Earth resistivity

The commercial value of a clay deposit depends on market trends, competitive materials, transportation faciflties, new machinery and processes, and labor and fuel costs. Naturally exposed outcrops, geological area and stmcture maps, aerial photographs, hand and power auger drills, core drills, earth resistivity, and shallow seismic methods are used ia exploration for clays (32). Clays are mined primarily by open-pit operation, including hydraulic extraction however, underground mining is also practiced. 

Guide for measuring earth resistivity, ground impedance and earth stirface potentials of a ground system. Part I Normal measurements... 

Figure 4-254. Attenuation of electromagnetic signals for 1 and 0 n m average earth resistivity (a) attenuation as a function of frequency (b) maximum depth reached versus frequency. (Courtesy Geoservices [109]. ...

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. 

Change in Earth Resistance with Multiple Ground Rods... 

Rectangular or circular plates should present an area of at least 2 ft2 to the soil. Electrodes of iron and steel shall be at least 1/4 inch in thickness electrodes of nonferrous metal should have a minimum thickness of 0.06 inch. Plate electrodes are to be installed at a minimum distance of 2.5 ft below the surface of the earth. Table 5.8 gives the earth resistance values for circular plates buried 3 ft below the surface in soil with a resistivity of 10,000 Q-cm. 

Due to its simple construction and the lack of galvanic isolation, a safety barrier shall be connected to the equipotential bonding system, which is stipulated imperatively in hazardous areas. As a rule, a minimum conductor cross-section of 4 mm2 (for copper) or an earthing resistance lower than 1 ohm shall be used. 

Lault loop impedance (TN systems) or earthing resistance (IT systems) is satisfactory X X X... 

Neutral earthing arrangements, e.g. solid, with a neutral earthing resistance (NER), with a common busbar, switches or circuit breakers for isolation. 

The overhead earthing conductor will divert some of the L-G fault current from entering the ground at the foot of the pole. The extent of diversion will be in proportion to the impedance of the overhead line compared with that of the earth resistance path back to the source. The calculations required for determining the fault current and its diverted amounts are shown in Appendix H by way of an example, and Figure 13.12. 

Assume that the screen is earthed at both ends at the same points as the signal source and the output load. Hence the common earth resistance Re is shared by both the screen and the core circuits. Again assume that Fin is zero for the same reason as above. 

Table H.la. Earth fault current as a function of earth resistance...

Earth resistance (ohms) Earth fault current (amps) Proportion of current diverted to the X-to-R ratio (pu)... 

Table H.lc. Data for earth resistance, touch voltage, ground potential rise and corner mesh voltage for different grid and rod designs...

The four-point probe resistivity measurement technique is well established. It was first proposed for semiconductor measurements in 1952 [46] although it had been used for earth resistivity measurements before that. The resistivity, p, for a collinear four-point probe with probe spacing, s, is given by... 

Earth resistivity lOOLlm Abnomial body 40iii 40m 40iii. 

Roman, I. 1934. Some interpretations of earth resistivity data. Transactions on American Institute of Mining and Metallurgical Engineers, 110 183. 

Barnes, H.E. 1952. Soil investigation employing a new method of layer-value determination of earth-resistivity interpretation. Highway Research Board Bulletin, 65 26-36. 

Fig. 10.49 Megger earth resistivity meter (courtesy Evershed and Vignoles)...

Kloukinas P, Mylonakis G (2011) Rankine Solntimi for seismic earth pressures on L-shaped retaining walls. In 5 ICEGE, Santiago, Chile, 10-13 January 2011 Lancellotta R (2007) Lower-bound approach for seismic passive earth resistance. Geotechnique... 

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. 

Another approach is shown in Fig. 10.223. This device incorporates a number of ports (holes) in the assembly. Moisture from the soil (and rain) is absorbed through the ports. The metallic salts subsequently absorb the moisture, forming a saturated solution that seeps out of the ports and into the earth-to-electrode hemisphere. Tests have shown that if the moisture content is within the required range, earth resistivity can be reduced by as much as 100 1. Figure 15.224 shows the measured performance of a typical chemical ground rod in three types of soil. 

Standard tests for evaluating stray currents are rare. ASTM G 165, Practice for Determining Rail-to-Earth Resistance, provides a test method for measuring the resistance-to-earth of the running rails, which are used as the conductors for returning the train operating current to the substation in electric mass transit systems. For more information, consult Refs 25, 26, and 48. 

Standard Practice for Conducting and Evaluating Laboratory Corrosion Tests in Soils Standard Practice for Determining Rail-to-Earth Resistance... 

Although the scaffold pole was a poor earth electrode, having a considerable earth resistance, it allowed sufficient current to flow through the labourer to cause fatal electrical injuries. Typical resistances would be 400 ohms for the earth connection and 2000 ohms for the labourer s body, resulting in a shock current of 230/(400 + 2000) = 0.096 A. A hand-to-hand current of 96 mA is sufficient to cause ventricular fibrillation. 

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