Fuse

Workingprinciple

Anelectricalappliancethatusesametalconductorasamelttobeconnectedinseriesinacircuit.Whenoverloadorshort-circuitcurrentpassesthroughthemelt,itwillfuseduetoitsownheat,therebybreakingthecircuit..Thefuseissimpleinstructureandconvenienttouse,andiswidelyusedasaprotectivedeviceinpowersystems,variouselectricalequipmentandhouseholdappliances.

Commontypes

Plug-infuse:Itisoftenusedattheendofalinewithavoltagelevelof380Vandbelow,asashort-circuitprotectionfordistributionbranchlinesorelectricalequipmentuse.

Screwfuse:Thereisafuseindicatorontheupperendcoverofthemelt.Oncethemeltisblown,theindicatorpopsupimmediatelyandcanbeobservedthroughtheglassholeontheceramiccapUntilnow,itisoftenusedinelectricalcontrolequipmentformachinetools.Screwfuse.Thebreakingcurrentislarge,anditcanbeusedforshortcircuitprotectionincircuitswithavoltagelevelof500Vandbelowandacurrentlevelof200A.

Closedfuses:Closedfusesaredividedintotwotypes:filledfusesandnon-filledfuses,asshowninFigure3andFigure4.Filledfusesgenerallyusesquareporcelaintubeswithquartzsandandmeltinside,andhavestrongbreakingcapacity.Theyareusedincircuitswithvoltagelevelsbelow500Vandcurrentlevelsbelow1KA.Theunfilledclosedfuseputsthemeltintoaclosedcylinderwithaslightlysmallerbreakingcapacity,andisusedinpowergridsorpowerdistributionequipmentbelow500Vandbelow600A.

Fastfuse:Fastfuseismainlyusedforshort-circuitprotectionofsemiconductorrectifiercomponentsorrectifierdevices.Becausetheoverloadcapacityofsemiconductorcomponentsisverylow.Itcanonlywithstandalargeoverloadcurrentinaveryshorttime,sotheshort-circuitprotectionisrequiredtohavetheabilitytoquicklyfuse.Thestructureofthefastfuseisbasicallythesameastheenclosedfusewithfiller,butthemeltmaterialandshapearedifferent.Itisavariablecross-sectionmeltwithaV-shapeddeepgroovepunchedfromasilversheet.

Self-resettingfuse:Usingsodiummetalasthemelt,ithashighconductivityatroomtemperature.Whenashort-circuitfaultoccursinthecircuit,theshort-circuitcurrentgeneratesahightemperaturetoquicklyvaporizethesodium,andthevaporizedsodiumpresentsahigh-resistancestate,therebylimitingtheshort-circuitcurrent.Whentheshort-circuitcurrentdisappears,thetemperaturedrops,andthemetalsodiumrestoresitsoriginalgoodconductivity.Theself-resettingfusecanonlylimittheshort-circuitcurrentandcannotreallybreakthecircuit.Theadvantageisthatthemeltdoesnotneedtobereplacedandcanbereused.

Structuralcharacteristics

Theratedcurrentofthemeltisnotequaltotheratedcurrentofthefuse.Theratedcurrentofthemeltisselectedaccordingtotheloadcurrentoftheprotectedequipment,andtheratedcurrentofthefuseshouldbegreaterthantheratedcurrentofthemelt,Determinedinconjunctionwiththemainelectricalappliance.

Thefuseismainlycomposedof3parts:melt,shellandsupport,amongwhichthemeltisthekeyelementtocontrolthefusingcharacteristics.Thematerial,sizeandshapeofthemeltdeterminethefusingcharacteristics.Meltmaterialsaredividedintotwotypes:lowmeltingpointandhighmeltingpoint.Lowmeltingpointmaterialssuchasleadandleadalloyshavealowmeltingpointandareeasytofuse.Duetotheirlargeresistivity,thecross-sectionalsizeofthemeltislarger,andmoremetalvaporisgeneratedduringfuse.Itisonlysuitableforfusewithlowbreakingcapacity.Device.Highmeltingpointmaterialssuchascopperandsilverhavehighmeltingpointsandarenoteasytofuse,butduetotheirlowresistivity,theycanbemadeintoasmallercross-sectionalsizethanlowmeltingpointmelts,andproducelessmetalvaporduringfuse,whichissuitableforhighbreakingCapablefuse.Theshapeofthemeltisdividedintotwotypes:filamentandribbon.Changingtheshapeofthevariablecross-sectioncansignificantlychangethefusingcharacteristicsofthefuse.Thefusehasavarietyofdifferentfusingcharacteristiccurves,whichcanbeadaptedtotheneedsofdifferenttypesofprotectionobjects.

