Domestic Electricty

• Kilowatt hours (kWh)


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


  • 3 wires: Live, Earth, Neutral
  • Live carries high voltage current into house
  • Earth normally carries no current
  • Neutral completes circuit with same current as live but lower voltage
• Earth wire


  Connected to casing of any device with metal case, so current travels safely to earth if live wire touches casing due to fault
• Advantages of ring main circuit

  • Current can travel two ways so wires can be thinner and have lower current
  • Can add more sockets anywhere on ring, each with same 230V voltage
• Calculating cost of electricity used

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

  2. Cost = Units used × Cost per unit
• Unit conversions


  • Seconds to hours: x 3600
  • Minutes to hours: ÷ 60
  • Watts to kilowatts: ÷ 1000
• Energy efficiency rating


  Indicates how efficient a device is, helps compare devices that do the same thing
• Alternating Current (AC)


  Voltage and current constantly change size and direction, supplied by National Grid
• Direct Current (DC)


  Voltage and current have constant size and direction, supplied by batteries
• Safety devices


  • Fuse: Stops current if too large by melting wire, must be replaced
  • Miniature circuit breaker (MCB): Stops current if too large, can be reset
  • Residual current circuit breaker (RCCB): Stops current if difference between live and neutral, reacts very quickly, can be reset
• Kilowatt hours (kWh)

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

  • 3 wires: Live, Earth, Neutral
  • Live carries high voltage current into house
  • Earth normally carries no current
  • Neutral completes circuit with same current as live but lower voltage
• Earth wire


  Connected to casing of any device with metal case, so current travels safely to earth if live wire touches casing due to fault
• Advantages of ring main circuit

  • Current can travel two ways so wires can be thinner and have lower current
  • Can add more sockets anywhere on ring, each with same 230V voltage
• Calculating cost of electricity used

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

  2. Cost = Units used × Cost per unit
• Unit conversions


  • Seconds to hours: x 3600
  • Minutes to hours: ÷ 60
  • Watts to kilowatts: ÷ 1000
• Energy efficiency rating


  Indicates how efficient a device is, helps compare devices that do the same thing
• Alternating Current (AC)


  Voltage and current constantly change size and direction, supplied by National Grid
• Direct Current (DC)


  Voltage and current have constant size and direction, supplied by batteries
• Safety devices


  • Fuse: Stops current if too large by melting wire, must be replaced
  • Miniature circuit breaker (MCB): Stops current if too large, can be reset
  • Residual current circuit breaker (RCCB): Stops current if difference between live and neutral, reacts very quickly, can be reset