When a high-voltage line shorts to ground, large currents can flow in the resistive soil and dangerous voltage gradients can occur as that current flows. This can produce a hazard because of the voltage between the two feet (a "step potential") or between the ground where a person is standing and a nearby metal structure that might be touched (a "touch potential"). For this reason, substations bury large copper conductors to make a ground electrode (a "ground mat") and all metal structures, including fences and sheds, are bonded together and to the ground mat by grounding conductors. Furthermore, a loose layer of coarse gravel is used to cover the ground. This increases the resistance along the step-and-touch potential circuits somewhat like an (imperfect) insulation on a wire.
Concrete slabs, especially when wet, can conduct dangerous amounts of current. Damp soil will as well. Concrete, therefore, is not a substitute for a layer of loose gravel in a substation yard. It is also desirable that the reinforcing steel in any concrete slab to be bonded to the local ground bus. In fact, the footings for transmission towers usually have the reinforcing bars bonded to the tower steel and to the ground mat. This results in a desirable reduction in the tower-to-ground resistance and tends to reduce the potential rise in the vicinity of the tower in the event of a flash-over.
Proper equipment grounding by connecting all noncurrent-carrying metal parts to ground is one of the most important factors in guarding personnel safety. This includes all metal conduits, metal raceway, metal cables armor, outlet boxes, cabinets, switchboxes, motor frames, transformer cases, switchgear enclosures, metal enclosures of controllers, and other frames, and metallic cases of all electric equipment and electrically operated equipment.
Remember that the purpose of grounding this equipment is to limit the voltage difference between non-current-carrying parts of the system and the earth under all operating conditions, normal and abnormal. The plant grounding system consists of connections to the earth, and interconnections of all ground connections of the building frame and the non-current-carrying parts of the electric system.
