The Earthing of a system is designed with the primary focus being the safety and security of the system by ensuring that the potential on each conductor is restricted to such a value that it is consistent with the level of insulation applied. Most high voltage public supply systems are earthed. Unearthed overhead lines can be found but this may be small as 11kV derived from 33kV mains, where the capacity of earth current is of 4A.

An effective earthing system consists of earth electrodes, buried main earth grid and equipment earth mats, interconnections of the earth grid to the structures and equipment where ever necessary. 

Before design process can begin, the soil resistivity has to be measured at the site. This is required when checking for the resistance of earth rod (electrodes) and earth grid (main earth mat). The resistance of one meter cube of soil measured between opposite surface is termed as soil resistivity. The 'four rod or the Wenner method' is the most followed method for measuring the soil resistivity. In this method, four rods are placed at an equal spacing (a) and equal depth, into the soil. The diameter of the rods need not be the same but the depth at which the rod is inserted into the ground must be equal. The voltage between the two inner electrodes is then measured and divided by the current between the two outer electrodes to get the value of resistance (R). The soil resistivity is then derived from the formula [ p = 2*pi*a*R ]; where p is the soil resistivity.

Two major factors considered in the design of an earthing system are the a) Touch Potential and  b) Step Potential

The potential difference between a grounded metallic structure and a point on earth's surface separated by a distance equal to one meter is termed as Touch Potential or Mesh Potential.

The potential difference between two points on earth's surface, separated by a distance of one meter is termed as Step Potential.

The design procedure includes the selection of the size of the earthing conductor depending on the nominal voltage level, system fault level, the fault current and the material that is to be used as the earthing conductor. Once the material to be used as earthing conductor is finalized, the touch and step potential of the soil at the site has to be determined, using the values of soil resistivity and soil resistivity at the top layer (as explained earlier in this post). Followed by this the touch and step potential that is attainable with the earthing system is to be calculated. 

The attainable value of Touch and Step potentials has to be lesser than the Tolerable Touch and Step potentials and also the combined resistance of the earth grid and the earth electrode has to be less than one ohm for a safe earthing design.