To ensure your electrical system functions properly, it's important your buried ground system has low impedance. So how do you achieve this goal while keeping safety in mind?
When designing and installing electrical power systems, proper grounding is not just a luxury, but a necessity. All good grounding systems should provide a low-impedance path for fault and lightning-induced currents to enter the earth, ensuring maximum safety from electrical system faults and lightning. More specifically, a properly installed grounding system not only helps safeguard buildings and equipment from damage caused by unintentional fault currents or lightning surges, but also protects a much more important investment: people.
Achieving an acceptable ground is challenging. Proper installation of grounding systems requires knowledge of national standards, conductor materials, and connections and terminations (Fig. 1, in original article). But that's not all. Don't forget to consider soil conditions where you install the ground rods (or grounding grid).
Impact of soil conditions on grounding. Though the overall effectiveness of a buried grounding system depends on many factors, the resistance of the earth (or earth resistivity) significantly impacts overall impedance of the buried conductor. Soil characteristics, such as moisture content, soil temperature and type, determine the overall resistivity of the earth. When grounding your system, always keep the following in mind:
- Moisture content.
The soil's moisture content is important because it helps chemicals in the soil that surround ground conductors carry the electrical current. In general, the higher the moisture content, the lower the soil's resistivity. When moisture content falls below 10%, resistivity increases significantly.
- Soil temperature.
Temperatures below freezing also increase soil resistivity. As soon as moisture turns to ice, resistivity increases sharply. In areas subject to freezing, driving a ground rod below the frost line is required to maintain a low-resistance ground.
- Soil type.
Black dirt, or soils with high organic content, are usually good conductors because they retain higher moisture levels and have a higher electrolyte level: leading to low soil resistivity. Sandy soils, which drain faster, have a much lower moisture content and electrolyte level. Therefore, they have a higher impedance. Solid rock and volcanic ash, such as that found in Hawaii, contain virtually no moisture or electrolytes. These soils have high levels of resistivity, and effective grounding is difficult to achieve. See Table 1 (in original article) for resistivities of different soils.
Measuring earth resistivity. The effectiveness of grounding rods largely depends on whether the soil surrounding the rods can conduct large electrical currents. To design a buried grounding system correctly, you must measure earth resistivity with a ground resistance testing instrument. This instrument should also have switches to change the resistance range. You can use various test methods to measure earth resistivity, but the three most common are:
Four-point method, the most accurate.
Variation in-depth method (three-point method).
After determining the soil resistivity, you are in a better position to determine what kind of buried grounding scheme will be most effective. Depending on the soil resistivity and grounding scheme requirements, the particular system can vary from a simple buried ground conductor to an extensive ground rod bed. The latter could include a grid system or a ground ring (Fig. 2, in original article). To decrease the grounding system impedance, you can use ground enhancement material or chemical-type electrodes.
How to achieve an acceptable ground. There are various options to lower soil resistivity. One method is to increase the moisture content of the soil. Topsoil resistivity may be reduced 800 ohm-m by increasing the moisture from 5% to 10%. An additional reduction in resistivity, although much smaller, can be obtained by increasing moisture from 10% to 20%. The problem with adding moisture to the soil is that it's not a practical option in most cases.
Another way to lower earth resistivity is to treat the soil with a salt, such as copper sulfate, magnesium sulfate, or sodium chloride. Combined with moisture, the salts leach into the soil to reduce earth resistivity. However, this inexpensive process can also cause problems. First, as the salts wash away, the soil reverts to its untreated condition. As a result, you must recharge the system periodically. Second, some salts may corrode the grounding conductors. Lastly, the salt may contaminate ground water. Local environmental regulations and the Environmental Protection Agency (EPA) may object to adding salts to the soil.
In many places, ensuring a low-resistance ground system is as simple as driving a ground rod into the subsurface soil layer that has a relatively permanent and conductive moisture content. Remember, the ground rod must extend below the lowest frost depth. You can also use ground enhancement material to achieve acceptable system resistance (Fig. 3, in original article).
What you should know when using ground enhancement material. Under almost all soil conditions, the use of a ground enhancement material will improve grounding effectiveness. Some are permanent and require no maintenance. You can use them in areas of poor conductivity, such as rocky ground, mountaintops and sandy soil, where you can't drive ground rods or where limited space makes adequate grounding difficult with conventional methods.
