Unit 1.4

• Kilowatt hours (kWh)

  Unit of energy used in the home, equal to the energy converted by a 1 kW device for 1 hour
• Wires in the ring main circuit

  • Live
  • Earth
  • Neutral
• Live wire

  • Carries a current into the house at a high voltage
• Earth wire

  • Normally carries no current
  • Connected to the casing of any device with a metal case to provide a safe path for current in case of a fault
• Neutral wire

  • Completes the circuit and has the same current as the live wire but a lower voltage
• The electrical circuits in your home are connected using a ring main circuit - as many plugs as you like can be places on a ring main circuit
• Advantages of a ring main circuit

  • The current can travel two ways so you can make the wires thinner and have a lower current in each part
  • You can add more sockets anywhere on the ring and each will have the same voltage (230V)
• Calculating cost of electricity used

  1. Units used (kWh) = Power (kW) × time (hours)

  2. Cost = Units used × Cost per unit
• Power is often given in Watts and time in seconds or minutes, so conversions are needed
• Efficiency rating

  Indicates how efficient a device is, the more efficient the better the rating
• Electricity can be supplied as Alternating Current (AC) or Direct Current (DC)
• Safety devices used in electrical circuits

  • Fuse
  • Miniature circuit breaker (MCB)
  • Residual current circuit breaker (RCCB)
• Fuse


  • Stops the current if it becomes too large by melting a wire, must be replaced once 'blown'
  • Prevents the device overheating
• Miniature circuit breaker (MCB)


  • Stops the current if it becomes too large, reacts more quickly and can be reset
  • Prevents the device overheating
• Residual current circuit breaker (RCCB)


  • Stops the current if the current in the neutral wire is different to the live wire
  • Reacts to a very small difference, reacts very quickly and can be reset
  • Protects the user from a serious electric shock
• Plug labelled

  A) Earth

  B) Neutral

  C) Fuse

  D) Live

  
• Types of fuses - if the devise works at 3A - go higher to 5A fuse

  • 3A
  • 5A
  • 13A
• AC


  • Voltage constantly changes size and the current constantly changes direction
  • Supplied by the National Grid

  
• DC


  • Voltage and current have a constant size and direction
  • Supplied by batteries