The document discusses types of circuit breakers, specifically air break circuit breakers. It describes the need for circuit protection from overloads and short circuits. Then it explains the construction, types including plain air and air blast, working principle, design, ratings, advantages, and disadvantages of air break circuit breakers. Air break circuit breakers use atmospheric air to extinguish arcs by rapidly lengthening and cooling the arc to increase resistance above the supply voltage.

1. SWITCHGEAR &
PROTECTION
SUBMITTED BY –
11510325[E5]
11510338[E5]
11510867[E7]
11510868[E7]
11510869[E7]
11510871[E8]
SWITCHGEAR & PROTECTION
TYPES OF CIRCUIT BREAKER

2. 1
TYPES OF CIRCUIT BREAKER
Table of contents
NEED OF CIRCUIT PROTECTION
TYPES OF CIRCUIT BREAKER
1. AIR BREAK CB
2. SF6 CB
3. MINIMUM OIL CB
4. VACCUM CB
5. BULK OIL CB
6. AIR BLAST CB

3. 2
Need for Circuit Protection
Current and Temperature Current flow in a conductor always generates heat. The greater the
current flow, the
hotter the
conductor.
Excess heat is damaging
electrical
Normal Current Flow

4. 3
components. High levels of heat cause the insulation to breakdown
and flake off, exposing conductors. For that reason, conductors have
a rated continuous current carrying capacity or ampacity.
Good Insulation
Excessive current is referred to as overcurrent. An overcurrent may
result from an overload, short circuit, or ground fault. Some circuit
breakers provide only short circuit protection, but most circuit
breakers provide protection against short circuits and overloads, and
some circuit breakers provide protection against all three types of
overcurrent. Overloads and short circuits are discussed in the
following paragraphs. Ground faults are discussed later in this
course.
Overloads An overloadoccurs whentoomany devicesare operatedonasingle
circuitor whenelectrical equipmentismade toworkbeyonditsrated
capabilities.Whenanoverloadoccurs,damage toconnectedequipment
or the conductors thatsupplythatequipmentcanoccur unless the circuit
isshut downbyan overcurrentprotectiondevice.Slightoverloadscanbe
allowedtocontinue forashort time,butas the currentmagnitude
increases,the circuitbreakermustopenfaster.

5. 4
Short Circuits
A short circuit isa low resistance pathforcurrentcreatedwhenbare
conductorstouch.Whena shortcircuit occurs withvoltage applied,the
decrease inresistance resultsinashort circuitcurrentthat can be
thousandsof timeshigherthannormal operatingcurrent.The heat
generated bythiscurrentwill cause extensive damage toconnected
equipmentandconductorsunlesscurrentisinterruptedimmediately.
Ohm’s Law describes the relationship of current, voltage, and
resistance. For example, a 240volt circuit with 24 Ω of resistance
draws 0 amps of current. When a short circuit develops, resistance
drops. If resistance drops to 24 milliohms, current increases to
0,000 amps.

6. 5
Types of Overcurrent Protection Devices
To protect a circuit against over current a protection device
must detect a fault and automatically disconnect the
electrical equipment from the voltage source. An
overcurrent protection device must be able to recognize
the difference between overloads and short circuits and
respond in accordingly. Small overloads can be allowed to
continue for a short time, but larger overloads require
quicker response, and short circuits must be interrupted
instantaneously.
Fuse A fuse isa one-shotdevice.The heatproducedbyovercurrentcausesthe
currentcarrying elementtomeltopen,disconnectingthe loadfromthe
source voltage.
Non-Time-DelayFuse Non-time-delayfusesprovide excellentshortcircuitprotection.Whenan
overcurrentsituationoccurs,heatbuildsuprapidlyinthe fuse.Non-time-
delayfusesusuallyhold500% of theirratingfor approximatelyone-fourth
second,afterwhichthe currentcarryingelementmelts.Thismeansthat
these fusescannotbe usedinmotorcircuits,whichoftenhave inrush
currentsgreaterthan 500%.
Time-DelayFuses
Time-delayfusesprovide overloadandshortcircuitprotection.Time-delay
fusesusuallyallowfive timesthe ratedcurrentforupto ten secondsto
allow motorsto start.
Circuit Breaker Circuit breakers provide a manual means of energizing and
de energizing a circuit and automatic overcurrent
protection. Unlike fuses, which must be replaced when they

7. 6
open, a circuit breaker can be reset once the overcurrent
condition has been corrected. Pushing the handle to the
“OFF” position then back to the “ON” position restores the
circuit. If a circuit reopens upon reset to the “ON” position,
the circuit should be checked by a qualified electrician.
All circuit breakers perform the following functions:
• SENSE when an overcurrent occurs.
• MEASURE the amount of overcurrent.
• ACT by tripping in a timely manner to prevent damage to the circuit
breaker and the conductors it protects.
Circuit Breaker Operation In the following illustration, an AC motor is connected
through a circuit breaker to a voltage source. When the
circuit breaker is closed, a complete path for current exists
between the voltage source and the motor allowing the
motor to run. Opening the circuit breaker breaks the path
of current flow and the motor stops. The circuit breaker
automatically opens when it senses a fault. After the fault
has been cleared, the breaker can be closed, allowing the
motor to operate.
Circuit Breaker
Circuit Breaker Closed
Motor Running
Circuit Breaker Open
Motor Stopped

8. 7
 AIR BREAK CIRCUIT BREAKER –
 INTRODUCTION
 CONSTRCUTION
 TYPES OF AIR BREAK CIRCUIT BREAKER
 WORKING PRINCIPLE OF AIR BREAK CIRCUIT BREAKER
 CIRCUIT BREAKER DESIGN
 CB RATING
 ADVANTAGES
 DISADVANTAGES
 APPLICATION

9. 8
 INTRODUCTION
The air at atmosphere pressure is used as an arc extinguishing medium in air break
circuit breakers. These circuit breakers employ the high resistance interruption
principle.
The arc is rapidly lengthened by means of the arc runners and arc chutes and the
resistance of the arc is increased by cooling lengthening and splitting the arc.
The arc resistance increases to such an extent that the voltage drop across the arc
becomes more than the supply voltage and the arc extinguished.
Air breaker circuit breakers are used in dc circuits and ac circuits up to 12kv system.
The air break circuit breakers are generally indoor types and installed on vertical panel
or indoor draw out type switchgear.
Air break circuit breaker are widely used in indoor medium voltage and low voltage
switchgear.
Typical reference values of ratings of air break circuit breakers are
460 volt 400-3500 A 40-75 KA
3. 3 KV 400-3500 A 13.1- 31.5KA
6 .6KV 400-2400 A 13.1- 20 KA
Magnetic field is utilized for lengthening the arc in high voltage air break circuit breakers.
There are several types of Air circuit breakers areavailable in the market today that is durable,
high-performing, easy to install and maintain. The air circuit breakers have completely replaced
oil circuit breakers.

