Earthing
The various standards for earthing provide design limits to be met and together with codes of practice explain how the earthing system can be designed to meet these. There are differences in the design limits applying to consumer installations and to supply industry installations and reference should be made to the correct standard to check the limits which apply in each situation.
In the past, it was normal practice to design an earthing system to achieve a certain impedance value. The earth electrodes being positioned near the equipment where a fault current was expected to pass. Recently, this has changed towards the approach used in North America. The most important difference is that the earthing system is designed to ensure that potentials in its vicinity are below appropriate limits. These potentials are referred to as:
In the past, it was normal practice to design an earthing system to achieve a certain impedance value. The earth electrodes being positioned near the equipment where a fault current was expected to pass. Recently, this has changed towards the approach used in North America. The most important difference is that the earthing system is designed to ensure that potentials in its vicinity are below appropriate limits. These potentials are referred to as:
- 1. Step Potential (the potential difference between two points on the surface of the soil which are 1m apart).
- 2. Touch Potential (the potential difference during fault conditions between exposed metalwork and a point on the soil surface).
- 3. Transfer Potential (the potential difference between as insulated cable connected to a remote earth reference and the earth rod). A number of factors will determine whether or not a person experiencing any of these potentials will be at risk, and the standards attempt to take these into account to ascertain the limits below which the design will be considered acceptable.