Ground resistivity

Each electric motor and connected control gear is grounded separately at least two points. The ground resistance should be checked to ensure continuity of ground conductors. Refer to Chapter 21 for more details on grounding requirements. 

From the above the resistance to ground of a plate grounding is inversely proportional to the square root of the linear dimension fA) of the plate. The variation in resistance with the size of the plate is shown in Figure 22.2. Considering the resistivity of soil as 10 k2m, since the ground resistance is proportional to the resistivity of soil, there would be different parallel curves for the ground resistance for different values of resistivity of soil. 

Figure 22.5 Likely variation in ground resistance with more than one electrode connected in parallel...

The same lest can also be conducted with the help of a battery, voltmeter and an ammeter, as illustrated in Figure 22.8. The voltmeter must now indicate the same reading at all three locations. When V becomes constant, read the current /. Then the ground resistance... 

Figure 22.7 Measuring the ground resistance with the help ot a ground tester...

When the generator and the switchyard grounding mats are interconnected the ground fault current will divide between the two, depending upon their ground resistances, in inverse proportions (Figure 22.11) such that... 

The ground resistance is a function of the area occupied by the grounding station and the stratification of the soil. The stratification of the soil is usually of a non-uniform nature and may vary the resistivity of soil vertically as well as horizontally, thus varying the resistance of soil. The minimum value of ground resistance (resistance of the grounding station) at a certain depth h from the ground surface may be expressed by... 

With an increase in the length of the buried ground conductors, the value of their ground resistance diminishes. It has been found that equation (22.13) is more accurate for a grid depth up to 250 mm. At greater depths of station grids, a moi e accurate representation is found in the following equation ... 

Total ground resistance of power plant iirid switchyard aretis interconiieeied in parallel (recommended practice) where... 

R = station ground resistance in p = average resistivity of soil in 2m... 

The validity of Eq. (3-15) further follows from the results given in Fig. 3-4. It represents the current-potential curve for a buried storage tank. The difference f/ n - I/off is proportional to the current /. The quotient R is equal to the grounding resistance of the tank. The (/) curve corresponds to the true polarization curve. 

Fig. 3-9 Ohmic voltage drop at defects in the pipe coating. (1) spherical field (2) voltage drop at the soil surface (3) cylindrical field (5) = voltage drop at the defect d) = voltage drop at the grounding resistance.

The Wenner method is chiefly used to determine the grounding resistance along the pipeline track and the installation positions for cathodic structures. Local limited soil resistivity is most clearly determined from the grounding resistance of an inserted Shephard rod (see Fig. 3-18). Soil stratification can be recognized from the apparent specific soil resistivity, p, by the Wenner method, if a is varied. 

Grounding resistances for the usual electrode and anode shapes as well as the form of their voltage cones on the soil surface are given in Section 9.1 and in Table 24-1. 

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