Earth resistivity effect

Earth resistance depends on the type of soil, its mineral composition, moisture content, and temperature. Table 5.2 provides the resistivity of various types of soils Table 5.3, the effect of moisture on soil resistivity and Table 5.4, the effect of temperature on soil resistivity. The information contained in the tables is used to illustrate the effect of various natural factors on soil resistivity. Table 5.5 shows the changes in earth resistance by using multiple ground rods. Note that, to realize the full benefits of multiple rods, the rods should be spaced an adequate distance apart. 

The basic measure of effectiveness of an earth electrode system is called earth electrode resistance. Earth electrode resistance is the resistance, in ohms, between the point of connection and a distant point on the earth called remote earth. Remote earth, about 25 ft from the driven electrode, is the point where earth electrode resistance does not increase appreciably when this distance is increased. Earth electrode resistance consists of the sum of the resistance of the metal electrode (negligible) plus the contact resistance between the electrode and the soil (negligible) plus the soil resistance itself Thus, for all practical purposes, earth electrode resistance equals the soil resistance. The soil resistance is nonhnear, with most of the earth resistance contained within several feet of the electrode. Furthermore, current flows only through the electrolyte portion of the soil, not the soil itself Thus, soil resistance varies as the electrolyte content (moisture and salts) of the soil varies. Without electrolyte, soil resistance would be infinite. 

Effect of earth resistivity on statistical overvoltage distribution curves. 

The effect of various earth resistivities was also investigated, and it appears that there is no significant difference between the calculated results with the 50-1000 Q m earth resistivity. 

Preventive medicine through vaccination continues to be the most cost-effective pubHc health practice, even with the drastic advance in modern medicine. Mass vaccination programs have eradicated smallpox from the earth. The World Health Organization (WHO) has a major campaign underway to eradicate poHo by the year 2000. The development of vaccines has saved millions of Hves and prevented many more from suffering. However, there are stiU many diseases without effective vaccines, such as malaria. With the recent emergence of antibiotic-resistance strains and exotic vimses, an effective vaccine development program becomes a top priority of pubHc health poHcy. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

With four-electrode measurements effected from the surface, an average soil resistivity over a larger area is obtained. The resistivity of a relatively localized layer of earth or pocket of clay can only be accurately measured by using a spike electrode. Figure 3-18 gives dimensions and shape factors, Fg, for various electrodes. 

The connection to earth can be made utilizing earth rods, earth plates, earth lattices or grids, or buried strip conductors. Selection depends on the locality and type of ground conditions at the site. The effectiveness of the earth connection is, in turn, dependent on the resistivity of the soil at the site, the type of soil and climatic conditions have a direct effect upon the resistance of the connection made. Typical values of resistivity — m) for different types of soil are ... 

Being of low resistivity it has the effect of increasing the anode size with resulting reduction in resistance to earth. 

Earth embanked structures can be used if space is available (Figure 4.6). When possible, advantage can be taken of the high blast resistance of earth-covered structures either above or below ground since this form of construction is extremely resistant to high blast overpressures. Disadvantages include additional space required, non-conventional appearance, and effects of site conditions such as high water table. 

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