melts and breaks the circuit if the current is too high. The thin wire melts before the cable can overheat.

In which wire is a fuse placed?

live. Then the appliance is disconnected from the high-voltage supply when the fuse blows.

What is the earth wire for?

to carry current away safely if the metal case becomes live. It stops the case giving anyone a shock.

An appliance normally takes 4 A. Which fuse should it have?

5 A. The fuse should be the next size above the normal current.

What is one advantage of a circuit breaker over a fuse?

It can be reset instead of replaced. A trip switch just needs switching back on once the fault is fixed.

Why is it dangerous to use mains appliances with wet hands?

Water lowers the resistance of the skin, so a larger current can flow through the body. Tap water conducts, which makes an electric shock much more likely.

Mains electricity and safety · Cambridge IGCSE Physics (9

In plain words

Mains electricity can kill, so appliances have protection built in. A fuse is a thin wire that melts if too much current flows, breaking the circuit. The earth wire gives a stray current an easy, safe path away from a metal case, so the case can't give you a shock.

Three things to know

A fuse or circuit breaker (trip switch) in the live wire cuts off the current if it gets too big, before the cable overheats. Choose the fuse rated just above the normal current.
The earth wire joins a metal case to earth. If the live wire touches the case, a large current flows to earth and blows the fuse. Double-insulated appliances have a plastic case and need no earth wire.
Dangers: damaged insulation, overheated cables, damp conditions, and too many plugs in one socket. Mains is alternating current (a.c.); a battery gives direct current (d.c.).

Worked example

A 920 W toaster runs on 230 V. Which fuse is right: 3 A, 5 A or 13 A?

Normal current = P ÷ V = 920 ÷ 230 = 4 A.
Choose the fuse just above that.
The 5 A fuse.

Tips and tricks

Choosing a fuse: work out the normal current, then pick the next size up.
The fuse and the switch both go in the live wire, so that the appliance is cut off from the high voltage.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Mains electricity and safety: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What does a fuse do?
- increases the current
- stores energy
- melts and breaks the circuit if the current is too high (the answer)
- reduces the voltage
The thin wire melts before the cable can overheat.
In which wire is a fuse placed?
- live (the answer)
- neutral
- earth
- any of them
Then the appliance is disconnected from the high-voltage supply when the fuse blows.
What is the earth wire for?
- to carry the normal current
- to carry current away safely if the metal case becomes live (the answer)
- to increase the power
- to complete the circuit
It stops the case giving anyone a shock.
An appliance normally takes 4 A. Which fuse should it have?
- 1 A
- 3 A
- 5 A (the answer)
- 13 A
The fuse should be the next size above the normal current.
What is one advantage of a circuit breaker over a fuse?
- It is cheaper.
- It lets more current through.
- It can be reset instead of replaced. (the answer)
- It needs no live wire.
A trip switch just needs switching back on once the fault is fixed.
Why is it dangerous to use mains appliances with wet hands?
- Water is a good insulator.
- Water lowers the resistance of the skin, so a larger current can flow through the body. (the answer)
- Water raises the voltage.
- Water makes the fuse stronger.
Tap water conducts, which makes an electric shock much more likely.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

4 questions, 10 marks. Write your answers on paper, then check them.

Mains electricity and safety

Cambridge IGCSE Physics (9–1) 0972 · 10 marks · papermunch.org

Name ______________________________ Date ______________

A kettle is rated 2300 W, 230 V. Fuses of 3 A, 5 A and 13 A are available. Choose the right fuse and explain why.[3]
Show answerHide answer
13 A. The normal current is 2300 ÷ 230 = 10 A, and the fuse must be rated a little above that.
I got this one
Explain how the earth wire and fuse protect someone using a metal-cased toaster if the live wire comes loose and touches the case.[3]
Show answerHide answer
A large current flows from the case through the low-resistance earth wire. This melts the fuse, which disconnects the live supply, so the case is no longer live.
I got this one
Explain why a hairdryer with a plastic case does not need an earth wire.[2]
Show answerHide answer
It is double insulated: the case is an insulator, so it cannot become live.
I got this one
State the difference between alternating current and direct current.[2]
Show answerHide answer
Direct current always flows in the same direction; alternating current keeps reversing direction.
I got this one

Answers: Mains electricity and safety

1. 13 A. The normal current is 2300 ÷ 230 = 10 A, and the fuse must be rated a little above that.
2. A large current flows from the case through the low-resistance earth wire. This melts the fuse, which disconnects the live supply, so the case is no longer live.
3. It is double insulated: the case is an insulator, so it cannot become live.
4. Direct current always flows in the same direction; alternating current keeps reversing direction.