10. 9
 CONSTRUCTION OF AN AIR BREAK CIRCUIT BREAKER
Air Circuit Breaker Construction (ABB E Max Low Voltage, Current Limiting Air
Circuit Breaker and Selective (Non-Current Limiting) Air Circuit Breaker)

11. 10
Labeling of ABB circuit breaker
1. OFF button (O)
2. ON button (I)
3. Main contact position indicator
4. Energy storage mechanism status indicator
5. Reset Button
6. LED Indicators
7. Controller
8. “Connection”, “Test” and “isolated” position stopper (the three-position latching/locking
mechanism)
9. User-supplied padlock
10.Connection “,” Test “and” separation “of the position indication
11.Connection (CE) Separation, (CD) Test (CT) Position indication contacts
12.Rated Name Plate
13.Digital Displays
14.Mechanical energy storage handle
15.Shake (IN/OUT)
16.Rocker repository
17.Fault trip reset button

12. 11
 Types of Air circuit breaker –
1. Plain air circuit breaker or Cross-Blast Air Circuit Breaker
2. Air blast circuit breaker
Plain air circuit breaker or Cross-Blast Air Circuit
Breaker:
The circuit breaker is fitted with a chamber surrounding the contact. The
chamber is known as “arc chute”. The arc is made to drive in it. The arc chute will
help in achieving cooling. Arc chute is made from some refractory material. The
inner walls of arc chute are shaped in such a way that arc is not only forced into
close proximity, but will drive into the serpentine channel projected on arc chute
wall.
The arc chute is divided into a number of small compartments by using metallic
separation plates. Metallic separation plates are arc splitters and each of small
compartments behave as a mini arc chute. Initial arc will split into a series of arcs
this will make all arc voltages higher than system voltage. They are preferable
choice in low voltage application.

13. 12
Air blast circuit breaker:
This type of circuit breaker is used for systemvoltage of 245 KV, 420 KV and even more. Air
blast circuit breaker has further divided into three categories
1. Axial blast breaker
2. Axial blast with sliding moving contact.
Axial blast breaker:
The moving contact is in contact. There is a nozzle orifice in fixed contact at
normal closed condition of breaker. When a fault occur high pressure is
introduced into the chamber. High-pressure air will flow through nozzle orifice
voltage is sufficient to sustain.

14. 13
Axial blast with sliding moving contact:
The moving contact is fitted over a piston supported by a spring. The blast transfers arc
to arcing electrode.

15. 14
 WORKING PRINCIPLE
Air circuit breakers operate with their contacts in free air. Their method of arc
quenching control is entirely different from that of oil circuit-breakers. They are
always used for a low-voltage interruption and now tends to replace high-voltage
oil breakers. The below-shown figure illustrates the principle of air breaker circuit
operation.
Air Circuit breakers generally have two pairs of contacts. The main pair of contacts (1)
carries the current at normal load and these contacts are made of copper metal. The
second pair is the arcing contact (2) and is made of carbon. When the circuit breaker is
being opened, the main contacts open first. When the main contacts opened the arcing
contacts are still in touch with each other. As the current gets a parallel low resistive
path through the arcing contact. During the opening of main contacts, there will not be
any arcing in the main contact. The arcing is only initiated when at last the arcing
contacts are separated. The each of the arc contacts is fitted with an arc runner which
helps. The arc discharge to move upward due to both thermal and electromagnetic
effects as shown in the figure. As the arc is driven upward it enters in the arc chute,
consisting of splatters.

16. 15
The arc in the chute will become colder, lengthen and split hence arc voltage becomes
much larger than the system voltage at the time of operation of air circuit breaker, and
therefore the arc is extinguished finally during the current zero.
The air brake circuit box is made of insulating and fireproof material and it is divided into
different sections by the barriers of the same material, as shown above, figure (a). At
the bottom of each barrier is a small metal conducting element between one side of the
barrier and the other. When the arc, driven upwards by the electromagnetic forces,
enters the bottom of the chute, it is split into many sections by the barriers, but each
metal piece ensures electrical continuity between the arcs in each section, the several
arcs are consequently in the series.
The electromagnetic forces within each and every section of the chute cause the arc in
that section to start the form of a helix, as shown above, figure (b). All these helices are
in series so that the total length of the arc has been greatly extended, and its resistance
is abundantly increased. This will affect the current reduction in the circuit.
Figure (a) shows the development of the arc from the time it leaves the main contacts
until it is within the arc chute. When the current next ceases at a current zero, the
ionized air in the path of where the arc had been being in parallel with the open contacts
and acts as a shunt resistance across both the contacts and the self-capacitance C,
shown in below figure with red as a high resistance R.
When the oscillation starts between C and L as described for the idealized circuit
breaker shown in Figure below, this resistance damps the oscillation heavily. Certainly, it
is usually so heavy that the damping is critical, the oscillation cannot then take place at
all, and the re-striking voltage, instead of appearing as a high-frequency oscillation,
rises dead-beat to its eventual value of peak generator voltage. This is shown below the
lower waveform.

17. 16
The working principle of this breaker is ratherdifferent from those in any other types of
circuit breakers. The main aim of all kind ofcircuit breaker is to prevent the reestablishment
of arcing after current zero by creating asituation where in the contact gap will
withstand the systemrecovery voltage. The aircircuit breaker does the same but in different
manner. For interrupting the arc, it creates an arcvoltage in excess of the supply voltage. Arc
voltage is defined as the minimum voltagerequired maintaining the arc. This circuit
breaker increases the arc voltage by mainlythree different ways.
It may increase the arc voltage by cooling thearc plasma. As the temperature of arc plasma
isdecreased, the mobility of the particle in arcplasma is reduced; hence more voltage
gradient is required to maintain the arc.It may increase the arc voltage by lengthening
the arc path. As the length of arc path isincreased, the resistance of the path is
increased, and hence to maintain the same arccurrent more voltage is required to be applied
across the arc path. That means arc voltage isincreased.
Splitting up the arc into a number of series arcsalso increases the arc voltage.

18. 17
 Circuit Breaker Design –
The following section presents some basics of circuit breaker design. Variations to these design
principles are discussed later in the course. Circuit breakers are constructed from the following
five major components:
• Frame (Molded Case)
• Contacts
• Arc Chute Assembly
• Operating Mechanism
• Trip Unit
Frame The frame provides an insulated housing to mount the circuit
breaker components. The construction material is usually a thermal set plastic, such as glass-
polymer. The construction material can be a factor in determining the interruption rating of the
circuit breaker. Typical frame ratings include: maximum voltage, maximum ampere rating, and
interrupting rating.
▪ Straight-Through Contacts –
The current flowing in a circuit controlled by a circuit breaker flows through the circuit
breaker’s contacts. When a circuit breaker is turned off or is tripped by a fault current, the
circuit breaker interrupts the flow of current by separating its contacts.
Some circuit breakers use a straight-through contact arrangement, so called because the
current flowing in one contact arm continues in a straight line through the other contact arm.

