Circuit Breakers for Engineering Students

Circuit Breakers

1. EE3601 Protection and
Switchgear

2. UNIT V CIRCUIT BREAKERS
Physicsofarcingphenomenonandarcinterruption-DCandAC
circuitbreaking–re-strikingvoltageandrecoveryvoltage-rateofriseof
recoveryvoltage-resistanceswitching-currentchopping-interruption
ofcapacitivecurrent-Typesofcircuitbreakers–airblast,airbreak,oil,
SF6andvacuumcircuitbreakers–comparisonofdifferentcircuit
breakers–RatingandselectionofCircuitbreakers.

3. Circuit Breakers
•Acircuitbreakerisapieceofequipmentwhich
can
(i)makeorbreakacircuiteithermanuallyorby
remotecontrolundernormalconditions
(ii)breakacircuitautomaticallyunderfault
conditions
(iii)makeacircuiteithermanuallyorbyremote
controlunderfaultconditions

4. Circuit breaker Operating principle
•Acircuitbreakeressentiallyconsistsoffixedand
movingcontacts,calledelectrodes.Undernormal
operatingconditions,thesecontactsremain
closedandwillnotopenautomaticallyuntiland
unlessthesystembecomesfaulty.
•Ofcourse,thecontactscanbeopenedmanually
orbyremotecontrolwheneverdesired.
•Whenafaultoccursonanypartofthesystem,the
tripcoilsofthecircuitbreakergetenergisedand
themovingcontactsarepulledapartbysome
mechanism,thusopeningthecircuit.

5. Circuit breaker Operating principle
•Whenthecontactsofacircuitbreakerare
separatedunderfaultconditions,anarcisstruck
betweenthem.Thecurrentisthusableto
continueuntilthedischargeceases.
•Theproductionofarcnotonlydelaysthecurrent
interruptionprocessbutitalsogenerates
enormousheatwhichmaycausedamagetothe
systemortothecircuitbreakeritself.
•Therefore,themainprobleminacircuitbreakeris
toextinguishthearcwithintheshortestpossible
timesothatheatgeneratedbyitmaynotreacha
dangerousvalue.

6. Arc Phenomenon
•Shortcircuit–largecurrent–contactsopen–
heatproduced–ionization–arcstruckbetween
contacts–arcprovideslowresistancepath–
potentialacrosscontactsislow.
•Duringthearcingperiod,thecurrentflowing
betweenthecontactsdependsuponthearc
resistance.
•Thegreaterthearcresistance,thesmallerthe
currentthatflowsbetweenthecontacts.

9. Arc Phenomenon
•Thearcresistancedependsuponthefollowing
factors:
•(i)Degreeofionisation—thearcresistance
increaseswiththedecreaseinthenumberof
ionisedparticlesbetweenthecontacts.
•(ii)Lengthofthearc—thearcresistance
increaseswiththelengthofthearci.e.,
separationofcontacts.
•(iii)Cross-sectionofarc—thearcresistance
increaseswiththedecreaseinareaofX-section
ofthearc.

10. Principles of Arc Extinction
•Before discussing the methods of arc extinction, it
is necessary to examine the factors responsible
for the maintenance of arc between the contacts.
These are :
•(i) p.d. between the contacts
•(ii) ionised particles between contacts

11. Principles of Arc Extinction
•Takingtheseinturn,
•(i)Whenthecontactshaveasmallseparation,thep.d.
betweenthemissufficienttomaintainthearc.Oneway
toextinguishthearcistoseparatethecontactstosucha
distancethatp.d.becomesinadequatetomaintainthe
arc.However,thismethodisimpracticableinhigh
voltagesystemwhereaseparationofmanymetresmay
berequired.
•(ii)Theionisedparticlesbetweenthecontactstendto
maintainthearc.Ifthearcpathisdeionised,thearc
extinctionwillbefacilitated.Thismaybeachievedby
coolingthearcorbybodilyremovingtheionised
particlesfromthespacebetweenthecontacts.

