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13-Alternatives to Copper GroundingCP91986

13-Alternatives to Copper GroundingCP91986 - COPPER...

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Unformatted text preview: _ COPPER GROUNDING ALTERNATIVES Alternatives to copper grounding Common bonding of underground ferrous structures to massive copper grounding grids creates problems for corrosion engineers in attempts to ca thodically protect these structures. Usual methods of electrics/grounding are discussed. Lack of effective communication be- tween engineering disciplines is recognized. Aller- natives to conventional copper grounding electrodes are discussed with consideration given to permanence ol_ the proposed electrodes. Examples of alternative grounding electrodes and systems employed over the past 25 y are presented. introduction MOST UNDERGROUND STRUCTURES have been bonded in common to reduce hazardous voltages associated with light- ning and manmade fault currents or induced currents in the earth. Common grounding provides an economical and low resistance ground for power systems; it also minimizes poten- tial differences in the earth and between individual metallic structures. Cathodic protection (GP) engineers have found them- selves in disagreement with power or grounding engineers electrical isolation and common bonding of metallic struc- tures to massive copper grounding grids or networks. While copper grounding has been the standard of the electrical in- dustry almost since inception, use of copper causes severe corrosion problems for connected ferrous structures. Copper is cathodic to other materials of construction. This accounts, in part, for copper's permanence since other materials will sacrifice themselves to protect the copper. Elimination of cop— per grounding can extend the life of the other commonly grounded underground strutstures. Generally, the corrosion control arguments, perceived as "black magic," are lost, to the requirements of established grounding practices. This is partially the result of the corro- sion engineer’s lack of effective communication with other engineering disciplines. The principles of galvanic corrosion and CP have not been effectively explained to engineers in the power industry. Over the years, some progress has been made toward pressing the case for selective electrical isolation. Earlier papers have dealt with isolation of cathodically Dro- * Presented during CORROSlON/efi, Paper No. 341, NACE, Houston, TX, 1986. ‘Richard B. Bender Corrosion Associates, Inc., PO. Box 11302, Fort Worth, TX 76110; and Kirk Engineering Company, Inc., 203 Sixth Street South, Oneonta. AL 35121. in sites requiring cathodic protection* Earl L. Kirkpatrick" tected structures from electrical grounding systems while maintaining AC electrical continuity.‘»2 When protecting a short, isolated, weillcoated, and small, diameter pipeline that has been provided with effective elec- trically insulating fittings at each end, all that may be required is a small sacrificial anode. As the structure becomes larger and more complex, current requirements increase. More and larger sacrificial anodes are required, or impressed current systems must be considered to meet these-increased needs. If dealing with a large, complex underground network, such as a pipeline compressor station or a power plant, many other underground structures enter the picture. At some point, it becomes imperative to consider common bonding of all underground structures to avoid deleterious cathodic inter- ference effects on nearby isolated metallic structures im fluenced by large impressed current CF systems. When this is done, the electrical grounding grid is tied invariably in com- mon with the structures to be protected. Electrical safety con- siderations concerning step potential, touch potential, and transfer potential may require common bonding of all struc- tures to the grid.3 Undoubtedly, this grid has been constructed of bare copper conductors and/or driven copper or copper clad ground rods. Conflict Copper-steel couples greatly accelerate the corrosion rate of the commonly bonded steel elements when CP is not applied.