19. 18
▪ Blow-ApartContacts –
As an improvement over the straight-through contact design, Siemens developed the blow-
apart contact design now commonly used by circuit breakers with higher interrupting ratings.
With this design, the two contact arms are positioned parallel to each other, as shown in the
following illustration. As current flows through the contact arms, magnetic fields develop
around each arm. Because the current flow in one arm is opposite in direction to the current
flow in the other arm, the two magnetic fields oppose each other. Under normal conditions, the
magnetic fields are not strong enough to force the contacts apart.
▪ Arc Chute Assembly –
When circuit breaker contacts open, current continues to flow for a short time
by arcing across the air space between the contacts. When the contacts open far
enough, the arc is extinguished and the current stops.
Minimizing the arc is important for two reasons. First, arcing can damage the contacts.
Second, the arc ionizes gases inside the molded case. If the arc isn’t extinguished quickly
the pressure from the ionized gases can cause the molded case to rupture.
Movable Contact Arm
Magnetic Field
Stationary Contact Arm

20. 19
 AIR BREAKCIRCUIT BREAKER RATING–

21. 20

22. 21
REFERENCE – HITACHI

23. 22
 ADVANTAGES
1. Air blast circuit breaker is a suitable option to use where frequent operation is
required because of lesser arc energy
2. The risk of fire is eliminated in the operation of Air blast circuit breaker.
3. Air blast circuit breaker is small in size, because of the growth of dielectric
strength is so rapid (which final contact gap needed for arc extinction is very
small).
4. Speed of circuit breaker is much higher during operation of the air blast.
5. Arc quenching is much faster
6. The duration of the arc is same for all values of current.
7. Stability of operation can be maintained and depends on speed operation of
circuit breakers.
8. It requires less maintenance.
 DISADVANTAGES–
1. The air supplier plant requires additional maintenance.
2. It contains high capacity air compressor.
3. There is a chance of air pressure leakage from the air pipes junction.
4. There is chance of a high rate rise of re-striking voltage and current chopping.
5. The air has relatively lower arc extinguishing properties.

24. 23
 APPLICATIONS –
1. It is used for protection of plants
2. It is used for common protection of electrical machines
3. It used for protection of transformers, capacitors and generators.
4. Air circuit breaker is also used in Electricity sharing system and NGD about 15kV
5. Also used in Low as well as High voltage and Current applications.

25. 24
SF6 CIRCUIT BREAKER

26. 25
SF6 (SULFUR HEXAFLUORIDE) CIRCUIT BREAKER
Outlines :
1. Introduction
2. Properties
3. Construction
4. Working principle
5. Types
6. Components
7. Application
8. Advantage
9. Disadvantage
10. Specifications
11. Conclusion

27. 26
1.INTRODUCTION
A circuit breaker in which SF6 under pressure gas is used to extinguish the arc is called
SF6 circuit breaker. SF6 (sulphur hexafluoride) gas has excellent dielectric, arc quenching,
chemical and other physical properties which have proved its superiority over other arc
quenching mediums such as oil or air.
The circuit breaker which used air and oil as an insulating medium, their arc extinguishing force
builds up was relatively slow after the movement of contact separation. In the case of high
voltage circuit breakers quick arc extinction properties are used which require less time for quick
recovery, voltage builds up. SF6 circuit breakers have to 760 kV, SF6 circuit breakers is good
properties in this regards compared to oil or air circuit breakers. So in high voltage up used.
In the past for higher transmission voltage Oil Circuit Breaker (OCB) and Air Blast Circuit
Breaker (ABCB) were used. These days for higher transmission voltage levels SF6 Circuit
Breakers are largely used. OCB and ABCB have almost become obsolete. In fact, in many
installations SF6 CB is used for lower voltages like 11 kV, 6 kV etc. i) Sulphur Hexafluoride
symbolically written as SF6 is a gas which satisfy the requirements of an ideal arc interrupting
medium. So SF6 is extensively used these days as an arc interrupting medium in circuit breakers
ranging from 3 kv up to 765 kv class. In addition to this SF6 is used in many electrical
equipments for insulation. Here first we discuss in brief, some of the essential properties of SF6
which is the reason of its extensive use in circuit breakers
SF6 gas has high dielectric strength which is the most important quality of a material for use in
electrical equipments and in particular for breaker it is one of the most desired properties.
Moreover, it has high Rate of Rise of dielectric strength after arc extinction.

28. 27
2.Properties ofSulphur hexafluoride Circuit Breaker
 It is colourless, odourless, non-toxic, and non-inflammable gas.
 SF6 gas is extremely stable and inert, and its density is five times that of air.
 It has high thermal conductivity better than that of air and assists in better cooling current
carrying parts.
 SF6 gas is strongly electronegative, which means the free electrons are easily removed from
discharge by the formation of negative ions.
 It has a unique property of fast recombination after the source energising spark is removed. It is
100 times more effective as compared to arc quenching medium.
 Its dielectric strength is 2.5 times than that of air and 30% less than that of the dielectric oil. At
high pressure the dielectric strength of the gas increases.
 Moisture is very harmful to SF6 circuit breaker. Due to a combination of humidity and SF6 gas,
hydrogen fluoride is formed (when the arc is interrupted) which can attack the parts of the circuit
breakers.
A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to
quench the arc. They are most often used for transmission-level voltages and may be
incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or
de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas.

29. 28
3.CONSTRUCTION OF SF6 CIRCUIT BREAKER
Like other circuit breaker viz. Vacuum Circuit Breaker, Air Blast Circuit Breaker etc.,
SF6 Circuit Breaker has fixed contact as well as moving contact. Theses fixed and moving
contacts are known as MAIN CONTACT. There exists one another contact which is known
as ARCING CONTACT. Arcing Contact is part of fixed contact. Basically, Arcing contacts are
only designed to withstand arcing. It is not designed for carrying load current. In spite, main
contacts are designed to carry load current and not the arcing.
Therefore, it can be said that, while closing of SF6 circuit breaker, first Arcing Contact will
close. Thereafter arcing contact will close. Similarly, while opening, first main contact will open
and then arcing contact will open. Notice here that, during opening operation of SF6 Circuit
Breaker, the order of opening main and arcing contact is revered as that in closing operation.
This is because, while opening if the main contact opens first, there will not be any arcing as the
current is getting path through the arcing contact. But if the arcing contact open first then during
opening of main contact there will be arcing and as discussed main contacts are not meant to
withstand arcing.
Interrupter Unit – This unit consists of moving and fixed contacts comprising a set of current-
carrying parts and an arcing probe. It is connected to the SF6gas reservoir. This unit consists slide
vents in the moving contacts which permit the high-pressure gas into the main tank.
Gas System – The closed circuit gas system is employed in SF6 circuit breakers. The SF6 gas
is costly, so it is reclaimed after each operation. This unit consists low and high-pressure
chambers with a low-pressure alarm along with warning switches. When the pressure of the gas
is very low due to which the dielectric strength of gases decrease and an arc quenching ability of
the breakers is endangered, then this system gives the warning alarm

30. 29
Apart from Fixed contact and moving contacts, SF6 Circuit Breaker has following main
components:
 Interrupter
 Insulating Nozzle
 SF6 Gas Chamber
An interrupt is a chamber which encloses the breaker contacts, insulating nozzle, SF6gas
chamber. Interrupter is made of porcelain. Figure below shows the basic parts of SF6 circuit
breaker.

