Earthing and bonding for protection

• Protective Earthing is required to provide a low impedance path, necessary for the protective device to operate should a fault between a line conductor and an exposed conductive part or a circuit protective conductor should occur 
• A low impedance path allows sufficient current to flow to operate the protective device quickly. 
• What would be the result of a low impedance fault path

  • Low impedance = high current = quick operation of protective device 
  • Low impedance = is resistance (is DC term )for AC
• What is 'source earthing'


  
  

  • Associated with the source of supply to an electrical installation.
  • Source earthing, sometimes called ‘supply system earthing’, is the provision of a connection between the source of energy - normally the distribution transformer secondary winding star point or generator winding 
• What is electrical equipment earthing

  • Associated with the electrical installation itself 
  • The connection of protective conductors to parts of the installation and to equipment and accessories (this includes tools and containment) 
• Electrical equipment earthing is applicable in installations where the measure for fault protection is Automatic Disconnection of supply (ADS), as used in the majority of installations in the United Kingdom. 

   
• What is the purpose of electrical equipment earthing

  The purpose of electrical equipment earthing, is to allow operation of the protective devices for protection against electric shock and fault current to earth. Permitting automatic disconnection of the supply to the faulty circuit in the event of an earth fault. 
• What are the intakes for earthing

  • TN-S
  • TNC-S
  • TT
• What does TN-S do

  
  

  • Separate earth conductor from the supplier transformer star point
  • (low impedance value = 0.8ze) 
• What does TNC-S do

  • where protective multiple earthing is provided, Neutral and earth conductor combined and separated at the intake position 
  • (low impedance value = 0.36 ze) 
• What does TT do

  • via an earthing conductor to an earth electrode.  
  • (low impedance value = 21ze) 
• How is electrical equipment earthing achieved

  1. Connecting a protective conductor (CPC) to exposed conductive part
  2. Connecting a protective conductor to extraneous conductive part
• What is a Exposed conductive part


  Part of the instillation not normally live but liable to become live under fault condition
• What is a Exposed extraneous part

  Not part of the instillation but introduces a zero potential into the instillation
• Examples of exposed conductive parts are:

  • Metal outlets
  • metal switcher
  • Electric cookers
  • Etc
• Examples of Extraneous conductive parts

  • Water insulation pipes
  • Gas insulation pipes
  • Central heating and air conducting systems
  • Etc