Practise Mains electricity. 13 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How appliances are connected to the mains with three-core cable: the colour, job and potential of the live, neutral and earth wires, and why the live wire is dangerous. Expect 1-mark recall of colours and 2 to 3 mark 'explain' questions about safety.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Recall the colours of the three wiresLive is brown, neutral is blue, earth is green and yellow stripes.
4
State the job of each wireLive carries the alternating pd from the supply, neutral completes the circuit, earth is a safety wire.
5
State the potential of each wireLive is about 230 V (compared with earth); neutral is at or close to 0 V; earth is at 0 V.
6
Explain how the earth wire keeps you safeIt stops the appliance becoming live and only carries a current if there is a fault.
7
Explain why touching the live wire is dangerousYour body is at 0 V, so there is a large pd across you and a current flows through you.
7
Explain dangers even when a switch is openThe live wire is still connected to the supply, so it is still at about 230 V.
Notes
Three-core cable
Most electrical appliances are connected to the mains using three-core cable: three wires, each covered in colour-coded insulation.
Live wire: brown. Neutral wire: blue. Earth wire: green and yellow stripes.
The colours make each wire easy to identify.
What each wire does
The live wire carries the alternating potential difference from the supply. The pd between the live wire and earth is about 230 V.
The neutral wire completes the circuit. It is at, or close to, earth potential (0 V).
The earth wire is a safety wire that stops the appliance becoming live. It is at 0 V and only carries a current if there is a fault.
Example: if a loose live wire touches the metal case of an earthed appliance, the current flows through the earth wire instead of through a person who touches the case.
Why the live wire is dangerous
Your body is at earth potential (0 V). If you touch the live wire, there is a pd of about 230 V across your body, so a current flows through you to earth. This is an electric shock, which can kill.
A live wire can be dangerous even when a switch is open. The switch breaks the circuit through the appliance, but the live wire is still connected to the supply and is still at about 230 V.
Any connection between the live wire and earth is dangerous. There is a pd of about 230 V across the connection, so a current flows through it. If the connection is a person, the current flows through their body. If the connection has a low resistance, the current is very large and can heat the wires enough to start a fire.
Cheatsheet
Live: brown, about 230 V, carries the alternating pd from the supply
Neutral: blue, at or close to 0 V, completes the circuit
Earth: green and yellow stripes, 0 V, safety wire
The earth wire only carries a current if there is a fault
Touching the live wire: 230 V across your body → current through you → shock
The live wire can be dangerous even when a switch is open
How to answer each type of question
Recall the colours and jobs of the wires
1 mark each3
Learn the three colours: brown, blue, green and yellow stripes.
Match each wire to its job: carries the pd (live), completes the circuit (neutral), safety (earth).
Example. (a) Give the colour of the insulation on the neutral wire. (b) Give the colour of the insulation on the earth wire. (c) Name the wire that carries the alternating potential difference from the supply.
Show the model answer
(a) Blue (1) (b) Green and yellow stripes (1) (c) The live wire (1)
Explain how the earth wire makes an appliance safer
2 marks6
Describe the fault: the live wire touches the metal case.
Say that the earth wire stops the case becoming live (keeps it at 0 V).
Say that the current flows through the earth wire, not through the person.
Example. A toaster has a metal case. The case is connected to the earth wire. Explain how the earth wire makes the toaster safer to use.
Show the model answer
If there is a fault and the live wire touches the metal case, the earth wire stops the case becoming live / keeps the case at 0 V (1). The current flows through the earth wire instead of through a person who touches the case (1).
Explain why a person could get an electric shock
3 marks7
Say that the live wire is at about 230 V (even with the switch open, it is still connected to the supply).
Say that the person is at earth potential (0 V), so there is a large pd across their body.
Say that a current flows through the person to earth.
Example. A light switch is off (open). A person touches a damaged part of the live wire that leads to the switch, while standing on the ground. Explain why the person could get an electric shock.
Show the model answer
The live wire is still connected to the supply, so it is still at about 230 V even though the switch is open (1). The person is at earth potential / 0 V, so there is a large potential difference across their body (1). A current flows through the person to earth, which causes the shock (1).
Shortcuts and memory tricks
Colours: brown = live (brown 'burns', it is the dangerous one); blue = neutral (cool and calm, near 0 V); green and yellow = earth (think of green grass on the earth).
