earth wire is green-and-yellow striped, and is a safety wire connected to the metal case of the appliance.

Other features of the plug aid safety too: the pins are made of brass (a good conductor that does not rust or corrode), the case and the insulation around each wire are made of tough plastic (a good insulator), and a cable grip clamps the outer cable so the wires cannot be pulled loose from the pins.

Wire	Colour	Role
Live	Brown	Carries the $\approx 230 \text{ V}$ alternating p.d. from the supply
Neutral	Blue	Completes the circuit; stays near $0 \text{ V}$
Earth	Green-and-yellow	Safety wire to the metal case; carries current to earth only if a fault occurs

Exam Tip: Memorise the three wire colours and roles: live = brown, neutral = blue, earth = green-and-yellow. A reliable hook is that the earth wire is the safety wire connected to the case — a common misconception is that it carries current all the time, when in fact it only does so if a fault occurs.

The Earth Wire and the Fuse: How They Protect You

The earth wire and the fuse work together to make an appliance with a metal case safe. Here is the danger they guard against, and how they remove it.

Suppose a fault develops inside an appliance — say the live wire works loose and touches the metal case. Without protection, the case would become live at $230 \text{ V}$ , and anyone touching it would receive a fatal shock as current flowed through their body to earth.

With the earth wire and fuse fitted, this is what happens instead:

Mains Electricity and the Domestic Supply

Mains Electricity and the Domestic Supply

Alternating Current and Direct Current

The UK Mains Supply

Power and Energy of Mains Appliances

Worked Example — Current, power and energy

The Three-Pin Plug

The Earth Wire and the Fuse: How They Protect You

More in Combined Science