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What is Substation Earthing?
The fundamental role of any substation earthing is to dissipate the short circuit current into the earth without drying out the area and to limit the potential gradient throughout the substation to maintain the step and touch voltages within safe values. There are two features considered during the design of earthing for a substation which is

Providing a low impedance path for the fault currents occurred during normal conditions and fault conditions
Avoiding fatal electric shocks to the person working in the vicinity of the grounded facilities
Earthing Standards
To calculate the substation earthing design parameters and the potential shock safety limits, a large variety of national and international guidelines are followed across the globe that is:
BS7354 -1990 Code of practice for Design of High Voltage Open Terminal Stations
EATS 41-24- Guidelines for the design, installation, testing, and maintenance of main earthing stations in substations
IEEE Standard 80-2000- Guidelines for AC substation grounding
Parameters Considered For Earthing Of Substation
GROUND POTENTIAL RISE:
An earthing mat is a grounding system formed by a grid of conductors buried horizontally and provides a low impedance path for the earth’s fault current to dissipate into the earth. The earthing grid present in the substation is an electrical connection to the earth at zero potential reference point. The connection where the earth mat is buried is not ideal due to the resistivity of soil that leads to the flow of current via the grid to the earth, during a typical earth fault condition leading to a potential rise in the system, creates a potential gradient within and around the substation ground area. Ground Potential Rise (GPR) can be defined as the product of ground electrode impedance with respect to zero earth and the current flowing through that electrode impedance.

GPR = I0 × Rg

GPR can be controlled by keeping the resistance of the earthing grid as low as possible, so the earth fault conditions are limited for maintaining the step and touch potential limits. Step potential, mesh potential, and transferred potential plays a vital role in the calculation of the earthing system and to ensure equipment safety as well as human safety.
STEP POTENTIAL:
The potential difference between two points on the surface of the earth separated by a distance of one pace that is normally assumed to be one meter in the direction of the maximum potential gradient is known as Step Potential. Considering a constant body impedance of 1000Ω Step potential can be calculated as
TOUCH POTENTIAL:
The potential difference between a grounded metallic structure and a point on the earth’s surface at a distance equal to the normal maximum horizontal reach is approximately one meter.
MESH POTENTIAL: