Soil Resistivity Testing

The experts at ComSite Construction Company, utilizing the Wenner 4-point Soil Resistivity Test Method, accurately model the electrical resistance characteristics of the soil at your site. Our electrical grounding experts know where and how to conduct the tests so the results are accurate and meaningful. Correct soil resistivity information is essential for designing a safe and cost-effective electrical grounding system.Our engineers can help you plan and execute an electrical grounding design that will provide the best performance available within the constraints of your available area, budget, and equipment being protected.To schedule your initial consultation, please contact us or call 864-480-7680.

What is soil resistivity and why does it matter? In essence, it’s the amount of opposition to current flow poised by the soil. Resistance is the inverse of conductance, and though we measure resistance what we’re really after is the conductance. How well will that soil conduct electricity? The conductance is a function of the concentrations of soluble salts and the moisture level in soil. As these increase, conductance does also (and the resistivity, which we measure, decreases). These salts and the moisture are corrosive, so a good grounding system is actually one that gets eaten away (which is why periodic ground testing is essential to maintaining a reliable grounding system).

In short, the lower the soil resistivity the better. But it varies widely from site to site and can vary even at the same site. You can’t know what it is without testing, and you can’t get accurate testing results unless you understand the theory and application of the testing (typically, a person doing ground testing doesn’t).

In the old television series, Star Trek: The Next Generation, the unrelenting Borg were known for their mantra “Resistance is futile.” But for a high performance electrical grounding system design, soil resistivity testing is far from futile. It’s critical that you get this done and done correctly. Accurate soil resistivity data are essential for precise designs and predictable results. Absent these data, the design is really just guesswork that may cost too much or provide too little protection.

For commercial and industrial substations including telecommunications sites the recommended resistance to ground is 5 ohms or less. Depending upon the site location and soil characteristics, achieving that 5 ohm resistance can be very difficult. You can’t just drive a bunch of ground rods to get this. If ground rods get too close to each other, they actually interfere with each other and resistance goes up. The solution is designed, rather than just resulting from driving more rods. The solution might not include rods at all.

According to the IEEE Standard 142-1991, the grounding electrode resistance of large electrical substations should be 1 ohm or less. Achieving that resistance generally requires installing electrode systems that require ongoing maintenance, part of which includes replenishing the electrolytes added around the electrodes.

The type of soil, moisture, electrolytic content, and temperature all affect soil resistance. Frost line depth, water table level, bedrock, and available space dictate some specifics of the grounding system design. We determine soil resistivity using the Wenner 4 Point test method, or by analyzing soil samples.

As part of our electrical grounding system design, the engineers at ComSite Construction Company provide a written recommendation along with a drawing of the site illustrating the recommended grounding configuration, quantity, and location of the electrical grounding electrodes. We detail out the recommended grounding electrode spacing, and we model the recommended installation. To schedule your free initial consultation, please do not hesitate to contact us or call 864-480-7680.

The Wenner 4 point soil resistivity test is the most reliable soil resistivity test possible. Unfortunately, the method is often misapplied, producing misleading results. Our trained technicians conduct the test in a manner that produces accurate, reliable results every time. Some points about how we do this resistivity testing:

  • The Wenner 4-point soil resistivity test uses 4 probes spaced at equal distances across the surface of the earth, in a straight line.
  • The probe spacing distance determines the testing depth into the soil.
  • As a matter of quality assurance, we typically conduct multiple tests at a variety of probe spacing and resolve for any inconsistencies.
  • We enter the data into our specialized computer software and run calculations on specific soil resistivity readings at various depths.


Some of the many reasons to ensure you have a well-designed grounding (and bonding) system:

  • A lightning protection system is pointless without it (see short discussion, below).
  • Human safety is critical during ground potential rise events in work place areas.
  • Federal law mandates mitigation of all known hazards in the workplace. Substations are always considered workplaces; step and touch potentials must be eliminated to ensure the safety of work personnel.
  • Although transmission and distribution towers or poles are not always considered work places and are often exempt from these requirements, sometimes they are. Install equipment that’s not related to the electric utility company and requires outside vendors for support, and it’s now considered a workplace.

Cellular telecommunications, environmental monitoring, and microwave relay equipment are good examples of equipment that, when installed on a high-voltage tower, turn the tower into a workplace. Federal regulation CFR 1910.269 specifically requires eliminating step and touch potentials on transmission and distribution lines that include any related communication equipment.

Let’s discuss lightning a little bit more.

A lightning strike analysis can save lives and protect property, if its recommendations are correctly implemented. Lightning is a random and unpredictable event. Globally, some 2,000 ongoing thunderstorms cause about 100 lightning strikes to the earth each second. Did you know that total equipment damage costs from lightning strikes is about 150 million dollars every year in the United States alone? A lightning strike analysis is a critical step in a comprehensive site grounding design.

The purpose of a lightning protection system for a building is to provide a preferred path for the lightning strike to follow (to the earth) without injuring people or damaging equipment in the building. The main components of a lightning protection system are:

  • Air terminals. These are mounted up high, typically directly on the roof. They provide a preferential target for the lightning.
  • Down conductors. These are special conductors along which the lightning travels to the grounding elements. Being millions of volts and high frequency, lightning travels along the skin of these conductors and can jump across bends in the conductors or leave the conductors entirely if the installer doesn’t strictly follow the installation standards.
  • Grounding elements. These are often copper-clad ground rods, but may be in some other configuration (e.g., counterpoise or mesh) depending on what will work in that location. The job of the grounding element is to diffuse the lightning into the earth. How well it can diffuse the energy is a function of soil resistivity versus the total surface area of the grounding element in contact with the soil. This is why good system designers rely so heavily on soil resistivity data (and why bad designers who “wing it” can leave the site unprotected).

Whether to install (and subsequently maintain) a lightning protection system is a critical decision. Its benefits include:


  • Safety of personnel.
  • Protection of property.
  • Reduction of insurance costs.
  • Reduction of downtime.


The engineers at ComSite Construction Company provide lightning stike analysis based on the requirements set out in NFPA 780 Risk Analysis Guidelines (National Fire Protection Association Standard for the Installation of Lightning Protection Systems). To schedule your free initial consultation, please do not hesitate to contact us or call 864-480-7680.

Qualified tower installers know to where to bond and where to ground. What’s the difference between these two concepts?

When you ground, you connect something to the earth (you can read more about this distinction in the IEEE-142, the Green Book and in the National Electrical Code, Article 250 and in other sources). So when you think of grounding, you could think of it as “earthing” to keep things straight in your mind. We ground for lightning protection purposes. We do not ground to create an equipotential plane, because the resistance of the earth is far too high to facilitate that. Anyone who’s done ground resistance testing can tell you this

When you bond, you create a conductive path between metallic objects. We bond for several purposes. For example, we bond metallic objects to prevent a flashover between them. For electrical systems, we bond to create a low-impedance path for fault current to flow.

Let’s address some common misconceptions about grounding:

  • Myth: When you ground something, a person touching it is at the same potential as the ground rod and so can’t be electrocuted. The problem here is Ohm’s Law. You can stand on a ground rod and be electrocuted if your other foot is on the dirt because the earth has resistance and so does your body. Draw out the circuit on paper, and do the math.
  • Myth: Grounding eliminates differences of potential. No, this is what bonding does. When you bond, you create an extremely low impedance path between metallic objects. That’s why lightning protection standards require bonding metallic objects on a roof to prevent flashover.
  • Myth: Grounding eliminates electrical noise. This misconception arises from the misuse of the word “grounding.” We talk about signal ground, but it’s not actually ground (earth). Airplanes are not connected to the earth, yet their electronics have signal “ground.” What’s meant here is a reference plane, not an actual ground.
  • Myth: Grounding creates a return path for electricity. This is a very dangerous myth to believe in. Suppose, for example, we eliminated the neutral wiring in homes and just used ground as a return path. This would make taking a shower a rather lethal experience. Similarly, consider the common 277V lighting system in commercial and industrial buildings. The return path is the neutral, not the bonding or grounding system. In cases where the installers have used the metallic raceway instead of a neutral (a code violation), the result has been undesired circulating current. Why? That brings us to our final myth….
  • Myth: Electricity seeks the path of least resistance. This directly contradicts Kirchoff’s Law of Parallel Circuits, upon which our entire electrical infrastructure and all of our electronic devices are based. The reality is that electricity flows along all available paths, in reverse proportion to the resistances.

So, does grounding actually serve any purpose? Yes. The National Electrical Code devotes considerable space to this topic, as do the lightning protection standards. Grounding provides a path to earth for lightning and other high voltage transients. For towers, this is critically important. Lighting strikes tall structures, preferentially. So a tower, being tall, is essentially a big lightning rod. Without a lightning protection systems (which relies on grounding), the tower can suffer catastrophic damage from the million plus volts it gets hit with. But deficiencies in how that system is designed and/or installed can result in currents flowing through structural connectors, overheating those connectors, and weakening them over time. The lightning protection system diverts this energy to ground, thereby protecting the tower.

The three grounding standards in the USA are:

  • IEEE-142, The Green Book.
  • NFPA 70 (NEC), Article 250.
  • IAEI Soares Book on Grounding.

The two lightning protection standards in the USA are:

  • LPI-175, the Lightning Protection Institute’s Standard of Practice for the Design, Installation, and Inspection of Lightning Protection Systems.
  • NFPA 780, the National Fire Protection Association’s Standard for the Installation of Lightning Protection Systems Article 250.