Ampere-secondcharacteristics:

Theactionofthefuseisrealizedbythefusingofthemelt.Thefusehasaveryobviouscharacteristic,thatis,thesafetySecondcharacteristic.

Forthemelt,itsoperatingcurrentandoperatingtimecharacteristicsaretheampere-secondcharacteristicsofthefuse,alsocalledinversedelaycharacteristics,thatis:whentheoverloadcurrentissmall,thefusingtimeislong;whentheoverloadcurrentislarge,Thefusingtimeisshort.

Fortheunderstandingoftheampere-secondcharacteristic,wecanseefromJoule'slawthatQ=I²*R*T,intheseriescircuit,theRvalueofthefuseisbasicallyunchanged,ThecalorificvalueisproportionaltothesquareofthecurrentI,andproportionaltotheheatingtimeT,thatistosay:whenthecurrentislarge,thetimerequiredforthemelttofuseisshorter.Whenthecurrentissmall,thetimerequiredforthemelttofuseislonger.Eveniftherateofheataccumulationislessthantherateofheatdiffusion,thetemperatureofthefusewillnotrisetothemeltingpoint,andthefusewillnotevenbeblown.Therefore,withinacertainrangeofoverloadcurrent,whenthecurrentreturnstonormal,thefusewillnotbeblownandcanbeusedcontinuously.

Therefore,everymelthasaminimummeltingcurrent.Correspondingtodifferenttemperatures,theminimummeltingcurrentisalsodifferent.Althoughthiscurrentisaffectedbytheexternalenvironment,itcanbeignoredinpracticalapplications.Generally,theratiooftheminimummeltingcurrentofthemelttotheratedcurrentofthemeltisdefinedastheminimummeltingcoefficient.Themeltingcoefficientofcommonlyusedmeltsisgreaterthan1.25,whichmeansthatthemeltwitharatedcurrentof10Awillnotfusewhenthecurrentisbelow12.5A.

Fromhere,itcanbeseenthattheshort-circuitprotectionperformanceofthefuseisexcellent,andtheoverloadprotectionperformanceisaverage.Ifyoureallyneedtouseitinoverloadprotection,youneedtocarefullymatchthelineoverloadcurrentwiththeratedcurrentofthefuse.Forexample:8Ameltisusedina10Acircuitforshort-circuitprotectionandoverloadprotection,buttheoverloadprotectioncharacteristicsatthistimearenotideal.

Thechoiceoffuseismainlybasedontheprotectioncharacteristicsoftheloadandthesizeoftheshort-circuitcurrent.Forsmall-capacitymotorsandlightingbranchlines,fusesareoftenusedasoverloadandshort-circuitprotection,soitishopedthatthemeltingcoefficientofthemeltshouldbeappropriatelysmall.RQAseriesfusesoflead-tinalloymeltareusuallyused.Forlarger-capacitymotorsandlightingtrunklines,short-circuitprotectionandbreakingcapacityshouldbeemphaticallyconsidered.UsuallychoosetheRM10andRL1seriesfuseswithhigherbreakingcapacity;whentheshort-circuitcurrentisverylarge,itisadvisabletousetheRT0andRTl2seriesfuseswithcurrentlimitingfunction.

TheratedcurrentofthemeltcanbeChooseaccordingtothefollowingmethods:

1.Whenprotectingasmoothloadwithoutstartingprocess,suchaslightingcircuits,resistors,electricfurnaces,etc.,theratedcurrentofthemeltisslightlygreaterthanorequaltotheratedcurrentintheloadcircuit.

