Introduction to circuit breakers and fuse

To learning a boat circuit breakers and fuse

14. High Resistance Method :
Fromthephenomenonofarc,itisclearthat
thecurrentflowinginthecircuitentirely
dependsuponthearcresistance.
Inthismethod,resistanceisincreasedtosuch
avaluethatthecurrentisdecreasedtoavery
lowvalue.Oncethecurrentisdecreasedthearc
isextinguished.
Theresistanceofarccanbeincreasedin
followingways

15. IncreasingtheLengthoftheArc:Weknow
that,resistanceR∝ρl/A.Ifthelengthofthearc
isincreasedthentheresistanceisincreased,once
theresistanceisincreased,thecurrentdecreases.
ByReducingtheAreaofCross-sectionofthe
Arc:Iftheareaofcross-sectionofthearcis
reduced,theresistanceincreasesasR∝ρl/A.
Whentheresistanceisincreased,thecurrentis
automaticallyinterruptedthereby,thearcis
extinguished.

16. CoolingtheArc:Byprovidinganycooling
agent,theionizedparticlespresentbetween
thecontactscanbereduced.Thereduction
ofionizedparticlesincreasestheresistance
betweenthecontactsandhencedecreases
thecurrent.
SplittingtheArc:Thearcismadetosplit
intoanumberofsections.Ineachsection,
resistanceisincreasedduetotheeffectof
coolingandlengtheningofarc.

17. LowResistanceMethodorCurrentZeroMethod:
Itisapplicableonlytoaccircuits.Weknowthatthe
currentbecomeszerotwiceacycleincaseofac.
Whenitbecomeszero,thearcextinguishesforalonger
durationoftime.
Duringthenexthalf-cycle,itreappearsagain,ifthe
dielectricstrengthoftheairgapislessthanthevoltage
gradientbetweenthecontacts.Usingthispropertyofac
circuits,thearccanbeextinguishedeasilywhenthe
currentbecomeszero.
Forsuchconditionstoprevail,thedielectricstrengthof
themediumsurroundingtheionizedparticlehastobe
improved.Itispossiblebyeithercoolingorsweeping
outtheionizedparticlesanddeionizingthemedium.

18. Thedifferentmethodsfordeionizationofthemediumfor
extinguishingarcare,
IncreasingtheLengthoftheGap-Thedielectric
strengthofthemediumcanbeincreasedbyincreasing
thelengthofthearcasthedielectricstrength∈isdirectly
proportionaltothelengthli.e.,∈∝l.Oncethedielectric
strengthofthemediumisincreased,thearcfailsto
restrikeagainandtheflowofcurrentisinterrupted,thus
extinguishingthearc.
Cooling-Duringcoolingtheionizedparticlesare
deionizedintoneutralmolecules.Bymeansofcoolingthe
dielectricstrengthofthemediumisincreasedandthearc
isextinguished.

19. HighPressure-Ifthepressurenearthecontacts
ismaintainedatahighvalue,ithelpsintherapid
deionizationoftheionizedparticlestoneutral
moleculestherebyincreasingthedielectric
strengthofthemedium.
BlastEffect-Inthistypeofeffecteitherhigh-
pressureoilisforcedtopassthroughthearcgap
orgasisusedinordertosweepawaytheionized
particlesandimprovethedielectricstrengthof
themedium.

24. Rate of Rise of Re-striking Voltage (RRRV):
The transient voltage that appears across the contacts of the circuit breaker
at the current zero period during arcing is known as Restrikingvoltage.
Theexpressionforthere-strikingvoltageisV
C
(t)=V
m
(1−cosω
n
t)
The maximum value of the re-striking voltage occurs att = π / ω
n
= π √ L C
The maximum value of re-striking voltage = 2 V
m
= 2 ×peak value of the
system voltage.
The rate of rise of re-striking voltage (RRRV)= d/dt[ Vm(1− cosω
n
t) ]
∴RRRV = V
mω
n
sin ω
n
t
The rate of rise of re-striking voltage is directly proportional to the natural
frequency.
R RRV ∝ω
n
∝1 √ L C
Where L is the inductance of the system
C is the capacitance of the system
So, the rate of rise of re-striking voltage (RRRV) is dependent upon both
the inductance and capacitance of the system.
Now, the maximum value of RRRV occurs when ω
n
t= π/2 i.e. when
t = π/2ω
n
∴The maximum value of RRRV = V
m
ω
n
= V
m
/ √ L C
RRRV
max
= V
m
/ √ L C
RRRV is measured in kV /μ-sec.