‘1 There are numerous instances of corrosion leaks oc- curring on plant piping in commonly bonded systems. This fre- quently happens before the plant is operated, particularly when CP was not implemented in the early stages of the prof- ect. Corrosion control design requirements are adversely im- ' pacted by the excessive current requirements necessary to at- fectively polarize a copper cathode;1 Alternatives There are acceptable alternatives to the use of bare cop- per conductors and ground rods including the following: Stain- less steel ground rods; sacrificial anodes in cast, rod, or rib- bon shapes; rebar or iron rods in concrete; galvanized steel ground rods and cables, as well as the use of cathodically pro tected less noble metals such as iron and steel.6 The National Electrical Code (NEC)7 does not require cop- per grounding; instead, it requires permanence in metal elec- trodes and conductors to be used for grounding. Romanoff presented data in 1957 that shows the performance of copper in highly reducing soils, and those containing sulfides is not appreciably better than the performance of iron.8 This observa- 0094—1492/86l000898l$3.00l0 September 1 986 © 1986 National Association of Corrosion Engineers 17 tion by Romanoff has been borne out in recent industry experi- ence where severe corrosion of copper concentric neutral wires has caused failure in Underground Residential Distribu- tion (UFiD) systems.9 Copper experiences rapid corrosion in some soils. Additionally, we now have definitive evidence that an alternating current flow between soil and copper can con- tribute to an increased corrosion rate on the copper grounding conductor)“ Even in soils where copper outperforms iron from a corrosion standpoint, copper may not have a sufficiently long service life to meet the life expectancy of the grounded structure or plant. Corrosion engineers must overcome the "mind set" by power industry and electrical engineers over the exclusive use of copper as the only corrosion free material of choice for grounding systems. Copper's low volume electrical resistivity makes it an effective conductor and/or grounding electrode. In isolated systems It may perform admirably as a grounding electrode. The principle corrosion problem arises when the copper grounding system is tied in common with other under- ground metals. Since copper, as a part of an iron-copper cou- ple, requires an inordinately large amount of direct current and is difficult to polarize, it places a burden on a CP system. This makes design of the GP system more difficult. Usually, the copper grid is placed in areas of the plant where it is already difficult to achieve CP on the underground piping because of the concentration of underground structures that increase the current required per unit volume in the soil. The net result is a requirement fora more elaborate CP system that may involve the expense of a distributed anode system. Copper conductors may be insulated and still serve as a grounding conductor without serving the dual function of a conductor and a grounding electrode. If a sufficiently low resistance grounding system can be obtained with driven ground rods (or galvanic anodes), the grounding contribution of the bare copper conductor is not necessary. Case histories illust‘ratealternative grounding systems. Serious considera- tion should be given to these approaches to the problem. Compressor stations The author began designing zinc electrode grounding systems with neoprene insulated connecting cables for pipe line compressor stations 25 y ago. Since the underground pip- ing was in common with the grounding system and was cathodically protected, there was no concern about galvanic corrosion losses on the zinc grounding electrodes. Use of the zinc grounding system made it easier to achieve effective levels of CF on the pressure and process piping close to the compressor building foundation. Similar systems have been used for years to ground pipeline motorized valves or to bleed off induced AC.11 Utility pier foundations During recent dealings with a major southern electrical utility on corresion control of power line structure footers. a review of the utility’s standards drawings revealed drawings resembling Figure 1. A series of grounding resistance calcula- tions was presented to show that very little grounding benefit was derived from installing a copper grounding plate beneath the poured pier. Pier diameters ranged from 2.5 to 7.5 ft (0.76 to 2.3 m) and to 10 to 85 ft (3.4 to 26 m) in length. Using Institute of Electrical and Electronics Engineers (lEEEWl formulas,12 the resistance to remote earth of a minimal column 2.5 ft (0.76 m) diameter by 20 ft (6.1 m) long with four number 15 (5.1 cm) rebars was calculated. When compared to the resistance to remote earth of a 1 ft (0.3 m) diameter copper plate, it was shown that the resistance of the four rebars was only 11 to 13% of the resist- ance of the copper plate, and the mutual interference (cou- pling) between the two closely spaced electrodes would fur- “)lEEE, New York. NY. 18 Vertical Reinforcing Bars