Breaker.docx

Breaker.docx

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Breaker

Circuitbreaker

FromWikipedia,thefreeencyclopedia

Forotheruses,see Circuitbreaker（disambiguation）.

Ithasbeensuggestedthat OilCircuitBreaker be merged intothisarticle.（Discuss） ProposedsinceApril2016.

Partofaserieson

Electricalwiring

Regulationofelectricalinstallations

∙BS7671

∙IEC60364

∙CanadianElectricalCode（CEC）

∙NationalElectricalCode（NEC）

Wiringmethods

∙Armoredcable

∙Cabletray

∙Electricalconduit

∙Knob-and-tubewiring

∙Multiwayswitching

∙Thermoplastic-sheathedcable

Circuitbreakers anddevices

∙AFCI

∙ELCB

∙GFCI/RCD

∙Receptacles/Sockets

∙SulfurHexafluoride

∙Switch

Electricaldistribution

∙Busbar

∙Distributionboard

∙Switchgear

∙Switchboard

∙Transformer

Wiringbyregionorcountry

∙HongKong

∙NorthAmerica

∙UnitedKingdom

∙v

∙t

∙e

CircuitBreaker

Electronicsymbol

Anaircircuitbreakerforlow-voltage（lessthan1,000volt）powerdistributionswitchgear

Atwo-poleminiaturecircuitbreaker

Fourone-poleminiaturecircuitbreakers

Molded-casecircuitbreaker

A circuitbreaker isanautomaticallyoperated electrical switch designedtoprotectan electricalcircuit fromdamagecausedby overcurrentoroverload or shortcircuit.Itsbasicfunctionistointerruptcurrentflowafterprotectiverelays detecta fault.Unlikea fuse,whichoperatesonceandthenmustbereplaced,acircuitbreakercanbereset（eithermanuallyorautomatically）toresumenormaloperation.Circuitbreakersaremadeinvaryingsizes,fromsmalldevicesthatprotectanindividualhouseholdapplianceuptolarge switchgear designedtoprotect highvoltage circuitsfeedinganentirecity.Thegenericfunctionofacircuitbreaker, RCD ora fuse,asanautomaticmeansofremovingpowerfromafaultysystemisoftenabbreviatedtoADS（AutomaticDisconnectionofSupply）.

Contents

  [hide] 

∙1Origins

∙2Operation

∙3Arcinterruption

∙4Short-circuit

∙5Standardcurrentratings

∙6Typesofcircuitbreakers

o6.1Low-voltagecircuitbreakers

o6.2Magneticcircuitbreakers

o6.3Thermalmagneticcircuitbreakers

o6.4Commontripbreakers

o6.5Medium-voltagecircuitbreakers

o6.6High-voltagecircuitbreakers

o6.7Sulfurhexafluoride（SF6）high-voltagecircuitbreakers

o6.8Disconnectingcircuitbreaker（DCB）

o6.9Carbondioxide（CO2）high-voltagecircuitbreakers

∙7Otherbreakers

∙8Seealso

∙9References

∙10Externallinks

Origins[edit]

Anearlyformofcircuitbreakerwasdescribedby ThomasEdison inan1879patentapplication,althoughhiscommercialpowerdistributionsystemused fuses.[1] Itspurposewastoprotectlightingcircuitwiringfromaccidentalshortcircuitsandoverloads.Amodernminiaturecircuitbreakersimilartotheonesnowinusewaspatentedby Brown,Boveri&Cie in1924.HugoStotz,anengineerwhohadsoldhiscompanyto BBC,wascreditedastheinventoronDRP（DeutschesReichspatent）458392.[2] Stotz'sinventionwastheforerunnerofthemodernthermal-magneticbreakercommonlyusedinhouseholdloadcenterstothisday.Interconnectionofmultiplegeneratorsourcesintoanelectricalgridrequireddevelopmentofcircuitbreakerswithincreasingvoltageratingsandincreasedabilitytosafelyinterrupttheincreasingshortcircuitcurrentsproducedbynetworks.Simpleair-breakmanualswitchesproducedhazardousarcswheninterruptinghighcurrents;thesegavewaytooil-enclosedcontacts,andvariousformsusingdirectedflowofpressurizedair,orofpressurizedoil,tocoolandinterruptthearc.By1935,thespeciallyconstructedcircuitbreakersusedatthe BoulderDamprojectuseeightseriesbreaksandpressurizedoilflowtointerruptfaultsofupto2,500MVA,inthreecyclesoftheACpowerfrequency.[3]

Operation[edit]

Allcircuitbreakersystemshavecommonfeaturesintheiroperation.Althoughdetailsvarysubstantiallydependingonthevoltageclass,currentratingandtypeofthecircuitbreaker.

Thecircuitbreakermustdetectafaultcondition;in lowvoltage circuitbreakersthisisusuallydonewithinthebreakerenclosure.Circuitbreakersforlargecurrentsorhighvoltagesareusuallyarrangedwith protectiverelay pilotdevicestosenseafaultconditionandtooperatethetripopeningmechanism.Thetrip solenoid thatreleasesthelatchisusuallyenergizedbyaseparate battery,althoughsomehigh-voltagecircuitbreakersareself-containedwith currenttransformers,protectiverelays,andaninternalcontrolpowersource.

