Earthing and Bonding

Earthing Paths

The term earth path tends to be defined by what is connected to it. It is therefore more logical to describe the earth by the circuit types from which it is gathering stray currents. Hence power, instrument, computer, structural and terrestrial earths are more meaningful names but their function requires clear definition in plant safety documentation so that their use is not misunderstood. There is little concern that the risk of ignition will be increased by incorrect earthing but the operational aspects of systems may be severely hampered.

Two common and more meaningful terms to distinguish different earth paths are

• dirty earth
  the return path conductor that can carry the prospective fault currents from electrical equipment. As a result the conductor is likely to shift in potential by a greater amount (with respect to the neutral of the supply system which is the ultimate reference). Fault currents are inseparable from the parasitic capacitance currents, which will flow along the same route, although they will be at a much lower level.
• clean earth
  the return-path conductors, which cannot carry fault currents from electrical equipment. This, by definition, is impossible to control or guarantee since any circuit to earth will conduct fault currents back to the return of the source of supply.

Earthing and bonding

There is a subtle but essential difference between earthing and bonding, which must be understood.

• Earthing
  is where a low-impedance path is provided in order for return currents to operate electrical protection devices such as fuses and overcurrent trips in an appropriately short time.
• Bonding
  is where voltage differences between electrical conducting parts are eliminated.

Definition of terms

• Bonding
  
  1. The electrical interconnecting of conductive parts, designed to maintain a common electrical potential.
  2. The permanent joining of metallic parts to form an electrically conductive path, which will assure electrical continuity and the capacity to conduct safely any current likely to be imposed.