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Home > Physics > Using electricity > Behind the wall


Behind the wall

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This bite deals with fuses and circuit breakers and the mains supply.

Fuses and circuit breakers

Fuses are used in 3-pin plugs and in the fixed wiring in a house.


  • Fuses in plugs protect the flex of the appliance, and in doing so, the appliance itself.
  • Fuses in the fixed wiring of a house protect the mains wiring.
  • If a fault develops and the current in the wire becomes dangerously large, the fuse melts and breaks the circuit.
  • Plug fuses come in different values. Although other values are available, the following rule is used to decide on the correct value of fuse to use:
    • for an appliance with a power rating of up to 700 watts, use a 3 ampere fuse
    • for an appliance with a power rating of 700 watts or greater, use a 13 ampere fuse

Circuit breakers

  • Circuit breakers are automatic switches in the mains supply which do the same job as fuses. They break the live wire if a fault causes the current to become too large.
  • (Credit) You should be able to say why fuses are in the live lead of an appliance. This is so that when a fuse blows it cuts off the live wire which is at a high voltage. The neutral wire is never at a high voltage.
  • (Credit) You should be able to give a reason why a circuit breaker is better than a fuse. Some reasons are:
    • they operate more quickly
    • they only need to be reset, not replaced
    • they cannot be replaced with the wrong size of wire


Video: How fuses and RCDs help protect us from electrocution

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