25. SulphurHexafluoride(SF6) circuit breaker
InSF6circuitbreakers,sulphurhexafluoride(SF6)gas
isusedasthearcquenchingmedium.
TheSF6isanelectro-negativegasandhasastrong
tendencytoabsorbfreeelectrons.
Thecontactsofthebreakerareopenedinahigh-
pressureflowofSF6gasandanarcisstruckbetween
them.
Theconductingfreeelectronsinthearcarerapidly
capturedbythegastoformrelativelyimmobile
negativeions.
Thislossofconductingelectronsinthearcquickly
buildsupenoughinsulationstrengthtoextinguishthe
arc.

27. Constructional Details
Itconsistsoffixedandmovingcontactsenclosedin
achamber(calledarcinterruptionchamber)
containingSF6gas.Thischamberisconnectedto
theSF6gasreservoir.
Whenthecontactsofbreakerareopened,thevalve
mechanismpermitsahigh-pressureSF6gasfrom
thereservoirtoflowtowardsthearcinterruption
chamber.
Thefixedcontactisahollowcylindricalcurrent
carryingcontactfittedwithanarcinghorn.
Themovingcontactisalsoahollowcylinderwith
rectangularholesinthesidestopermittheSF6
gastoletoutthroughtheseholesafterflowing
alongandacrossthearc.

28. Thetipsoffixedcontact,movingacontact
andarcinghornarecoatedwithcopper-
tungstenarc-resistantmaterial.
SinceSF6gasiscostly,itisreconditioned
andreclaimedbyasuitableauxiliarysystem
aftereachoperationofthebreaker.

29. WorkingOperation:
IntheclosedpositionoftheSF6circuit
breaker,thecontactsremainsurroundedbySF6
gasatapressureofabout2·8kg/cm2.
Whenthebreakeroperates,themovingcontactis
pulledapartandanarcisstruckbetweenthe
contacts.
Themovement ofthemovingcontactis
synchronisedwiththeopeningofavalvewhich
permitsSF6gasat14kg/cm2pressurefromthe
reservoirtothearcinterruptionchamber.

30. Thehigh-pressureflowofSF6rapidly
absorbsthefreeelectronsinthearcpathto
formimmobilenegativeionswhichare
ineffectiveaschargecarriers.
Theresultisthatthemediumbetweenthe
contactsquicklybuildsuphighdielectric
strengthandcausestheextinctionofthe
arc.
Afterthebreakeroperation(i.e.,afterarc
extinction),thevalveisclosedbytheaction
ofasetofsprings.

31. Advantages:
(i) Due to the superior arc quenching property of
SF6, such circuit breakers have very short arcing
time.
(ii) Since the dielectric strength of SF6 gas is 2 to 3
times that of air, such breakers can interrupt
much larger currents.
(iii) TheSF6 circuit breakergives noiseless
operation due to its closed gas circuit and no
exhaust to atmosphere, unlike the air blast circuit
breaker.
(iv) The closed gas enclosure keeps the interior dry
so that there is no moisture problem.
(v) There is no risk of fire in such breakers because
SF6 gas is non-inflammable.

32. (vi) TheSF6 Circuit breakershave a low
maintenance cost, light foundation requirements
and minimum auxiliary equipment.
(vii) Since SF6 breakers are totally enclosed and
sealed from the atmosphere, they are particularly
suitable where explosion hazard exists e.g., coal
mines.
Disadvantages:
(i)SF6 breakers are costly due to the high cost of SF6.
(ii) Since SF6 gas has to be reconditioned after every
operation of the breaker, additional equipment is required for
this purpose.

33. Applications:
AtypicalSF6circuitbreakerconsistsof
interrupterunitseachcapableofdealingwith
currentsup-to60kAandvoltagesintherange
of50—80kVAnumberofunitsareconnectedin
seriesaccordingtothesystemvoltage.SF6
circuitbreakershavebeendevelopedfor
voltages115kVto230kV,powerratings10
MVAto20MVAandinterruptingtimelessthan
3cycles.

34. Minimum Oil Circuit Breaker (Low oil
Circuit Breakers) (MOCB)
In case of bulk oil circuit breaker, the oil is used for two
functions.
-Firstly it acts as an arc-quenching medium and
-Secondly it insulates the live parts from earth.
But for arc extinction purpose a very small amount of oil is
sufficient while the major part is utilized for insulation
purposes.
Thus the oil required in case of bulk oil circuit breaker is
large and need frequent replacement.
This increasesthe overall cost; tank size and weight of or
the breaker.
Above drawbacks of bulk oil circuit breakers are removed
in minimum oil circuit (also known as MOCB) breaker in
which a small quantity of oil is used for arc extinction and
for insulation purpose.