2.Themeltcurrenttoprotectasinglemotorthatworksforalongtimecanbeselectedaccordingtothemaximumstartingcurrent,orcanbeselectedasfollows:

IRN≥(1.5～2.5)IN

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whereIRN---meltratedcurrent;IN---motorratedcurrent.Ifthemotorstartsfrequently,thecoefficientintheformulacanbeappropriatelyincreasedto3～3.5,whichshouldbedeterminedaccordingtotheactualsituation.

3.Protectmultiplelong-termworkingmotors(powersupplymains)

IRN≥(1.5～2.5)INmax+ΣIN

INmax-maximumcapacityTheratedcurrentofasinglemotor.ΣINtherest.Thesumoftheratedcurrentofthemotor.

Mainclassification

Accordingtothevoltageused,fusescanbedividedintohigh-voltagefusesandlow-voltagefuses.Accordingtotheprotectionobject,itcanbedividedintofusesforprotectingtransformersandgeneralelectricalequipment,fusesforprotectingvoltagetransformers,fusesforprotectingpowercapacitors,fusesforprotectingsemiconductorcomponents,fusesforprotectingmotors,andfusesforprotectinghouseholdappliances.Fuses,etc.Accordingtothestructure,itcanbedividedintoopentype,semi-closedtype,tubetypeandspraytypefuse.

Theopen-typefusehasasimplestructure,themeltiscompletelyexposedtotheair,andissupportedbyaporcelaincolumnwithoutasupport.Itissuitableforlow-voltageoutdooruse.Whenthecurrentisinterrupted,largesoundandlightaregeneratedintheatmosphere.

Themeltofthesemi-enclosedfuseismountedonaporcelainframeandinsertedintoaporcelainboxwithmetalsocketsatbothends,suitableforlow-voltageindooruse.Whenthecurrentisinterrupted,thesoundandlightgeneratedareblockedbytheporcelainbox.

Themeltofthetubefuseisinstalledinthefusebody.Thenplugitintothesupportordirectlyconnecttothecircuitforuse.Thefuseisacompletelysealedinsulatingtubewithmetalcapsorcontactbladesatbothends.Iftheinsulatingtubeofthisfuseisfilledwithquartzsand,itwillhaveacurrentlimitingeffectwhenbreakingthecurrent,whichcangreatlyimprovethebreakingcapacity,soitisalsocalledahighbreakingcapacityfuse.Ifthetubeisvacuumed,itiscalledavacuumfuse.IfthetubeisfilledwithSF6gas,itiscalledanSF6fuse,anditspurposeistoimprovethearcextinguishingperformance.Sincequartzsand,vacuumandSF6gashavegoodinsulatingproperties,thisfuseisnotonlysuitableforlowpressurebutalsoforhighpressure.

Thejetfuseistoinstallthemeltinaninsulatingtubemadeofsolidgas-producingmaterials.Thesolidgas-generatingmaterialscanbeelectricalreversewhitecardboardorplexiglassmaterials.Whentheshort-circuitcurrentpassesthroughthemelt,themeltimmediatelymeltstoproduceanarc.Thehigh-temperaturearcquicklydecomposesthesolidgas-generatingmaterialtoproducealargeamountofhigh-pressuregas,therebyejectingtheionizedgaswiththearcatbothendsofthetube,emittinggreatsoundandlight.Andwhenthealternatingcurrentcrosseszero,thearcisextinguishedandthecurrentisinterrupted.Theinsulatingtubeisusuallyinstalledonaninsulatingsupporttoformthewholefuse.Sometimestheupperendoftheinsulatingtubeismademovable,anditwillfalloffimmediatelyafterbreakingthecurrent.Thistypeofjetfuseiscommonlyknownasadropfuse.Generallysuitableforoutdooroccasionswherethevoltageishigherthan6kV.

