Soil resistivity testing literally collects data on how well (or how poorly) the local earth conducts electrical current. Since we expect the grounding system to dissipate electrical current to the earth, it makes sense that we would utilize resistivity data of the local soil to design the ground system that serves as the connection between both the electrical distribution system and equipment to earth.
Grounding system design has several benefits. All of them accrue from the fact that the engineering design process results in predictable grounding system performance. A major benefit is that this predictability removes the guesswork from ground system performance. You will know prior to the ground system installation what performance will be achieved. The design tells you how many electrodes should be used, where they are to be installed, how deep they must be and how large the grounding ring (if required) must be to meet the performance objective.
An effective grounding system design also avoids the unnecessary costs of overkill or having to fix/enhance the grounding system when the performance objective is not met. In our experience we have seen the extremes of both. A few years ago, an electrical contractor in Indiana (with excellent soil) installed a huge buried ring/grid of driven rods to ensure that his telecom customer did not need XIT systems. A single 10 XIT system would have met their 5Ω requirement. The cost to this customer was about 25 times higher than necessary.
The reverse situation is installing the grounding system and not meeting the performance requirement. It is always far more expensive to fix what was not done right the first time. No one would be pleased to discover that the just completed 50 X 50 foot buried ground ring did not meet the performance objective and, had it been 60 X 60 feet, the objective would have been met.
Grounding system resistance is determined by the soil resistivity and the size of the grounding system. The scientific but simple formula is R = ρ (L/A), with R representing the resistance of the grounding system, ρ the soil resistivity, L the conducting length of the path, and A the surface area of the grounding system. The formula can be simplified by dropping the L (important when considering conductors, not grounding systems), making the formula R = ρ/A.
In some situations, soil resistivity is so high that a 5Ω grounding resistance is not possible to achieve or certainly not economically feasible. When this happens, it means that ρ is so large that A cannot be made large enough to overcome the high resistivity and the grounding resistance objective R cannot be met. Even in this case, using a valid grounding system design approach demonstrates to the interested parties that the best effort has been made.
Soil resistivity is the resistance to electrical current flow across a cubic meter of earth. The current source is bonded to one square meter plates on each side. This ensures the complete volume of earth is available for the current to flow in. The unit of measure is generally ohms-meter and sometimes ohms-centimeter (ohms-meter X 100).
Resistivity is a characteristic of a material. It is measured in ohms-meters. It differs from resistance in that it is a measure of resistance per specific volume of material. Generally, the resistivity of materials does not change.
Resistance is a point-to-point measurement. For example, the resistivity of the copper material making up a 4/0 AWG conductor will remain the same along a 100-foot section. However, the resistance of the 4/0 AWG conductor when measured from one end to the 50-foot point would be one half the value when measured along the entire 100-foot section.
The first factor that determines soil resistivity is the type of soil. Generally speaking, sand and gravel conduct poorly and clays conduct well. IEEE documents show a very broad range of resistivities for a specific type of soil. For instance, clays vary from 200 to 10,000 ohms-centimeter, or a factor of 50. Limestone can vary between 500 and 400,000 ohms-centimeter or a factor of 800.
With such ranges, just knowing the soil type is not sufficient information for the design of a grounding system. The soil itself serves as the starting point for resistivity. From that point, the resistivity varies as a function of the moisture content, the electrolyte content and the temperature. In general, the more of each, the lower the resistivity will be. Each of these quickly reaches a point of diminishing returns, at which point more provides little or no additional benefit. When soil freezes, resistivity becomes exponentially higher.
Essentially, none of these factors can be changed in the field, so soil resistivity testing tells you what you need to know. It does not matter why the resistivity is what it is; it only matters that we define the number as precisely as possible.
Reaching the Water Table
Much is often made about “reaching the water table” with the grounding system. Sometimes that might improve grounding resistance and sometimes it might not. One thing is always true; you can only depend on real data provided by real testing and a design based on real data. One might spend a fortune reaching the water table and get no benefit.
Years ago, one of our customers purchased a 400-foot XIT system to reach the water table. This was a classified government project and the cost was significant. While we were not involved in the design, I suspect we would have suggested something else with an engineered design based on soil resistivity data.
Another time we were asked to analyze/test the grounding at a telehotel on the Florida coast. The local consultant had designed a ground ring of about 70 feet with several driven rods. He was convinced that it was less than 1 ohm resistance because of the “brackish” water table at five feet. We found the ground system resistance to be in the 45-50 ohm range. Soil resistivity testing showed high-resistivity soil, not good soil as the consultant thought.
