PRINCIPLES OF GROUNDING JD Delancy, K1ZAT/3 The main purpose of equipment, facility, and system grounding is to provide for the safety of personnel. This is accomplished by insuring that all equipment configurations, antenna, or support structures, as well as all metal structures, motor, and generator frames, cable armor, control equipment enclosures, conduits, and all portable electrical equipment cabinets and housing are at ground potential thereby reducing possibility of electrical shock of personnel coming in contact with metal parts of the equipment and towers. The secondary function of all grounds is to improve the operation and continuity of service of all equipment configurations. Faulty ground returns are detrimental to these functions and can result in intermodulation effects and noise voltage build-up with their associated service interruptions, false signals, equipment damage, or signal distortion. Considering that the characteristics of the soil (earth) and the weather vary greatly at the locations in which the installation are planned, it is practically impossible to develop an earth grounding system which can be utilized as a standard for all locations. During the planning stages of an extensive electrode system for substations or other electrical power distribution system, consideration must be given to the potential variations which can occur over the area of the ground connections. A ground connection, regardless of its application, must meet certain specifications. The electrodes buried in the ground to form an electrical connection to the earth must themselves be capable of withstanding mechanical abrasion and have sufficient area in contact with the soil so that the ground resistance is within the rated limits. The resistance of this earth path must remain reasonable constant throughout the seasons of the year and must be unaffected by unexpected circulating currents resulting from the equipment configuration to which the connection is made. In short, ground connections should be durable, have low D-C resistance, low A-C impedance, have adequate current carrying capacity, and be of such a design that they can be readily installed and maintained. Driven ground electrodes, more commonly referred to as ground rods or pipes, are utilized where bedrock is beyond a depth of 10 feet. Ground rods are commercially manufactured in 1/2", 5/8", 3/4" and one inch diameters, and in lengths of 6', 8', 10', 12', and 16'. The National Electric Code (NEC) specifies that ground rods of steel or iron shall be at least 5/8" in diameter and that rods of non-ferrous materials shall not be less than 1/2" diameter. Although galvanized steel rods are used, the more commonly utilized material, copper clad steel provides an excellent means of obtaining the lowest possible resistance contact with the earth. The NEC requires that any water metering equipment be bypassed by a jumper of a size not less than that required for the ground ing conductor. The ground conductor shall bypass the meter and service unions. The water piping system must be made electrically continuous by bonding together all parts which may become disconnected. As with other ground connections, the resistance should be measured before deciding on this type of ground installation. It should be noted that where cast iron screw type joints are utilized for joining together lengths of pipe, they usually provide metallic connections of low resistivity. However, if joints are made of "leadite" or similar types of cement, the resistivity values of these connections may be several hundred ohms, rendering the water system useless as a suitable ground system; therefore, tests should be conducted to insure continuity of ground circuits. Multiple driven electrodes will not always provide an adequate low resistance to earth. In such instances, it is generally possible to reduce the resistivity of the soil immediately surrounding the driven electrode by treating the soil with a substance which, when in solution, is highly conductive. There are several substances, however, the better known, in order of preference are: a. Magnesium sulphate (common name: Epsom Salts) b. Copper Sulphate (common name: Blue Vitriol) c. Calcium Chloride d. Sodium Chloride (common name: Common Salt) e. Potassium Nitrate (common name: Saltpeter) Preference is given to use of magnesium sulphate, which is the most common material used. It combines low cost with high electrical conductivity and low corrosive effect on a ground electrode or plate. All electrodes used in the soil treatment should be of copperweld type. Large reductions in the ground contact resistance of the individual ground electrodes may be expected after chemical treatment of the earth where low resistances are difficult to obtain without chemical treatment. The initial effectiveness of chemical treatment is greatest where the soil is somewhat porous because the solution permeates a considerable volume of earth, and expanded ground contact thereby increases the effectiveness of the electrode. When soil of compact texture is encountered, the chemical treatment is not as effective at first because the solution tends to remain in its original location for a longer period of time. Chemical treatment limits the seasonal variation of resistance and lowers the freezing point of the surrounding soil. Chemical treatment of the earth around a driven electrode utilizing the Magnesium Sulphate and water solution is described as follows: a. A 4-foot length (approx) of 8 inch tile pipe is buried in the ground approximately four inches from the ground electrode, and filled to within one foot of the ground level with the Magnesium Sulphate and water solution. The 8-inch pipe should have a wooden cover with holes, and be located at ground level. b. Forty to Ninety pounds of chemical will initially be required, and will retain its effectiveness for two or three years. Each replenishment of chemical will extend its effectiveness for a longer period, so future retreatment occurs less and less frequently. The use of Common Salt or Saltpeter is not recommended as it will require greater care to be given to protection against corrosion. Additionally, any metal enclosure nearby and unrelated to grounding, should also be treated to prevent damage by corrosion. Therefore, Common Salt or Saltpeter should be utilized only when absolutely necessary to reduce the resistivity of the soil. When Common Salt must be utilized, the amount necessary to treat the earth around a driven electrode depends upon the the available water supply. A decrease in resistivity of the earth can be achieved by adding more water. Additional water dissolves the salt and also aids in carrying the salt solution throughout the conducting soil hemisphere. Therefore, a minimum treatment of earth per ground electrode would contain at least five pounds of salt and as much water as is required to initially flood the area. The rate at which chemical treatment will lower the resistivity of the soil depends upon the rate at which the solution will seep through the soil. Commercial tests have shown that an initial chemical treatment retains its effectiveness for at least one year, however, porous soil and excessive rainfall or drainage would reduce the period appreciably. In some cases, treatment has remained effective for three to six years. If the station and equipment is located on a rocky mountain top, this system could not be utilized since soil treatment would be in-effective. end-of-file