12. Methods of Arc Extinction
•There are two methods of extinguishing the arc in circuit
breakers viz.
•1. High resistance method.
•2. Low resistance or current zero method

13. Methods of Arc Extinction
•1. High resistance method. In this method, arc resistance
is made to increase with time so that current is reduced
to a value insufficient to maintain the arc. Consequently,
the current is interrupted or the arc is extinguished. The
principal disadvantage of this method is that enormous
energy is dissipated in the arc. Therefore, it is employed
only in d.c. circuit breakers and low-capacity a.c. circuit
breakers.

14. Methods of Arc Extinction
•Theresistanceofthearcmaybeincreasedby:
•(i)Lengtheningthearc.Theresistanceofthearcisdirectlyproportionaltoits
length.Thelengthofthearccanbeincreasedbyincreasingthegapbetween
contacts.
•(ii)Coolingthearc.Coolinghelpsinthedeionisationofthemediumbetween
thecontacts.Thisincreasesthearcresistance.Efficientcoolingmaybeobtained
byagasblastdirectedalongthearc.
•(iii)ReducingX-sectionofthearc.IftheareaofX-sectionofthearcisreduced,
thevoltagenecessarytomaintainthearcisincreased.Inotherwords,the
resistanceofthearcpathisincreased.Thecross-sectionofthearccanbe
reducedbylettingthearcpassthroughanarrowopeningorbyhavingsmaller
areaofcontacts.
•(iv)Splittingthearc.Theresistanceofthearccanbeincreasedbysplittingthe
arcintoanumberofsmallerarcsinseries.Eachoneofthesearcsexperiences
theeffectoflengtheningandcooling.Thearcmaybesplitbyintroducingsome
conductingplatesbetweenthecontacts.

15. Methods of Arc Extinction
•LowresistanceorCurrentzeromethod.Thismethodisemployedforarc
extinctionina.c.circuitsonly.Inthismethod,arcresistanceiskeptlowuntil
currentiszerowherethearcextinguishesnaturallyandispreventedfrom
restrikinginspiteoftherisingvoltageacrossthecontacts.Thede-ionisationof
themediumcanbeachievedby:
•(i) lengthening of the gap. The dielectric strength of the medium is
proportional to the length of the gap between contacts. Therefore, by opening
the contacts rapidly, higher dielectric strength of the medium can be achieved.
•(ii) high pressure. If the pressure in the vicinity of the arc is increased, the
density of the particles constituting the discharge also increases. The increased
density of particles causes higher rate of de-ionisation and consequently the
dielectric strength of the medium between contacts is increased.
•(iii) cooling. Natural combination of ionised particles takes place more rapidly if
they are allowed to cool. Therefore, dielectric strength of the medium between
the contacts can be increased by cooling the arc.
•(iv) blast effect. If the ionised particles between the contacts are swept away
and replaced by unionised particles, the dielectric strength of the medium can
be increased considerably. This may be achieved by a gas blast directed along
the discharge or by forcing oil into the contact space.r a.c. circuit breakers
employ this method for arc extinction.

16. Fault clearing time of a circuit breaker

17. Arcvoltage:Itmaybedefinedasthevoltagethatappears
acrossthecontactduringthearcingperiod,whentheelectric
currentflowismaintainedintheformofanarc.
Thevoltagedropacrossthearciscalledarcvoltage,Atypical
valuemaybeabout3%ofratedvoltage

18. Restrikingvoltage:Itmaybedefinedasthetransientvoltagethat
appearsacrossthebreakingcontactattheinstantofarcextinction.
Recoveryvoltage:Itmaybedefinedasthevoltagethatappearsacross
thebreakercontactafterthecompleteremovaloftransientoscillations
andfinalextinctionofarchasresultedinallthepoles.

19. Arc Interruption
There are two methods of arc interruption.
1.High resistance Interruption
Cooling of arc
Lengthening of arc
Splitting of arc
Constraining of arc
2.Low resistance interruption (or) current zero interruption
Arc resistance is kept low until current zero

20. There are two theories to explain the zero current interruption
of the arc.
1.Recovery rate theory (Slepain’sTheory)
2.Energy balance theory (Cassie’s Theory)
Recovery rate theory
The arc is a column of ionised gases.
To extinguish the arc, the electrons and ions are to be removed from the
gap immediately after the current reaches a natural zero
Ions and electrons can be removed either by recombining them into
neutral molecules or by sweeping them away by inserting insulating
medium (gas or liquid) into the gap.
The arc is interrupted if ions are removed from the gap at a rate faster
than the rate of ionisation.