31. 30
Figure-1
Carefully observe the figure and notice the different parts, though some parts like SF6gas
chamber, nozzle, valve etc. are not shown in the above figure but they will be shown while
discussing the working principle.
An sulphurhexaflouride (SF6) circuit breaker consists of fixed and moving contacts enclosed in a
chamber. The chamber is called arc interruption chamber which contains
the sulphurhexaflouride(SF6) gas. This chamber is connected to sulphurhexaflouride (SF6) gas
reservoir. A valve mechanism is there to permit the gas to the arc interruption chamber. When
the contacts of breaker are opened, thevalve mechanism permits a high
pressure sulphurhexaflouride (SF6) gas from the reservoir to flow towards the arc interruption
chamber.

32. 31
The SF6 Circuit Breakers are available as in following designs.
1. live tank,
2. dead tank,
3. or grounded tank designs.
The live tank means the interruption happens in an enclosure that is at line potential. Such SF6
CB has an interrupter chamber that is mounted on insulators and is at line potential. An
interrupter with such a modular design can be connected in series to operate at higher-voltage
levels.

33. 32
The dead tank means that interruption takes place in a grounded enclosure and CTs are
located on both sides of the break (i.e., interrupter contacts). In such CBs, the interruption
maintenance takes place at ground level and its seismic withstand is better than CBs with the live
tank designs.
However, they require more insulating gas in order to provide the proper amount of
insulation between the interrupter and the grounded tank enclosure. The modular dead tank CB
has been especially developed for integration of SF6 insulated substation systems.
The grounded tank means that interruption happens in an enclosure that is partially at line
potential and partially at ground potential. The evolution of the grounded tank CB design is the
result of installing a live tank CB interrupter into a dead tank CB design.

34. 33
4.WORKING PRINCIPLE OF SF6 CIRCUIT BREAKER
In the normal operating conditions, the contacts of the breaker are closed. When the fault occurs
in the system, the contacts are pulled apart, and an arc is struck between them. The displacement
of the moving contacts is synchronised with the valve which enters the high-pressure SF6 gas in
the arc interrupting chamber at a pressure of about 16kg/cm^2.
The SF6 gas absorbs the free electrons in the arc path and forms ions which do not act as a charge
carrier. These ions increase the dielectric strength of the gas and hence the arc is extinguished.
This process reduces the pressure of the SF6 gas up to 3kg/cm^2 thus; it is stored in the low-
pressure reservoir. This low-pressure gas is pulled back to the high-pressure reservoir for re-use.
Now a day puffer piston pressure is used for generating arc quenching pressure during an
opening operation by mean of a piston attached to the moving contacts.
The contacts of SF6 Circuit Breaker are surrounded in an environment of SF6 gas at some
pressure. Actually, the dielectric strength of SF6 gas is directly proportional to its pressure. In
220 kV, 400 kV and 765 kV applications, the gas pressure is maintained at 6.5 bar. Let’s
consider breaker opening operation for better understanding of operating mechanism. First have
a look at the contacts when the breaker is in fully close position as shown in Figure-1.
Now we will open the breaker and will observe its mechanism step by step
.
Step-1: Main ContactOpen

35. 34
As discussed earlier in the post, main contact will open first. This is shown in the figure above.
Observe in figure that, though main contacts are open, arcing contacts are still close. As main
contacts open, the piston in the cylinder moves causes the SF6 gas to compress due to reduction
of volume Vp.
Step-2: Arcing Contacts Open
As soon as arcing contacts separates from contact 1, an arc is strikes. Due to this arcing, heat is
produced. This heat of arc further increases the pressure of SF6 in the chamber Vt. Mind that, the
pressure of arc extinguisher i.e. SF6 is increased by the heat of arc. This is the reason; such

36. 35
breaker is called self-compensating type. Here self-compensating means that, the capacity of
breaker to interrupt the fault is proportional to fault current.

37. 36
Step-3: Arcing Contact separates from Nozzle
When arcing contact separates from the insulating Nozzle, the pressurized SF6 gas in volume Vt
is released in the arc. This causes the arc to extinguish at the moment the current passes though
the natural zero. Thus, the pressurized SF6 gas extinguishes the arc and hence circuit is
interrupted.
In case of small current like in unloaded transformer or reactor, the thermal energy of arc is not
enough to pressurize the SF6 gas. In such case the pressure developed in the SF6 gas chamber
Vp in Step-1 is extinguishes the arc.

38. 37
5.TYPES OF SF6 CIRCUIT BREAKER
As discussed, in 220 kV, 400 kV and 765 kV applications, the SF6 gas pressure is maintained at
6.5 bar. You will be amazed that, even though voltage level is increasing, same pressure of SF6
i.e. 6.5 bar is used for 220, 400 and 765 kV applications. Actually as we go up at higher voltage
level, the number of contacts increases in SF6 circuit Breaker. Based on this philosophy, SF6
circuit breaker can be classified into following types:
1. Single Breaker Circuit Breaker
2. Double Break Circuit Breaker
3. Multi Break Circuit Breaker
1. Single Break SF6 Circuit Breaker
In Single Break Circuit Breaker, only one moving and fixed contacts are present. This means
that, there will only be one interrupter unit in such breaker. Single break SF6 circuit breaker is
used for 220 kV applications.
2. Double Break SF6 Circuit Breaker
In such type of breaker, there are two set of moving and fixed contacts connected in series.
Therefore, to enclose two set of contacts, there must be two interrupt unit in series. This type of
breaker is used in 400 kV applications. In double break circuit breaker, grading capacitors are
used to equalize the voltage distribution across each contact. Thus for 400 kV application, the
voltage across each contact will be 200 kV. Therefore, it is logical to use SF6 gas at a pressure
same as used in 200 kV application.

39. 38
3. Multi Break SF6 Circuit Breaker
In multi break circuit break, more than two set of fixed and moving contacts are used. Such type
of breaker is used in 765 kV applications
6.COMPONENTS OF SF6 CB
An SF6 circuit breaker can be divided into two components, namely
 Interrupter unit
 The gas system
INTERRUPTER UNIT
 This unit consists of moving contacts and fixed contacts in a chamber filled with SF6
 The fixed contact is hollow cylindrical contact comprising an arcing horn.
 The moving contact is also a hollow cylindrical contact with holes in its sides known as side
vents. Side vents in the moving contact permit the high-pressure gas into the main tank after
flowing along and across the arc.
 When moving contact is withdrawn from fixed contact an arc is struck between contacts. The
SF6 gas is blown axially along the arc by the gas system of the breaker. The gas is made to flow
from high-pressure zone to a low-pressure zone through a nozzle. The nozzle is located such that

40. 39
the gas flows axially over the arc length. The heat is removed from the arc by axial convection
and radial dissipation. This reduces the arc diameter and
 the arc is extinguished at current zero instant.

