# Lesson.5: Fuses and Safe Plugs

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Lesson.5: Fuses and Safe Plugs
Objectives State the colour, function and position of the three wires in a domestic plug. Describe the function, operation and safety purposes of fuses, circuit breakers and earth wires. Explain the meaning of the term, double insulated and why such an appliance does not require an earth wire.

Plug Questions

Wiring a Plug Practical

Questions; Plugs and Fuses
The diagram shows a correctly wired three-pin plug. 1. a.) Name the terminals marked A, B and C. b.) What colour is the wire connected to each of A, B and C? Complete if/when finish prac early.

c.) Which of the wires A, B, C carry current in normal use?
d.) Name D and explain its purpose.

current in appliance when working normally
2. Why does a double insulated device not need an earth wire? 3. What fuse should be fitted in the plug connected to the following appliances? (3 A, 5 A, 10 A and 13 A fuses are available.) appliance current in appliance when working normally (A) fuse rating electric kettle 11.5 hair dryer 4.0 table lamp 0.25 electric iron 6.0

Answers 1.a.) A = live B = neutral C = earth b.) A = brown B = blue C = green/yellow stripe c.) A d.) Fuse; wire in fuse melts if current becomes too large, breaking the circuit and so protecting the appliance.

current in appliance when working normally
2. Double insulated appliances have a plastic case (an insulator) with no electrical connections to it so the case cannot become live. 3. appliance current in appliance when working normally (A) fuse rating electric kettle 11.5 13 hair dryer 4.0 5 table lamp 0.25 3 electric iron 6.0 10

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Questions; Fuses and Circuit Breakers
1. a.) What is the main purpose of a wire fuse or circuit breaker? b.) Explain how a wire fuse protects an appliance if it develops a fault. c.) Explain how a wire fuse and an earth connection stop you getting an electric shock if an appliance develops a fault.

2. In a modern home, each wiring circuit is. protected
2. In a modern home, each wiring circuit is protected by a circuit breaker rather than a wire fuse. a.) Give one advantage of a circuit breaker compared with a wire fuse. b.) When should you also use a residual current device (RCD)? c.) Why is an RCD better than a wire fuse or circuit breaker?

Answers 1. a.) To switch off the current to an appliance if a fault develops. b.) Wire in fuse melts if current gets too large, stopping current; this prevents overheating, which can cause a fire, and protects the appliance from further damage. c.) If a live wire touches the case of the appliance, a large current flows in the earth and live wires and so the fuse ‘blows’.

2. a. ) Can be re-set at flick of a switch or it has no wire
2. a.) Can be re-set at flick of a switch or it has no wire to be replaced. b.) When using power tools. c.) Stops current immediately as a fault develops, (such as accidentally cutting a cable).

Complete Higher ‘Charge in Circuits’ page 137 and ‘Fuses and Safe Plugs’ page 138 and ‘Resistance’ page 139. Foundation Charge in Circuits’ page 126 and ‘Resistance’ page 128.

Foundation MARK SCHEME

Learning Objectives State the colour coding for live, neutral and earth wires: live – brown; neutral – blue; earth – green/yellow. Describe and explain the functions of the live, neutral and earth wires: live – carries the high voltage; neutral – the second wire to complete the circuit; earth – a safety wire to stop the appliance becoming live. State that an earthed conductor cannot become live. Describe that an earthed conductor cannot become live. Describe reasons for the use of fuses/circuit breakers (as re-settable fuses).

Explain the reasons for the use of fuses/circuit breakers as re-settable fuses (structure and mode of operation not required). Describe how a wire fuse works: if the current becomes too large; wire fuse melts, breaking the circuit. Explain how a wire fuse reduces the risk of fire: If the appliance develops a fault: too large a current causes the fuse melt; preventing flow of current; prevents flex overheating and causing fire; prevents further damage to appliance. Describe and recognise that “double insulated” appliances do not need earthing Explain why “double insulated” appliances do not need earthing: case of appliance is a non conductor and cannot become live. Explain how a wire fuse and earthing protects people.