Earthing provides protection to personnel and equipment by ensuring operation of the protective control-gear and isolation of the faulty circuit in the following case.
- Insulation puncture or failure.
- Accidental contact between high and low voltage lines.
- The breakdown in insulation.
- Lighting strokes
Equipment earthing is different from neutral point earthing. Equipment earthing means connecting the non-connecting carrying metallic parts in the neighborhood of electrical circuit to earth.
Necessity of earthing
- To protect the operating personnel from the danger of shock in case they come in contact with charged frame due to defective insulation.
- To maintain the line voltage constant under unbalanced load condition.
- Protection of the equipment.
- Protection of large buildings and all machines fed from overhead lines against lighting.
Plate type of equipment (plate electrodes)
Plate electrodes when of the galvanization iron or still shall not be less than 6.30mm in thickness plate electrode of copper shall not be less than 3.15mm in thickness. Plate electrodes shall be of size at least 30 cm*60mm. plate electrode shall be buried such that it stop edge is at a depth not less than the surface of the grounded.
Where the resistance of one plate electrode is higher than the required value, two or more plates shall not be used in parallel. In such a case two plates shall be separated from each other by not less than 8.0m plates shall preferably be set vertically use of plate electrode is recommended only where the current carrying capacity is the prime consideration, for example, in generating stations and substances.
The following device is usually used to provide the necessary protection.
- Fuses [Re-wearable [kit Kat fuses]\ HRC cartridge fuse.
- Miniature circuit breakers [MCB]
- Earth leakage circuit breakers[ELCB].