Onceafaultisdetected,thecircuitbreakercontactsmustopentointerruptthecircuit;somemechanicallystoredenergy（usingsomethingsuchasspringsorcompressedair）containedwithinthebreakerisusedtoseparatethecontacts,althoughsomeoftheenergyrequiredmaybeobtainedfromthefaultcurrentitself.Smallcircuitbreakersmaybemanuallyoperated,largerunitshave solenoids totripthemechanism,and electricmotors torestoreenergytothesprings.

Thecircuitbreakercontactsmustcarrytheloadcurrentwithoutexcessiveheating,andmustalsowithstandtheheatofthearcproducedwheninterrupting（opening）thecircuit.Contactsaremadeofcopperorcopperalloys,silveralloysandotherhighlyconductivematerials.Servicelifeofthecontactsislimitedbytheerosionofcontactmaterialduetoarcingwhileinterruptingthecurrent.Miniatureandmolded-casecircuitbreakersareusuallydiscardedwhenthecontactshaveworn,butpowercircuitbreakersandhigh-voltagecircuitbreakershavereplaceablecontacts.

Whenacurrentisinterrupted,an arc isgenerated.Thisarcmustbecontained,cooledandextinguishedinacontrolledway,sothatthegapbetweenthecontactscanagainwithstandthevoltageinthecircuit.Differentcircuitbreakersuse vacuum,air, insulatinggas,or oil asthemediumthearcformsin.Differenttechniquesareusedtoextinguishthearcincluding:

∙Lengtheningordeflectingthearc

∙Intensivecooling（injetchambers）

∙Divisionintopartialarcs

∙Zeropointquenching（contactsopenatthezerocurrenttimecrossingofthe AC waveform,effectivelybreakingnoloadcurrentatthetimeofopening.Thezerocrossingoccursattwicethelinefrequency;i.e.,100timespersecondfor50 Hzand120timespersecondfor60 HzAC.）

∙Connecting capacitors inparallelwithcontactsin DC circuits.

Finally,oncethefaultconditionhasbeencleared,thecontactsmustagainbeclosedtorestorepowertotheinterruptedcircuit.

Arcinterruption[edit]

Low-voltageminiaturecircuitbreaker（MCB）usesairalonetoextinguishthearc.Thesecircuitbreakerscontainso-calledarcchutes,astackofmutuallyinsulatedparallelmetalplateswhichdivideandcoolthearc.Bysplittingthearcintosmallerarcsthearciscooleddownwhilethearcvoltageisincreasedandservesasanadditionalimpedancewhichlimitsthecurrentthroughthecircuitbreaker.Thecurrent-carryingpartsnearthecontactsprovideeasydeflectionofthearcintothearcchutesbyamagneticforceofacurrentpath,although magneticblowout coilsor permanentmagnets couldalsodeflectthearcintothearcchute（usedoncircuitbreakersforhigherratings）.Thenumberofplatesinthearcchuteisdependentontheshort-circuitratingandnominalvoltageofthecircuitbreaker.

Inlargerratings,oilcircuitbreakersrelyuponvaporizationofsomeoftheoiltoblastajetofoilthroughthearc.[4]

Gas（usually sulfurhexafluoride）circuitbreakerssometimesstretchthearcusingamagneticfield,andthenrelyuponthedielectricstrength ofthesulfurhexafluoride（SF6）toquenchthestretchedarc.

Vacuum circuitbreakershaveminimalarcing（asthereisnothingtoionizeotherthanthecontactmaterial）,sothearcquencheswhenitisstretchedaverysmallamount（lessthan2–3 mm（0.079–0.118 in））.Vacuumcircuitbreakersarefrequentlyusedinmodernmedium-voltageswitchgearto38,000volts.

Aircircuitbreakersmayuse compressedair toblowoutthearc,oralternatively,thecontactsarerapidlyswungintoasmallsealedchamber,theescapingofthedisplacedairthusblowingoutthearc.

Circuitbreakersareusuallyabletoterminateallcurrentveryquickly:

typicallythearcisextinguishedbetween30msand150msafterthemechanismhasbeentripped,dependinguponageandconstructionofthedevice.Themaximumcurrentvalueandlet-throughenergydeterminethequalityofthecircuitbreakers.

Short-circuit[edit]

Circuitbreakersareratedbothbythenormalcurrentthattheyareexpectedtocarry,andthemaximumshort-circuitcurrentthattheycansafelyinterrupt.Thislatterfigureisthe ampereinterruptingcapacity （AIC）ofthebreaker.

Undershort-circuitconditions,thecalculatedmaximum prospectiveshortcircuitcurrent maybemanytimesthenormal,ratedcurrentofthecircuit.Whenelectricalcontactsopentointerruptalargecurrent,thereisatendencyforan arc toformbetweentheopenedcontacts,whichwouldallowthecurrenttocontinue.Thisconditioncancreateconductiveionizedgasesandmoltenorvaporizedmetal,whichcancausefurthercontinuationofthearc,orcreationofadditionalshortcircuits,potentiallyresultingintheexplosionofthecircuitbreakerandtheequipmentthatitisinstalledin.Therefore,circuitbreakersmustincorporatevariousfeaturestodivideandextinguishthearc.

Themaximumshort-circuitcurrentthatabreakercaninterruptisdeterminedbytesting.Applicationofabreakerinacircuitwithaprospectiveshort-circuitcurrenthigherthanthebreaker'sinterruptingcapacityratingmayresultinfailureofthebreakertosafelyinterruptafault.Inaworst-casescenariothebreakermaysuccessfullyinterruptthefault,onlytoexplodewhenreset.

Typicaldomesticpanelcircuitbr