Potentials: live about 230 V, neutral about 0 V, earth 0 V. Only the live wire is at a high potential.
A shock needs a pd across you. You are at 0 V, so the danger is anything at a high potential: the live wire.
Where marks are lost
Saying the neutral wire carries no current. It carries current in normal use; it is the earth wire that only carries a current when there is a fault.
Saying the neutral or the earth wire is at 230 V.
Saying the earth wire 'protects the appliance'. It protects the user by stopping the case becoming live.
Thinking that a wire is safe because the switch is off: the live wire can still be at 230 V.
Writing just 'green' for the earth wire instead of 'green and yellow stripes'.
Exam technique
In shock questions, give both potentials: the live wire at about 230 V and the body at 0 V, so there is a large pd and a current through the body.
Use the key words 'fault', 'becoming live' and 'earth potential'.
If a question describes a fuse or a circuit breaker, use the information it gives you in your answer.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Most electrical appliances are connected to the mains using a three-core cable. Give the colour of the insulation on the neutral wire.
Blue
State the potential difference between the live wire and earth.
230 V
State the colour of the insulation on the live wire and on the earth wire.
Live: brown. Earth: green and yellow stripes.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Most electrical appliances are connected to the mains using a three-core cable.
(a) What colour is the insulation on the earth wire? Tick (✓) one box.[1]
blue
brown
green and yellow stripes
red
(b) Which wire carries the alternating potential difference from the supply? Tick (✓) one box.[1]
earth wire
live wire
neutral wire
(c) Give the colour of the insulation on the neutral wire.[1]
(d) State the potential of the earth wire.[1]
Show the answer and mark scheme
(a)Answer: green and yellow stripes
(b)Answer: live wire
(c)Answer: Blue
blue
(d)Answer: 0 V
0 (V)
Question 2Medium7 marks
A student is replacing the lamp in a ceiling light. The light switch, which is in the live wire, is open (off).
(a) State the potential difference between the live wire and earth.[1]
(b) The wire between the mains supply and the switch is still live. Explain why the student could get an electric shock by touching this wire, even though the switch is open.[3]
(c) The earth wire is a safety wire. It only carries a current if there is a fault. What is the purpose of the earth wire?[1]
(d) Explain why it is dangerous to provide any connection between the live wire and earth.[2]
Show the answer and mark scheme
(a)Answer: 230 V
(about) 230 (V)
(b)Answer: The wire is still at about 230 V while the student's body is at (or close to) 0 V (earth potential), so there would be a large pd across the student's body and a current would flow through the student to earth.
the live wire is still at (about) 230 V / a high potential
the student is at 0 V / earth potential
so there is a (large) potential difference across the student and a current flows through them (to earth)
(c)Answer: To stop an appliance (its metal case) becoming live.
to stop the appliance / metal case becoming live
(d)Answer: There is a large pd (230 V) between the live wire and earth, so a large current would flow through the connection; this could give an electric shock (if the connection is a person) or cause overheating and a fire.
(there is a pd of 230 V so) a large current would flow (through the connection)
which could cause an electric shock / overheating / a fire
Question 3Hard7 marks
A washing machine has a metal case. A fault occurs: the live wire becomes loose and touches the inside of the metal case.
(a) Explain what would happen if a person touched the case and the case was not connected to the earth wire.[3]
(b) The case is connected to the earth wire. Explain how this protects the person.[2]
(c) Explain why the earth wire does not normally carry a current.[2]
Show the answer and mark scheme
(a)Answer: The case would be at 230 V (live). The person is at 0 V, so there would be a pd of about 230 V across their body; a current would flow through the person to earth, giving an electric shock that could be fatal.
the case would be live / at 230 V
the person is at 0 V / earth potential, so there is a (large) potential difference across the person
a current would flow through the person (to earth) causing an electric shock
(b)Answer: The earth wire connects the case to earth (0 V) with a very low resistance, so the case cannot become live; a large current flows to earth through the earth wire instead of through the person.
the case is kept at (or close to) 0 V / cannot become live, so there is no (dangerous) potential difference across the person
the (large) current flows to earth through the (low-resistance) earth wire rather than through the person
(c)Answer: Normally the case is not connected to the live wire, so the case and the earth wire are both at 0 V; there is no pd between them, so no current flows.
normally the case is not connected to the live wire / the case is at 0 V
so there is no potential difference between the case and earth (so no current)