35. Constructional Details
TheMOCBhasthreechambers;thearcchamberandthe
supportingchamber,TopChamber.
Thearcchamberincludesfixedandmoving
contactfilledwithoil.Thischamberisusedfor
extinguishingthearc.
Itismadefromporcelainwithbakelisedpaper
encasing.Italsoincludesventsforgasreleaseat
thetop.
Thesupportingchamberprovidessupporttothe
arcchamber.Itismadeofporcelainfilledwithoil
forinsulation.
Itinsulatesthearcchamberthatismountedon
topofit.Itincludesanarmoperatedbya
mechanismtomovethemovingcontactupward
anddownwardusingarod.

36. Themovingcontactisalsoattachedtoapiston
thatdevelopsthepressurenecessaryforarc
quenching.
Therefore,MOCBisatypeofForcedBlastoil
circuitbreakerwherethepressureisdevelopedby
externalmeansindependentofthearccurrent.
Thisarrangementpermitstwoadvantages.
-Firstly,thecircuitbreakingchambers
requiresasmallvolumeofoilwhichisjustenough
forarcextinction.
-Secondlytheamountofoiltobereplaced
isreduced,astheoilinthesupportingchamber
doesnotgetcontaminated bythearc.

38. WorkingOperation:
•Undernormaloperatingconditions,themoving
contactremainsengagedwiththeupperfixed
contact.
•Assoonasfaultoccursonthesystem,themoving
contactispulleddownbythetrippingspringsand
arcisstruckbetweenthetwocontacts.
•Theheatofarccausesvaporisationoftheoiland
producesgasesunderhighpressure.
•Duetothisaction,theoilispassthrougha
centralholeinthemovingcontactandcreates
turbulanceoftheoil.
•Thiscausessuccessivelyquenchingofarcbythe
effectofseparatestreamofoilmovingacrosseach
sectioninturnandbearingawayitsgases.

39. Advantages:
1. It requires a very little amount of oil.
2. Less amount of oil means the risk of fire hazards
is low.
3. It has low weight.
4. It has small size and takes up small space
5. It is cheaper than BOCB.
6. It has easier maintenance and oil can be easily
replaced.
7. Best for installation in places where it is not
frequently used.
Disadvantages:
1.Lesser oil is more affected by the amount of
carbonization from the contacts.
2.The oil quickly losses its dielectric strength..
3.It requires more frequent maintenance.

40. ApplicationsofOilCircuitBreaker:
Theoilcircuitbreakersareusedinhigh
voltageapplicationssuchasinpowergrids,
substations,powerlinesandtransmission
anddistributionsystemsforupto220kV.

41. Vacuum Circuit Breaker
1.Thevacuum circuitbreaker(alsoknown
asvacuum interrupterorVCB)usesthe
arcquenchingprocessinthevacuumasa
switchingmedium.
2.Thevacuumhasthehighestinsulatingstrength.
So,ithasfarsuperiorarcquenchingproperties
thananyothermedium.
3.Thedegreeofvacuumisintherangeof10^
-7
to
10^
-5
Torr.Itissuitableformedium-voltagepower
distributionrangingfrom22kVto66kV.

43. ConstructionalDetails:
1.Thevacuumcircuitbreakerconsistsoffixed
contact,movingcontactandanarcshieldmounted
inavacuuminterrupter.
2.Themovingcontactisconnectedtothecontrol
mechanismbystainlesssteelbellow.
3.TheArcshieldsaresupportedtotheinsulating
housingsuchthattheycovertheseshieldsandis
preventedfromcondensingontheinsulating
enclosure.
4.Thepossibilityofaleakiseliminatedduetothe
permanentsealingoftheVacuumchamberforthat
aglassvesselorceramicvesselisusedasthe
outerinsulatingbody.

44. WorkingofVacuumCircuitBreaker:
Whenthebreakeroperates,themovingcontact
separatesfromthefixedcontactandanarcis
struckbetweenthecontacts.
Theproductionofthearcisduetotheionization
ofmetalionsanddependsverymuchuponthe
materialofcontacts.
Thearcisquicklyextinguishedbecausethe
metallicvapors,electrons,andionsproduced
duringarcarediffusedinashorttime.
Sincevacuumhasaveryfastrateofrecoveryof
dielectricstrength,thearcextinctionina
vacuumbreakeroccurswithashortcontact
separation(say0.625cm).

