Earthing System Design in AC Substations: IEEE STD 80 Requirements

Additional High Resistivity Layer
I. Earthing Design

Within electrical substations, a distance should be maintained between any energised conductor and any other part of the substation, including the earth. The minimum distance, known as a “safety distance”, is calculated with regards to the characterisation of the substation components and other factors that consider the operator’s movement across the site.

Having complete information about the substation helps in the design of an appropriate earthing installation. Indeed, the position of each component and associated equipment is considered with respect to the safety distance. Figure 1 illustrates a representation of the earthing system surrounding the equipment.

The earth grid design is influenced by many other parameters and factors such as the characterisation of fault sources and soil electrical parameters. From a safety aspect, designing a safe protective earthing system is essential in order to provide the shortest path for the effective flow of fault currents without exceeding the set operations and equipment limits.

Representation of Earthing System and Equipment Position (Not a real case)
Figure 1. Representation of Earthing System and Equipment Position (Not a real case)

In this light, national and international standards, as well as local regulations have been developed to identify the requirements of the earth grid design and define the relevant parameters for structures, electrical equipment, and systems therein. Figure 2 shows a simplified flowchart from the IEEE Std 80. This chart groups the design procedure into four main stages.

Several iterations are usually performed to achieve the requirements of safety given by the standard regulations and rules. Accurate field data (e.g., soil resistivity) as well as acceptable ranges for design variables (e.g., covered area, cost, . . .) are required for the considered earthing system.
Design Procedure of Earthing Systems
Figure 2. Design Procedure of Earthing Systems
II. Design Initial Stage

In the first stage, details of the project are essential. For instance, the general location plan of the substation should provide good estimates of the area to be covered by the earth grid.

Multiple visits to the substation site are required to conduct the soil resistivity measurements. A series of measurements should be established using the four-pin method as represented in Figure 3.

In each measurement, the distance between electrodes is changed for vertical depth in the soil, while changing the horizontal position helps to check the change across the site. This process helps determine the soil structure as well as estimate an equivalent model of the soil resistivity.

A uniform soil resistivity model is the simplest representation that can be considered, which provides a single value of soil resistivity. However, this is not the representative of real life. For more accurate interpretation of soil resistivity measurements, other models can be used, namely: 1D multi-layer, 2D and 3D models.

Measurement Using Wenner Configuration
Figure 3. Measurement Using Wenner Configuration
III. Design Basic Computations

Corrosion (of different types) is one of the most important aspects that must be considered in the design of an earth grid. It can damage both the earthing conductors (conductor cross section) and the connection accessories. Consequently, fault and/or lightning currents cannot be dissipated effectively into the soil, adversity affecting the safety of people and the system operation.

For this reason, the conductor material should be selected so that soil conditions are not corrosive, maintaining the integrity of the earthing system for years (if the conductors are of adequate size). The shape of the conductor can be cylindrical, plate or a combination of both types as shown in Figure 4.
Representation of Different Earthing Conductors
Figure 4. Representation of Different Earthing Conductors

Earthing conductors should be also able to conduct the maximum fault currents during the occurrence of the faults (clearing time and backup). Various factors are included in the sizing of conductors such as the material thermal properties, current capacity, and impedance as well as the soil characterisation.

The next phase of the initial stage consists in determining the touch and step criteria. The maximum driving voltage of any accidental circuit should not exceed the limits defined by IEEE Std 80. The tolerable touch and step voltages are determined according to the characterisation of the surface material and the maximum expected fault current (magnitude and duration). 

Usually, a protective surface layer of high resistivity (e.g., gravel) is used (as represented in Figure 5), to minimise the current passing through the human body, providing a safety to individual inside the substation.

Additional High Resistivity Layer
Figure 5. Additional High Resistivity Layer

At this stage a preliminary design can be determined. According to IEEE Std 80, this design should include a conductor loop surrounding the entire grounded area, plus adequate cross-conductors to provide convenient access for equipment grounds, etc.

It is worth noting that the conductor spacing distance and earth rod positions can be estimated in accordance with the fault current in the substation.

III. Verification Stage

The verification stage basically consists of controlling the safety measures within the substation. For example, Earth Potential Rise (EPR), also known as GPR, should be below the tolerable touch voltage computed in the initial stage. If this condition is verified, no further analysis is necessary at this level. In most cases, additional conductors are required in specific locations to bring voltages below tolerable limits. The design should also be rectified if the computed touch and step voltages are higher than the tolerable voltages.

EPR is a function of the earthing resistance and characteristics of fault current passing through the system into the earth. Therefore, earthing resistance, surface potential and current distribution in the earthing system should be estimated. In addition, the mesh voltage should be below the tolerable touch voltage, or the preliminary design should be modified. Figure 6 shows the potential profile on the substation surface for two types of earthing grid.
Additional High Resistivity Layer
Figure 5. Additional High Resistivity Layer

Resistance to earth of the grid as well as the mesh and step voltages of the preliminary design can be estimated using a uniform soil model. However, more accurate estimates can be made by using computer tools and considering different aspects (e.g., frequency of the electric current). Analysis based on modelling the components of the earthing system and a multi-layer soil resistivity profile can determine the outcome with high precision.

IV. Modification Stage

The earth grid design can be reviewed many times before delivering the final version. In principle, these revisions aim to meet all the requirements and regulations for general safety of individuals and installations. A revision of the grid design is required when either the step or touch tolerable limits are exceeded.

In practical cases, engineers may not reach the desired resistance of the earthing system because it may be affected by different aspects such as the seasonal variation of the soil resistivity, difficulties raised from subsurface objects, mechanical deformation in the vicinity, and so on.

According to the IEEE Std 80, the earthing design should  be reviewed to eliminate hazards due to transferred potential and hazards associated with special areas of concern such as Communication circuits, rails, piping, fences and so on.

V. Final Stage

The final stage comes after fulfilling the requirements for the step and touch voltages. It consists in the enhancement of the earthing system through additional grid/rods in selected locations (e.g., rods in the earth grid corners). Additional rods can be driven in the area near the base of surge arresters and transformer neutrals (if applicable). Further earth grids/horizontal conductors may be required if the main earthing system does not include certain parts of the substation.

For any further questions, please don’t hesitate to reach out to us at [email protected]. Or contact us through our website to submit a query.


[1] IEEE Std 367:2012, IEEE Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault.
[2] IEEE Std 80:2013, IEEE Guide for Safety in AC Substation Grounding.
[3] IEEE Std 81:2012, IEEE Guide for measuring earth resistivity, ground impedance, and earth surface potentials of a ground system.
[4] Zhang et al., “Research Advances of Soil Corrosion of Grounding Grids”, Micromachines 2021, 12, 513. DOI: 10.3390/mi12050513.

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