Ground conductors

For an accurate measurement using a field voltmeter, the meter must be caUbrated (or zeroed) before a test. The charge on a grounded conductor is measured. A grounded conductor should have a charge equal to zero, which means the voltage would be zero. Therefore, the field meter should read zero (Fig. 2a). 

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. 

The ground conductor can be of aluminium, GI or copper, as discussed earlier. A humid or a chemically contaminated location is corroding in nature. Aluminium has a rapid reaction and is fast corroding. At such locations, use of GI or copper conductor would be more appropriate. Table 22.4 suggests the ground conductor sizes for aluminium conductor power cable.s for small and medium-ratine feeders when aluminium is used for the ground... 

Table 22.4 Size of aluminium ground conductor for different sizes of power cables for a grounding system...

For large feeders and FIT system.s the ground fault current would be controlled naturally through the ground circuit impedance and a smaller ground conductor may suftlce. 

In LT systems, where the neutral is groiintled, the neutral as well as the ground conductor tnay have to carry unbalanced currents up to hall the rating of the line currents due to single-phase loads. A ground conductor should also be rated for the same size as the neutral, irrespective of the setting of the relay. 

S = cross-sectional area of a bare ground conductor in mm . 

A. To calculate the main ground conductor size, assume that the system is protected through HRC fuses. Then, based on the previous assumptions. 

We discussed in Section 21.1.1 the maximum tolerable currents through a human body and their duration. The potential difference in a ground conductor at any point where a human body may come into contact with it during the course of a ground fault should be such that the resultant current through the human body will remain within these tolerable limits. 

The likely positions, in which a human body may come into contact with a phase or a ground conductor, and the corresponding potential differences, that he may be exposed to, are illustrated in Figure 22.9. 

Figure 22.9 Likely positions in which a human body may come in contact with phase and the ground conductors and the corresponding voltage gradients (illustrated through the layout of Figure 13.21)...

The flow of circulating currents in the grounding conductors or ground of region two caused between two or more interconnected grounding stations, for a fault occurring in region one is termed the telluric effect. 

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 ... 

To determine the minimum size of ground conductor, consider a station grid made of Z coated steel, having the following parameters ... 

Consider GI for grounding, and the same parameters of Section 22.14.1, leading to a minimum size of grounding conductor as 80 A/mm, based on IS 3043. Fault level for a 400 kV power station as in Table 13.10 is 40 kA (envisaging no further rise in the fault level, /q = /g)... 

Estimating the value of ground conductor length to design the grounding grid... 

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