45. Advantages:
1.Compactandreliable.
2.Longerlife.
3.Noriskoffire.
4.EnvironmentalfriendlythanSF6CB.
5.Highdielectricstrength.
6.Easymaintenance.
7.Arcextinctionisveryfast.
8.Userfriendly.
Disadvantages:
1.ThecostofVCBwillincreaseifthevoltage
exceeds38kV.
2.ItisUneconomicalatvoltagesexceeding
38kv.

46. ApplicationsofVCB:
1.Highvoltagecircuits
2.Usedinsubstationsandgenerators
3.Interruptsthedoubleearthfaultsandout
ofphasecurrents
4.Applicationsthatrequirehighswitching
sequencesuseVCBs
5.Railwayapplicationsusethiscircuit
breakerfortractioncurrentandelectric
supplyswitching.
6.Toswitchmotordrives.

47. Air blast circuit breaker
Airblastcircuitbreakeremploysahigh-
pressureairblastasanarcquenchingmedium.
Airblastcircuitbreakersareusingcompressed
airorgasasthecircuitbreakingorinterrupting
medium.Gasessuchasnitrogen,carbondioxide,
hydrogenorFreoncanbeusedasarcinterrupting
medium.
ConstructionalDetails:
Themajorcomponentsoftheairblastcircuit
breakerare,
1.Airreservoir
2.Hallowinsulatorassembly
3.Arcextinctionchamber
4.Valves5.Currentcarryingconductors.

49. HallowInsulatorsAssembly:
Thesearemountedontheairreservoirthatconnectsthe
airreservoirtothearcextinctionchamberforthesupplyof
high-pressureairfromtheairreservoirtothearcextinction
chamber.
Valvesareprovidedatthebaseofthehallowinsulators
thatcontroltheflowofhigh-pressureair.
ArcExtinctionChamber:
Thearcextinctionchamberismountedonthehollow
insulatorassembly.
Themakingandbreakingofthecircuitarecarriedoutin
thearcextinctionchamber.
Itconsistsoffixedandmovingcontactswithaspring
mechanism.Themovingcontactmovesawayandclosesto
thefixedcontactdependingupontheairpressure.

50. Valves:
Theseareprovidedatthebaseoftheinsulators
thatregulatetheflowsofhigh-pressureairfrom
theairreservoirtothearcextinctionchamber.
Whenthereisafaultinthecircuitthevaluesget
openedwiththehelpofanoperatingrodconnected
tothepneumaticoperatingmechanism.
CurrentCarryingConductors:
Thecurrentcarryingconductorslinkallthearc
extinctionchambersinseriesandthepolesof
neighboringequipment.

51. WorkingOperationofABCB:
Undernormalconditions,thebreakercontacts
presentinthearcextinctionchamberarein
contactwitheachother.
Underfaultconditions,thebreakerreceivesa
trippingsignalandthismakesthevalvesopenby
operatingrodconnectedtoit.Whenthevalvesget
opened,thehigh-pressureairentersintothearc
extinctionchamberthroughthehallowinsulator
assembly.
Oncethehigh-pressureairentersintoarc
extinctionchamber,itcausestoincreasethe
pressureonthemovingcontactinsidethearc
extinctionchamber.Hencethemovingcontactgets
separatedfromthefixedcontact.

52. Atthetimeofseparation,anarcisdrawn
betweenthetwocontacts.Thearcisformeddueto
theionizationofairpresentbetweenthecontacts
andthisionizedairistakenoutbythehigh-
pressureairthroughtheairoutletvalves.
Thusthearcformedbetweenthetwocontacts
getsextinguished.
TypesofAirBlastCircuitBreakers:
Airblastcircuitbreakersareclassifiedonthebasisthe
directionofairblasttothearc.Theyareclassifiedinto:
AxialBlastType–airblastisdirectedalongthearcpath.
CrossBlastType–airblastisdirectedatrightanglesto
thearcpath.
RadialBlastType–airblastisdirectedradially.

54. Axial Blast Circuit Breaker:
Under Normal Condition
The fixed and moving contacts are held in a closed
position with the help of spring pressure.There is
an airreservoirconnected to the arcing chamber
through an air valve.
Theairvalvecontrolstheflowofairintothearcing
chamber.Thevalveisclosedundernormalconditions.

55. UnderFaultyCondition
Whenafaultoccursatrippingimpulseisproduced
whichcausestheopeningoftheairvalve.
Sincetheairvalveconnectstheairreservoirand
thearcingchamber,ahigh-pressureairenters
thearcingchamber.Thisairpushesawaythe
movingcontactagainstthespringpressure.
Themovingcontactisseparatedandanarcis
struck.Atthesametime,high-pressureairblast
flowsalongthearcandtakesawaytheionized
gasesalongwithit.
Consequently,thearcisextinguishedandthe
currentflowisinterrupted.

