Design of Grounding System

Design of Grounding System

Copyright: © 2018 |Pages: 46
DOI: 10.4018/978-1-5225-3853-0.ch002
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This chapter contains the types of grounding systems, single ground rod, single strip end connected, single strip center connected, radial grounding, single radials, equipotential mesh electrodes, radial grounding, multiple radials, and grid electrodes grid with ground rods, ring electrode, and grounding chain. It contains also the method of calculating single grounding electrode system resistance, length/depth of the ground electrode, diameter of the ground electrode and number of ground electrodes to obtain the required grounding system resistance. The chapter contains the grounding nomograph, example worksheet using nomograph, design of multiple grounding electrode in straight line and ring systems, examples layout and graph, multiple grounding electrode system are presented. It contains also calculating methods of multiple grounding electrode system resistance of different configurations. Color code technique to calculate the design parameters of the grounding rods is also presented.
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Types Of Grounding Systems

The basic philosophy of any ground electrode installation should be an attempt to maximize the surface area contact with the surrounding soil. Not only does this help to lower the resistance of the grounding system, but it also greatly improves the surge impedance of the grounding system due to the large capacitive coupling which is achieved. The actual layout of the grounding system, and the number, length and depth of ground rods will vary with the usages, type of soil and space availability. Some common designs include:

  • Single Ground Rod: One single ground electrode may be sufficient for an electrical installation in a built-up area where the local supply authority utilizes a multiple or common multiple earth neutral system. However, it may not provide adequately low impedance for lightning current injection. See (Figure 1).

  • Single Strip End Connected: This is a common option for installations where, because of rock, driving an electrode is impractical. It is not recommended for lightning protection systems as there is only one path. Very high ground voltages will be experienced at the injection point. Single strip end connected arrangement is given in (Figure 2).

  • Single Strip Center Connected: Since the connection to the strip is at the center, any fault/injection current travels in two directions. This layout, as given in (Figure 3) has lower impedance, but it is generally not adequate for lightning protection systems.

  • Radial Grounding, Single Radials: A design that is well suited to lightning protection in areas of medium resistivity. The radials can run to 100 feet in length. See (Figure 4).

  • Equipotential Mesh Electrodes: Minimize the risk of step and touch potential hazard by positioning a mat and bonding it to the structure or operating handle at locations where personnel may be required to operate switchgear or stand in the course of their duties. Low ground impedance can be obtained using mesh electrodes grounding given in (Figure 5).

  • Radial Grounding, Multiple Radials: Radial grounding has multiple radials design, as shown in (Figure 6), well suited to lightning as it allows energy to diverge as each conductor takes a share of the current, offering lower impedance. Voltage gradients leading away from the injection point will be lower, reducing danger from step potentials.

  • Grid Electrodes: Grounding for installations where there is concentration of electrical equipment, such as electrical substations, is often designed to meet a specific value of resistance (typically 1 ohm). Under fault conditions, a grid can dissipate currents over a large area. (See Figure 7).

  • Grid With Ground Rods: It may be advantageous to add ground rods to the grid. In doing so, it may be possible to access a low resistivity soil layer. Care must be taken to ensure each ground rod is spaced at least twice the installation depth. See (Figure 8).

  • Ring Electrode: Installations, including communications huts, pad mount transformers and fences surrounding high voltage installations, are generally surrounded by a ground ring. This practice also reduces the hazard of step and touch potential. See (Figure 9).

Figure 1.

Single ground rod

Figure 2.

Single strip end connected

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