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This formula, together with the IEEE Standard No. 80-1986 111, is used in the design of a steel grounding system for a large heavy industrial plant. Formulas are also discussed on how to design proper cathodic protection methods to minimize corrosion of a steel grounding system. A numerical example is discussed to enhance the understanding. Based on the National Institute of Standards and Technology statistical data on the corrosion of steel, an equation is developed to estimate the corrosion rate of underground steel that varies with resistivity, pH value, moisture content and aeration of the soil.
IEEE Standard No. 80-1986 entitled "IEEE Guide for Safety in AC Substation Grounding" discusses in detail the design of ground grid for AC sub-station with primarily copper as the ground grid material.
The selection of material used in the electrical ground grid depends primarily on the following factors:
* Fusing or current carrying capabilities
* Conductor resistance
* Availability & cost of conductor material
Copper, in addition to high conductivity, has the advantage of freedom from underground corrosion and is the most commonly used metal in the grounding grid design. However, bare copper conductors buried in the ground form a galvanic cell with the buried steel in the vicinity and corrodes steel. Unless some corrosion reduction technique is adopted, damage may extend to all steel or galvanized members, such as conduit, structure footings, metallic sheathed cables and pipes.
A relationship is obtained from the National Institute of Standards and Technology data 171 to represent the corrosion rate of steel (Bessemer Steel) . These data were produced from actual experimental data taken in 44 different soils over a period of 12 years. These tests used l½ inch and 3 inch diameter samples. It was noted that the 3 inch samples corroded 13% more than the l½ inch samples, with an error of± 10%. A general equation including the effects of the above factors is provided in the following form.
|Design of Steel grounding System in a Heavy Industrial Plant|