- Earthing Definition: Earthing in substations involves connecting specific points to an earthing grid to ensure safety and functionality.
- Substation Earthing Methods: Substation earthing uses corrosion-resistant MS rods and flats, buried at specific depths, to connect various parts to the earthing grid.
- Equipment Earthing: Each substation equipment, like transformers, insulators, and isolators, has specific methods and points of connection to the earthing grid.
- Multiple Risers: Ensuring at least two risers from different directions for all steel structures and equipment enhances reliability and safety.
- Special Connections: Certain equipment, like lightning arrestors and transformers, require additional connections for surge protection and fault detection.
Earthing of substation equipment bonds exposed metal and specified neutrals to an earth grid so fault current has a path and touch voltage stays limited. The points we must earth in an electrical substation include:
- The neutral point of different voltage levels
- The metallic enclosure of all current carrying equipment
- The framework of all current carrying equipment
- All the metallic structure even not associated with current carrying equipment
Method of Earthing
In typical Indian AIS yards, those points go to an earthing grid of corrosion-resistant mild steel rods, buried at least 600 mm below ground. IEEE 80 often uses a shallower mesh and copper or copper-clad conductor. If these rods cross a cable trench, road, underground pipework or rail track, they should be at least 300 mm below the barrier.
Mild-steel rods form the buried grid. Mild-steel flats form the above-ground risers. A riser is the run from an earthing point down to the grid. Rods below ground match the main earth-grid conductors.
Each steel structure should have at least two risers to the grid. One riser should come from a grid rod in the x direction and the other from the y direction, so a single conductor cut does not isolate the structure.
Equipment earthing points follow the same two-riser pattern.
Each isolator mechanism box sits on its own auxiliary earth mat. Each auxiliary mat ties to the main grid and sits about 300 mm below ground in this construction practice.
Riser flats bolt to the equipment earthing pads. Paint those bolted joints with anticorrosive paint. Leave the pad bolted, not welded, so the equipment can be swapped.
Weld the riser lead to the earth mat. Weld the above-ground flats to the buried rod conductors. Paint welded joints with red lead and bitumen.
Earthing of Gantry Tower
The shield wire comes down along one leg of the gantry. That drop is the downcomer. Clamp the downcomer to the leg members at about 2 metre intervals. Connect the downcomer to an earthing lead from a pipe earth electrode. Connect a diagonally opposite leg of the same structure to the main earthing grid through a riser.
Earthing of Bus Post Insulator
Each bus post insulator or BPI ties to the main earthing grid through two risers. A 50 mm × 10 mm MS flat comes down the BPI support from each of the two earthing points on the metallic base. Those flats join the risers from the x and y conductor of the main earthing grid.
Earthing of Current Transformer
One 50 mm × 10 mm MS flat comes down one leg of the current transformer support from the metallic base of the CT. That flat joins the main earthing grid through a riser. A diagonally opposite vertical leg joins the main grid through a second riser. If the first riser comes from an x-direction conductor of the ground grid, the second riser should come from a y-direction rod.
The CT junction box should also join the main earthing grid at two points with 50 mm × 10 mm MS flats.
Earthing of Circuit Breaker
The supporting structure of each pole of a circuit breaker, with the metallic base of the poles, joins the main earthing grid through two risers, one from the x direction and one from the y direction where the grid allows. The pole structures join together with 50 mm × 8 mm MS flat. The mechanism box of each pole also joins the main grid with 50 mm × 10 mm MS flat.
Earthing of Isolator
The base of each isolator pole should be bonded together with one 50 mm × 10 mm MS flat. That flat joins the main earthing grid through two risers, one from the x-direction earth-mat conductor and one from the y-direction conductor where the grid allows. The isolator mechanism box should join the auxiliary earth mat, and that auxiliary mat should join the main earthing grid at two different points.
Earthing of Lightning Arrestors
The base of the lightning arrestors must join the main earthing grid through one riser. The structure of the lightning arrestors must join the main grid through another riser. A third connection runs from the arrestor through the surge counter to a treated earth pit. That pit may include a test link.
Earthing of Capacitive Voltage Transformer
The base of the CVT or capacitive voltage transformer joins the main earthing grid through a riser. The special earthing point on the CVT base joins a pipe earth electrode with 50 mm × 8 mm MS flat. The bottom of the support structure also joins the main grid through a riser. Two opposite earthing points of the CVT junction box should also join the main earthing grid.
Earthing of Cable Sealing System
The supporting structure of a cable sealing system should join the main earthing grid through two risers. A 50 mm × 10 mm MS earthing strip must come down from the top of the supporting structure.
Earthing of Bay Marshalling Kiosk
Two protected leads sit on opposite sides of the bay marshalling kiosk. Those two points must join the main earthing grid through two risers. The links sit on the lower portion of the marshalling kiosk or box.
Earthing of Earthing Transformer
The base of the earthing transformer must join the main earthing grid through two risers. The neutral point of the earthing transformer must join a pipe earth electrode through a test link. The neutral-to-ground connection should pass through a neutral current transformer so earth-fault protection can measure residual current.