21. In this method , the rate at which the gap recovers its dielectric
strength is compared with the rate at which the restriking voltage
(transient voltage) across the gap rises.
If the dielectric strength increases more rapidly than the restriking
voltage, the arc is extinguished.
If the restriking voltage rises more rapidly than the dielectric strength,
the ionization persists and breakdown of the gap occurs, resulting in
an arc for another half cycle.

22. Energy balance theory
The space between the contacts contains some ionised gas immediately
after current zero and hence, it has a finite post-zero resistance.
At the current zero moment, power is zero because restriking voltage is
zero.
When the arc is finally extinguished the power again becomes zero, the
gap is fully de-ionised and its resistance is infinitely high.

23. Inbetweenthesetwolimits,firstthepowerincreasesreachesa
maximumvalue,thendecreasesandfinallyreacheszerovalueas
showninFig.5.4.
Duetotheriseofrestrikingvoltageandassociatedcurrent,energyis
generatedinthespacebetweenthecontacts.
Theenergyappearsintheformofheat.Thecircuitbreakeris
designedtoremovethisgeneratedheatasearlyaspossibleby
coolingthegap,givingablastofairorflowofoilathighvelocity
andpressure.
Iftherateofremovalofheatisfasterthantherateofheatgeneration
thearcisextinguished.Iftherateofheatgenerationismorethanthe
rateofheatdissipation,thespacebreaksdownagainresultinginan
arcforanotherhalfcycle.

24. Expression for Restriking Voltage and RRRV

27. The maximum value of restriking voltage occurs at t=π/ω
n
The maximum value of restriking voltage =2 V
m
The rate of rise of restriking voltage,
RRRV=
The maximum value of RRRV occurs at t=π/2ω
n
The maximum value of RRRV =ω
nV
m

28. Resistance Switching
Toreducetherestrikingvoltage,RRRVandseverityofthe
transientoscillations,aresistanceisconnectedacrossthecontacts
ofthecircuitbreaker
Thisisknownasresistanceswitching.Theresistanceisin
parallelwiththearc.
Apartofthearccurrentflowsthroughthisresistanceresultingin
adecreaseinthearccurrentandincreaseinthedeionizationofthe
arcpathandresistanceofthearc.
Thisprocesscontinuesandthecurrentthroughtheshunt
resistanceincreasesandarccurrentdecreases.Duetothedecrease
inthearccurrent,restrikingvoltageandRRRVarereduced

29. Theanalysisofresistanceswitchingcanbemadetofindoutthe
criticalvalueoftheshuntresistancetoobtaincompletedampingof
transientoscillations.Figureshowstheequivalentelectricalcircuit
forsuchananalysis.

35. Problem:
In a 220 kV system, the reactance and capacitance up to the location
of circuit breaker is 8Ω and 0.025 μF, respectively. A resistance of
600 ohms is connected across the contacts of the circuit breaker.
Determine the following:
1. Natural frequency of oscillation
2. Damped frequency of oscillation
3. Critical value of resistance which will give no transient oscillation
4. The value of resistance which will give damped frequency of
oscillation, one-fourth of the natural frequency of oscillation.

36. Current Chopping
•Whenlowinductivecurrentisbeinginterruptedandthearc
quenchingforceofthecircuitbreakerismorethannecessaryto
interruptalowmagnitudeofcurrent,thecurrentwillbeinterrupted
beforeitsnaturalzeroinstant.
•Insuchasituation,theenergystoredinthemagneticfieldappearsin
theformofhighvoltageacrossthestraycapacitance,whichwill
causerestrikingofthearc.
•Theenergystoredinthemagneticfieldis½Li
2
,ifiisthe
instantaneousvalueofthecurrentwhichisinterrupted.Thiswill
appearintheformofelectrostaticenergyequalto½cv
2
.Asthese
twoenergiesareequal,theycanberelatedasfollows.