56. CrossBlastCircuitBreaker:
Inthistypeofbreaker,anair-blastisdirectedat
rightanglestothearc.Thecrossblastlengthens
andforcesthearcintoasuitablechuteforarc
extinction.

57. Whenthemovingcontactiswithdrawn,anarcis
struckbetweenthefixedandmovingcontacts.
Thehigh-pressurecross-blastforcesthearcintoa
chuteconsistingofarcsplittersandbaffles.The
splittersservetoincreasethelengthofthearcand
bafflesgiveimprovedcooling.
Theresultisthatarcisextinguishedandtheflow
ofcurrentisinterrupted.
RadialairblastCircuitBreaker:
Theairblastflowsradiallyintospacebetweenthe
contacts.Thecontactsareseparatedandanarcis
formedbetweenthem.Theairflowsatagreat
speedaxiallyalongthearc.

58. Advantages:
Thereisnoneedforoilinairblastcircuitbreakers.This
eliminatestheoilmaintenancerequirementascompared
totheoilcircuitbreaker.
Itiseasilyaccessibleandtheweightisrelativelyless.
Theairblastcircuitbreakerhasahighrupturingcapacity.
High-speedoperation(rapidreclosingatsuccessive
operation)ispossible.
Deteriorationofoiliseliminatedandthearcingproducts
producedarecompletelyremoved.
Fireriskincaseofairblastcircuitbreakeriseliminated.
Thebreakerallowsthehigh-speedinterruption.
Thesetypesofcircuitbreakersarealmostmaintenance-
free.

59. Disadvantages:
1.Therecanbeapossibilityofairleaksat
thefittingsofthepipeline.
2.ThevariationsinRRRV(RateofRisingof
Recovery Voltage)willaffectthe
performanceoftheairblastcircuit
breaker.
3.The breaker requiresadditional
equipmentliketheelectricmotor,air
compressors,etc.,whichincreasesthe
cost.

60. CircuitBreakerRatingsandSpecifications:
Thecircuitbreakerneedstoworkundersevere
electrodynamicforces,thermalstress,etc.
Theseareveryhighundershortcircuitfault
conditions.Hence,itsratingwillbedifferent
whencomparedwithotherequipments.Itsrating
specifiesallthecharacteristicsvaluesthatdefine
theworkingconditionsforwhichithasbeen
designed.
Afewoftheimportantspecificationsare,
•Ratedvoltage
•Ratedcurrent
•Ratedfrequency
•Makingcapacity
•Breakingcapacity

61. RatedVoltage:
Theratedvoltageofacircuitbreakerisdefined
asthehighestrmsvalueofvoltageforwhichitis
designed.
Usually,theratedvoltagewillbegreaterthanthe
ratednominalsystemvoltage.
RatedCurrent:
TheRatedcurrentofacircuitbreakerisdefined
asthehighestrmsvalueofcurrentthatitcan
carrycontinuouslywhilemaintainingthe
temperature(invariousparts)withinthe
prescribedlimits.

62. RatedFrequency:
Theratedfrequencyofacircuitbreakeristhe
frequencyatwhichithasbeendesignedtooperate.
Ifthecircuitbreakerisintendedtooperateat
someotherfrequency,thenitseffectslike
temperaturerise,breakingtime,etc.,mustbe
takenintoconsideration.
Thestandardratedfrequencyis50Hz.

63. BreakingCapacity:
Thebreakingcapacitiesaredividedintotwo
typessymmetrical breakingcapacityand
asymmetricalbreakingcapacity.
Thesymmetricalbreakingcapacityofacircuit
breakerisdefinedastheproductofsymmetrical
breakingcurrent(rmsvalueofaccurrent)and
recoveryvoltageV
R
.
Theasymmetricalbreakingcapacityofacircuit
breakerisdefinedastheproductofasymmetrical
breakingcurrent(rmsvalueoftotalcurrent)and
recoveryvoltageV
R.

65. MakingCapacity:
Themakingcurrentormakingcapacityofa
circuitbreakerisgivenby,
=1.8x√2xSymmetricalbreakingcurrentor
breakingcapacity
Factor√2convertsrmsvaluetomaximumvalue
andfactor1.8takesthedoublingeffectofshort-
circuitcurrent(duetodccomponent)intoaccount.