Inaddition,fusescanbedividedintogeneral-purposefuses,backupfusesandfull-rangefusesaccordingtothebreakingcurrentrange.Thebreakingcurrentrangeofgeneral-purposefusesreferstotherangefromtheoverloadcurrentgreaterthan1.6to2timestheratedcurrenttothemaximumbreakingcurrent.Thistypeoffuseismainlyusedtoprotectpowertransformersandgeneralelectricalequipment.Thebreakingcurrentrangeofthebackupfusereferstotherangefromtheoverloadcurrentbeing4to7timestheratedcurrenttothemaximumbreakingcurrent.Thistypeoffuseisoftenusedinserieswithacontactor.Whentheoverloadcurrentislessthan4to7timestheratedcurrent,thecontactorwillrealizethebreakingprotection.Mainlyusedtoprotectthemotor.

Withtheneedsofindustrialdevelopment,specialfusessuitableforvariousrequirementshavealsobeenmanufactured,suchaselectronicfuses,thermalfuses,andself-resettingfuses.

Inter-levelcoordination

Inordertopreventover-levelfusingandexpandthescopeofaccidents,thereshouldbegoodcoordinationbetweenthefusesoftheupperandlowerlevels(ie,mainsandbranchlines).Whenselecting,themeltratedcurrentoftheupperlevel(powersupplymainline)shouldbe1to2levelslargerthanthatofthelowerlevel(powersupplybranchline).CommonlyusedfusesincludetubefuseR1series,screwfuseRLlseries,filler-enclosedfuseRT0seriesandfastfuseRSO,RS3seriesandsoon.

Maintenance

Thefuseinthelow-voltagepowerdistributionsystemisakindofelectricalappliancethatplaysaroleofsafetyprotection.Thefuseiswidelyusedintheprotectionofthepowergridandtheprotectionofelectricalequipment.Intheeventofashort-circuitfailureoroverloadofelectricalequipment,thecircuitcanbeautomaticallycutofftoavoiddamagetoelectricalequipmentandpreventthespreadofaccidents.

Thefuseconsistsofaninsulatingbase(orsupport),contacts,melts,etc.Themeltisthemainworkingpartofthefuse,andthemeltisequivalenttoaspecialwireinseriesinthecircuit.Whenthecircuitisshort-circuitedoroverloaded,thecurrentistoolargeandthemeltmeltsduetooverheating,therebycuttingoffthecircuit.Themeltisoftenmadeintofilaments,gridsorflakes.Themeltmaterialhasthecharacteristicsofrelativelylowmeltingpoint,stablecharacteristics,andeasyfusing.Metalssuchaslead-tinalloy,silver-platedcoppersheet,zinc,silver,etc.aregenerallyused.Intheprocessofmeltingandcuttingoffthecircuit,anarcwillbegenerated.Inordertoextinguishthearcsafelyandeffectively,themeltisgenerallyinstalledinthefusehousingandmeasuresaretakentoquicklyextinguishthearc.

Thefusehastheadvantagesofsimplestructure,convenientuse,lowprice,etc.,andiswidelyusedinlow-voltagesystems.

Precautions

Inviewoftheexcellentshort-circuitprotectionperformanceofthefuse,itiswidelyusedinhighandlowvoltagepowerdistributionsystemsandcontrolsystemsandelectricalequipmentasshort-circuitandover-currentprotectionThedeviceisoneofthemostcommonlyusedandmostimportantprotectiondevices.

Intheapplication,attentionshouldbepaidtotheuseoffuseprecautions,dailyinspectionsandmaintenance.

Precautionsfortheuseofthefuse:

1.Theprotectioncharacteristicsofthefuseshouldbecompatiblewiththeoverloadcharacteristicsoftheprotectedobject,takingintoaccountthepossibleForshort-circuitcurrent,selectafusewithcorrespondingbreakingcapacity;

2,theratedvoltageofthefuseshouldadapttothelinevoltagelevel,andtheratedcurrentofthefuseshouldbegreaterthanorequaltotheratedcurrentofthemelt;

3.Theratedcurrentofthemeltofeachlevelofthefuseinthelineshouldbematchedaccordingly,andtheratedcurrentofthepreviousmeltmustbegreaterthantheratedcurrentofthenextmelt;

4,themeltofthefuseItisnecessarytousematchingmeltsasrequired,anditisnotallowedtoincreasethemeltatwillorreplacethemeltwithotherconductors.

