Earthing of Substation Equipmenton 24/2/2012 & Updated on 27/7/2018
- The neutral point of different voltage levels.
- Metallic enclosure of all current carrying equipment.
- Framework of all current carrying equipment.
- All the metallic structure even not associated with current carrying equipment.
Method of EarthingWe connect all the points to be earthed with earthing grid with corrosion resistance mild still rods. We bury the connection rods minimum 600 mm below the ground level. If these horizontally buried rods cross a cable trench, road, underground pipework, or rail track the rods should cross the barriers through at least 300 mm below the bottom of the barriers. Although we use MS rods for connecting earth grid below the ground level but we normally use ms flats for same above ground level. We call the connection between different earthing points and earning grid as riser. We normally use ms flats in portion of the risers above ground level. The rod portion of the risers below the ground level is identical to the rod conductors use for creating main earth grid. We should connect all the steel structures with earth grid by at least two risers. In this case one riser must come from the rod of earthing grid in x direction and other from y direction.
We also connect the earthing points of all equipment in same fashion. We connect all the isolator mechanism boxes with individual auxiliary earth mat and each auxiliary earth mat to main earth grid. We place each auxiliary earth mat just 300 mm below the ground level. We connect all the raisers flats to earthing pads of the equipment by nut bolts and we should paint the bolted connections by anticorrosive paints. This point of earthing can not be welded to facilitate equipment replacement as when required. The leads come as riser from the earth mat should be welded with earth grid. The flats above ground should also be welded to the rod conductors below the ground. We must paint the welded points with red lead and bitumen.
Earthing of Gantry TowerThe shield wire comes down along the a leg of gantry structure. The shield wire which comes down along the a leg of gantry structure is referred as down comer. The downcomer is clamped with the leg members of the structure at every 2 meters intervals. This downcomer is connected with a earthing lead coming directly from a pipe earth electrode. Diagonally opposite leg of the same structure should directly be connected to the main earthing grid via riser.
Earthing of Bus Post InsulatorEach bus post insulator or BPI is connected to the main earthing grid via two risers. A 50 mm × 10 mm ms flat comes down along the BPI support structure from each of the two earthing points of BPI metallic base. These ms flats from base of the BPI are connected to the risers come from x and y conductor of main earthing grid.
Earthing of Current TransformerOne 50 mm × 10 mm ms flat comes down along the a leg of current transformer support structure from metallic base of the CT. This is connected to the main earthing grid via riser. Diagonally opposite vertical leg members of the structure is connected to the main earthing grid via another riser. If first riser comes from x conductor of the ground grid then second riser must come from the rod conductor of y direction.
The CT junction box should also be connected to the main earthing grid from two points by means of 50 mm × 10 mm ms flats.