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5.2 Cables and Plugs

GCSE Physics · AQA · Chapter 5: Electricity in the Home

A free GCSE Physics lesson for AQA (8463) from OBA School: three-core cables, three-pin plugs, earthing, fuses, circuit breakers and double insulation. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe a three-core cable and a three-pin plug, and state the colour and job of each wire
  • Explain the choice of material for plug cases, pins, wires and cable coverings
  • Explain how the earth wire and the fuse work together to protect a user when a fault happens
  • Describe how circuit breakers and double insulation make appliances safer
  • Explain why touching the live wire is dangerous, even when an appliance is switched off

Key words

  • Three-core cable: A cable with three insulated wires (live, neutral and earth) inside a flexible outer layer.
  • Earth wire: A safety wire at 0 V connected to the metal case of an appliance. It only carries current if there is a fault.
  • Fuse: A thin wire inside a cartridge, in series with the live wire. It melts and breaks the circuit if the current is too big.
  • Circuit breaker: An automatic switch that opens (trips) when the current is too big. It can be reset.
  • Double insulated: An appliance with a plastic case and no exposed metal parts, so it needs no earth wire.
  • Short circuit: A fault where the current takes a path of very low resistance, so a very large current flows.
  • Electric shock: The effect of a current passing through the body. A large enough current can kill.

Mains cables

A mains cable contains two or three wires. Each wire is a bundle of thin copper strands, because copper is an excellent electrical conductor and is flexible. Each wire has its own coloured plastic insulation, and the whole set is wrapped in an outer layer of flexible plastic or rubber. The insulation stops the wires touching each other and stops anyone touching a conductor.

  • Live; Colour brown; Potential alternates (230 V); Job carries the alternating pd from the supply to the appliance
  • Neutral; Colour blue; Potential close to 0 V; Job completes the circuit back to the supply
  • Earth; Colour green and yellow stripes; Potential 0 V; Job safety wire; carries current only if there is a fault
  • Two-core cables (live and neutral only) are used for appliances with plastic cases, such as a hairdryer or a phone charger.
  • Thicker cables are used where the current is bigger, for example for an electric cooker or the socket circuit of a house. Thicker wires have less resistance, so they do not overheat.

Inside a three-pin plug

  • Case: stiff plastic or rubber, because these are good electrical insulators and are tough. The inside is shaped so the wires and pins cannot touch each other.
  • Pins: brass, because brass is a good conductor, is harder than copper and does not rust or corrode.
  • Earth pin: the longest pin, so it connects first when the plug is pushed in and the case is earthed before the live connection is made.
  • Fuse: fitted between the live pin and the live wire.
  • Cable grip: clamps the outer layer of the cable so the wires cannot be pulled out of their terminals.

Exam tip: Live = brown, on the right next to the fuse. Neutral = blue, on the left. Earth = green and yellow, at the top to the longest pin. A memory hook: BRown goes Right, BLue goes Left.

How the earth wire and fuse protect you

Many appliances, such as toasters, kettles and washing machines, have a metal case. The earth wire connects this case to the ground through the longest pin of the plug. Normally no current flows in the earth wire.

The safety sequence

1. A fault makes the live wire touch the metal case.

2. The earth wire gives a path of very low resistance from the case to the ground.

3. A very large current flows from the live wire, through the case and the earth wire.

4. This current is much bigger than the fuse rating, so the fuse wire heats up and melts.

5. The live wire is disconnected, so the case is no longer live and is safe to touch.

Without the earth wire, the case would stay at the potential of the live wire. A person touching it would provide a path to earth, and a current would pass through their body. The fuse also protects the cable: if too much current flows for any reason (for example a short circuit between the live and neutral wires), the fuse melts before the cable can overheat and start a fire.

Key fact: The fuse and the earth wire work as a team. The earth wire makes a large current flow; the large current makes the fuse melt.

Circuit breakers and double insulation

A circuit breaker does the same job as a fuse but uses an automatic switch. When the current gets too big, an electromagnet (or a sensor) opens the switch. A residual current circuit breaker compares the current in the live and neutral wires and trips if they are different, which happens when some current is leaking to earth, perhaps through a person.

  • How it works; Fuse thin wire melts; Circuit breaker switch opens automatically
  • Speed; Fuse slower; Circuit breaker much faster, so better at preventing a shock
  • After a fault; Fuse must be replaced; Circuit breaker can be reset once the fault is fixed

An appliance with a plastic case and no metal parts that can be touched is double insulated. Even if a wire inside comes loose, the case cannot become live, so no earth wire is needed and a two-core cable is used. Such appliances carry this symbol:

Why the live wire is dangerous

Your body is at about 0 V (earth potential). If you touch the live wire, there is a large pd across your body, so a current flows through you to the ground. This is an electric shock and can be fatal. Any connection between the live wire and earth is dangerous for the same reason.

  • A live wire can still be dangerous when an appliance is switched off: the wire between the socket and the switch is still connected to the supply.
  • Damaged or worn cables are dangerous because the live wire may be exposed.
  • Sockets in a house are connected in parallel: each one gets the full 230 V, and each appliance can be switched on and off on its own.

Safety: Always switch off at the socket and unplug an appliance before touching its wiring or changing its fuse.

Common mistakes

  • Swapping the live and neutral colours. Live is brown; neutral is blue.
  • Saying the fuse is in the neutral or earth wire. It is in series with the live wire, so it disconnects the dangerous wire.
  • Saying the earth wire carries current all the time. It only carries current when there is a fault.
  • Saying an appliance with a plastic case needs an earth wire. It is double insulated, so it does not.
  • Saying the fuse “stops the shock”. The earth wire creates the large current; the fuse then melts and cuts off the live wire.

Revision questions

Try these, then check your answers below each one.

Question 1. Why are the pins of a three-pin plug made of brass? (Choose from: Brass is a good conductor, hard and does not rust, Brass is a good insulator so it cannot give a shock, Brass is a better conductor than copper, Brass melts easily if the current is too big.)

Answer: Brass is a good conductor, hard and does not rust

Working: Brass conducts well, is harder than copper and does not corrode. (Copper conducts slightly better but is too soft.)

Question 2. True or false? The fuse in a plug is connected in the neutral wire.

Answer: False

Working: False. The fuse is in series with the live wire, so when it melts the dangerous live wire is disconnected.

Question 3. Which appliance can safely be connected with a two-core cable (no earth wire)? (Choose from: A hairdryer with a plastic case, A toaster with a metal case, A washing machine with a metal case, A metal desk lamp.)

Answer: A hairdryer with a plastic case

Working: A plastic-cased (double-insulated) appliance has no exposed metal parts, so it needs no earth wire.

Related lessons

Continue in the full course

This lesson is section 5.2 (AGCPH0502) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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