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3.5.3Electricity

Fuses

5 min read 5 parts Lesson 16 of 16

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Part 1 of 5Why Fuses Are Used

3.5.3 Fuses

A fault can cause an unsafe current that overheats wires and starts fires.

Overheating
  • A very large current heats the cable because of its resistance, which can melt the insulation.
The fuse wire
  • A fuse contains a thin wire with a low melting point that heats up first when current rises.
Breaking the circuit
  • When the current exceeds the rating, the wire melts and the circuit is broken, stopping the current.
Position
  • The fuse is connected in the live wire so that the appliance is isolated from the mains when it blows.

Part 2 of 5Mains Plug Safety

Fuses work alongside other safety features in a three-pin plug.

Mains Plug Safety
Live wire
  • Brown and carries the alternating voltage to the appliance.
Neutral wire
  • Blue and completes the circuit back to the supply.
Earth wire
  • Green and yellow and connects the metal casing to the ground in case of a fault.
Double insulation
  • Appliances with plastic casings do not need an earth wire as the user cannot touch live parts.
WireColourFunction
LiveBrownCarries current to the appliance
NeutralBlueReturns current to the supply
EarthGreen and yellowSafety connection to the casing

Part 3 of 5Choosing a Fuse

The correct fuse is rated just above the normal working current.

1
Find the current drawn by the appliance using I = P ÷ V with the mains voltage of 230 V.
2
List the available fuse ratings, commonly 3 A, 5 A and 13 A in a UK plug.
3
Choose the smallest standard fuse rating that is higher than the normal current.
4
A rating that is too low blows when the appliance is working normally.
5
A rating that is too high will not blow quickly enough to protect the wiring.

Part 4 of 5Worked Example

Question

  • A toaster is rated at 1100 W and is connected to the 230 V mains.
  • Which fuse should be fitted: 3 A, 5 A or 13 A?

  1. Current I = P ÷ V = 1100 ÷ 230 = 4.78 A.
  2. The fuse rating must be just above this value.
  3. 3 A would blow; 5 A is just above 4.78 A; 13 A is much too high.

Answer A 5 A fuse should be fitted.

Part 5 of 5Exam Checklist & Tips

3.5.3 – Key Points for the Exam

  • Fuse A safety device containing a thin wire that melts when the current is too large.
  • Fuse rating The maximum current a fuse can carry before it melts.
  • Live wire The wire that carries the alternating voltage to an appliance.
  • Earth wire A safety wire that connects the metal case to the ground.

Common Mistakes

  • Choosing a fuse with a rating well above the normal current, so it gives no protection.
  • Putting the fuse in the neutral wire instead of the live wire.
  • Choosing a rating equal to or below the normal current so that the fuse blows immediately.
  • Forgetting that a blown fuse should be replaced only with one of the same rating.
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