Fuseinspection:

1.Checkwhethertheratingsofthefuseandthemeltarecompatiblewiththeprotectedequipment;

2,checktheappearanceofthefusefordamage,deformation,andwhetherthereareflickeringanddischargetracesontheporcelaininsulationpart;

3,checkwhetherthecontactpointsofthefuseareintact,closeincontact,andwhetherthereisoverheating;

4.Whetherthefusingsignalindicatorofthefuseisnormal.

Useandmaintenanceofthefuse:

1.Whenthefuseisblown,carefullyanalyzethecauseoftheblow.Thepossiblereasonsare:

1)Normalfusingduetoshort-circuitfailureoroverloadoperation;

2)Themeltisusedfortoolong,andthemeltisoxidizedorthetemperatureishighduringoperation,whichcausesthemeltcharacteristicstochangebymistake;

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3)Thereismechanicaldamagewhenthemeltisinstalled,whichreducesitscross-sectionalareaandcausesfalsedisconnectionduringoperation.

2.Whendisassemblingandreplacingthemelt,itisrequiredtodothefollowing:

1)Beforeinstallinganewmelt,findoutthecauseofthemeltfuse.Ifthecauseofthefuseisnotdetermined,donotreplaceMelttrialdelivery;

2)Whenreplacinganewmelt,checkwhethertheratingofthemeltmatchestheprotectedequipment;

3)Whenreplacinganewmelt,Itisnecessarytochecktheinternalburnsofthefusetube.Ifthereisaseriousburn,thefusetubeshouldbereplacedatthesametime.Whentheporcelainfusiontubeisdamaged,itisnotallowedtoreplaceitwithanothermaterialtube.Whenreplacingthemeltofthefillertypefuse,payattentiontothefiller.

3.Thefuseshouldberepairedatthesametimeasthepowerdistributiondevice:

1)Cleanthedustandcheckthecontactconditionofthecontactpoints;

2)CheckthefuseWhethertheappearanceofthedevice(removethefusetube)isdamaged,deformed,andwhethertheporcelainhasdischargeflickertraces;

3)Checkthefuse,whetherthemeltmatchestheprotectedcircuitorequipment,ifthereisaproblemInvestigatepromptly;

4)PayattentiontochecktheNwireintheTNgroundingsystem,thegroundingprotectionlineoftheequipment,anditisnotallowedtousefuses;

5)MaintainandcheckthefuseAtthetime,thepowersupplyshouldbecutoffaccordingtotherequirementsofsafetyregulations,anditisnotallowedtoremovethefusetubewhenthepowerison.

Themaindifference

Thedifferencebetweenafuseandacircuitbreaker:

Thesamepointisthattheshortcircuitprotectioncanbeachieved.Theprincipleistousethecurrentflowingthroughtheconductortomaketheconductorheatup,andtheconductormeltswhenthemeltingpointoftheconductorisreached,sothecircuitisdisconnectedtoprotecttheelectricalappliancesandlinesfrombeingburned.Itisanaccumulationofheat,sooverloadprotectioncanalsobeachieved.Oncethemeltisburned,themeltmustbereplaced.

Thecircuitbreakercanalsorealizetheshortcircuitandoverloadprotectionofthecircuit,buttheprincipleisdifferent.Itrealizesthecircuitbreakerprotectionthroughthecurrentbottommagneticeffect(electromagnetictripper),andrealizestheoverloadprotectionthroughthethermaleffectofthecurrent(notFuse,usuallydonotneedtoreplacethedevice).Inpractice,whentheelectricalloadinthecircuitisclosetotheloadofthefuseusedforalongtime,thefusewillgraduallyheatupuntilitblows.Asmentionedabove,thefusingofthefuseistheresultofthecombinedactionofcurrentandtime,whichprotectstheline.Itisaone-off.Thecircuitbreakerisasuddenincreaseinthecurrentinthecircuit.Whentheloadofthecircuitbreakerexceedstheload,itwillautomaticallyopen.Itisaprotectionforaninstantaneouscurrentincreaseinthecircuit,suchaswhentheleakageislarge,orshortcircuit,orinstantaneouscurrentGreatprotection.Whenthereasonisfoundout,itcanbeswitchedonandusedagain.Asmentionedabove,thefusingofthefuseistheresultofthecombinedeffectofcurrentandtime,andthecircuitbreakerwilltripaslongasthecurrentexceedsitssetvalue,andtheeffectoftimecanhardlybeconsidered.Circuitbreakersarecommonlyusedcomponentsforlow-voltagepowerdistribution.Therearealsosomeplaceswherefusesaresuitable.

