Traditionally, the trial-and-error method was used and could be enhanced by an individual’s experience. This consists of designing the ground during the process of installation, and repeatedly testing it in progress, until the desired spec has been met.

For example, a rod can be driven then tested. A second rod can be coupled to the first, driven deeper, and tested again. This procedure is repeated until spec is met. Similarly, a second rod can be added in parallel (driven into the soil separately and connected by a conductor rather than coupled end-to-end) and tested. Additional rods can then be added to form a ground bed until a sufficiently low resistance is obtained.

The trial-and error method is still frequently used and often works well. But its limitation is that it is subject to the Law of Diminishing Returns: more and more work for less and less reward.

In optimal soil conditions, a satisfactory ground can be achieved with only a few retests. But in more difficult environments, one can end up wasting the day without realizing the goal.

Best practice in power substation grounding

The ground grid

The substation grounding system is an essential part of the every electrical system. The proper grounding of a substation is essential and very important for the following two reasons. First, it provides a means of dissipating electric current into the earth without exceeding the operating limits of the equipment.

Second, it provides a safe environment to protect personnel in the vicinity of grounded facilities from the dangers of electric shock under fault conditions.

The grounding system includes all of the interconnected grounding facilities in the substation area, including the ground grid, overhead ground wires, neutral conductors, underground cables, foundations, deep well, etc.

The ground grid consists of horizontal interconnected bare conductors (mat) and ground rods. The design of the ground grid to control voltage levels to safe values should consider the total grounding system to provide a safe system at an economical cost.

The following information is mainly concerned with personnel safety. The information regarding the grounding system resistance, grid current, and ground potential rise can also be used to determine if the operating limits of the equipment will be exceeded.

Safe grounding requires the interaction of two grounding systems:

The intentional ground: consisting of grounding systems buried at some depth below the earth’s surface

The accidental ground: temporarily established by a person exposed to a potential gradient in the vicinity of a grounded facility

It is often assumed that any grounded object can be safely touched. A low substation ground resistance is not, in itself, a guarantee of safety. There is no simple relation between the resistance of the grounding system as a whole and the maximum shock current to which a person might be exposed.

(a) Exposure to touch voltage; (b) Exposure to step voltage

A substation with relatively low ground resistance might be dangerous, while another substation with very high ground resistance might be safe or could be made safe by careful design.

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.