Substation Grounding Grid: Design and Testing

Short answer

The grounding grid is the network of buried conductors that bonds all equipment and structures of a substation or plant to earth. Its main purpose is safety: limiting step and touch voltages during a ground fault. It also provides the return path that ground protection needs and the reference for surge arresters and electronic equipment.

Short answer. The grounding grid (earthing grid) is the network of buried conductors that bonds all the equipment and structures of a substation to earth. Its job is to keep the potential difference between points a person can touch or stand on within tolerable values during a ground fault. It is designed with IEEE 80 and measured with IEEE 81, and the design is adapted to the local code and the utility’s requirements.

What a grounding grid is

The grounding grid is a network of bare conductors buried under the whole area of a substation or plant, completed with vertical rods and connected to every piece of equipment, structure, fence and shield wire. It has three functions:

Safety of people. During a ground fault, the current entering the soil raises the potential of the grid. The grid must spread that potential so that the difference between hand and feet (touch voltage) or between the two feet (step voltage) stays within tolerable values.
Fault return path. Ground protection needs a current large enough and predictable enough to operate.
Common reference for surge arresters, cable screens, instrument transformers and electronic equipment.

Main parts

Horizontal conductors in a grid pattern, normally copper or copper-clad steel, buried at shallow depth.
Vertical ground rods, which reach deeper and more stable soil layers, especially at the perimeter.
Permanent connections, with exothermic welds or qualified compression connectors (IEEE 837 covers their qualification).
Down conductors that connect each piece of equipment, structure and cabinet to the grid.
Surface layer of crushed rock, which increases the contact resistance between the feet and the soil.
Grounding of the perimeter fence, coordinated with the grid to avoid dangerous touch voltages from outside.
Inspection wells for measurements and connection checks.

How it works

When a ground fault occurs, only part of the total current enters the soil through the grid; the rest returns through shield wires, neutrals and cable screens. That fraction, multiplied by the grid impedance, gives the ground potential rise (GPR).

The IEEE 80 guide sets out the most widely used method:

Grid design method according to IEEE 80

Soil resistivity. It is measured, normally with the four-electrode Wenner method (IEEE 81), and a soil model is built, often a two-layer model
Tolerable step and touch voltages. They depend on the assumed body weight (50 kg or 70 kg), the fault duration and the resistivity of the surface layer
Design mesh and step voltages. They are calculated and compared with the tolerable ones. If they do not comply, the grid is made denser, rods are added, the area is enlarged or the surface layer is improved

The fault current and the clearing time come from the short-circuit and protection coordination studies. For that reason, when the short-circuit level of the network grows or the protection times change, the grid design must be reviewed.

Most jurisdictions require an earthing system designed to control step and touch voltages and set requirements for materials, connections and measurements. The grid design is adapted to the local code and to the requirements of the utility.

Types

Extra-high and high voltage: large grids with high fault currents, where control of the GPR and of transferred voltages is critical. Teleprotection and outgoing communications usually run on optical fiber so that potential is not transferred.
Medium voltage: distribution substations and switchgear, with smaller grids and, sometimes, high-resistivity soils that call for solutions such as deep rods.
Industry: the substation grid is integrated with the grounding of the plant, the motors, the cable trays and the lightning protection system. Good practice is to keep all the grounding of an installation interconnected.
Solar plants: grids of large area with many kilometers of conductor, where care is taken with the interconnection between the substation, the transformer stations and the structures; the whole is covered in grounding of a solar plant.

Relationship with protection

The grid and the ground protection work together. The current seen by ground overcurrent 50N/51N and the directional ground element depends on the return path, and the system’s zero-sequence current depends on how the neutrals are grounded, for example with a grounding transformer. In addition, CT and voltage secondary circuits must be grounded at a single point to avoid circulating currents during a fault.

Commissioning and maintenance tests

Soil resistivity before the design, by the Wenner method, in several directions.
Grid impedance by fall of potential, with the current electrode far enough away (several times the grid dimension). On large grids, injection at a frequency different from the power frequency is used to separate the signal from noise.
Measurement of step and touch voltages by injecting current and measuring at the critical points, to validate the design.
Continuity between down conductors and grid, with high-current injection and measurement of resistance or voltage drop between each piece of equipment and a reference point.
Inspection of connections, down conductors and surface layer; in maintenance, sampling excavations to check for corrosion.

These measurements are complemented by tests of the connected equipment, such as those for surge arresters. For these measurements there are earth testers such as the Metrel MI 3152 and MI 3155 and, for large grids, analyzers with more current and step and touch voltage measurement, such as the Metrel MI 3290.