41. 40
THE GAS SYSTEM
The closed circuit gas system is used in the SF6 circuit breaker. SF6 is a costly gas. Therefore, it
is renovated and recycled, after each operation of the breaker. The necessary auxiliary system is
made for such purpose. The gas is stored in a high-pressure chamber at 16 atmospheres whereas
the gas pressure at the low side is 3 atmospheres. The breaker also has an alarm system. The
alarm system gives a warning if the gas pressure drops below a certain value and safety system
immobilize the breaker if gas pressure reaches to a danger limit. Because at low pressure the
dielectric strength of SF6 gas is reduced, which decreases its arc quenching ability. To prevent
gas leakages at joints, sealing is done very carefully. To prevent the liquefaction of gas at low
temperature a heater is provided in the high-pressure chamber which maintains its temperature at
200C.
7.APPLICATION OF SF6 CIRCUIT BREAKER
Industrial SF6 High Volt Circuit Breakers
SF6 industrial circuit breakers use contacts that are surrounded by the sulfur hexafluoride gas
which quenches the resulting arc. Industrial SF6 high volt circuit breakers are mostly used in
transmission level volts where large currents are involved. They can be incorporated into gas-
insulated compact switchgears for more effectiveness. Supplemental heating of these circuit
breakers or a calculated de-rating of their capacities may become necessary in cold climates to
counter the liquefaction of SF6 gas under low temperatures.

42. 41
8.SF6 CIRCUIT BREAKERS ADVANTAGES
 Excellent insulating, arc extinguishing, physical and chemical properties of SF6 gas is greater
advantage of SF6 circuit breakers
 The gas is non-inflammable and chemically stable. The decomposition products are non-
explosive i.e., there is no risk of fire or explosion
 Electrical clearances are very much reduced because of high dielectric strength of SF6
 Outdoor EHV SF6 circuit breaker has less number of interrupters per pole in comparison to the
air-blast circuit breaker and minimum oil breaker. Outdoor SF6 circuit breaker is simple,
comparatively cheaper in cost, maintenance free and compact
 Its performance is not affected due to variation in atmospheric conditions
 It gives noiseless operation it does not make sound like air-blast circuit breaker during operation
 No frequent contact replacement-arcing time is small owing to outstanding arc quenching
properties of SF6 and therefore contact erosion is less. Hence contacts do not suffer oxidation
 Therefore, is no reduction in dielectric strength of SF6 since no carbon particle is formed during
the arcing
 Minimum maintenance. The breaker may require maintenance once in four to ten years
 The sealed construction avoids the contamination by moisture, dust, sand etc. No costly
compressed air system is required as in the case of air blast circuit breaker
 Same gas is re-circulated into the circuit thereby reducing the requirement of SF6 gas.
 No over voltage problem. The arc is extinguished at natural current zero without the current
chopping and associated over-voltages originating across the circuit breaker terminals
 The SF6 gas circuit breaker can perform various duties like clearing short line faults, opening
unloaded transmission lines, capacitor switching, transformer reactor switching etc. without any
problem
 Ample overload margin. For the same size of the conductors the current carrying capacity of the
SF6 circuit breakers is about 1.5 times that of the air blast circuit breakers because superior heat
transfer capability of the SF6 gas

43. 42
9.Disadvantages of SF6 circuit breaker
 Imperfect joints leading to leakage of the SF6 gas. Continuous monitoring devices are required
 SF6 gas is suffocating to some extent. In case of leakage in the breaker tank SF6 gas being
heavier than the air settles in the surroundings and may lead to suffocation of the operating
personnel. However, it is not poisonous
 Arced SF6 gas is poisonous and should not be inhaled
 The internal parts need thorough cleaning during periodic maintenance under clean and dry
environment. Dust of Teflon and sulphides should be removed
 Special facilities are required for transportation of gas, transfer of gas and maintenance of quality
of the gas. The deterioration of quality of gas affects the performance and hence reliability of the
SF6 circuit breaker

44. 43
10.Circuit Breaker specifications of SF6, 132kV Breaker
Following figure shows the circuit breaker specifications of SF6 type which we will be
discussing.We will analyze each information individually.
Rated Voltage:
Rated voltage specified on the nameplate of breaker shows the rated voltage of the breaker not
the rated voltage of the system. It is very important to remember that the voltage on the name
plate of breaker should always be greater than rated voltage of the system. Given breaker which
is 145KV is to be installed at 132KV system. There is no specific relation between the rated
voltage of breaker and system however the following table shows the recommended breaker
rated voltage for each standard voltage level.
Rated voltage of the system IEC Recommended rated voltage of the breaker
11KV —-12.5kV
132KV —-145kV

45. 44
220KV —-245kV
500KV —-525kV
Rated Frequency.
Rated frequency on the name plate of the system should exactly be equal to the frequency of the system.
There are normally two standard of frequencies followed around the world that is 50Hz and 60Hz.
Rated Normal Current.
Above figure mentions rated maximum current which can flow through the breaker continuously
without any damage. The rated current of the line or system (on which breaker is installed) must
be equal or less than the rated current of the breaker. The value given above is 2000A. It means
that continuous current which flow through the breaker in normal condition must be equal or less
than2000A.
RatedShort circuit breaking current:
It is the maximum current which the breaker can break without any damage to the breaker or
quenching media. At rated short circuit breaking current, the arc produced by current can safely
be quenched by the breaker. For current higher than this value, the arc may not be completely
quenched and therefore breaker can be damaged. On given name plate, the rated short circuit
breaking current is 40KA. At 40KA or below, the breaker contacts can open without any damage

46. 45
Rated short circuit making current:
It is the maximum current on which the breaker contacts can close without any damage. This
value is important in reclosing of breaker. At instant if the breaker is closed and the fault is still
in the system, the breaker will be safe if the current is less or equal to the rated short circuit
making current. This value has specific relation with the rated short circuit breaking current.
Rated SC making current=2.5 ×Rated SC breaking current
In the given case
Rated SC making current=2.5 ×40 kA=100 kA.
This value is intentionally kept higher because when fault occurs in healthy system, the current
builds up from zero till breaker is tripped. However if the breaker is reclosed and load is
detached than the fault current after reclosing will be more than initial fault current therefore this
current is kept 2.5 times the rated SC breaking current.
Circuit Breaker specifications related to Arc Quenching media.
Every circuit breaker has some insulating and arc quenching media. Small breakers use air for
this purpose. However large breakers use, oil, vacuum and SF6 gas for purpose of insulating as
well as arc quenching. Specifications related to quenching media are very important to
consider for correct operation and maintenance. Since the given name plate is of SF6 breaker
therefore properties of SF6 breaker are mentioned.
SF6 nominal filling pressure at 20ᵒC (6 Bar).
This value shows the normal pressure of SF6 gas which must be achieved while filling the gas.
The standard temperature for this pressure is 20ᵒC. Although the tank can withstand high

47. 46
pressure however that tolerance is left for the increase in pressure due to rise in external
temperature.
Signal loss of SF6 at 20ᵒC. (5.2 bar):
If the gas pressure inside breaker is reduced to 5.2 bars, alarm signal will be given to the operator
about the decrease in gas pressure.
Lock out of SF6 at 20ᵒC.(5 bar).
If the gas pressure is further reduced to 5 bars or below, the breaker will be tripped and cannot be
re closed. Re closing will be locked until the gas pressure is increased. The circuit breaker cannot
quench the arcs during breaking of current if the SF6 gas pressure is reduced therefore
monitoring the gas pressure inside SF6 breaker is very important.
11.Conclusion:
Sf6 circuit breakers are mostly employed for high voltage applications. For protection and
reliability of electrical substation of an electricity generation, transmission and distribution
system where voltages transformed high to low or reverse using transformers.
The countless advantage of this CB gas as to offer will take us to new heights in the prevalent
power system.
The adoption of sf6 in switch gear for all operating conditions has brought advantages in
performance, size, weight, global cost and reliability.

