Domestic Electricity

This lesson covers domestic (mains) electricity — as required by the Edexcel GCSE Physics specification (1PH0). You need to understand the UK mains supply, the wiring in a three-pin plug, the functions of fuses, circuit breakers and RCDs, and the dangers of electricity.

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The UK Mains Supply

The UK mains electricity supply is an alternating current (AC) supply.

Property	UK Mains Value
Type of current	Alternating current (AC)
Voltage	~230 V
Frequency	50 Hz

AC vs DC

Feature	AC (Alternating Current)	DC (Direct Current)
Direction of current	Constantly reverses direction	Flows in one direction only
Source	Mains supply, generators	Batteries, cells, solar cells
Voltage	Alternates between positive and negative	Constant value
Frequency	50 Hz in the UK (50 complete cycles per second)	0 Hz (no alternation)

Why AC Is Used for Mains Supply

• AC can easily be stepped up (increased) to very high voltages using transformers for efficient long-distance transmission through the National Grid.
• It can then be stepped down (decreased) to safe voltages for use in homes.
• DC cannot be easily transformed to different voltages using simple transformers.

Exam Tip: The mains supply is 230 V AC at 50 Hz. These values come up frequently in exam questions. Remember: 50 Hz means the current direction changes 50 times per second (i.e. 50 complete cycles per second).

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The Three-Pin Plug

A three-pin plug connects an appliance to the mains supply. Inside the plug, there are three wires, each with a specific colour and function.

The Three Wires

Wire	Colour	Function	Voltage
Live	Brown	Carries the current to the appliance; alternates between +230 V and −230 V	~230 V (alternating)
Neutral	Blue	Completes the circuit; carries current away from the appliance	~0 V
Earth	Green and yellow stripes	Safety wire; provides a low-resistance path to earth if there is a fault	0 V (only carries current during a fault)

Important Points

• The live wire is the most dangerous wire because it is always at ~230 V, even when the switch is off (if the switch only disconnects the neutral wire, the live wire remains live).
• The neutral wire is at approximately 0 V relative to earth.
• The earth wire is normally at 0 V and carries no current unless there is a fault.

graph TD
    A["Mains Supply<br/>230 V AC"] --> B["Three-Pin Plug"]
    B --> C["Live Wire<br/>(Brown)<br/>~230 V"]
    B --> D["Neutral Wire<br/>(Blue)<br/>~0 V"]
    B --> E["Earth Wire<br/>(Green/Yellow)<br/>0 V — safety"]
    C --> F["Appliance"]
    D --> F
    E --> G["Connected to<br/>metal casing<br/>of appliance"]

    style A fill:#2c3e50,color:#fff
    style C fill:#8B4513,color:#fff
    style D fill:#2980b9,color:#fff
    style E fill:#27ae60,color:#fff
    style F fill:#7f8c8d,color:#fff
    style G fill:#27ae60,color:#fff

Exam Tip: You must know the colour of each wire and its function. A common exam question shows a diagram of an opened plug and asks you to identify or label the wires. Remember: Brown = Bottom right (live); Blue = Bottom left (neutral); striped = top (earth). This refers to the standard UK plug layout.

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The Earth Wire and Earthing

The earth wire is a safety feature. It provides a low-resistance path for current to flow safely to earth (ground) if a fault occurs.

How Earthing Works

1. If a fault develops (e.g. the live wire comes loose inside an appliance and touches the metal casing), the casing becomes live.
2. Without an earth wire, a person touching the casing would receive an electric shock (current would flow through their body to earth).
3. With an earth wire connected to the metal casing, the current instead flows through the earth wire to the ground — a much easier (low-resistance) path.
4. This large current through the earth wire causes the fuse to blow (or circuit breaker to trip), disconnecting the live wire and making the appliance safe.

Double Insulation

Some appliances have a plastic casing and are said to be double insulated. They do not need an earth wire because:

• Plastic is an electrical insulator — it cannot become live.
• There is no exposed metal that a user could touch.
• These appliances use a two-core cable (live and neutral only) and have the symbol ⬜⬜ (a square inside a square) on them.

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Fuses

A fuse is a thin wire (inside a ceramic or glass tube) that is designed to melt and break the circuit if the current exceeds a certain value.

How a Fuse Works

1. The fuse is connected in series with the live wire.
2. Under normal operation, the current through the fuse is below its rating — it remains intact.
3. If a fault causes the current to become too high, the fuse wire heats up and melts.
4. This breaks the circuit, stopping current flowing and preventing damage or fire.

Common Fuse Ratings

Fuse Rating	Used For
3 A	Low-power devices: lamps, phone chargers, radios, TVs
13 A	High-power devices: kettles, heaters, ovens, washing machines

Choosing the Right Fuse

To choose the correct fuse:

1. Calculate the normal operating current using I = P / V.
2. Choose a fuse with a rating just above the normal current.

Example: A 700 W microwave on a 230 V supply.

I = P / V = 700 / 230 = 3.04 A

Choose a 13 A fuse? No — a 3 A fuse would blow at normal operation. The correct choice is a 5 A fuse (if available) or a 13 A fuse (using standard UK fuse values of 3 A or 13 A).

Actually, since the operating current is 3.04 A, a 3 A fuse is too small (it would blow during normal use). Therefore, choose a 13 A fuse from the standard UK options. However, if 5 A fuses are available, select 5 A as it is the next rating above 3.04 A.

Exam Tip: When choosing a fuse, always calculate the current first using I = P/V. The fuse rating must be above the normal operating current but as close as possible — a fuse that is too high will not blow quickly enough during a fault.

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Circuit Breakers

A circuit breaker is an automatic switch that trips (opens) when the current exceeds a set value.

Advantages Over Fuses

Feature	Fuse	Circuit Breaker
Speed	Relatively slow (wire must melt)	Very fast (electromagnetic trip)
Reset	Must be replaced after blowing	Can be reset (switched back on)
Cost	Cheap to buy	More expensive initially
Convenience	Less convenient (need spare fuses)	More convenient (just flip the switch)

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RCDs (Residual Current Devices)

An RCD is a safety device that detects a difference between the current in the live wire and the neutral wire.

How an RCD Works