IEEE 80: Safety in Substation Grounding

Short answer. IEEE Std 80-2013, Guide for Safety in AC Substation Grounding, is an international guide for designing the grounding of an AC substation with one main goal: that no person receives, during a ground fault, more current through the body than they can withstand. It does not stop at setting a grid resistance. It starts from the human body, defines which potential differences are tolerable and proposes a procedure so that the grid does not exceed them.

What it covers and what it leaves out

The scope (clause 1.1) is outdoor AC substations, conventional or gas-insulated. It includes distribution, transmission and generating-plant substations. With due care, its methods also work for the indoor parts of those substations or for fully enclosed substations.

DC substation problems and the quantitative analysis of lightning are left out. The guide focuses on power frequencies of 50 Hz to 60 Hz. Even so, a grounding system designed with it offers some protection against steep-front waves entering the substation.

What it was written for

Clause 1.2 lists its purposes:

1. To establish, as a design basis, the limits of potential difference that can exist during a fault between points that the human body can touch.
2. To review grounding practices with attention to safety and to define design criteria.
3. To give a procedure for designing practical grounding systems with those criteria.
4. To develop analytical methods that help to understand and solve typical voltage gradient problems.
5. To provide reference cases for comparing the guide’s equations with commercial software.

One idea that organizes the whole guide

Clause 4.1 sets two objectives for a safe grounding system. The first is to carry currents into the earth, in normal operation and in a fault, without exceeding equipment limits or affecting service continuity. The second is to reduce the risk that a person near the grounded facilities is exposed to a critical electric shock.

To achieve this, the guide separates two interacting systems. The intentional system is the buried electrodes. The accidental system is the one temporarily formed when a person is exposed to a potential gradient. A direct consequence: a low grid resistance, by itself, does not guarantee safety. The guide says so plainly, because there is no simple relation between the resistance of the system and the maximum current that could flow through a person.

How it is organized

The standard groups its content in three blocks, plus annexes:

The annexes include the bibliography, worked calculation examples, the current division analysis, simplified equations for step and mesh voltages, the equivalent uniform soil, a parametric analysis, grounding methods for high-voltage substations with a grounded neutral and the reference cases (benchmarks).

The guides in the series, by topic

Each IEEE 80 topic has its own guide. This list groups them in the order in which they are usually consulted during a design.

Safety of people.

• Electric shock in a substation: how it happens and why: how a ground fault ends up producing a shock.
• Tolerable body current according to IEEE 80: how much current the body withstands as a function of time.
• Accidental ground circuit and body resistance: body resistance and the touch and step circuit.
• Gravel layer in substations: Cs factor and safety: the gravel and the Cs factor.
• Fault clearing time and grid safety: the clearing time that enters the limits.
• Ground potential rise (GPR): what it is and how to calculate it: the potential the grid reaches.
• Transferred voltage: cables, pipes and fences: the potential that leaves through cables, pipes and fences.

Soil and grid resistance.

• Soil resistivity: factors and chemical treatment: what resistivity depends on.
• Soil resistivity measurement: Wenner method: how it is measured with four electrodes.
• Two-layer soil model from measurements: when a single value is not enough.
• Ground grid resistance: IEEE 80 formulas: the formulas of the standard.

Design and calculation.

• Substation grounding design: the IEEE 80 method: the complete procedure, step by step.
• Maximum grid current (IG) and split factor: what part of the fault enters the soil.
• Decrement factor Df and fault asymmetry: the effect of asymmetry.
• Which ground fault sizes a substation grid: which fault sizes the grid.
• Grid conductor size: thermal capacity: thermal capacity of the conductor.
• Ground grid materials and connections: copper, steel, aluminum and their joints.
• IEEE 80 grid calculation example: square grid without rods: a calculation worked from start to finish.

Special cases, construction and verification.

• Substation fence grounding: bonded to the grid or separate.
• GIS substation grounding according to IEEE 80: enclosures, ground bonding jumpers and foundations.
• Grounding of control cabinets, surge arresters and service areas: control cabinets, arresters and service areas.
• Building a ground grid: trenching and plowing: trenching, plowing and work sequence.
• Potential map and integrity of the installed grid: how the installed grid is checked.

Field measurement of resistivity and grid resistance is covered in detail in the IEEE 81 guide and in the ground resistance test guide. The effects of step and touch voltage on an installation in service are in step and touch voltage and in substation ground grid.

The design method at a glance

Chapter 16 brings the procedure together. The site data that weigh most are the maximum grid current, the duration of the fault and of the shock, the soil resistivity, the surface layer resistivity and the grid geometry. The area occupied by the grid is the most influential geometric factor in its resistance: the larger the area, the lower the resistance and the lower the potential rise.

The procedure compares the voltages that the grid produces with those that the body tolerates. Since in design the mesh voltage is usually the worst possible touch inside the substation, the guide takes it as the reference and then checks the step voltage. If the grid does not comply, it is modified (spacing, conductor length, depth, rods) and the check is repeated. The guide specifies the steps and the equations; this article does not reproduce them because they are applied with each project’s own data.

What you can and cannot do with the guide

The guide gives the criterion and the equations. It does not replace the judgment of the designer. Its equations assume uniform soil and defined geometries, and the standard itself warns when more precise methods are advisable, for example with highly non-uniform soils, irregularly shaped grids or large areas.

Nor does it replace the local code. The local code and the utility’s requirements set requirements of their own, which are verified together with it. For a first calculation with your data you can use our free grounding resistance tool, which applies the method of the guide to a rectangular grid.

We carry out your grounding study

At Tiza Group we design the grounding of medium, high and extra-high voltage substations and of industrial plants. We measure or receive the soil resistivity, calculate the grid current with your short-circuit study and verify the step and touch voltages against the tolerable limits. See the engineering design and studies service or write to us through contact.

Related guides

• Substation grounding design: the IEEE 80 method
• IEEE 80 grid calculation example: square grid without rods
• Substation ground grid: what it is, design and tests
• Step and touch voltage: difference and IEEE 80
• IEEE 81: how a grounding system is measured

References

• IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 1.1 and 1.2 (scope and purpose), 4.1 (basic problem), 16 (grounding system design) and annexes A to H.
• IEEE Std 81, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.

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