Ground-beds design

Probably the most important factor in estabhshing an adequate ground bed is the soil resistivity. We will briefly review resistivity before discussing ground bed design. 

Safety Ground Bed Design Proper design and construction of a safety groimd bed involves fom distinct steps ... 

Ground Beds and Ground Bed Design 426 Electrical Safety Programs 432 Summary 435 References 435... 

The ground-bed design is an essential component of a cathodic protection system. The following are where the salient points which need consideration while selecting a site for the ground-bed installation ... 

Design of ICS primarily involves anode selection, rectifier selection, ground-bed selection, and calculation of the I-ground-bed resistance and other main parameters. 

The ground-bed resistance is similar to that of the total circuit resistance calculated for the design of a sacrificial system (Section 15.6.2). In this case, multiple anodes are used and the resistance of anode to ground is calculated from Sunde equation [88]. 

Design Requirements Adequate performance of the station ground bed can usually be achieved by observing the foUowing guidelines (IEEE Std. 80 1986 Cooley and King 1980). 

Such anodes are used in areas where the resistivity of the soil is very high, such as in deserts. They are also suitable for areas where otherwise a large ground-bed electrically remote is required to keep the resistance to minimum. If the surrounding deep soil has a low resistivity, excellent distribution of current is obtained. The designs vary according to soil condition. 

Figure 5.49 Typical horizontal anode design chart for impressed current ground beds. (From TEXACO Cathodic Protection - Design and application school, Texaco Houston Research Center, Training Manual. Reproduced by kind permission of Cheveron, USA)...

The above procedure can be used to determine the anode to earth resistance. Design charts can be constructed for vertical anodes and horizontal anode ground-beds for both impressed and galvanic anode systems. The development of design curves is discussed in Section 5.31.2. 

Conducting soil resistivity surveys is a primary step in designing of a cathodic protection system for pipelines. The methods of determination of soil resistivity have been described in an entire section. Pipelines in low resistivity soils would require a greater amount of current for protection than pipelines in a high resistivity soils, because of a higher magnitude of corrosion in the former. Hence, low soil resistivity areas are selected to install the anode ground-bed. 

The following example shows how the design curve can be used to obtain the resistance of impressed current and galvanic anodes. Separate charts are constructed for impressed current ground-beds and horizontal ground-beds. With each chart the following design information is provided ... 

Design an impressed current system to protect a coated pipeline 4 mile long, 6(5/8)" OD in a soil of 2000 ohm-cm resistivity. Graphite anodes 3" X 5 are to be used. The back voltage between the pipeline and ground-bed is 3.0 V. 

A unique condition encountered on land that has been built up from coral deposits is the presence of blowholes, fissures and caves, which augments the penetration of seawater to areas remote from the actual seashore. Knowing that seawater makes for a very extensive, uniform, low resistivity "ground bed" for cathodic protection anodes, the above condition facilitates the design of unique cathodic protection systems. 

The current requirement of the protected object basically determines the design of the anode bed. For example, for a pipeline requiring 10 A with horizontal anodes laid in soil with p = 45 H m, according to Fig. 9-14, eight anodes are necessary. The grounding resistance of one anode amounts to Rq = 14 H. From Fig. 9-8, the grounding resistance of the anode bed with an interference factor F= 1.34 for 8 anodes spaced at 5 m comes to R = 2.34 Q.. 

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