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1、9576_C02.fmPage36Monday,October17,20057:15AM36ElectricPowerDistributionEquipmentandSystems(c)FIGURE2.1Continued.(c)Single-phase,4.8-kVcircuit.(d)FIGURE2.1Continued.(d)13.2-kVspacercable.Copyright©2006Taylor&FrancisGroup,LLC9576_C02.fmPage37Monday,October17,20057:15AMOverheadLines374
2、3−83−8441−6892−0PositionofguyNeutralFIGURE2.2Examplecrossarmconstruction.(From[RUS1728F-803,1998].)Woodisthemainpolematerial,althoughsteel,concrete,andfiberglassarealsoused.Treatedwoodlastsalongtime,iseasytoclimbandattachequipmentto,andalsoaugmentstheinsulationb
3、etweentheenergizedcon-ductorsandground.Conductorsareprimarilyaluminum.Insulatorsarepintype,posttype,orsuspension,eitherporcelainorpolymer.TheNationalElectricalSafetyCode(IEEEC2-2000)governsmanyofthesafetyissuesthatplayimportantrolesinoverheaddesignissues.Polesmusthavespaceforcrewsto
4、climbthemandworksafelyintheair.Allequipmentmusthavesufficientstrengthtostandupto“normal”operations.Conduc-torsmustcarrytheirweight,theweightofanyaccumulatedice,pluswith-standthewindpressureexertedonthewire.Wearenotgoingtodiscussmechanicalandstructuralissuesinthisbook.Formoreinformati
5、on,seetheLineman’sandCableman’sHandbook(Kurtzetal.,1997),theMechanicalDesignManualforOverheadDistributionLines(RUS160-2,1982),theNESC(IEEEC2-2000),andtheNESCHandbook(Clapp,1997).Overheadconstructioncancost$10,000/mito$250,000/mi,dependingonthecircumstances.Someofthemajorvariablesare
6、laborcosts,howdevel-opedthelandis,naturalobjects(includingrocksinthegroundandtreesintheway),whetherthecircuitissingleorthreephase,andhowbigtheconductorsare.Suburbanthree-phasemainsaretypicallyabout$60,000to$150,000/mi;single-phaselateralsareofteninthe$40,000to$75,000/mirange.Constru
7、ctionisnormallylessexpensiveinruralareas;inurbanareas,crewsmustdealwithtrafficandsetpolesinconcrete.AsWillis(1997)notes,upgradingacircuitnormallycostsmorethanbuildinganewline.Typicallythisworkisdonelive:theoldconductorhastobemovedtostandoffbracketswhilethenewconductorisstrung,andthep
8、olesmayhavetoberein
