The calculated value of a grounding system and the measured one rarely match. IEEE 81 lists five causes: the calculation method, the resistivity on the day of the measurement, an insufficient resistivity survey, buried metallic structures and the error of clamp meters. The gap shrinks by measuring resistivity and resistance under similar weather conditions.
Short answer. A grounding system is calculated with a model of the soil and the electrodes, and measured with an instrument on a particular day. It is normal for the two values not to match. IEEE Std 81-2012 (clause 8.1.2) lists five causes: the limitations of the calculation method, the soil resistivity on the day of the measurement, an insufficient resistivity survey, buried metallic structures, and the error of clamp meters. Understanding them makes it possible to tell whether the difference is a defect in the grid or an effect of the measurement.
What the impedance of a ground is
The guide defines ground impedance as the vector sum of resistance and reactance between an electrode, a grid or a system and remote earth. Ground resistance is that same impedance without the reactance. Remote earth is a theoretical concept: a zero-impedance electrode located at an infinite distance. In practice it is reached when the mutual resistance between the ground under test and the test electrode is negligible.
Ground connections have resistive, capacitive and inductive components. At power frequency, resistance dominates if the system is small. Capacitance and inductance matter at high frequencies, as in radio communications and lightning, or in very large grounding systems. Isolated pole grounds and most distribution substations, with typical soil resistivities, are predominantly resistive. Large systems, especially in low-resistivity soils, have a significant reactive component. The reactive component increases with the size of the grid and, in particular, when it is interconnected with grounded neutrals and shield wires. It must be determined when the analysis involves impulse currents.
What determines the value
The impedance depends on the resistivity of the surrounding soil and on the extent and configuration of the buried electrode. The ground can combine dry soil, clay, gravel, slate or sandstone, with very different resistivities. It can be homogeneous over a large area or layered, so that it is almost isolated from its surroundings. That is why impedance varies with the season: temperature, moisture and soil density change.
According to the guide, in a given soil, the effectiveness of a grid depends mainly on its total size and on the soil resistivity. Adding conductors and rods to an existing grid helps somewhat to reduce its impedance, but each addition reduces it less and less. The guide also observes that the resistance of a grid seems to have little relation to the resistivity at the burial depth, especially if it has long rods in contact with deep soil. This will not be so if the grid lies over a high-resistivity stratum, such as bedrock, or in permanently frozen soil. A practical consequence: the resistivity data must include the deep layers, not only the surface one.
The five causes of the difference
| Cause (clause 8.1.2) | What happens |
|---|---|
| a) The analytical calculation methods are insufficient | The model does not represent the grid geometry or the real soil well |
| b) The resistivity on the day of the measurement is not the one assumed | The soil changes with the weather and the real value is not the one used in the calculation |
| c) The resistivity survey was insufficient | Few measurements, poorly distributed, with short probe spacing or inadequate instruments |
| d) There are metallic structures and ground wires near the test site | They divert an important part of the test current |
| e) Clamp meter error | The reading is wrong if the reactance of the test circuit is significant compared with its resistance, or if the filters do not remove the 50 Hz or 60 Hz of stray currents |
On cause e), the guide specifies that clamp meters are especially sensitive to reactance because their test frequencies range from 1 kHz to 3.4 kHz. If there are ground loops, the clamp may not measure the impedance that was intended.
How to reduce the difference
The guide states that the difference between the measured and calculated value is minimized if the soil resistivity and grid resistance measurements are made under similar weather conditions. For grids that change with the seasons, it is prudent to make the resistivity measurements under adverse weather conditions, and so obtain conservative values for the site.
Two other observations in the guide help with interpretation. To compare with calculated values, the impedance can be measured before interconnecting external shield wires, metallic pipes and other interference. And if the grid is large, with an impedance below 1 Ω, it may have a relevant reactance, so it is measured at a frequency slightly different from power frequency.
What to do with a large difference
A large difference does not by itself mean the grid is wrong. The reasonable order is to review the causes in the table first. If they still do not explain it, test the continuity of the conductors and connections, because a poor connection raises the reading. If the grid turns out not to be the cause, the important comparison is another one: whether the real step and touch voltages are within the tolerable limits.
The reference values for ground resistance depend on the local code and on the utility’s requirements; the guide on the ground resistance test covers what the test measures. To estimate the theoretical value of a rectangular grid from your data, you can use the free grounding resistance tool. The sources of measurement error are described in Ground measurement errors. To cross-check the value there are several methods: fall of potential, the slope method for large grids and the computer-assisted ground multimeter. The resistivity that feeds the calculation is measured with the Wenner method or with Schlumberger and other methods.
We measure your grounding system
At Tiza Group we measure the resistance and impedance of your grounding system, in Colombia, or elsewhere on request, compare it with the design and explain the difference when there is one. See the 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
- Ground measurement errors: stray currents and coupling
- Fall-of-potential method and the 62 % rule explained
- Grounding grid resistance: IEEE 80 formulas
- Soil resistivity: Schlumberger and other methods
- Ground resistance: what the test measures and IEEE 81
References
- IEEE Std 81-2012, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System: definitions (clause 3) and clauses 6.5, 8.1.1 and 8.1.2.
- IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding.
Frequently asked questions
Is it normal for the measured resistance to differ from the calculated one?
Yes. IEEE 81 lists five reasons why the theoretical value can differ from the measured one: the limitation of the calculation methods, the real resistivity on the day of the measurement, an insufficient resistivity survey, buried metallic structures nearby, and clamp meter errors.
Is the impedance of a grounding system the same as its resistance?
Not always. Impedance is the vector sum of resistance and reactance between the electrode and remote earth. Resistance is the impedance without the reactance. In small grounds the reactance is negligible; in large grids, especially if they are interconnected with neutrals and shield wires, it can be significant.
Does the resistance of the grid change over time?
Yes. After installation, the settling of the soil through the annual weather cycles tends to lower it during the first year or two. After that it does not usually vary much from one year to the next, although the seasons change the moisture, temperature and density of the soil.