GROUNDING AND LIGHTNING PROTECTION
GENERAL
It is most important that equipment, facilities, and systems be properly grounded and protected from lightning to prevent injury to personnel and damage to equipment. Electromagnetic Interference (EMI) problems at some installations have also indicated the necessity for more efficient grounding and bonding systems
This theory, principles, and practices of grounding and lightning protection procedures for communication-electronic equipment, facilities, and systems. It provides facts and methods deemed most applicable to determine the type and amount of grounding and lightning protection required.
It is most important that equipment, facilities, and systems be properly grounded and protected from lightning to prevent injury to personnel and damage to equipment. Electromagnetic Interference (EMI) problems at some installations have also indicated the necessity for more efficient grounding and bonding systems
This theory, principles, and practices of grounding and lightning protection procedures for communication-electronic equipment, facilities, and systems. It provides facts and methods deemed most applicable to determine the type and amount of grounding and lightning protection required.
PURPOSE
1. The data provided which will enable installation engineering personnel and directed toward qualified personnel such as communications officers, technical advisers to determine the adequacy of existing protective devices.
2. The type and amount of protection required at an initial installation is also supplied, as well as the criteria necessary for determining the applicable procedures and the protective devices that should be installed.
SCOPE
This publication is detailed and broad enough to serve as a reference for equipment, facility, or system grounding procedures and lightning protection considerations. Those applications are to radio; radar, microwave, forward scatter, meteorological equipments, facilities, systems, power distribution and emergency power grounding and/or lightning protection fundamentals. They are compost of the following contents:
- BASIC PRINCIPLE OF GROUNDING are compost of Characteristics, Requirement of a satisfactory ground connection, Factors affecting earth resistance and ground resistance, Measurement, calculation and measure devices of ground resistance, Resistance to earth of a ground connection , location of a good ground connection , type of ground electrodes , treating soil artificially , grounds techniques , equipment grounding , system ground and common ground for system and equipment.
- BASIC PRINCIPLE OF LIGHTNING PROTECTION are compost of Lightning discharges, Materials, basic principles of protection for structures , Integral system of lightning protection , Installation requirements for integral system , Separately mounted shielding system of lightning protection, Basic principles of protection for electrical power distribution systems, Power station and substation protection ,Primary and secondary service protection, Protection of lines ,Protective devices used for lightning protection ,Lightning protection of antenna
- APPLICATION
- GENARAL MAINTENANCE PROCEDURES
- GROUND SYSTEMS VS. UNGROUND SYSTEMS
BASIC PRINCIPLES OF GROUNDING
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 supporting structures, as well as all metal structures, motor and generator frames, cable armor, control equipment enclosures, conduits, and all portable electrical equipment cabinets and housings are at ground potential thereby reducing possibility of electrical shock to 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 inter modulation effects and noise voltage buildup 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 made or planned, it is practically impossible to develop an earth grounding system which can be utilized as a standard for all locations. During the pre-engineering and 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.
CHARACTERISTICS OF GROUNDS
1. GROUNDS OR GROUND CONNECTIONS Connections to the earth, called grounds or ground connections, are made utilizing metal rods; T or channel iron (galvanized or copper plated), buried water pipes, buried plates, wires or cables.
2. GROUND RESISTANCE Ground resistance is the electrical resistance to current flow between the earth and the equipment configuration. When two or more ground rods, buried plates, etc., (ground electrodes) are connected in parallel, ground resistance is the approximate combined resistance computed for a parallel arrangement.
3. COMPONENTS OF GROUND RESISTANCE Ground resistance is the result of the following factors:
a. Resistance of the ground wire and its connection to the ground electrode.
b. Resistance of the electrode itself
c. Resistance of the contact between the electrode and the soil.
d. Earth (soil) resistance
EARTH (SOIL) RESISTANCE. Earth resistance is the inherent electrical resistance of the soil used as a grounding source. This resistance may vary greatly, depending upon the type of soil, its moisture content, and temperature. Normally, when speaking of grounding procedures or techniques, it is common to refer to the ground resistance. Earth resistance is only one factor affecting ground resistance.
4. REQUIREMENTS OF A SATISFACTORY GROUND CONNECTION
A ground connection, regardless of its application, must meet certain specification. The electrodes buried in the ground to form an electrical connection to the earth must themselves be good electrical conductors and be capable of resisting corrosion while in contact with the soil. In addition, the electrodes must 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 reasonably 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, a-c impedance, have adequate current-carrying capacity, and be of such a design that they can be readily installed and maintained.
5. FACTORS AFFECTING EARTH RESISTANCE The range of earth resistance may vary from several ohms to several million ohms. This variation is due to the electro-chemical action in the soil and is dependent upon the moisture content and temperature as well as composition of the soil. The factors affecting earth resistance are discussed in the following paragraphs.