48. 47
.Minimum oil circuit breaker.

49. 48
MINIMUM OIL CIRCUIT BREAKER
About ..
In this type of circuit breaker minimum oil is used as an arc
quenchingmedium and it is mounted on a porcelaininsulator
to insulate it from the earth. The arc chamber of such type of
circuit breaker is enclosed in a bakelisedpaper. The lower
portion of this breaker is supported by the porcelainand the
upper porcelainenclosed the contacts.

50. 49
This circuit breaker is of the single breaker type in which a
moving contact tube moves in a vertical line to make or break
contact with the upper fixed contacts mounted within the arc
control devices.
A lower ring of fixed contacts is in permanent contact with the
moving arm to provide the other terminal of the phase unit.
Within the moving contact, the tube is a fixed piston. When the
moving contact moves downwards, it forces the insulatingoil to
enter into the arc control devices . Thus, the arc gets quench.
Minimum oil circuit breaker requires less space as compared to
bulk oil circuit breaker which is an important feature in large
installations.But it is less suitable in places where the frequent
operationis required because the degree of carbonisation
produced in the small volume of oil is far more dangerous than in
the conventionalbulkoil circuit breakers and this also decreases
the dielectric strength of the material.
The low oil circuit breakers have the advantages of a requirement
of the lesser quantity of oil, smaller space requirement, smaller
tank size, smaller weight, low cost, reduced risk of fire and
reduced maintenanceproblems. Minimumoil circuit breaker

51. 50
suffers from the following drawbacks when compared with the
bulk oil circuit breakers
1 . Construction of Minimum Oil Circuit
Breaker
The simplified constructional diagram of a Minimum Oil Circuit Breaker
(MOCB) is shown in the figure. It consists of two oil filled chambers namely
upper chamber and lower chamber, which are separated from each other.
The are extinction process is carried out in the upper chamber. So, it is
called as an arc extinction chamber or current interruption chamber of
Minimum Oil Circuit Breaker (MOCB).This chamber houses an arc control
device, an upper fixed contact and a ring shaped lower fixed contact. The
are control device is fitted to the upper to the upper fixed contact.The
moving contact slides through the lower fixed contact such that a physical
(or electrical) maintained between them. The entire assembly of upper
fixed contact. lower fixed contact and arc control device is enclosed in a
glass fiber enclosure which is surrounded by oil.

52. 51
The oil present in the lower chamber does not involve the arc extinction
process and instead it is used only for insulation purpose. So, the lower
chamber is also known as dielectric supporting chamber. Both the upper and
lower chambers are individually enclosed with the cylindrical shaped synthetic
resin bonded papers within the porcelain insulators.
The operating rod which is permanently fixed to the moving contact is
connected to the operating mechanism which provides vertical motion in
order to make and break the circuit.
Figure contents:
1 vent valve 6 separating piston
2 terminal pad 7 terminal pad
3 oil level indicator 8 upper drain valve
4 moving contact 9 lower drain valve
5 lower fixed contact

53. 52

54. 53
2. Operating Principle of Minimum Oil Circuit
Breaker
Whenever the moving contact is drawn out of the. hollow
structure of fixed contact, under current carrying conditions, an
arc is drawn between them. As the contacts are present in the
dielectric oil, the oil surrounding the arc attainsa high
temperature and thus decomposes the releasing gases. The gases
formed will expand and so the pressure inside the chamber
rises.Hence,the gases will move upwards by which the contacts
cools down and the arc splits.
The gap between the contacts will be filled with fresh oil. This oil
will also be decomposed and the released gases will expand. So.
the pressure will be increased again. Hence, the gases will move
upwards and again fresh oil will enter.
Minimum Oil Circuit Breakers (MOCB)-Contruction,Working
This flow of fresh oil through the gap between the contacts will
continue untilthe final arc extinction which occurs near . current
zero instant. In order to achieve sufficiently high dielectric
strength immediatelyafter current zero instant. either of the
following method is implemented.

55. 54
(i) Forcing fresh oil into the gap between the contacts with the
help of piston action produced by the piston attached to the
moving contact.
(ii) Maintaininghigh pressure on the oil inside the circuit breaker
with the help of an inert gas. By this the fresh oil will enter into
the gap between the contact from all directionsand the gases
moves upwards.

56. 55
arc intreuption in oil-
On separation of the moving contact from the fixed contact in the arc
chamber, the current
continues to flow through the vaporizing metallic current paths. The high
temperature occurring
under such conditions,decomposes the oil (which boilsat 658°K), in the
immediate vicinityand a
gas bubbleis formed (under high pressure).
It consists of (from outside inward): wet oil vapour, superheated oil vapour,
hydrocarbons

57. 56
(C2H2 at around 4000°K), the arc (approximate temperature 7000°K) as
shown in Figure 1.
Explanation
 CF4 – Carbon Tetrafluoride
 CuF2 – Copper Difluoride
 HF – Hydrogen Fluoride
 H2O – Water
 SF4 – SulphurTetrafluoride
 SF6 – SulphurHexafluoride
 SO2 – SulphurDioxide
 SO2F2- SulphurylFluoride
 SOF2 – ThionylFluoride
 WF6 – Tungsten Hexafluoride
 WO3 – Tungsten Trioxide
As can be seen, the arc runs in a mixture of hydrogen (in both molecular
and atomic states),
carbon and copper vapour. The thermal conductivityis high due to the
dissociationof hydrogen
molecules into atoms. The thermal energy generated in the arc is
primarily dissipatedoutward
through the surrounding gas envelope to the oil.

58. 57
Also, the gas in the arc chamber escapes to the gas expansionchamber, so
that a type of heat
dissipationby convectionis created, thus the rate at which heat is
dissipatingis increasing. Near
current zero, the thermal power generated by the current (in the arc)
approaches zero.
If the heat dissipationoutwards is sufficiently large, the temperature in the
arc zone can be
reduced in such a manner that the arc would lose conductivityand
extinguish. An arc in hydrogen
as a short thermal time constant, so that the conditionsare favourablefor
quenching.There are
two other situationsthat may occur under certain conditions:thermal
Restriking of Arc, reignition.
Thermal restriking is when the post-arc current rises again and passes into
the next half cycle of
SCC, as the arc plasma heats up due to the insufficiency of heat dissipation
to make conductance
of the arc zone equal to zero. Reignitionhappenswhen restriking voltage of
the system causes
a renewed formation of the arc, (after completionof the first interruption)
and continuationof flow

59. 58
of current. The arcing chamber designs are either of axialor radialventing
type. Often, a combinationof both are used in the design of minimum oil,
MV CB’s.
The axial venting process generates high gas pressures and has high
dielectric strength. This is
used mainly for interruption of low currents. The radialventing is used for
high current
interruptions, as the gas pressures developedare low and the dielectric
strength is low.
The higher the current to be interrupted, the larger the gas pressure
developed.
Minimum Oil Circuit Breaker company in india:
Mpower Electric Solutionsis the Sole Proprietorship based organizationthat
was established in the year 2005 at Hyderabad,Telangana,India. Our
organizationis engaged in manufacturing, trading, exporting and supplying
of a wide range of Power Meters, Actuator Sensor Interface, Conductor
Cable, Circuit Protection Devices, Load Break Switches, Safety Relays, Signal
Isolators, Electrical Switchgear and many more. We also provide Electrical
Switchgear AMC Services, Electrical ConsultancyServices and many more.

