Explain the concept of 'touch potential' and 'step potential' in grounding, and how are they mitigated in substation design?

Touch potential is the voltage a person could be exposed to when touching a grounded object in a substation during a ground fault. It's the potential difference between the grounded object (e.g., equipment enclosure) and the point where the person is standing. Step potential is the voltage a person could be exposed to by stepping across the ground during a ground fault. It is the potential difference between a person's feet, typically assumed to be one meter apart. Both touch and step potentials pose a significant electrical shock hazard to personnel in a substation during a ground fault. Substation design incorporates several measures to mitigate these hazards. A primary method is to create a low-impedance ground grid that effectively distributes fault currents and minimizes ground potential rise (GPR). A dense grid of interconnected conductors buried beneath the substation reduces the voltage gradient across the surface. Surface grading is used to provide a uniform surface resistivity. This can involve adding a layer of crushed rock or gravel to the surface to increase the surface resistance and reduce the current density. Equipotential planes are created by bonding together all metallic objects within reach of personnel, such as equipment enclosures, fences, and handrails. This minimizes the touch potential by ensuring that all objects are at approximately the same potential. Grounding mats are placed at frequently accessed locations, such as switch operating handles, to provide a low-resistance path to ground and minimize the touch potential. Finally, warning signs and training are provided to personnel to educate them about the hazards of touch and step potentials and the importance of following safe work practices.