fuses and earthing

• How does earth wire and fuse prevent an electric shock?

  • if live wire touches metal case, because case is earthed a large current flows through live wire, through case, and out down earth wire
  • surge in current melts fuse when amount of current is greater than fuse rating
  • fuses connected to live wire so breaking fuse breaks the circuit and cuts of live supply
  • isolates the whole appliance, preventing electric shock from case and fire caused by heating effect of large current
• Fuse rating and cable thickness

  • the larger the current, the thicker the cable needed to carry it to stop cable getting too hot and melting
  • fuse rating needed for cables increases with cable thickness
• Blowing a fuse in a toaster

  • fault allows live wire to touch metal case
  • large current flows out (surges) though earth wire to the earth
  • large surge in current melts the fuse which isolates the appliance and makes it safe
• Household fuses

  • blow when a light bulb goes
  • protects wiring in house, not just appliance
• Circuit breakers

  • large current may turn off a circuit breaker
  • turn off quicker than time taken for fuse to melt
  • can be reset, easier than having to replace a fuse
  • more expensive than fuses
• Earthing meaning

  case must be attached to an earth wire
• Earthed conductor

  can never become live
• Why are appliances with metal cases usually earthed?

  to reduce the danger of electric shock
• Double insulated

  • if appliance has a plastic casing and no metal parts showing
• What do double insulated appliances only need?

  only live and neutral wire, no earth wire
• Two-core cables

  cables that only carry live and neutral wires