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The concept of low resistance path to earth is fundamental to electrical theory and practice. When designing and installing electrical systems, proper grounding is a necessity. If lightning or power fault induced currents have no easy path to earth, they will find unintended paths that may cause damage to equipment or, worse yet, injury to personnel. Good grounding is essential to prevent damage to industrial facilities where millions of dollars of equipment and products can be ruined in a flash—a large thunderstorm can produce over 100 lightning strikes a minute, and even a modest storm cloud can generate tens of kiloamps.

Fortunately, lightning is one of the only forms of natural disaster with an impact that can be controlled. Well-designed surge protection devices coupled with a low resistance path to earth are the most important factors in providing protection for both personnel and equipment.

All lightning protection systems must terminate in a connection to earth, where the lightning strike current will finally travel into the soil and dissipate. Grounding systems must provide low earth impedance as well as low resistance. The most important factor affecting grounding resistance is the soil resistivity where the electrode is installed. In areas where resistivity is high, special steps must be taken to ensure a low resistance path to earth.

Grounding resistance and soil resistivity are, by definition, always proportional.

When the dimensions of the electrode are known, resistance can be expressed as follows:

R = ρ x f

where:

R = Grounding Resistance

ρ = Soil Resistivity

f = A function determined by the shape and size of the electrode

Soil resistivity is a measure of how much the soil resists the flow of electricity. An understanding of soil resistivity and how it varies is necessary in order to design an effective grounding system. The resistivity of soil is, in fact, influenced by many factors and fluctuates constantly; it is typically lower in summer and higher in winter.

There are two ways to determine the resistivity of the soil at a certain site. The first is to actually measure the resistivity itself with specialized equipment. The second is to drive a ground rod of known length and diameter into the ground and to measure its grounding resistance. That reading can then be used to calculate the resistivity of the surrounding soil.

Soil characteristics including moisture content, temperature, and composition (Fig. 1) determine the overall resistivity at any particular location. Soil with high organic content tends to be a better conductor because it retains a higher moisture level. Some varieties of soil and earth are highly resistive and can act like insulators. Typical soil does not permit electric current to flow when it is completely dry (although soil is almost never found completely dry in its natural state). Sandy soils have much lower moisture content and tend to have higher resistivity.

Fig. 1: Soil type and resistivity (Source: SAN EARTH Technical Review)

The soil's moisture content is important because it helps chemicals in the soil that surround ground conductors to carry electrical current. In general, the higher the moisture content, the lower the soil's resistivity. When moisture content falls below 10%, resistivity increases significantly (Fig.2).

Fig. 2: Percentage of Moisture in Soil vs. Resistivity (Source: SAN EARTH Technical Review)

After moisture, the factor that has the biggest effect on the resistivity of soil is temperature. Figure 3 below shows how the resistivity of a soil sample varies with changes in temperature and the rate of its increase as temperature declines.

Temperatures below freezing increase soil resistivity dramatically so grounding systems should be installed below the frost line to maintain a low-resistance ground. When moisture turns to ice, resistivity increases sharply.

Fig. 3: Soil Temperature vs. Resistivity (Source: SAN EARTH Technical Review)

Ground Enhancement Materials

Ground Enhancement Materials are used in areas with high soil resistivity to reduce grounding resistance. SAN-EARTH M5C is a very fine powder packaged in 25 kg (55 lb.) bags that provides an environmentally safe, long term solution to many grounding problems. It was originally developed to aid the grounding of electric power transmission lines in mountainous areas where construction is difficult and soil resistivities tend to be high. SAN-EARTH M5C reduces resistance to ground by up to 50%, lowers surge impedance significantly, is environmentally safe, and reduces corrosion in grounding conductors.

SAN-EARTH M5C is frequently used as a grounding material because of its convenience and effectiveness. It can be deployed in two forms, as a powder that can be spread over the ground or in a grout or slurry that can be poured. In general, no water is required when grounding with SAN-EARTH M5C. It is designed to solidify by absorbing the moisture from the surrounding soil. This makes it perfect for use in grounding at sites where a supply of water is not readily available.

SAN-EARTH M5C grounding electrodes are easily installed by spreading the dry powder in a strip over and around a conductor in a horizontal trench. One bag (Fig. 4) is enough to build a two-foot-wide, ten-foot-long electrode. When the trench is refilled, SAN-EARTH M5C absorbs moisture from the surrounding soil and hardens to become part of the grounding electrode. The surface area of the electrode is dramatically increased, resistance is substantially reduced and surge impedance is lowered significantly. The hardened SAN-EARTH M5C also acts as a conductor theft-deterrent.

Fig. 4: SAN-EARTH M5C electrodes provide the low resistance ground essential to any lightning protection system.

In normal conditions, an electrolytic reaction occurs when any metal buried in the ground is exposed to a positive electric current. That reaction, caused by ionic conductivity, can result in serious corrosion of the metal. This condition can be avoided through the use of SAN-EARTH M5C. Covering the metal with SAN-EARTH M5C creates conduction between the metal and the ground enhancement material, reducing the electrolytic reaction and preventing the metal from corroding. Studies show that corrosion is reduced by a factor of ten in electrodes that are encased in SAN-EARTH M5C.

SAN-EARTH M5C offers many advantages over Bentonite, a material discovered by Wilbur C. Knight in the 19^th century near Fort Benton, Montana. Bentonite is a term now used as a general description of water-absorbing clay and is known by such names as Wyo-Ben or Lynconite II. In its powder or dry form Bentonite is a very poor grounding backfill material due to its high resistivity. Consequently, it is typically mixed with large amounts of water before installation. Then it is poured as a liquid or gel into the grounding system. Bentonite cracks and shrinks dramatically and returns to its non-conductive state as it dries. Unlike Bentonite, SAN-EARTH M5C has a low resistivity in its powder form, slightly wet form, very wet form, and hardened form—it remains conductive and can be installed in many different ways.

SAN-EARTH M5C is manufactured in accordance with Sankosha's ISO 9001 Quality Standards and conforms to IEC Standard 62561-7, which specifies the requirements and tests for earthing enhancing compounds that produce low resistance in an earth termination system. The tests required by this standard include leaching, sulfur content, resistivity and corrosion. IEC 625561 also details the requirements on the structure and content of the test report.

One IEC 62561-7 requirement for earthing enhancement materials is that they must be chemically and physically stable. They must be chemically inert and they must not leach into the soil over time.

If an earthing enhancement material contains a significant amount of sulfur, it can corrode the ground rod electrode. IEC 62561-7 requires that any earthing enhancement material contains less than 2% sulfur. Although the IEC does not require a minimum resistivity value for earthing enhancement materials, it does prescribe that all manufacturers of materials used for earthing enhancement test the resistivity in accordance to the ASTM G57 “Standard Test Method for Field Measurement of Soil Resistivity.”

There are many productive uses of SAN-EARTH M5C. Common applications include but are not limited to electric transmission and distribution towers, microwave towers, substation ground systems, central office switches, emp shielding, cellular phone systems, radio transmission towers and satellite ground stations.

More information about SAN-EARTH M5C conductive grounding cement is available on the Sankosha website.