CIRCUIT BREAKERS

 Definition
 A circuit breaker is defined as a piece of equipment
which can do any one of the following tasks:

 Makes or breaks a circuit either manually or by remote

control under normal conditions.

 Breaks a circuit automatically under fault conditions.

 Makes a circuit either manually or by remote control

under fault conditions.
 Advantage over Fuse

 A fuse operates once and then has to be replaced.

 The main advantage associated with the use of

circuit breaker is that a circuit breaker can be
reset(either manually or automatically)to resume
normal operation
 Circuit Breaker Contacts
 There are two types of contacts:

 Primary contact

 Arcing contact
 Circuit Breaker Contacts

 The primary contact is always made of a high

conductive material such as copper (Cu).

 The arcing contact is made of arc resistance material

such as tungsten or molybdenum, which has a much
lower conductivity than those used for primary
contacts.
 Normal Condition
 Under the normal operating conditions, these
contacts remain closed and are not open
automatically until and unless the system becomes
faulty and it receives an command from the Relay.
 Faulty Condition

 When a fault occurs on any part of the system, the

trip coils of the circuit breaker get energized ( on
receiving an command from Relay ) and the moving
contacts are pulled apart by some mechanism, thus
opening the circuit.
 Arc Phenomenon
 When the contacts of a circuitbreaker are separated
under fault conditions, an arc is struck between
them. The current is thus able to continue until the
discharge ceases.

 Arc is Useful?

 The arc is useful in a way as it provides alow

resistance path for the current aftercontact
separation. It prevents currentchopping and
associated abnormalswitching over-voltages in the
system.

 The arc provides a gradual, but quick,transition

from the current-carrying to thecurrent-breaking
states of the contacts
 Arc Extinction
 The arc depends upon the following factors:
 Degree of Ionization
 Length of the Arc
 Cross-section of the Arc
 1.Based on Voltage
 On the basis of the voltage levels forwhich they are used,
the circuit breakersare classified as:
 CategoryRange of VoltageLow voltage
 Less than 1 kV
 Medium voltage
 1 kV to 52 kV
 High/Extra Highvoltage
 66 kV to 765 kV
 Ultra High voltage
 Above 765 kV
 Based on Location
 Circuit breakers are, based upon where they are
located, classifiedas:

 Indoor type

 Outdoor type
 Based on External Design
 From the point of view of their physical structural design,
outdoorcircuit breakers can be identified aseither:

 dead tank type (the switching unit is located within a

metallic container which is kept a earth potential)

 live tank type ( the circuit breaker the switching unit is

located in an insulator bushing which is live at line
voltage .Live Tank circuit breakers are cheaper than
dead tank and require less space ).
Live Tank Circuit Breaker
Dead Tank Circuit Breaker
 Based on InterruptingMediaMedia
 The circuit breakers may be classifiedinto following categories:

 Oil Circuit Breakers

 Air Circuit Breakers

 Air-blast Circuit Breakers

 Sulphur Hexafluoride (SF
 6
 ) CircuitBreakers

 Vacuum Circuit Breakers
 1.Oil Circuit BreakersOil Circuit Breakers

 The circuit breakers in which some insulating oil

(i.e.,transformer oil) is used as an arc quenching
medium
 Arc Extinction in OCBs

 The arc extinction is facilitatedmainly by
two processes:

 Firstly, the hydrogen gas hashigh heat conductivity
andcools the arc, thus aiding thede-ionization of the
mediumbetween the contacts
 Secondly, the gas sets up turbulence inthe oil and
forces it into the spacebetween contacts, thus
eliminating thearcing products from the arc path.
Thisresults in extinguishing the arc and asa result
the circuit current isinterrupted.
 Advantages

 The oil provides insulation for thelive exposed
contacts .

 The hydrogen produced duringarcing has excellent
coolingproperties and helps extinguish thearc.

 The oil close to the arc regionprovides cooling
surface
 Disadvantages

 Oil is inflammable and may cause firehazards.

 The hydrogen, when combined with air,may form an
explosive mixture.

 During arcing, oil becomes polluted bycarbon particles,
which reduces itsdielectric strength. Hence, it
requiresperiodic maintenance and replacement
 a

 Types of Oil Circuit Breakers

 They can be classified with thereference to the
quantity of oilused.

 Bulk Oil Circuit Breakers

 Minimum Oil Circuit Breakers
 Bulk Oil Circuit Breakers

 These circuit breakers usea large quantity of oil.
 The oil has to serve two purposes:

 It extinguishes the arc during openingof contacts.

 It insulates the current conducting partsfrom one
another and from the earthedtank.
 Minimum Oil Circuit Breakers

 These circuit breakers use asmall quantity of oil.

