Earthing System is an integral part of Lightning Protection System which has been designed around the structure to protect it from the dangers of lightning strikes by allowing the current to flow to the earth. For an efficient earthing system, it is essential that a low electrical resistance to the earth is achieved by the use of good quality conductors having sufficient cross sectional area to carry the expected current flow.
Several factors are needed to be considered while designing an earthing system. To have an efficient earthing system it is necessary to determine the ground resistivity which is dependent on factors such as chemical composition of the soil and presence of certain salts and moisture content in it. Excess of moisture or presence of certain chemicals can reduce the resistivity of the soil. The survey is incomplete without details regarding the metallic framework like tanks, pipes, rails, etc. which may need to be bonded into the earthing system to prevent danger of side flashing.
An earth electrode should be connected to each down conductor which constitutes to the Lightning Protection System. An Earth Inspection Pit is required for earth electrode to enable periodic inspection, testing and maintenance of earth resistance.
The maximum resistance value for an earthing system is specific to area and application. In case a single earth rod is not sufficient to achieve the required resistance, multiple earth rods may be used. In such case the combined resistance of the rods is proportional to reciprocal of individual rod resistance. Normally, the minimum spacing between the rods should not be less than their driven length. In geographical locations where it is impossible to use earth rods with longer lengths it is advisable to use rods with larger diameter, earth plates or earth mats. A lower resistance value can be obtained by maximizing the utilization of the rods, increasing the length of the rods to drive them deeper or by using rods of larger diameter.
It is very necessary to keep periodic check on Earthing System to ensure that it retains its ability to conduct the same current carrying capacity as it did when it was installed. Any exposed or underground earth system is subject to corrosion due to environmental conditions. A properly installed and maintained earth system can last for several years.