There are many parameters that have an effect on the voltages in and around the substation area. Since voltages are site-dependent, it is impossible to design one grounding system that is acceptable for all locations. The grid current, fault duration, soil resistivity, surface material, and the size and shape of the grid all have a substantial effect on the voltages in and around the substation area. If the geometry, location of ground electrodes, local soil characteristics, and other factors contribute to an excessive potential gradient at the earth surface, the grounding system may be inadequate from a safety aspect despite its capacity to carry the fault current in magnitudes and durations permitted by protective relays.
During typical ground fault conditions, unless proper precautions are taken in design, the maximum potential gradients along the earth surface may be of sufficient magnitude to endanger a person in the area.
Moreover, hazardous voltages may develop between grounded structures or equipment frames and the nearby earth.
The circumstances that make human electric shock accidents possible are:
- Relatively high fault current to ground in relation to the area of the grounding system and its resistance to remote earth
- Soil resistivity and distribution of ground currents such that high potential gradients may occur at points at the earth surface
- Presence of a person at such a point, time, and position that the body is bridging two points of high potential difference
- Absence of sufficient contact resistance or other series resistance to limit current through the body to a safe value under the above circumstances
- Duration of the fault and body contact and, hence, of the flow of current through a human body for a sufficient time to cause harm at the given current intensity
The relative infrequency of accidents is due largely to the low probability of coincidence of the above unfavorable conditions.
To provide a safe condition for personnel within and around the substation area, the grounding system design limits the potential difference a person can come in contact with to safe levels.