Performancecomparisonoffuseandcircuitbreaker:

Fuse:

1,fusingThemainadvantagesandcharacteristicsofthedevice

1)Goodselectivity.AslongastheratedcurrentofthefuselinkoftheupperandlowerfusemeetstherequirementsoftheovercurrentselectionratiospecifiedbythenationalstandardandtheIECstandardof1.6:1,thatis,theratedcurrentoftheupperfuseisnotlessthan1.6timesofthevalueofthelowerlevel,itisregardedastheupperandlowerlevelenergy.Selectivelycutoffthefaultcurrent;

2)Goodcurrent-limitingcharacteristics,highbreakingcapacity;

3)Relativelysmallsize;

4)RelativelyexpensiveCheap.

2.Themainshortcomingsandweaknessesofthefuse

1)Thefusemustbereplacedafterthefaultisblown;

2)Theprotectionfunctionissingle,onlyone-stageovercurrentInversetimelimitfeature,overload,shortcircuitandgroundfaultusethisprotection;

3)Whenone-phasefuseoccurs,itwillcausetheundesirableconsequencesoftwo-phaseoperationforthethree-phasemotor.Ofcourse,analarmsignalcanbeusedThefusecanmakeupforit,andone-phasefusecandisconnectthree-phase;

4)Remotecontrolcannotberealized,anditneedstobecombinedwithelectricknifeswitchesandswitches.

Non-selectivecircuitbreaker:

1.Mainadvantagesandfeatures

1)Afterthefaultisdisconnected,itcanbeoperatedmanuallyReset,thereisnoneedtoreplacecomponents,unlessthelargeshort-circuitcurrentneedstoberepaired;

2)Thetwo-stageprotectionfunctionoflong-timereleaseandinstantaneouscurrentreleasewithinversetimecharacteristicsareusedasoverloadandForshort-circuitprotection,eachperformitsownduties;

3)Remotecontrolcanbeachievedwhentheoperatingmechanismispoweredon.

2.Mainshortcomingsandweaknesses

1)Itisdifficulttoachieveselectivecut-offbetweentheupperandlowernon-selectivecircuitbreakers.Whenthefaultcurrentislarge,itiseasytocausetheupperandlowercircuitbreakerstobeInstantaneousdisconnection;

2)Therelativepriceisslightlyhigher;

3)Thebreakingcapacityofsomecircuitbreakersissmall,suchasacircuitbreakerwithasmallerratedcurrentinstallednearalarge-capacitytransformerIntheposition,thebreakingcapacitywillbeinsufficient.Existingproductswithhighbreakingcapacitycanbesatisfied,butthepriceishigher.

Selectivecircuitbreaker:

1.Mainadvantagesandfeatures

1)Withtheabove-mentionednon-selectivecircuitbreakersAdvantages;

2)Ithasavarietyofprotectionfunctions,includinglongtimedelay,instantaneous,shorttimedelayandgroundfault(includingzerosequencecurrentandresidualcurrentprotection)protection,whichcanrealizeoverload,opencircuitdelay,Largeshort-circuitcurrentinstantaneousactionandgroundfaultprotection,highprotectionsensitivity,convenientadjustmentofvariousparameters,easytomeetvariousprotectionrequirementsofpowerdistributionlines.Inaddition,itcanhavecascadeprotectionfunction,whichhasbetterselectiveactionperformance;

3)Today'sproductsaremostlyintelligent.Inadditiontoprotectionfunctions,therearepowermeasurement,faultrecords,andcommunicationexcuses.,Toachievecentralizedmonitoringandmanagementofpowerdistributiondevicesandsystems.

2.Mainproblems

1)Thepriceisveryhigh,soitshouldonlybeusedattheheadendofpowerdistributionlinesandsub-trunklinesinparticularlyimportantplaces;

2)Largersize.