In the field of electrical engineering for power,substations and transmission lines earth grids are designed to prevent people from electrocution.
When looking more close at the behaviour of the earth (soil) in case of a current flowing into the earth grid resulting from a fault the undermentioned situations are of importance.
Earth potential rise (EPR) or ground potential rise (GPR):
Since the earth impedance is not zero a current through earth creates a potential difference between the point where the current enters earth (via the earth grid) and a distant point. Therefore the potential difference is the highest between a distant point and the point where the current enters the ground. This effect is called Earth potential rise (EPR) or Ground potential rise (GRP).
GPR is a concern in the design of electrical power systems because the high potential rise may be a hazard to people and/or equipment. Since the current flowing trough a person determines the level of hazard many factors are to be taken into account such as: available fault current, soil type, soil moisture, temperature, underlying rock layers, and clearing time to interrupt a fault.
If the ground connection between a metal object and the earth has a high impedance (common with some soil conditions), the object itself and any conductive item (a person) touching the object can be energized. The touch voltage is the potential between the energized object and the feet of a person standing on the soil touching the energized object.
When current is flowing from an energizes object to earth, the ground potential rises at the object and a voltage gradient will occur based on the resistivity of the earth, resulting in a potential difference between any two points on the ground. This is called a Step Potential as it can cause voltage difference between a person’s feet. The further away the person is standing from the energized object (point where the current flows into the earth) the less is the step voltage and therefore the risk of personal hazard.
At (high)power and substations measures are taken to reduce the risk of (high)touch and step voltages by placing some insulation material (pebbles) on top of the soil. This reduces the danger from electrocution by a current flowing trough a person resulting from touch or step voltages.