 In such circuit breakers, oil isused only for arc
extinction;the current conducting partsinsulated by
air or porcelain ororganic insulating material
 Air-Blast Circuit Breakers

 These circuit breakers employa high pressure air-
blast as an

 The contacts are opened in aflow of air-blast
established bythe opening of the blast valve.arc
quenching medium.

 Arc Extiction

 The air-blast cools the arc andsweeps away the
arcingproducts of the atmosphere.Consequently, the
arc isextinguished and flow of current is interrupted
 Advantages

 The risk of fire is eliminated.

 The arcing products are completely removed by the blast,so the
expenditure of oil replacement is avoided.

 The size of these breakers is reduced.

 Due to the rapid growth of the dielectric strength, thearcing time is also
very small. It causes less burning of oil.

 The arc energy is also very small fraction of that in oilcircuit breakers.

 The arc extinction is facilitate by the high pressure air,and is independent
of the fault current to be interrupted.
 Disadvantages

 These circuit breakers are verysensitive to the
variation s in therate of rise of restriking voltage.

 The air-blast is supplied by thecompressor plant
that needsconsiderable maintenance.
 Sulphur Hexafluoride (SF
 66
 ))Circuit BreakersCircuit Breakers

 In these circuit breakers, Sulpherhexafluoride gas (SF
 6
 ) is used as the arcquenching medium.

 The SF
 6
 is an electronegative gas and has astrong tendency to absorb free electrons.

 This loss of conducting electrons in the arcquickly builds up enough
insulation strengthto extinguish the arc.
 Uses of SF
 66
 CBsCBs

 These circuit breakers areavailable for complete
range of medium voltage and highvoltage application
up to 800kV and above.

 This medium is most suitablefor metal-clad and
hybrid HVsub-stations.
 Advantages

 Due to the low energy the contact erosion is small.

 The gaseous medium SF
 6
 possesses excellent dielectric andarc quenching properties.

 Due to the superior arc quenching property of the SF6 gas,such circuit breakers
have very short arcing time.Furthermore, they can interrupt much larger current.

 These breakers give noiseless operation due to its closedgas circuit and no exhaust
to atmosphere unlike the air-blast circuit breaker.

 The SF6 gas is not inflammable, so there is no risk of fireand explosion in SF6
breakers
 Disadvantages

 These circuit breakers are expensive due to thehigh cost of
SF6 gas.

 Since SF6 gas has to be reconditioned afterevery operation of
the breaker, additionalequipment is required for this purpose.

 The SF
 6
 gas has been identified as a greenhousegas, so can be harmful
for population in its areaof operation.
 Vacuum Circuit Breakers

 In these circuit breakers, thevacuum is used as the
arcquenching medium.

 The degree of vacuum in thesecircuit breakers is in the
rangefrom 10
 -7
 to 10
 -5
 torr
 Advantages

 The vacuum circuit breakers are compact in size andhave longer lives.

 Because of the very low voltage across the metalvapor arc, energy is very
low.

 There is no generation of gases during and after thecircuit breaker
operation.

 The outstanding feature of these breakers is that itcan break any heavy
fault current perfectly just beforethe contacts reach a definite open
position.

 They can successfully withstand lightning surges
 Air Circuit Breakers

 Air circuit breaker is defined asa circuit breaker, in
which thecontacts open and close in airat
atmospheric pressure.
 Arc Interruption

 The principles of arc interruption used in anair circuit breaker are
rather different fromthose in any other type of circuit breaker.

 This can be achieved in three ways:
 1.
 Intense cooling of the arc plasma, so thatthe voltage gradient is very
high
 2.
 Lengthening the arc path to increase thearc voltage
 3.
 Splitting up the arc into a number of series arcs
 Circuit Breakers Ratings
 There are three ratings forbreakers as:

 Breaking Capacity

 Making Capacity

 Short-time Rating
 Breaking Capacity

 Breaking capacity is defined asthe r.m.s. current that
a circuitbreaker is capable of breakingat given
recovery voltage andunder specified conditions
(i.e.power factor, rate of rise of restriking voltage)
 It is a common practice to express the breakingcapacity in MVA by taking
into account the ratedthe rated breaking current and rated servicevoltage.

 Thus if I is the rated breaking current in Amperesand the rated service
voltage is V in volts, thebreaking capacity for three-phase circuit is:
 Breaking Capacity = sqrt 3
 x
 V
 x
 I
 x
 10
 -6
 MVA

 Making Capacity

 It is the peak value of current(including d.c.
component) duringthe first cycle of current wave
afterthe closure of circuit breaker.

 Mathematically stated, makingcapacity is given as:
 Making Capacity = 2.55 x symmetricalbreaki
ng capacity
 Short-time Rating

 The period for which the circuitbreaker is able to carry
faultcurrent while remaining closed isknown as short-
time rating.

 The short-time rating of a circuitbreaker depends upon
its ability towithstand:
 1.
 The electromagnetic force effects
 2.
 The temperature rise