Three pin plug

Three pin plug. Image credit

ZIMSEC O Level Combined Science Notes: Electrical safety

  • The following concepts and terms have to be understood:
  • Short circuit
  • Overloading
  • Earthing
  • Fuses
  • Three pin plug
  • Two pin plug

Short circuit

  • this is when a wire breaks and comes in contact with the metal casing of an appliance e.g. of a stove
  • Current will flow to the casing making it live
  • Current flows along the path of least resistance (short circuit)
  • Metal casings and appliances must be earthed
  • to prevent current from flowing through the person holding the appliance and electrocuting the person


  • Is when too many appliances are plugged into one power-point/plug
  • drawing too much current
  • The circuit gets overloaded and
  • the wires carrying the current might get hot
  • A fuse can be used to prevent overloading


  • A lot of household and industrial appliances are earthed
  • They are connected with a three-core flex wire
  • Sockets have a third socket to which the earth wire is attached
  • This wire is connected to the metal casing of the appliance
  • If there is a fault in the appliance and a short circuit occurs current will flow to the earth via this earth wire
  • More current will flow through the earth wire than the neutral wire
  • And the fuse will blow due to overloading
  • This breaks the circuit
  • The wires are run in PVC conduits/pipes
  • The earth wire is grounded using a thick wire in the earth
  • When steel conduits are used (this is now very rare) they are earthed by grounding them as well


  • Fuses are made up of a thin wire
  • Each wire have its amperage rating for example 13A
  • The fuse is then attached to the live wire in a circuit
  • In the event of a power surge or an overload the fuse wire blows as it cannot handle any current that exceeds its rating
  • For example a 13A fuse will only be able to handle 13A and any current exceeding that will result in the fuse blowing
  • When it blows it breaks the circuit

Three pin plug

Three pin plug. Image credit

Three pin plug. Image credit

  • These plugs carry power from a household socket to the appliance
  • EAch wire in the power circuit makes contact with wires in the appliance
  • The live wire is connected to the live wire in the appliance and
  • The neutral wire makes contact with the neutral wire in the appliance
  • the wires are also colour coded
  • The live wire is brown
  • The neutral wire is blue
  • the earth wire is green and yellow
  • Three pin plugs also have a plug

Two pin plug

A two pin plug

A two pin plug

  • Some electrical appliances have two pin plugs instead
  • These only have the live and neutral wires
  • They have no fuse
  • They usually draw only a little current
  • They are also well insulated and usually made of plastic
  • There is no risk of the user being electrocuted

To access more topics go to the Combined Science Notes page.

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