lA-Zl To Be Seced \ Evenly Around Circle / \ Anchor bolt circle V \ A'Z/ \J Section A-A Approx t“half round drain groove Grout after pole is set and plumbed Rough foundation height for access to leveling nuts _i_ T Z' - ‘5‘: Copper round :2 pl! 6 In: :0): no Prollie View FlGUFtE 1 — Utility pole pier footing. ther reduce the contribution of the grounding plate to the total resistance of the couple. Clearly, there is little grounding benefit from providing the copper grounding plate under the poured pier. However, the copper grounding plate accelerates the corrosion rate on the gaIVanized steel pole that is in contact with the earth elec- trolyte. Because it requires approximately two orders of magnitude higher current density per unit area to protect bare copper in the soil compared to the current density required for iron in concrete, it can be seen that the copper represents a much greater liability from the corrosion standpoint than benefit from the grounding standpoint.‘3 Foundation grade beams The NEC recognizes the use of rebar not less than 0.5 in. (13 mm). 20 ft (6 m) or more in length as a grounding electrode with the proviso that it is encased by at least 2 in. (5 cm) of con— crete that is in direct contact with the earth. Steel below grade that is completely encapsulated in chloride free concrete may be expected to perform satisfactorily for long periods of time. Because of the large quantities of rebar that goes into the average commerciai or industrial project, the grounding engi- neer should not want for an adequate ground. The biggest con- cern may be to limit the extent of the grounding network. Substation grounding grid Galvanized steel ground rods supplemented by GP have long been employed by the Rural Electrification Administra- tion (REA) and others. More recently, entire grounding grids have been fabricated from galvanized steel rods and cable.” Materials Performance Communications Effective communications and education require more than talking or presenting your point of view. Effective com- munication with others requires understanding of the other person's viewpoint and problems. Engineers involved in foren- sic testimony have discovered that the successful trial lawyer has become far better versed in the engineer's specialty than the engineer would ever hope or desire to become in trial law. When a corrosion problem arises from a grounding con- flict. ask where and when there was a failure to communicate or educate. Corrosion engineers familiar with electrical grounding practices and principles can work effectively with other disciplines to meet the dual. and sometimes conflicting requirements of effective electrical grounding and corrosion control. By educating our electrical engineering counterparts about galvanic corrosion problems and offering acceptable alternatives to copper grounding electrodes. grounding net- works can be obtained that are compatible with CP systems. References 1. E. Kirkpatrick, CORROSION/74. Paper No. 6. NACE. Hous- ton, TX. 1974. 2. E. Kirkpatrick. CORROSION/79. Paper No. 53. NACE. Hous- ton, TX. 1979. 3. IEEE Std. 80-1976, “IEEE Guide for Safety in Substation Grounding." The Institute of Electrical and Electronics Engineers. Inc., New York. NY. 0, 8, 1976. 10. 12. . Babolan. France, Rowe, Ftynewicz. “Galvanic Fitting & Corrosion—Field & Laboratory Studies," ASTM. Phila- delphia, PA. p. 55. 1976. L. Applegate, Cathodic Protection, McGraw-Hill Book Company, Inc... New York, NY. p. 17, 1960. 1-3-7 Year Test Program, Final Report. NACE Work Group T-1OC-2a. NACE. Houston. TX. 1971. NFPA 70-1984. National Electrical Code. National Fire Pro- tection Association, Article 250-81. 1984. M. Romanoff. ”Underground Corrosion." US National Bureau of Standards. Circular 579. US Government Print- ing Office. Washington, DC. p. 80. 1957. K. Compton, Materials Performance. Vol. 21, No. 3, p. 31, 1982. J. Hanck, G. Nekoksa. Materials Performance, Vol. 23, No. 3, p. 43, 1984. . NACE Standard FlP-01-77 (1983 Revision). “Mitigation of Alternating Current and Structures and Corrosion Control Systems." NACE. Houston. TX. 1983. IEEE Std. EIO Reaffirmed 1971. "IEEE Guide for Safety in Alternating-Current Substation Grounding." The institute of Electrical and Electronics Engineers. Inc, New York, NY,p.54. 1971. P. Fidjestol, B. Flonning. B. Floland, Materials Perform— ance. Vol. 24, N0. 7. p. 12, 1985. J. Nelson. W. Holm, IEEE Transactions on Industry Appli- cationS. Vol. 1A-21. No. 2, The Institute of Electrical and Electronics Engineers. Inc.. New York. NY. 1985. ——_._—___—_——— September 1986 19 ...
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