60. 59
The developmentof these products is occurred under the supervision of our
experts. High review material is sourced by us from the solid, qualitycognizant
and affirmed sellers of the business. We utilize this material as a part of the
assembling procedure of our products in agreeabilitywith the decently
characterized qualitystandardsand measures. Our productsare currently
accessible to little, medium and expansive estimated commercial ventures.
3.APPLICATIONS:
Minimum oil circuit breakers: Minimumoil circuit breakers operate very
fast and are used in transmission networks and substations. These types of
circuit breakers are of the live-tank design and require only a small amount
of oil.
The oil is utilizedprimarily as the interrupting media, unlikein a bulk oil
circuit breaker where the oil is utilisedas an insulatingmedia as well. In a
minimum oil circuit breaker, the interrupter is enclosed in an oil-filled
chamber at live potential.

61. 60
Insulatingoil in electrical power apparatusserves two primary roles, as an
insulatorand a coolant.As an insulatingliquid,oil’smost important
property is a high dielectric strength and as a cooling liquid,a low viscosity
is paramount. The principalnemeses of insulatingoil are oxidation,
contamination(particularlymoisture, which tends to lower the dielectric
properties of insulatingoil), and excessive temperature. These enemies are
managed through design, regimented installationprocesses and prudent
operationof the oil-filledpower apparatus. At all stages of a power asset’s
life, testing must be conducted to evaluatethe conditionof the insulating
oil. In fact, for some assets, it can be argued that the oil is tested more
frequently than any other component.
Z Tests that are concerned with the immediate conditionand acceptability
of the insulatingoil includedielectric breakdown tests, moisture
measurement by the Karl Fischer (KF) method, and dielectric dissipation
factor. Megger provides laboratorygrade oil test equipment, backed by
experience reaching back to the early 20th century, to test these
fundamentalcharacteristics of insulatingoil.

62. 61
4.Advantages and Disadvantages-
Advantages :
 Requires smaller space
 Maintenanceis less
 Cost per breaking capacity in MVA is less
 Suitablefor both manualand automatic operation
Disadvanatges: Possibility of fire and explosion.
 Difficult to remove gases from the space between contacts.
 Oil deteriorates rapidly due to carbonization
 Smaller quantityof oil, so carbonization increases.

63. 62

64. 63
VACUUM CIRCUIT BREAKER
WHAT IS VACUUME CIRCUIT BRAKER –
A circuit breaker is a device that, interrupts an electric circuit to prevent unwarranted
current, caused by a short circuit, typically resulting from an overload. Its basic
functionality is to interrupt current flow after a fault is detected. A vacuum circuit breaker
is a kind of circuit breaker where the arc quenching takes place in vacuum medium. The
operation of switching on and closing of current carrying contacts and interrelated arc
interruption takes place in a vacuum chamber in the breaker which is called vacuum
interrupter.
Vacuum Circuit Breaker
The Vacuum interrupter technology was first introduced in the year of 1960. But still, it is
a developing technology. As time goes on, the size of the vacuum interrupter has
reduced from its early 1960’s size due to different technical developments in this field of
engineering.

65. 64
CONSTRUCTION OF VCB –
➢ The vacuum circuit breaker comprises a steel arc chamber in the center-symmetrically
arranged ceramic insulators. The pressure inside the vacuum interrupter is maintained
below 10^-4 torr.
➢ The material used for current carrying contacts plays an important role in the
performance of the vacuum circuit breaker. The alloys like, Copper-bismuth or copper-
chrome are the ideal material to make VCB contacts.
Construction of Vacuum Circuit Breaker

66. 65
CONSTRUCTIONAL FIGURE OF VACUUM CIRCUIT BREAKER-
Vacuum Circuit Breaker consists of Enclosure, Contacts, Vapor Condensing Shield,
Metallic Bellows and Seal.
Enclosure -
The enclosure is made of impermeable insulating material like glass. The enclosure
must not be porous and should retain high vacuum of the order of 10-7 torr.

67. 66
Contacts-
There are two types of contacts, moving and fixed. The moving contact is connected
with large stem connected to operating mechanism of breaker. Contacts of Vacuum
Circuit Breaker have generally disc shaped faces. The disc is provided with symmetrical
grooves in such a way that the segments of the two contacts are not in the same line.
The magnetic field set-up by the components of currents with such geometry causes the
plasma of the arc to move rapidly over the contacts instead of remaining stable at one
point. The concentration of the arc is thus prevented and the arc remains in diffused
state. The sintered material used for contact tip are generally copper-chromium or
copper bismuth alloy.
Vapor Condensing Shield -
These metallic shields are supported on insulating housing such that they cover the
contact region. The metal vapor released from the contact surface during arcing is
condensed on these shields and is prevented from condensing on the insulting
enclosure.
Metallic Bellows-
One end of the bellows is welded to the enclosure. The other end is welded to the
moving contact. The bellows permit the sealed construction of the interrupter and yet
permit movement of the contact. Stainless steel bellows are generally used in vacuum
interrupters. Carefully observe every component of Vacuum Circuit Breaker as shown in
figure below.

68. 67
➢ From the figure shown above, the Vacuum circuit breaker consists of a fixed contact, a
moving contact and a vacuum interrupter. The moving contact is connected to the
control mechanism by stainless steel bellow. The arc shields are supported o the
insulating housing such that they cover on these shields and is prevented from
condensing on the insulating enclosure. The possibility of a leak is eliminated due to
permanent sealing of vacuum chamber for that a glass vessel or ceramic vessel is used
as the outer insulating body.

69. 68
WORKING PRINCIPLE -
The sectional view of vacuum circuit breaker is shown in the figure below when the
contacts are separated due to some abnormal conditions, an arc is struck between the
contacts, the arc is produced due to ionization of metal ions and depends very much on
the material of contacts.
view of Vacuum Circuit Breaker Sectional

70. 69
➢ The arc interruption in vacuum interrupters is different from other types of circuit
breaker. The separation of contacts causes the release of vapor which is filled in the
contact space. It consists positive ions liberated from contact material. The vapor
density depends on the current in the arc. When the current decreases, the rate of
vapor release decreases and after current zero, the medium regain its dielectric
strength if the vapor density is reduced.
➢ When current to be interrupted is very small in a vacuum, the arc has several parallel
paths. The total current is divided into many parallel arcs which repel each other and
spread over the contact surface. This is called diffused arc which can be interrupted
easily.
➢ At high values of current, the arc gets concentrated in a small region. It causes rapid
vaporization of the contact surface. The interruption of the arc is possible if arc remains
in diffused state. If it is quickly removed from the contact surface, the arc will be re-
strike.
➢ Arc extinction in vacuum breakers is greatly influenced by material and shape of the
contacts and the technique of considering metal vapor. The path of the arc is kept
moving so that temperature at any one point will not be high.
➢ After the final arc interruption, there is rapidly building up of dielectric strength which is
peculiar of the vacuum breaker. They are suitable for capacitor switching as it will give a
re-strick free performance. The small current is interrupted before natural current zero,
which may cause chopping whose level depends on the material of contact.

