The earthing or grounding is nothing but the connection of neutral point if the supply system to the general mass of earth in such a way that immediate discharge of electricity can take place without danger.
When grounding is provided then it ensures the safety of personnel against electrical shocks and avoids accidents. The equipment is also protected against lighting and voltage surges. The voltage stress on lines is reduced along with that on the equipments with respect to earth under abnormal conditions. With earthing, the earth fault currents are controlled for protective relays.
These are two ways in which the three phase systems can be operated. These are viz. with isolated neutral and with earthed neutral.
But presently isolated neutral system is not used as with such system during fault, large transient voltages with magnitude several times that of normal value is produced which may cause breakdown of insulation. This results in damage of the concerned equipment and interruption of the supply system. Inspite of this the advantage of this system is under earth fault on one of the phases, the remaining two healthy phases will continue supplying load for a shorter period.
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