IEEE Std 81-2012 is the guide to the methods for measuring soil resistivity, the impedance of a grounding system and the potentials on the earth surface. It sets no acceptance values: it explains which method suits each goal, which errors to expect and how to check the integrity of the grid.
Short answer. IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System, brings together the methods used to measure a grounding system. It covers three quantities: soil resistivity, the impedance of the ground system with respect to remote earth, and the potentials on the earth surface, including step and touch voltages. It is the measurement guide that goes with IEEE 80, the design guide.
What the guide covers
Clause 1.1 lists nine topics:
- Establishing safe test conditions.
- Measuring soil resistivity.
- Measuring the power-frequency resistance or impedance of the ground system with respect to remote earth.
- Measuring the transient (impulse) impedance with respect to remote earth.
- Measuring step and touch voltages.
- Verifying the integrity of the grounding system.
- Reviewing the usual test methods and procedures.
- Reviewing the characteristics and limits of the instruments.
- Reviewing the factors that distort measurements.
The purpose (clause 1.2) is to present practical instrument-based methods for electric utility facilities. Those grounding systems range from a few rods to large grids with many rods, buried conductors and external connections: shield wires, cable shields and neutrals, counterpoise, metallic pipes and other additional paths to remote earth. The guide also helps interpret the data and describes tests that protect people and property.
What it is measured for
Clause 4 separates two groups of goals.
Soil resistivity is used to estimate the impedance of the grounding system and the potential gradients, including step and touch voltages. It is also used to calculate inductive coupling between power and communication circuits, design cathodic protection, design AC mitigation between lines and pipelines, and carry out geological studies.
Impedance and potential gradients are used to verify that a new grounding system is adequate and to detect changes in an existing one. They are also used to identify hazardous step and touch voltages and to determine the ground potential rise (GPR), with which power and communication circuits are protected.
Which method goes with which goal
| What you need to know | Method in the guide | Clause |
|---|---|---|
| Soil resistivity over a large volume | Four-point, with equal (Wenner) or unequal (Schlumberger-Palmer) spacing | 7.2.3 |
| Resistivity near a rod, at different depths | Driven-rod depth variation (three-point) | 7.2.2 |
| Resistance of a small electrode | Two-point or three-point | 8.2.2.1 and 8.2.2.2 |
| Impedance of a grid or a large electrode | Fall-of-potential (62 % rule, slope method) | 8.2.2.4 |
| Resistance of an electrode within a system with many grounds in parallel | Clamp-on (stakeless) or a combination of fall-of-potential and clamp-on | 8.2.2.5 and 8.2.2.6 |
| Impedance with noise and interconnected systems | Computer-aided grounding multimeter | 8.2.2.7 |
| Step, touch and transferred voltages | Staged fault, current injection, conventional meter or computer-aided multimeter | 9.4 |
| Continuity between conductors and down-leads | High current, resistance between down-leads, computer-aided multimeter | 10.2 to 10.4 |
| How current divides between shield wires and neutrals | Current division measurement with split-core current transformers | 11 |
| Behavior under lightning | Transient impedance with an impulse generator or a wideband meter | 12 |
How it is organized
The guide has twelve clauses and six informative annexes. Clauses 1 to 6 cover scope, definitions, goals, safety and general measurement problems. Clause 7 deals with resistivity; 8, impedance; 9, step and touch voltages; 10, integrity; 11, current division; and 12, transient impedance. The annexes develop nonuniform soils (A), determining a soil model (B), fall-of-potential theory (C), the resistivity of surface materials (D) and instruments (E). The last is the bibliography (F).
What to keep in mind
Two ideas run through the whole guide. The first is that there is no single correct method: the choice depends on the size of the grounding system, on what is buried nearby, on the electrical noise at the site and on the data you want to obtain. The second is that each method has assumptions. The 62 % rule, for example, only holds with uniform soil and an electrode with no external connections. A result without those assumptions in view can be misread.
The guide also warns that a lethal voltage can exist between the electrode under test and a distant ground, even under normal operating conditions. That is why it devotes clause 5 to safety, before any method.
The limit values are not here. The safety of a grid is judged with the tolerable voltages of IEEE 80 and with the applicable code and the utility’s requirements. The measurement provides the real data for that comparison.
A practical way to use the guide
If the question is about design, start with the substation grounding grid guide, which builds on IEEE 80, and with the ground resistance test, which summarizes what the test measures. If the question is about the safety of people, read the guide on step and touch voltage. For a first calculation with field measurements, the grounding resistance tool fits a soil model to Wenner readings. If the reading does not match the design calculation, see Calculated and Measured Earth Resistance: Why They Differ. Verifying the integrity of the installed grid is covered in Potential Map and Integrity of the Installed Grid.
We measure your grounding system
At Tiza Group we measure resistivity, grid impedance, step and touch voltages and continuity in substations, industrial plants and solar plants, from low to extra-high voltage, in Colombia, or elsewhere on request. We choose the method according to your installation and deliver the interpreted result. See our testing and commissioning service or write to us through contact. In the quotation we recommend the test and the equipment your case needs.
Related guides
- Safety When Testing an Energized Grounding System
- Earth Measurement Errors: Stray Currents and Coupling
- Soil Resistivity Measurement: Wenner Method
- Soil Resistivity: Schlumberger and Other Methods
- Two-Layer Soil Model from Measurements
- Substation Gravel Resistivity: How It Is Measured
- Fall-of-Potential: the 62 % Rule Explained
- Slope Method for Large Grids (Tagg)
- Measuring a Ground with Two or Three Points: When It Works
- Clamp-On Ground Measurement, Without Stakes: Limits
- Computer-Aided Grounding Multimeter
- Calculated and Measured Earth Resistance: Why They Differ
- Measuring Step and Touch Voltages in a Substation
- IEEE 80: Safety in Substation Grounding
- Ground Resistance Test: Fall-of-Potential and IEEE 81
- Substation Grounding Grid: Design and Testing
- Step and Touch Voltage: Difference and IEEE 80
- Grounding of Distribution Substations: Design and Tests
- Industrial Grounding: Design, Testing and Codes
- Substation Primary Equipment Testing: Guide by Equipment
- Standards for Testing Primary Substation Equipment
References
- IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System: clauses 1.1 and 1.2 (scope and purpose), 4.1 and 4.2 (goals), 7 to 12 (methods) and annexes A to F.
- IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding (normative reference of IEEE 81).
Frequently asked questions
Does IEEE 81 set a maximum ground resistance value?
No. It is a guide to measurement methods. The value your installation must meet is set by the applicable code, the utility or the grounding design, which is done with IEEE 80. The measurement only tells you what value the ground has today.
Is the guide only for substations?
No. It is written for electric utility facilities, but its methods apply to pole grounds, tower grounds, industrial plants and generating stations. Each method indicates which size of grounding system it suits.
Is a single resistance measurement enough to accept a grid?
No. Resistance gives a quick idea of the potential rise, but it does not say whether the step and touch voltages are tolerable or whether all the grid conductors are connected. The guide describes other measurements for that.