71. 70
MANUFACTURERS -
➢THOMTECH
➢SEIMENS

72. 71
➢CROMTON GREAVE
➢LARSEN AND TURBO

73. 72
Advantages of VCB -
Vacuum offers the utmost insulating strength. So it has extreme superior arc quenching
properties than any other medium.
➢ The vacuum circuit breaker has a long life.
➢ Unlike Oil Circuit Breaker (OCB) or air blast Circuit Breaker (ABCB), the explosion of
VCB is avoided. This enhances the safety of the operating personnel.
➢ No fire hazard
➢ The vacuum CB is fast in operation so ideal for fault clearing. VCB is suitable for
repeated operation.
➢ Vacuum circuit breakers are almost maintenance free.
➢ No exhaust of gas to the atmosphere and Noiseless operation.
Disadvantages of VCB -
➢ The main disadvantage of VCB is that it is uneconomical at voltages exceeding 38
kVolts.
➢ The cost of the breaker becomes excessive at higher voltages. This is due to the fact
that at high voltages (above 38 kV) more than two numbers of the circuit breaker are
required to be connected in series.
➢ Moreover, the VCBs production is uneconomical if produced in small quantities.
Applications of Vacuum Circuit Breaker -
The vacuum circuit breaker is today recognized as most reliable current interruption
technology for medium voltage switchgear. It requires minimum maintenance compared
to other circuit breaker technologies.
The technology is mainly suitable for mainly medium voltage application. For higher
voltage vacuum technology has been developed, but it is not commercially feasible.
Vacuum circuit breakers are used in metal clad Switchgear and also in porcelain
housed circuit breaker

74. 73
LARSEN AND TURBO VACUUME
CIRCUIT BREAKER -
L&T OFFERS half panels and sub-cubicles for 11kV. The VCB is ‘draw out’ type
andcan be racked out directly on the floor. VCBs are type tested according to
International Standards IEC-62271 at renowned laboratories. They are compact, safe
and reliable. The sub-cubicle is VCB with its compartment having all safety interlocks.
The half panel is an assembly of the medium voltage compartments except for low
voltage compartment.

75. 74
Features-
 Half panel for system voltage 11kV and short time current rating of 26.3kA and 40kA
 Half panel comes with an option of Copper/Aluminium bus-bar
 VCB current rating upto 3150A
 Provision of manual and electrical spring charging in VCB
 Spring charging motor voltage 230V AC and 110V DC/220V DC
 Closing/Tripping coil voltage 24V DC/110V DC/220V DC
 VCB comes with inbuilt anti-pumping feature

76. 75
Bulk Oil and Minimum Oil Circuit Breaker
Bulk Oil Circuit Breaker: A breaker which
uses a large quantity of oil for arc extinction is called a bulk oil
circuit breaker. Such type of circuit breaker is also known as dead
tank-type circuit breaker because their tank is held at ground
potential. The quantity of oil requires in bulk oil circuit
breaker depends on the system voltage. If the output rating of the
voltage is 110 KV, then it requires 8 to 10 thousand kg of oil, and
if their output rating is 220 KV, then breakers need 50 thousand
Kg of oil.
In bulk oil circuit breaker, oil performs mainly two functions.
Firstly, it acts as an arc extinguishing medium and secondly, it
insulates the live parts of the breaker from earth. The quantity of
oil requires for arc extinction is only about one-tenth of the total
and the rest being used for the insulation.

77. 76
These large quantities of oil are subject to the
carbonisation, sludging, etc., which occurs due to arc interruption
and other causes reducing the insulating properties and requires
regular maintenance.
Bulk oil circuit breaker needs a large tank which increases
expenses and also increases the weight of the circuit breaker.
Because of the following disadvantage the low oil circuit breaker
is developed which use minimum oil for arc extinction.
Working Principle of Oil Circuit Breaker
When the contacts of the oil circuit breaker are
opened under oil and an arc is struck between them. The heat of the arc
evaporates the surrounding oil and dissociates it into a substantial volume
of gaseous hydrogen at high pressure.

78. 77
The hydrogen gas occupies a volume about one thousand times that of the
oil decomposed. The oil is, therefore, pushed away from the arc and an
expanding hydrogen gas bubble surrounds the arc region and adjacent
portions of the contacts (See Figure).
The arc extinction is facilitated mainly by two processes.
 Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus
aiding the de-ionisation of the medium between the contacts.
 Secondly, the gas sets up turbulence in the oil and forces it into the space
between contacts, thus eliminating the arcing products from the arc path.
The result is that arc is extinguished and circuit
current interrupted.

79. 78
Advantages and Disadvantages of Oil CB
Advantages Disadvantages
1. It absorbs the arc energy to
decompose the oil into gases
which have excellent cooling
properties.
2. It acts as an insulator and
permits smaller clearance
between live conductors and
earthed components.
3. The surrounding oil presents
cooling surface in close
proximity to the arc.
1. It is inflammable and there is a
risk of a fire.
2. It may form an explosive mixture
with air
3. The arcing products (e.g.,
carbon) remain in the oil and its
quality deteriorates with
successive operations. This
necessitates periodic checking
and replacement of oil.

80. 79
CONSTRUCTION of BOCB
 The basic construction of bulk oil circuit breaker is
quitsimple.Here all moving contacts and fixed contacts are
immerged in oil inside closed iron vessel or iron
tank.Whenever the current carrying contacts are being
open within the oil the arc is producrd in between the
separated contacts.
 The large energy will be dissipated from the arc in oil
which vaporizes the oil as decomposes it.Because of that a
large gaseous pressure is developedinside the oil which
tries to displacethe liquidoilfrom surrounding of the
contacts.The inner wall of the oil tank has to withstand
this large pressure of the displacedoil.
 This the oil tank of bulk oil circuit breaker has to be
sufficiently strong in construction.
 An air cushion is necessary between the oil surface and
tank roof to accommodate the displacedoil when gas
forms around the arc.That is why the oil tank is not totally
filled up with oil ,it is filled up to certain level above which
the air is tight in the tank.
 The breaker tank top cover should be securely boltedon
the tank body and total breaker must be properly locked
with foundationotherwise it may jump out during
interruptionof fault current.

81. 80
 In these type of equipmentwhere expansibleoil is
enclosed in an air tight vessel(oil tank) there must be gas
vent fitted on the tank cover.Naturallysome form of
gasvent alwaysis providedon the cover.Naturallysome
form of gas vent alwaysis providedon the cover of bulk oil
circuit breaker.

82. 81