38. •Ifthevalueofvismorethanthewithstandingcapacityof
thegapbetweenthecontacts,thearcappearsagain.
•Sincethequenchingforceismore,thecurrentisagain
chopped.
•Thephenomenoncontinuestillthevalueofvbecomes
lessthanthewithstandingcapacityofthegap.

39. Acircuitbreakerinterruptsthemagnetisingcurrentofa100
MVAtransformerat220kV.Themagnetisingcurrentofthe
transformeris5%ofthefullloadcurrent.Determinethe
maximumvoltagewhichmayappearacrossthegapofthe
breakerwhenthemagnetisingcurrentisinterruptedat53%ofits
peakvalue.Thestraycapacitanceis2500μF.Theinductanceis
30H.

40. Interruption of capacitive current
•Theinterruptionofcapacitivecurrentproduceshighvoltage
transientsacrossthegapofthecircuitbreaker.
•Thisoccurswhenanunloadedlongtransmissionlineora
capacitorbankisswitchedoff.
•Thefigureshowsanequivalentelectricalcircuitofasimple
powersystem.Crepresentsstraycapacitanceofthecircuit
breaker.C
Lrepresentslinecapacitance.ThevalueofC
Lis
muchmorethan

42. •AttheinstantM,thecapacitancecurrentiszeroandthe
systemvoltageismaximum.Ifaninterruptionoccurs,the
capacitorsC
Lremainschargedatthemaximumvalueofthe
systemvoltage.AfterinstantM,thevoltageacrossthebreaker
gapisthedifferenceofV
candV
CL.AtinstantN,i.e.half-cycle
fromA,thevoltageacrossthegapistwicethemaximumvalue
ofV
c.Atthismoment,thebreakermayrestrike.
•Ifthearcrestrikes,thevoltageacrossthegapbecomes
practicallyzero.Thus,thevoltageacrossthegapfallsfrom
2V
cmaxtozero.Aseverehighfrequencyoscillationoccurs.

43. •AtinstantP,thesystemvoltagereachesitspositivemaximum
shownbythepointTinthefigure,andatthismomentthe
voltageacrossthegapbecomes4emax.Thecapacitivecurrent
reacheszeroagainandtheremaybeaninterruption
•Inthisway,thevoltageacrossthegapgoesonincreasing.But
inpractice,itislimitedto4timesthepeakvalueofthesystem
voltage.Thus,itisseenthatthereisaproblemofhigh
transientofhightransientvoltagewhileinterruptinga
capacitivecurrent.

44. Classification of Circuit Breakers
•Classification based on Voltage
–Low voltage Circuit Breaker (less than 1 kV)
–Medium voltage Circuit Breaker ( 1 kV to 52 kV)
–High Voltage Circuit Breaker (66 kV to 220 kV)
–Extra High voltage circuit breaker ( 300 kV to 765 kV
–Ultra High voltage Circuit breaker ( above 765 kV)
Classification based on Location
Indoor type
Outdoor type
•Classification based on medium used for arc quenching
–Air break circuit breaker
–Oil circuit breaker
–Air blast circuit breaker
–Sulphur hexafluoride SF
6circuit breaker
–Vaccumcircuit breaker

45. Oil circuit breakers
Types of oil circuit breakers
•Bulk oil circuit breakers
Plain break oil circuit breakers
Arc control oil circuit breakers
•Low oil circuit breakers

46. Bulk Oil Circuit Breaker

47. Plain break oil circuit breakers

48. Disadvantages of Plain break oil circuit breakers
•There is no special control over the arc other than
the increase in length by separating the moving
contacts.
•These breakers have long and inconsistent arcing
times
•These breakers do not permit high speed
interruption.

