买3008记者帮砍价汽车周刊记者

1BranchCircuitsChapter2汽车防盗器•Thepurposeofbranchcircuitsistocarrythecurrentfromtheserviceentrancepanel(SEP)totheelectricaldevices.•Threecommontypesofcurrentareusedinagriculturalbuildings:–120volt(108–125)–240volt(220–250)–ThreephaseWhyarearangeofvoltageslisted?Whathappenswhenthevoltagedropsbelow108V?Whathappenswhenthevoltagegoesabove124V?3ServiceEntrancePanel(SEP)•Theserviceentrancepanel(loadcenter)istheentrypointfortheelectricityintothebuilding.•Thesize(ampcapacity)oftheloadcenterisdeterminedbythenumberofcircuitsandtotalamploadforthebuilding.•CurrentNECregulationsrequirethattheloadcenterhaveamasterdisconnect.•TheentrancepanelmustbegroundedwithaNECapprovedearthconnection.WhatisanNECearthconnection?4ServiceEntrancePanel[SEP]“LoadCenter”MasterDisconnect120/240VServiceService“Hot”ConductorsMetalBoxNon-conductingbaseNon-conductingAttachmentbarsConductingAttachmentbars120VBranchCircuitGround(bare)Conductor240VCircuit120VBranchCircuit“Hot”(black)ConductorGroundNeutral120V120V120VBranchCircuitNeutral(white)ConductorEarthGroundBondingScrewCircuitNeutral&GroundConnectionsGroundingBarBreakerServiceEntranceNeutral5SEP--cont.•The120/240serviceisattachedtothemasterdisconnect(breaker).•Frommasterbreakereachhotconductorisconnectedtooneoftheconductingbreakerbars.•The120/240neutralconductorisattachedtothegroundingbar.•A120voltbreakerattachesbysnappingontooneconductingandonenon-conductingbarintheloadcenter.•Fora240voltcircuittwoindividualbreakersmaybeusedandtheleversarepinedtogetheroracombinationbreakermaybeused.Grounding•Branchcircuitshavetwodifferenttypesofgrounds.–System–Equipment6Systemgroundingisaccomplishedbyoneofthetwocurrentcarryingconductors(white).Whatisanothertermforthesystemground?Whatistheinsulationcolorofthesystemground?Whatistheinsulationcoloroftheequipmentground?Grounding-Equipment•EquipmentgroundingisthebondingofallnoncurrentcarryingmetalcomponentsbacktotheSEP.•Theequipmentgroundisdesignedtoprovidealowresistantcircuittotheearthintheeventofashortfromtheenergizedconductortoanymetalliccomponent.•Mustmakeacomplete,lowresistancecircuitfromallmetallicelectricaldevicesinthesystemtotheearth.7EquipmentgroundWhathazardiscreatediftheequipmentgroundisinterrupted?120VCircuits•120Vcircuitshave3or4conductors:–oneenergized(hot)conductorBlackorred–oneneutralconductorWhite–onegroundconductor.Bareorgreen8WhatothercommonbuildingcomponentismadefromPVCWhatdoesPVCstandfor?240VCircuits•240Voltcircuitshavethreeconductors:–Twohot–Equipmentground•Neutralcircuitisnotrequiredunlessboth240and120circuitsaresuppliedbythedevice.•The240VoltelectricalservicetotheSEPwillhaveaneutralsoboth240and120Voltbranchcircuitscanbeused.9ThreeCircuitTypes•Generalpurposebranchcircuits•Individualbranchcircuits•Motor10GeneralPurposeBranchCircuits•DesignedfortemporaryloadssuchaslightsandDCOs(DuplexConvenienceOutlets)under1500W.•Minimum12AWG•Fusedat20amps•Nomorethanten(10)DCOsorlightfixturespercircuit.(Fig.2-3,pg21)•RecommendedlocationforDCOs(Table1-12,pg18).11SpecialPurposeBranchcircuits•Usedforknownspecificloads–Stationarymotors–Stationaryappliances•SPOs(SpecialPurposeOutlets)–Usuallyusedforloadsgreaterthan20amps—240V.12WhatwouldbeanexampleofanSPOinanagriculturebuilding?MotorCircuits•Use240Vwheneverpossible.–Reducesamperageloadoncircuit–Reducesstrayvoltagepotential•Five(5)horsepowerandlargershouldbe3phase.13MotorCircuits—cont.•Branchcircuitsforelectricmotorshavefour(4)requirements(Fig2-6through9,pg23-26):1Branchcircuit,shortcircuitprotection2Adisconnectingmeans3Acontroller4Overloadprotection•SummaryTable2-1,pg2614MotorCircuits—ShortCircuitProtection•Fuseorcircuitbreaker–Formotorcircuitstheymusthavegreatercapacitythanfullloadcurrent.•Motorstartingloadishigherthantherunningload—SCPdevicesmustbeabletohandletemporaryoverload.–Inversetimebreaker–Timedelayfuses•Maximumsize–Inversetimebreaker=2.50timesfullloadcurrent–Timedelayfuses=1.75timesfullloadcurrent.15MotorCircuits—ShortCircuitProtection--example•DeterminetherequiredSCPfora120Vcircuitfora½horsepower,singlephasemotor.16Table2-3.1/2hp=9.8VBreaker:9.8x2.5=24.5AFuse:9.8x1.75=17.15•DeterminetherequiredSCPfora240Vcircuitfora1/6horsepower,singlephasemotor.Table2-3:1/6hp=2.2VBreaker:2.2x2.5=5.5AFuse:2.2x1.75=3.8A•Smallestbreakeris10A•Solution—Use10AbreakerintheSEPandinstalla4to6ampfuseinlinewiththemotor.MotorCircuits—DisconnectingMeans•Eachmotorormotorcircuitmusthaveanindividualdisconnectingmeans.•Thedisconnectingmeansmustdisconnectallhotwires.•TheDMmustclearlyindicatewhetheritisonoroff.17MotorCircuits—DisconnectingMeans-cont.•Mustbelocatedwithinsightandwithin50feetofthecontrollerandthemotor.•DisconnectingMeans–Stationarymotorscanusethecircuitswitchaslongascorrectsize.–Portablemotorstheplugandreceptacleisacceptable.•Thecircuitswitchcanbeasnapswitchaslongasthemotoris2hporlessanditscapacityisequaltoor1.25timesgreaterthanthemotorfullloadrating.18Whatisasnapswitch?MotorCircuits—Controller•Acontrollerisadeviceusedtoautomaticallystartandstopamotor.•Onlyrequiredtoopenenoughconductorsto