IEEE 80 safety limits

IEEE 80 sets, as the design basis, the safe limits of potential difference that can exist in a substation during a fault between points a person can touch, and gives a practical design procedure with those criteria. These are the points most often misapplied:

One person and two grounding systems. The guide treats the interaction between the intentional system (the buried electrodes) and the accidental one that forms temporarily when a person is exposed to a potential gradient near a grounded installation.
Body current limit (Dalziel). The current that 99.5 % of people can withstand without ventricular fibrillation is 0.116/√t for 50 kg body weight and 0.157/√t for 70 kg, with t the duration of the current in seconds. The calculation takes 1000 Ω for the body resistance and a resistance of each foot equal to 3 times the surface resistivity. For physiological effects, the guide gives perception from 1 mA, inability or difficulty to let go of an object between 9 and 25 mA, and ventricular fibrillation between 60 and 100 mA, for sustained current.
Tolerable voltages. Touch voltage is (1000 + 1.5·Cs·ρs) multiplied by the body current limit, and step voltage is (1000 + 6·Cs·ρs) multiplied by the same limit, with ρs the resistivity of the surface layer in Ω·m and Cs a reduction factor. With no protective surface layer, Cs = 1 and ρs is the soil resistivity. Because time is in the denominator, a slower fault clearing lowers the tolerable voltage significantly: the operating time of the protection is a design input of the grid, not a later detail.
Cs factor. It depends on the resistivity of the surface material, its thickness and the resistivity of the soil below, through a reflection factor. That is why the rock layer is only worth what its design thickness and resistivity are, and it is easily contaminated or removed.
Transferred voltage and GPR. The GPR is the maximum potential the grid reaches relative to a theoretical remote earth, at infinite distance and zero potential. Transferred voltage is the voltage carried to a remote point outside the substation. Mesh voltage is the maximum touch voltage within a mesh of the grid.
What the guide calculates, and its limits. It includes approximate equations for grid resistance, as a function of area, depth and total conductor length. It also includes equations for the current in the grid, with current division curves, and for step and mesh voltages, with spacing factors. All of them have geometric limits and assume uniform soil. The design criterion is that the actual mesh and step voltages stay below the tolerable ones; if not, the design is redone. In stratified soils or irregular grids, the design is usually supported by software that models arbitrary geometries.

For field diagnosis when measured voltages are high, see high step and touch voltages in the grid and earth resistance test. For the concept, step and touch voltage.

Measurements according to IEEE 81. The guide covers the measurement of soil resistivity, grounding system impedance and surface potentials. For resistivity, the Wenner method uses four equally spaced electrodes in a line, with apparent resistivity ρa = 2π·a·V/I, where a is the spacing. For the resistance of a grid, the fall-of-potential method places the current probe far away and moves the potential probe along the line between the grid and that probe. See also industrial grounding.

Common failures

Theft of copper conductors from down conductors and fences, frequent at unattended sites.
Broken or corroded connections, especially mechanical joints in aggressive soils.
New equipment not connected to the grid after an expansion.
Contaminated or removed rock layer, which lowers the tolerable voltages.
Undersized grid for the present short-circuit level.
Transferred voltages through pipes, rails or metallic cables leaving the substation.
Poorly made measurements: electrodes too close together or interference from other grounding.

We support you with your grounding grid

At Tiza Group we measure and verify grounding systems in substations, industry and solar plants, and relate them to the short-circuit study and protection coordination. See our testing and commissioning and primary engineering services, or contact us if you need a measurement or a review. Field measurements are done in Colombia, or elsewhere on request.

References

IEEE 80: guide for safety in AC substation grounding.
IEEE 81: guide for measuring earth resistivity, ground impedance and earth surface potentials of a grounding system.
IEEE 837: standard for qualifying permanent connections used in substation grounding.
IEEE 142: grounding of industrial and commercial power systems.
IEC 61936-1: power installations exceeding 1 kV ac.
IEC 60479-1: effects of current on human beings and livestock.

Frequently asked questions

Is a low grid resistance enough to make a substation safe?

No. Safety is defined by the step and touch voltages compared with the tolerable ones, which depend on the current actually injected into the soil, the fault clearing time and the surface layer. A low resistance helps, but a poorly distributed grid can have dangerous touch voltages at the corners.

Why is a clamp-on earth tester not used to measure a substation grid?

The clamp method needs a return path through other earthing in parallel and measures the loop, not the grid alone. On a large grid, use the fall-of-potential method with distant electrodes, or a current injection at a frequency different from the power frequency.

Why does the clearing time of the protection affect the grid?

Because the current the human body tolerates depends on the exposure time. If the protection clears slower than assumed in the design, the tolerable voltages drop and the grid may stop being safe.