49. Arc control oil circuit breakers
•Self blast oil circuit breakers
Plain explosion pot
Cross jet explosion pot
Self-compensated explosion pot
•Forced blast oil circuit breakers

50. Plain explosion pot

51. Cross jet explosion pot

52. Self-compensated explosion pot

53. Low oil (or) minimum oil circuit breakers

54. Practical View of oil CB

56. Air break CB
ACBs are still preferable choice up to voltage 15 KV

57. Air Blast CB

58. Axial blast air circuit breaker

59. Cross blast air circuit breaker

60. Advantages of Air blast circuit breaker
•Thereisnochanceoffirehazardcausedbyoil.
•Thebreakingspeedofcircuitbreakerismuchhigherduring
operationofairblastcircuitbreaker.
•Arcquenchingismuchfasterduringoperationofairblastcircuit
breaker.
•Thedurationofarcissameforallvaluesofsmallaswellashigh
currentsinterruptions.
•Asthedurationofarcissmaller,solesseramountofheatrealized
fromarctocurrentcarryingcontactshencetheservicelifeofthe
contactsbecomeslonger.
•Thestabilityofthesystemcanbewellmaintainedasitdependson
thespeedofoperationofcircuitbreaker.
•Requiresmuchlessmaintenancecomparedtooilcircuitbreaker.

62. SF
6CB
sulphur hexafluoride gas (SF6)gas at a pressure ofabout 2.8 kg/cm2

63. AdvantagesofSF
6circuitbreaker
•DuetothesuperiorarcquenchingpropertyofSF6,suchcircuit
breakershaveveryshortarcingtime.
•SincethedielectricstrengthofSF6gasis2to3timesthatofair,such
breakerscaninterruptmuchlargercurrents.
•TheSF6circuitbreakergivesnoiselessoperationduetoitsclosedgas
circuitandnoexhausttoatmosphereunliketheairblastcircuitbreaker.
•Theclosedgasenclosurekeepstheinteriordrysothatthereisno
moistureproblem.
•ThereisnoriskoffireinsuchbreakersbecauseSF6gasisnon-
inflammable.
•Therearenocarbondepositssothattrackingandinsulationproblems
areeliminated.
•TheSF6breakershavelowmaintenancecost,lightfoundation
requirementsandminimumauxiliaryequipment.
•SinceSF6breakersaretotallyenclosedandsealedfromatmosphere,
theyareparticularlysuitablewhereexplosionhazardexistse.g.,coal
mines.

67. Vacuum CB

68. Cross sectional view of vaccum CB

69. •Service life of vacuum circuit breaker is much longer than other
types of circuit breakers.
•There is no chance of fire hazard as oil circuit breaker.
•It is much environment friendly than SF
6Circuit breaker.
•Beside of that contraction of VCB is much user friendly.
•Replacement of vacuum interrupter (VI) is much convenient.

70. Vaccum VI Interrupter

71. References:
http://www.openelectrical.org/wiki/index.php
?title=Circuit_breakers

72. Miniature Circuit Breaker (MCB)
•AMCBisanelectromechanicaldevicewhich
makesandbreaksthecircuitinnormaloperation
anddisconnectsthecircuitundertheabnormal
conditionwhencurrentexceedsapresetvalue.
•MCBisahighfaultcapacitycurrentlimiting,trip
free,automaticswitchingdevicewiththermal
andmagneticoperationtoprovideprotection
againstoverloadandshortcircuit.

73. Miniature Circuit Breaker (MCB)

74. Miniature Circuit Breaker (MCB)

75. Miniature Circuit Breaker (MCB)

76. Features of Miniature Circuit Breaker (MCB)
•Itsoperationisveryfastandopensinlessthanone
millisecond
•Notrippingcircuitisnecessaryandtheoperationis
automatic
•Providesprotectionagainstoverloadandshortcircuit
withoutnoise,smokeorflame.
•Itcanberesetveryquicklyaftercorrectingthefault,just
byswitchingabutton.Norewiringisrequired.
•Itcannotbereclosediffaultpersists.
•Themechanicallifeisuptoormorethanonelakh
operatingcycle.

77. Features of Miniature Circuit Breaker (MCB)
•GenerallyMCBsareratedfora.c.voltageof240Vfor
singlephase,415Vforthreephaseor220Vd.c.
•Thecurrentratingavailableisform0.5Ato63A.
•Itisavailableassinglepole(SP),DoublePole(DP),
TripplePole(TP)withshortcircuitbreakingcapacityform
1kAto10kAwitharatedfrequency.

