Before 1960s the design criterion of substation earthing system was “low earth resistance.” (Earth Resistance< 0.5 ohms for High Voltage installations).
During 1960s, the new criteria for the design and evaluation of Substation Earthing System were evolved particularly for EHV AC and HVDC Substations.
- Low Step Potential
- Low Touch Potential
- Low Earth Resistance.
The earthed points are held near-earth potential by low resistance conductor connections with earth-mat.
Substation Earthing System Design
Underground Horizontal Earth Mesh (Mat/Grid)
Earthing Electrodes (Spikes)
- The number of Earth-Electrodes (Spikes) Ns for soil resistivity 500 ohm meter and earth fault current Is is :
- The number of Earth-Electrodes (Spikes) Ns for soil resistivity 5000 ohm meter is
33 kV sub stations : 25000 to 31000 A
These are used for final connection (bolted/welded/clamped) between the Earthing Riser and the points to be earthed.
The earthing strips are ﬁnally welded or bolted or clamped to the Earthed Point.