5.1 RANGE OF RESISTIVITY FOR DIFFERENT KINDS OF SOIL
Two layers of soil that are usually found together. The top soil, although usually rich in minerals, is dry and therefore relatively non-conducting. The thickness of the top soil stratum varies from 0 to about 6 inches. The solum (true soil) underlies the top soil and, although usually moist, may not contain sufficient conducting material to make it a rood conductor. The numerous types of soil prevent a simple classification. However, a study of certain recognizable types reveals a definite trend in resistivity. Data collected by the Bureau of Standards on the resistivity of different soils. They measured value of ground resistance of 5/8-inch by 5 foot driven electrodes, as reported by observers in widely separated parts of the US. This demonstrates that a grounding system which would be adequate in clay soil might be unsatisfactory in sandy soil. Observation has also shown that similar samples of soil from different locations sometimes vary by a factor of from 200 to 300 percent.
Two layers of soil that are usually found together. The top soil, although usually rich in minerals, is dry and therefore relatively non-conducting. The thickness of the top soil stratum varies from 0 to about 6 inches. The solum (true soil) underlies the top soil and, although usually moist, may not contain sufficient conducting material to make it a rood conductor. The numerous types of soil prevent a simple classification. However, a study of certain recognizable types reveals a definite trend in resistivity. Data collected by the Bureau of Standards on the resistivity of different soils. They measured value of ground resistance of 5/8-inch by 5 foot driven electrodes, as reported by observers in widely separated parts of the US. This demonstrates that a grounding system which would be adequate in clay soil might be unsatisfactory in sandy soil. Observation has also shown that similar samples of soil from different locations sometimes vary by a factor of from 200 to 300 percent.
5.2 EFFECT OF MOISTURE CONTENT 0N SOIL RESISTIVITY
The effect of moisture upon soil resistivity is illustrated by the typical curve. Moisture content is expressed in percent by weight of dry soil. Below 22 percent, the resistivity increases abruptly with a slight decrease in moisture content, indicating the critical difference moisture makes in the value of ground resistance. The soil tested in this case was red clay. It should be considered that moisture alone is not the predominant factor in the low resistivity of soils. For example, a ground rod driven into the bed of a mountain stream may present a high ground resistance if the water is low in metallic content and the soil does not contain conducting elements. Consequently, high moisture content does not necessarily solve the problem of providing low-resistance ground connections. Although salt water is ordinarily a contributing factor to the establishment of a proper ground, the type of soil may still make it impossible to establish a low resistance ground.
The effect of moisture upon soil resistivity is illustrated by the typical curve. Moisture content is expressed in percent by weight of dry soil. Below 22 percent, the resistivity increases abruptly with a slight decrease in moisture content, indicating the critical difference moisture makes in the value of ground resistance. The soil tested in this case was red clay. It should be considered that moisture alone is not the predominant factor in the low resistivity of soils. For example, a ground rod driven into the bed of a mountain stream may present a high ground resistance if the water is low in metallic content and the soil does not contain conducting elements. Consequently, high moisture content does not necessarily solve the problem of providing low-resistance ground connections. Although salt water is ordinarily a contributing factor to the establishment of a proper ground, the type of soil may still make it impossible to establish a low resistance ground.
The variation of resistivity in two samples of soil when the moisture content of each type is increased. The same samples, when thoroughly dry, have a resistivity of 109 ohms per centimeter cube. The resistivity of the samples vary greatly until, with 30-percent moisture content, one sample measures 6,400 ohms and the other 4,200 ohms.
5.3 EFFECT OF TEMPERATURE ON SOIL RESISTIVITY
Below 0°C (32°F), water in the soil freezes and introduces a tremendous increase in the temperature coefficient of resistance for soil. As the temperature decreases. the resistivity of the soil rapidly increases. A variation of soil resistivity with changes in soil temperature. The effect of temperature on the resistivitv of sandy loam, of 15.2-percent moisture and in changes of from 20°C (68°F) to -15°C (5°F). In this range, the resistivity varies from 7,200 to 330,000 ohms per centimeter cube. The variation of resistivity of red clay soil containing 18.6-percent moisture. From temperatures of -15°C (5°F) to 1.11°C (34°F) the resistivity changed from 35,000 to 1,000 ohms per centimeter cube. In both samples of soil, a pronounced change in resistivity is noted below the freezing point.
Below 0°C (32°F), water in the soil freezes and introduces a tremendous increase in the temperature coefficient of resistance for soil. As the temperature decreases. the resistivity of the soil rapidly increases. A variation of soil resistivity with changes in soil temperature. The effect of temperature on the resistivitv of sandy loam, of 15.2-percent moisture and in changes of from 20°C (68°F) to -15°C (5°F). In this range, the resistivity varies from 7,200 to 330,000 ohms per centimeter cube. The variation of resistivity of red clay soil containing 18.6-percent moisture. From temperatures of -15°C (5°F) to 1.11°C (34°F) the resistivity changed from 35,000 to 1,000 ohms per centimeter cube. In both samples of soil, a pronounced change in resistivity is noted below the freezing point.
