The earth resistance value is a major concern when dealing with earthing of any structure. Earth resistance measurements and the use of earthing compounds helps to identify and achieve a recommended earth resistance for a structure. We know that soil resistivity is the prime factor contributing to the earth resistance value. Therefore, the best solution to maintain the ideal earth resistance value is by maintaining the soil resistivity at the desired level. Soil treatment is one of the widely recommended methods for this purpose.
Earthing compounds are a mixture of minerals used to reduce soil resistivity and absorb and retain moisture in the soil for longer periods to maintain the soil’s conductivity. They are used as a backfilling material covering the ground electrode for lowering the earth resistivity. Earthing compounds are also called conductive backfills, ground enhancement backfills, earth bonding materials, earth bond enhancement compounds, soil conductivity amendments, earthing compounds, conductive concrete, conductive cement, grounding fills, and chemical earthing. The use of earthing compounds has been tested and evolved over the years.
Requirements for Earthing Compound
There are some requirements for an ideal earthing compound. Earthing compounds should be compatible with grounding electrode materials and environment-friendly. It should not lead to corrosion of the electrode material, which can result in poor earthing. It should not contain any toxic agents and should not drain out into the surrounding soil.
IEC 62561 part 7 mentions a list of requirements for an ideal earthing compound. IEC suggests that the earthing compound should not cause any harm to people or the environment. It should provide a stable environment in terms of physical and chemical properties and have low resistivity. The tests mentioned in IEC include the following:
a) Leaching test: This test is to ensure that the earthing compound is chemically stable and must not leach over time. A leaching test shall be performed to determine the presence of iron, copper, zinc, nickel, cadmium, cobalt, and lead. The test is to be performed based on EN 12457-2.
b) Sulphur test: A significant amount of sulphur in the earthing compound can lead to corrosion of earth electrodes. IEC 62561 states that an earthing compound passes the sulphur test only if the measured value is less than 2%. The test needs to be performed according to ISO 4689-3 or ISO14869-1.
c) Resistivity test: The resistivity test is to be performed in accordance with the ASTM G57, using the Wenner Four-electrode method. The resistivity can be measured using three earthing compound samples in the four-electrode soil box. The testing apparatus for this test includes:
- Earth Resistance Meter: It has two current and two voltage terminals or low-frequency AC source, a high input impedance voltmeter and an ammeter.
- Four-electrode Soil Box: This box is made of inert non-conductive material with four permanently mounted electrodes manufactured from mild or stainless steel.
IEC states that the measured resistivity value must be less than or equal to the value mentioned on the package of the product by the manufacturer.
d) Corrosion test: The corrosion test is important as the earthing compound needs to be inert both physically and chemically to avoid corrosion of the earth electrode. The corrosiveness of the earthing compound is determined by the potentiodynamic polarization resistance method specified in ASTM G59-97. The test apparatus needed for the test consists of a three-terminal potentiostat. The earth electrode (GI rod or copper-bonded rod) is inserted in the earthing compound sample which is being tested along with two other electrodes. These electrodes are then connected to a potentiostat, and the polarization resistance values are measured.
For copper plated electrodes, the polarization resistance shall be greater than 4 Ω·m2 for non-aggressive environments and greater than 8 Ω·m2 for aggressive environments. For galvanized earth electrodes, the polarization resistance shall be greater than 3 Ω·m2 for non-aggressive environments and greater than 7.6 Ω·m2 for aggressive environments.
It is insisted that all earthing compounds that comply with IEC 62561-7 should be marked with the manufacturer’s name and trademark. In addition, the serial of the batch of the earthing compound, installation instructions, the resistivity value and test apparatus used to obtain this value, and a conformity statement to IEC 62561-7 should be marked on the product.
Common Earthing Compounds
Charcoal and salt: Charcoal and salt are the most conventional and common earthing compounds used. Charcoal and salt layers are alternately filled surrounding the earthing electrode to increase the conductivity of the soil, thus ensuring quick ground of fault current. Filling the earth pit with charcoal and salt and then watering excessively increases the moisture content of the soil, and thus the soil resistance lowers considerably. Charcoal is highly porous and can absorb moisture and retain the water content of the soil. Salt forms ions in the presence of water. Thus the combination of charcoal and salt increases the conductivity of the soil and acts as a good earthing compound. The main drawback of using charcoal and salt as earth enhancement compounds is that they get drained after a period of time. Thus the soil resistivity begins to increase gradually, which can result in dangerous situations resulting from step and touch potential. Therefore, a periodic soil resistance check is crucial to avoid accidents.
Bentonite: Bentonite is a moisture-retaining clay which is used as an earthing compound. Naturally occurring Bentonite compound consists of montmorillonite. It is available as sodium bentonite or calcium bentonite. It is a naturally occurring substance and can contain some amount of impurities. But it is considered to cause no significant corrosion to the earth electrodes. The resistivity of bentonite is much lower in wet conditions as compared to in dry conditions. So, it is used as a backfill in the earth pit, after mixing with water. The lowering of resistivity happens due to the formation of electrolytes when bentonite is ionized by the addition of water. This electrolyte is formed due to bentonite’s ability to absorb and retain water by swelling. Though bentonite remains unaffected for the long term, it can sometimes get leached in the presence of contaminants in the soil. When compared to the conventional use of charcoal and salt, bentonite serves as a better earthing compound.
Marconite: Marconite is a very effective, synthetic earthing compound manufactured specifically for long-lasting low soil resistivity. They do not get leached or pollute the surroundings. They can be used for highly resistive soils for attaining better conductivity and thus help in greater fault dissipation. They are non-corrosive and so do not cause damage to the earth electrodes, thus ensuring a constant resistance value. The resistivity offered by marconite is significantly lower than bentonite. Hence it is recommended for structures which deal with high power handling, sensitive equipment, greater fault possibilities etc. They can be used in substations, industrial, commercial and residential buildings for efficient earthing. Marconite can be used with cement and forms a permanent solid structure within hours of installation. The cost of marconite is comparatively on the higher side but considering that it requires no maintenance or replacement for almost 50 years, makes it is cost-effective in the long run.
Conclusion
Earthing compounds are crucial for maintaining soil resistivity at the desired level. They prevent accidents due to step and touch potential hazards and related accidents. Earthing compounds should be selected and used as per the requirements of the soil and the earthing system. When choosing an earthing compound, ensure that it meets the IEC-mentioned requirements. Periodic checking of soil resistivity is always recommended for additional safety.
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