AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Mains electricity: earth wire – green and yellow stripes.
The earth wire in a three-core mains cable is covered in green and yellow striped insulation.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The earth wire in a three-core mains cable is covered in green and yellow striped insulation. It is a safety conductor, not part of the normal current path. It connects the metal case of an appliance to earth, so its potential stays at zero volts. If a fault lets the live wire touch the metal case, a large current flows through the earth wire to earth. That surge melts the fuse or trips the circuit breaker, disconnecting the live supply before anyone can receive a shock. A metal-cased kettle is the classic example: its earth wire protects the user if a live conductor works loose inside. Appliances with plastic cases are often double insulated and need no earth connection.
What earns the marks
- States that the earth wire is identified by green and yellow striped insulation.
- Explains that the earth wire is a safety conductor connecting a metal case to earth at zero volts.
- Describes the fault path: live touching the metal case sends a large current through the earth wire.
- Links the large earth current to the fuse melting or the circuit breaker tripping, cutting off the live supply.
- Recognises that double-insulated appliances with plastic cases do not require an earth connection.
Common mistakes and how to put them right
Saying the earth wire carries current all the time.
Put it right: It carries current only during a fault, so the correction is that it is normally dormant.
Confusing the earth colour with the neutral colour.
Put it right: Green and yellow stripes mean earth, blue means neutral, so the correction is to keep the two codes distinct.
Thinking the earth wire protects by absorbing the shock.
Put it right: It protects by causing a large current that operates the fuse or breaker, so the correction is to describe that mechanism.
Exam tips
- Write green and yellow stripes in full, since the specification names both colours.
- When describing protection, sequence the events: fault, large current, fuse melts or breaker trips, supply disconnected.
- Mention zero volts for the earthed case to show why touching it is safe in normal use.
Try the lesson
The start of the Mains electricity: earth wire – green and yellow stripes. lesson, made from checked teaching notes for this specification point.
Have a go first
An electrician is inspecting a damaged three-core cable and needs to identify the earth wire. What colour is the insulation on the earth wire?
Key words
Say what each word means before you turn the card.
Electrical safety
Try answering aloud or in your head before turning the card.
Fuses and circuit breakers
Try answering aloud or in your head before turning the card.
Double insulation
Try answering aloud or in your head before turning the card.
See every definition
- Electrical safety
- The earth wire is a safety conductor, not part of the normal current path, connecting the metal case of an appliance to earth.
- Fuses and circuit breakers
- The surge from a fault melts the fuse or trips the circuit breaker, disconnecting the live supply before anyone can receive a shock.
- Double insulation
- Appliances with plastic cases are often double insulated and need no earth connection.
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