78. Moulded Case Circuit Breaker (MCCB)
•Itisusedforcircuitshavingcurrentrangesform63Ato
3000A.
•Itsworkingisbasedonthermalmechanism.Ithasa
bimettaliccontactwhichexpandsandcontractswhen
therearechangesintemperature.
•Undernormalcondition,thecontactsareclosedallowing
currenttopass.
•Underoverloadorshortcircuitcondition,current
exceedsitssafevalue.Duetothis,heatisgeneratedand
thecontactsareopenedtointerruptthecircuit.

79. Moulded Case Circuit Breaker (MCCB)
•Duetotheinterruptionofhighcurrent,there
isarcformation.HenceinMCCBtherearearc
extinguisherswhichsuppressthearc.
•Thereisadisconnectionswitch,withthehelp
ofwhich,theMCCBcanbeoperated
manually.

80. Moulded Case Circuit Breaker (MCCB)
•Practicallyithasadjustabletripsettingsand
henceitcanbeusedforhighcurrent
application.
•Itcanbeeasilyresetafterthefault
rectification.Thusitprovidesoperational
safetyandconvenience.
•AlltheoperatingpartsofMCCBarecovered
withinaplasticmouldedhousingmadeintwo
halves.

81. Moulded Case Circuit Breaker (MCCB)
•Thetwohalvesarejoinedtogethertoform
thewholestructure.
•ThebasicdifferencebetweenMCBandMCCB
isthecurrentrating.
•HenceMCCBsareusedforindustrialand
commercialapplicationssuchasmainfeeder
protection,generatorandmotorprotection,
capacitorbankprotection,welding
applicationsandapplicationswhichrequire
adjustabletripsetting.

82. Moulded Case Circuit Breaker (MCCB)

83. Comparison of MCB and Fuse
S.No.Fuse MCB
1 Theoperationoffuseishighly
dependentonselectionofitsproper
rating.Iffusewireisnotselected
properlythenitresultsinnonoperation
offuseevenincaseofshortcircuit
MCBinstantlydisconnectsthesupply
automaticallyintheeventofshortcircuit
oroverload.Itthuseliminatestheriskof
fireandpreventsdamagetowiring
system
2 Ifthefusewireafteroperationis
replacedwithaneweronebutgoloose
thenitmaybedangerous.Alsoto
replaceablownfuseinbetweencurrent
carryingpointsisdangerousspeciallyin
dark.
Restartingpowersupplyaftertripping
duetooverloadorshortiseasy
3 Duringreplacementoffusewire,the
exactsizeoffusewiremaynotbe
available.Alsoforreplacementakitof
handtoolshastobekeptready
Nomaintenanceandrepairsisrequired
forMCB.Thedistributionsystem
employingMCBprovidessatisfactory
operationandlastsforyears.
4 Theboardemployingfuseisnot
compact
TheboardemployingMCBsgivebeautiful
lookasitiscompactandelegant

84. Rating of circuit breaker
A circuit breaker has to perform the following major duties under short-
circuit conditions.
1. To open the contacts to clear the fault
2. To close the contacts onto a fault
3. To carry fault current for a short time while another circuit breaker is
clearing the fault
Therefore, in addition to the rated voltage, current and frequency, circuit
breakers have the following important ratings
(i)Breaking Capacity
(ii)Making Capacity
(iii)Short-time Capacity

85. Breaking Capacity
•Itiscurrent(r.m.s.)thatacircuitbreakeris
capableofbreakingatgivenrecoveryvoltage
andunderspecifiedconditions(e.g.,power
factor,RRRV).

86. Breaking Capacity
Thebreakingcapacityofacircuitbreakerisoftwotypes
(i)Symmetricalbreakingcapacity
(ii)Asymmetricalbreakingcapacity
Symmetricalbreakingcapacity
ItistheRMSvalueoftheaccomponentofthefaultcurrentthat
thecircuitbreakeriscapableofbreakingunderspecified
conditionsofrecoveryvoltage.
Asymmetricalbreakingcapacity
ItistheRMSvalueofthetotalcurrentcomprisingofbothacand
dccomponentsofthefaultcurrentthatthecircuitbreakercan
breakunderspecifiedconditionsofrecoveryvoltage.