There are several kinds of ground enhancement material available. But use care when choosing the material. It should be compatible with the ground rod, conductor, and connection material. Some options include bentonite clay, coke powder, and specially engineered substances.
Bentonite is a clay substance used in areas with high soil resistivity. However, conduction in bentonite clay only takes place via the movement of ions. Ionic conduction can only occur in a solution, which means the bentonite clay must be moist to provide the required resistance levels. When bentonite clay loses moisture, its resistivity increases and volume decreases. This shrinkage results in a discontinuity in the contact between the bentonite clay and surrounding soil, which further increases system resistance.
Coke powder is another choice. A predominantly carbon substance, coke powder is highly conductive. However, groundwater can wash it away.
A noncorrosive low-resistance enhancement substance is a conductive cement that you can install wet or dry. Depending on the substance, it will not leach into the soil and meets EPA requirements for landfill. The railroad and utility industries have successfully used this material. When installed dry, it absorbs moisture from surrounding soil and hardens, retaining moisture within its structure. When used dry, no mixing is required, and you achieve maximum efficiency in a matter of days. This is because it absorbs enough water from the surrounding soil. You can also premix it with water to a heavy slurry. You can add this to the trench containing the grounding conductor or use it around a ground rod in an augered hole. The material binds the water into a cement making a permanent, highly conductive mass.
Some products offer a test-proven resistivity of 0.12 ohm-m or lower, compared with 2.5 ohm-m for bentonite clay. Unlike bentonite clay, the cement-like material does not depend on the continuous presence of water; nor does it require periodic charging treatments/replacement.
An ideal ground enhancement material should not require maintenance. When designing or installing a buried grounding system, look for materials that do not dissolve or decompose over time, require periodic charging treatments or replacements, or depend on the continuous presence of water to maintain conductivity.
Installation of ground enhancement materials. After selecting the material, consider the method of installation. Placement of ground enhancement material is quick and easy. For installation around a ground rod (Fig. 4, in original article), auger a 3 in. to 6 in. diameter hole to a depth equal to 6 in. less than the rod length. Drop the rod down the hole with the lower end centered and driven into the earth at least 12 in. Make the connection of the grounding conductor to the ground rod. Then, fill most of the hole using ground enhancement material. Lastly, fill the remainder of the hole with the soil removed during augering.
The installation of a conductor in a trench involves six steps as listed below. Refer to Fig. 5, for more guidance. Should you use a conductive-type cement for ground enhancement, see the estimated amount of linear feet obtainable from a bag of material for use as ground conductor covering in Table 2 (on page 64P, in original article).
Dig a trench at least 4 in. wide by 30 in. deep, or below the frost line, whichever is deeper.
Spread out enough ground enhancement material (either dry or in a slurry) to cover the bottom of the trench, about 1 in. deep.
Place the conductor on top of the ground enhancement material.
Spread more ground enhancement material on top of the conductor to completely cover the conductor, about l in. deep.
Carefully cover the ground enhancement material with soil to a depth of about 4 in., making sure not to expose the conductor.
Tamp the soil down, and fill in the trench.
Chemical-type electrodes are another option for difficult grounding situations. These consist of a copper tube filled with salts installed in an augered hole or trench. The electrode is backfilled with a ground enhancement material. The copper tube has holes in it near the top and bottom, and the top of the electrode remains exposed to the atmosphere. Water slowly dissolves the salts, which enter the tube from the top holes exposed to the atmosphere. Highly conductive salt solution leaches into the soil from the holes near the bottom of the tube.
The backfill material is usually bentonite clay or a combination of bentonite clay at the bottom and the cement slurry described above at the top. Chemical-type electrodes require periodic recharging of the salts. Although more expensive than a cement slurry encased ground rod, several long-term tests indicate a chemical-type electrode provides about the same effectiveness.
Measuring installed grounding systems. After installation, you may be required to measure the ground resistance of the installed system. Be aware that the 1996 NEC, Sec. 250-84, requires a single electrode consisting of rod, pipe or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of the type listed in Section 250-81 or 250-83. Always install multiple electrodes so they are more than 6 ft apart.
Maintenance of the grounding system. You need an effective inspection and periodic maintenance program to ensure continuity exists throughout the grounding system. Be sure to regularly inspect it, using an approved ground-testing instrument to test electrical resistance and continuity.