Mains Electricity

• Mains electricity


  Production of electrical energy on large scale
• Sources of mains electricity

  • Hydroelectric power - water
  • Geothermal energy - Soil/earth
  • Coal or diesel
  • Trees/Fossil
  • Wind energy - wind
  • Tidal waves in seas
  • Nuclear energy
• Grid system


  A system of power cables connecting all power stations in a country
• Grid system ensures power is available to consumers even when one of the power stations is shut down
• Power is stepped up to high voltage before transmission


  At high voltage, current is minimum and thus less energy is lost in form of heat
• Ways to minimize resistance in power transmission cables

  • Using thicker cables
  • Use cables made of materials that are good conductors of electricity
• Reasons why aluminum is preferred for power transmission cables

  • They are good conductors of electricity
  • They are light
  • They are cheaply available
• Dangers of high voltage transmission

  • Electric shock in case poles collapse or cables hang
  • Fire on nearby structures and vegetation when cables get a close
  • Harmful effects of strong electric field
• Step down transformer


  Transformer that reduces the voltage
• Central cable


  Cable that is earthed at the load transformer
• Live cable


  Cable that is not earthed from the local transformer
• Safety devices in domestic wiring system

  • Mains switch
  • Fuse
  • Earth connection
• Circuits in common fuse box

  • Lighting circuit
  • Cooker circuit
  • Ring mains circuit
• Lighting circuits are not supplied with an earth connection as most modern light fittings have insulated casings
• Two-way switch circuit


  A circuit in which a lamp can be operated by any one of the two switches
• Areas where two-way switch circuit is commonly installed

  • Lighting of staircase and corridors
  • Lighting of rooms
• Cooker circuit


  Circuit that carries large currents
• Cooker circuit is provided with an earth connection to protect the user from electric shock in case the live wire accidentally touches the casing
• Ring mains circuit


  A circuit on which sockets are installed parallel to each other
• Ring mains circuit has a loop arrangement which effectively increases the thickness of the wires and reduces the risk of overloading the circuit when several sockets are in use
• Ring mains circuit is provided with an earth connection to protect the user from electric shock in case the live wire accidentally touches the casing
• Parts of domestic wiring system

  • Switch
  • Fuse
  • Ring mains circuit
  • Neutral wire
  • Earth
  • Live wire
• Switch and fuse are connected to the live wire because it carries electrical energy at high potential
• Colors for wires in domestic wiring

  • Neutral - Blue or black
  • Earth - Green or yellow
  • Live - Red or brown
• Parts of a three-pin plug

  • Neutral wire
  • Earth wire
  • Live wire
  • Fuse
• Fuse is connected to the live wire to protect the appliance from excess current
• Earth pin is longer than live and neutral pins so that it is the first to connect and last to disconnect, protecting the user from electric shock
• Connecting the plug with these wiring mistakes would cause a short circuit to the earth through the live wire
• Fuse


  Protects appliances from excess current by melting and breaking the circuit
• Fuse working


  Made of a short thin wire of low melting point that melts and breaks the circuit when current exceeds the fuse rating
• Fuse material


  Tinned copper has low melting point
• Advantages of circuit breaker over fuse

  • Breaks the circuit instantaneously unlike the fuse that takes time to melt
  • Can be reset for use again once the fault has been corrected unlike the fuse where the wire must be replaced
• Fuse rating


  The fuse wire will melt and break the circuit if a current of 15A or more flows through the circuit in which it is connected