System earthing is primarily required to ensure that the potential of each conductor of the system restricts to its desired value for which the insulation is provided.The system earthing is so designed that it should facilitate the efficient and quick operation of protective relays in case of any earth fault.
T N System of Earthing
In T N System of earthing the source is directly earthed at one or more point. The conductive parts of the installations are connected to the earth through the earth points of the source by means of conductor. So, that any earth fault current created in the installations will pass through the earth through the earthing point of the source. Here, one conductor runs along with the supply line where the earthing points of the installation are directly connected. In overhead system, this conductor may be a separate conductor but in underground system, the cable sheath or armors is used for the purpose.
T T System of Earthing
In T T System of earthing, the source is earthed. But the conductive parts of the installations are connected to the earth through one or more local earth electrodes. These local electrodes do not have any direct connection to the earthing system of source. This T T system of earthing is applicable for both three phase and single phase installations.
I T System of Earthing
I T System of earthing is generally used in unearthed three phase network. Here, the three phase source is isolated from earth or connected to earth through a high impedance of suitable value. The conductive parts including metal body of the installations are connected to the earth through one or more local earth electrodes. These local electrodes does not have any direct connection to the source.
T N S System of Earthing
T N S earthing system is similar of T N system of earthing. In both T N and T N S system, the neutral wire and earth wire run separately along the network. The conductive parts of the installations are connected to the earth wire to provide earthing. But in T N S system, in addition to continuous earth wire from source, there are local earth pits installed at consumer premises. The installations are also connected to the local earth pits through earth electrodes. The local earth electrodes are inter connected to the earthing system of the source by means of earth wire runs along the network. That means each local earth electrode is individually connected to the earth wire.
T N G System of Earthing
T N G earthing system is similar of T N S system of earthing. In later, the neutral wire and earth wire do not run separately, rather they are combined together to form a PEN wire which runs along the network. The neutral and earth points of installation are connected to the same PEN wire in this system.