We have found this situation on several occasions and believe that the brackish water is often stripped of electrolytes when filtered through the earth. Remember that distilled water has no electrolytes and therefore is very resistive. Again, this is the reason we believe in testing to define the actual resistivity.
Testing to determine resistivity of the soil is a simple concept. It takes a special purpose meter, probes, hammer, coils of conductors, measuring tape and paper and pencils.
The soil resistivity test meter is known as a “4-Pole Meter” and functions as a current source. It generates enough voltage (most up to a max of 48) to generate the current the meter or operator selects. The resistance the meter sees exists in the conductors (minimal) and the probes-to-earth interface (lots).
Typically, probes are 18 inches long and made of stainless steel. Their purpose is to establish an electrical contact between the meter and earth. They allow the meter to inject the current and measure the resulting voltages. Probes are driven into the earth in a straight line, in equal spacing. Generally, we start at five feet of spacing so probes would be installed at 0, 5, 10 and 15 feet.
The meter forces current through the first probe, the earth, the last probe and back to the meter. Because the earth is semi conductive, a voltage drop develops. The meter also measures this voltage across the second and third probe. The meter then shows the amount of current flowing and the voltage that the current develops (two of the three unknowns in ohm’s law) and simply reads out the resistance in ohms.
Since the result is in ohms (a resistance) and we are looking for ohms-meter (a resistivity), a characteristic of the soil, a conversion must be performed. The following formula converts the resistance reading to ohms-meter: ρ = Probe spacing (feet) * meter reading * 1.915.
For example, if our reading was 9.80 ohms at the five feet probe spacing, then our resistivity would be 93.835 ohms-meter. An important concept to remember is that the 93.835 ohms-meter is the average resistivity between the surface and a depth of five feet (equivalent to the probe spacing).
It should also be mentioned that the testing current in all meters is a reversing DC. True DC polarizes the grounding and provides inaccurate readings. The frequency of most meters is 128 hertz. This frequency was selected because it is not a harmonic of power frequencies. The better meters have variable test frequencies so that adjustments can be made when interference is encountered.
As the current leaves probe number one, it forms a reverse funnel. All the current leaves the small area of the probe and immediately starts expanding, flowing through the widest cross-sectional area available (more parallel paths) before it starts “funneling” in to collect at the fourth probe on its way back to the meter. The area with the widest cross-sectional is where we want to measure the voltage drop because the current is flowing through not only the shallow earth, but also the deepest it can reach with the probe spacing being used.
Now we know the average resistivity down to five feet, but we still need a lot more information on other depths. Because a large grounding system will utilize soil at depths much greater than typical electrode depths, information much deeper than the five-foot depth needs to be known.
After performing the five-foot test, we repeat the test with probe spacing of 10, 15, 20, 30, 40, 60, 80 and out to a minimum of 100 feet. At each probe spacing we determine the average soil resistivity between the surface and a depth equivalent to the probe spacing.
One line (probe spacing of 5, 10, 15, 20, etc. feet) of testing is not enough. We prefer a minimum of three lines within the typical site. Starting in one corner, we would test down one side, then at a 45-degree angle, then at a 90-degree angle.
One reason for this test procedure is to have enough data for the prediction of grounding performance. Secondly, if a metallic pipe had been buried down one side of the site, the results will be seriously compromised on that line of testing. Remember that current will take the path inversely proportional to the impedance it sees. If a metallic pipe is available, not much current will flow through the earth that we are testing.
An important reason not yet mentioned for performing soil resistivity testing is that soil resistivity never remains consistent with depth. In our experience, the resistivity of the soil will always get either better or worse with depth. If the soil resistivity is better with depth, the ground system resistance can be improved by installing deeper electrodes. If the soil resistivity gets worse with depth, deeper electrodes will be unlikely to lower the resistance.
When designing grounding systems, soil resistivity testing is a necessity. Any design without specific soil resistivity testing is nothing more than a guess.
About the Author: John Howard, Vice President of Business Development at Lyncole XIT Grounding, has been with Lyncole since 1991. Utilizing his training in electricity and electronics, he has worked with both national and international telecommunications carriers and municipalities, developing and refining their grounding procedures, testing methods and electrical protection designs. Howard is also a frequent speaker at conferences and companies on grounding system design and proper testing.
Editor’s Note: This is the first article in a two-part series in Incident Prevention. Part 2 will focus on available options for the ground system design process and the design result.