Mains Electricity and Safety

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, the fuse and the circuit breaker — is essential knowledge, both for the exam and for staying safe. This lesson, part of Topic P3 (Electricity) of OCR Gateway 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, circuit breaker 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, explain how the earth wire, fuse, circuit breaker and double insulation keep users safe, and identify common electrical dangers.

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 it is easy to change its voltage 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 calculation and recall questions.

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. The earth pin is the longest so that it connects first.

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 memory hook is that the colours are listed L–N–E (live, neutral, earth) and the earth wire is the safety wire connected to the case.

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:

The earth wire connects the metal case to earth (the ground), providing a path of very low resistance.
If the live wire touches the case, a very large current flows from the live wire, through the case, down the earth wire to the ground — because the earth path has such low resistance.
This large current (a "surge") melts the fuse (or trips the circuit breaker), which breaks the circuit and cuts off the supply to the appliance.
The case is now safe — it is no longer connected to the live supply — and the appliance will not work until the fault is fixed and the fuse replaced.

A fuse is simply a thin piece of wire that melts and breaks if the current through it exceeds its rated value, isolating the appliance. Fuses come in standard ratings (commonly $3 \text{ A}$ , $5 \text{ A}$ and $13 \text{ A}$ ), and the correct fuse is one rated just above the normal operating current of the appliance, so it carries the normal current safely but melts quickly if a fault causes a surge.

Worked Example — Choosing the right fuse

A $920 \text{ W}$ appliance runs on the $230 \text{ V}$ mains. Fuses of $3 \text{ A}$ , $5 \text{ A}$ and $13 \text{ A}$ are available. Which should be fitted?

Step 1 — find the normal operating current from $I = \dfrac{P}{V}$ : $I = \dfrac{920}{230} = 4 \text{ A}$ .

Step 2 — choose the fuse rated just above this current: the $3 \text{ A}$ fuse would blow in normal use, so the correct choice is the $5 \text{ A}$ fuse.

Answer: fit the $5 \text{ A}$ fuse — the smallest standard fuse rated above the normal $4 \text{ A}$ current.

Exam Tip: The earth wire and fuse work together: a fault sends a large current to earth, which melts the fuse and breaks the circuit, making the case safe. To choose a fuse, calculate the normal current with $I = P/V$ and pick the standard fuse just above it.

Circuit Breakers and Double Insulation

There are two other important safety features:

Mains Electricity and Safety

Mains Electricity and Safety

Alternating Current and Direct Current

The UK Mains Supply

The Three-Pin Plug

The Earth Wire and the Fuse: How They Protect You

Worked Example — Choosing the right fuse

Circuit Breakers and Double Insulation

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