88. Breaking capacity:
Thebreakingcapacityofacircuitbreakerisgenerallyexpressedin
MVA.Forathree-phasecircuitbreaker,itisgivenby
Breakingcapacity=√3xratedvoltageinkVxratedcurrentinkA.
Thebreakingcapacitywillbesymmetricaliftheratedcurrentinthe
aboveexpressionissymmetrical.

89. Making Capacity
•Thepeakvalueofcurrent(includingd.c.
component)duringthefirstcycleofcurrentwave
aftertheclosureofcircuitbreakerisknownas
makingcapacity.
Making current = √2 x 1.8 x symmetrical breaking current.
Making capacity = √2 x 1.8 x symmetrical breaking capacity

90. Short-time rating
•Itistheperiodforwhichthecircuitbreakerisable
tocarryfaultcurrentwhileremainingclosed.
•Theshort-timecurrentratingisbasedonthermal
andmechanicallimitations.
•Thecircuitbreakermustbecapableofcarrying
short-circuitcurrentforashortperiodwhile
anothercircuitbreaker(inseries)isclearingthe
fault.

91. Short-time rating
•Theshort-timeratingofacircuitbreakerdepends
uponitsabilitytowithstand
(a)theelectro-magneticforceeffects
(b)thetemperaturerise.

92. Normal current rating
•Itisther.m.s.valueofcurrentwhichthecircuit
breakeriscapableofcarryingcontinuouslyatits
ratedfrequencyunderspecifiedconditions.The
onlylimitationinthiscaseisthetemperaturerise
ofcurrent-carryingparts.

93. Rated operating duty:
Theoperatingdutyofacircuitbreakerprescribesitsoperations
whichcanbeperformedatstatedtimeintervals
Forthecircuitbreakerswhicharenotmeantforautoreclosing,there
aretwoalternativeoperatingdutiesasgivenbelow:
(i)O-t-CO-t’-CO
(ii)O-t”-CO
WhereOdenotesopeningoperation,COdenotesclosingoperation
followedbyopeningwithoutanyintentionaltimelag,andt,t’andt”
aretimeintervalsbetweensuccessiveoperations

94. Selection of Circuit Breaker
•Thefollowingparametersarerequiredtobe
knownforselectingproperratingcircuitbreaker
atagivenlocationonapowersystem:
•(a)themaximumfaultcurrentwhichistobe
interruptedbythebreaker
•(b)themaximumcurrenttobecarried
momentarily.
•Thefaultcurrentconsistsofbotha.c.andd.c.
componentsanditscorrectcalculationisvery
complex.Asimplifiedmethodisrecommendedby
IEEEcommitteeisgivenbelow.

95. Selection of Circuit Breaker
•Todeterminefirstlytherequiredinterruption
capacityofcircuitbreakerthehighestvalueof
initialr.m.salternatingcurrentforanytypeand
locationoffault.
•Itcanbeconsideredasthreephasefaultasit
carriesmaximumfaultcurrentexceptinsome
cases.
•Thiscurrentcanbeobtainedbyusingsub-
transientreactanceforgeneratorsandtransient
reactanceforsynchronousmotorswhileinduction
motorsaretobeneglected.

96. Selection of Circuit Breaker
•Followingmultiplyingfactorscanbeappliedto
takeintoaccountthed.c.componentsofcurrents
anddecrementsofbotha.c.andd.c.components.
8cycleorslowbreaker1.0
5cyclebreaker 1.1
3cyclebreaker 1.2
2cyclebreaker 1.4
beforeapplyingmultiplyingfactor,allthegiven
factorsareincreasedby0.1forthebreakersonthe
generatorbuswhere3phaseshortcircuitKVA
exceed5,00,000.

97. Selection of Circuit Breaker

100. HVDC Breaking
The additional circuit creates artificial current zeros which are utilised
for arc interruption
C
pand L
pare connected in parallel to produce artificial current zero
after the separation of the contacts in the main circuit breaker MCB.