One of the main concerns when designing grounding systems is to ensure that no electrical hazards exist outside or within the substation during normal and fault conditions. In most cases, there are no safety concerns during steady-state normal conditions because very little current flows in the neutral and grounding system. This current, called residual current, rarely exceeds 10% of the nominal load current in most electrical distribution systems. Therefore, safety is usually a concern only during phase-to-ground faults.
In practice, most electric substations are fenced and the fence is quite often placed 1 m (3.28 feet) inside the outer conductor loop of the grounding system. This way, a person contacting the fence from the outside will be standing above or close to a ground conductor which will normally result in lower touch voltages than in the case where the fence is not surrounded by such a ground conductor loop. In this study, the fence at the East Central Substation is located 1 m inside the outer loop of the grounding system. Furthermore a large portion of the fence is not metallic (concrete or bricks).
Therefore, unless there are concerns for transferred potentials to remote locations via overhead or metallic paths, such as gas, oil or water pipes, railway tracks, etc., only the area delimited by the grounding system outer loop conductor needs to be examined with respect to unsafe touch voltages. However, step voltages must be explored everywhere inside and outside the substation site. In general, however, step voltages are rarely a concern inside electric power substation grounding grids, when touch voltages have been made satisfactory; outside the grid perimeter, however, step voltages need to be checked.
Now that we have specified an initial design for our grounding grid and that we have entered the data defining the soil structure and the characteristics of the circuit connected to the main grounding grid, we are ready to evaluate if our proposed design is safe and adequate.
The first step consists in determining suitable safety criteria to evaluate the performance of the grounding grid.