4. Construction
At the bottom, there is a tank which is called air reservoir,
and this air reservoir is connected with an air valve.
On the air reservoir tank, there are three hollow insulator
columns. On the top of each insulator column there is
double arc extinction chamber.
The current carrying parts are connected to the arc
extinction chamber in series.
The assembly of entire arc extinction chamber is mounted
on insulators as there exists large voltage between the
conductors and air reservoir.
The main arc extinction phenomenon is done in the double
arc extinction chamber
5. Working
An auxiliary compressed air system is required by this type of
circuit breaker. This system will supply air to the reservoir of the
breaker.
During the fault operation, the air is allowed to enter in the
extinction chamber which pushes away the moving contacts.
As we know that when contacts get separated then an arc is
struck between them. This arc has to be interrupted as soon as
possible.
So, in this circuit breaker the contacts are separated under the
action of air blast, this air blast will take away the ionized gases
and arcing products with it and finally helps in extinguishing the
arc.
This is very frequent operation, hence the arcing time is very less
in air blast circuit breakers.
6.
7. Types of air blast circuit breaker
1) Axial blast type – In this type, the blasting of air is
done directly along the path of arc,
2) Cross blast type-in this type, the blasting of air is
directed at right angle to the arc path,
9. Construction:
The arcing chamber is connected to the air reservoir
with an air valve as shown. The fixed contact and
moving contact are held in closed position by the
piston and the spring pressure under normal
conditions.
The air valve remains closed under normal condition,
but it opens automatically by the tripping impulse
when any fault occurs in the system.
A series isolator is also connected in the axial blast
type to give the sufficient voltage clearance after the
breaking operation.
10. Working:
When any fault occurs in the system, the tripping impulse
causes the air valve to open automatically and it connects
the air reservoir to arc extinction chamber.
The high pressure compressed air now move into the
extinction chamber where fixed and moving contacts are in
closed position with the spring pressure.
Now, as the arc extinction chamber is now filled with
compressed air so, it will exert pressure on moving contact
against the spring pressure.
When the air pressure becomes more than the spring
pressure then, moving contact will separate and an arc will
be struck between moving and fixed contact.
If you see closely then you will find that the pressure of air
blast is acting axially to the arc and it will decrease the area
of cross-section of the arc.
11. Cont….
We know that resistance is inversely proportional to area of
cross-section, it means when area of cross section
decreases then the resistance will increase, in other words
dielectric strength of the path will increase.
As, the current is flowing through the arc but due to
increase in resistance, current flow will stop, also the
arcing products (ionized gases) are taken away with the
blast of air to the atmosphere. Hence, arc is extinguished.
In such circuit breakers, the contact separation required for
interruption is very small (1.75 cm or so). Such a small gap
may constitute inadequate clearance for the normal service
voltage.
13. Construction
Air reservoir filled with compressed air is connected to
the arc extinction chamber with the help of air valve.
The Arc extinction chamber contains fixed and
moving contacts. Arc splitters and baffles are also
connected as shown in the figure.
These Arc splitters are used to increase the length of
the arc whereas baffles are used to provide necessary
cooling.
14. Working
when the moving contacts are withdrawn due to fault in the
system, an arc is struck between the moving and fixed contacts.
Now, the high pressure air which is coming at right angle to the
arc will force the arc into a chute consisting of arc splitters and
baffles.
These arc splitters will increase the length of arc as shown in
figure. As we know that resistance is directly proportional to the
length. It means if the length of arc increases, then resistance (or
dielectric strength) of the path will also increase.
The increment in length of arc will increase the dielectric
strength of the path and finally it will interrupt the flow of
current. Hence, the arc is extinguished.
Since the blast pressure is independent of value of fault current,
so it is very efficient at low currents also.
Here in cross blast air circuit breaker, a series isolating switch is
not necessary, because the final gap for interruption is good
enough to give normal insulation clearance.
15. Advantages
Cheapness and free availability of the interrupting
medium, chemical stability, and inertness of air
High speed of operation
Elimination of fire hazard
Short and consistent arching time and therefore less
burning of contacts
Less maintenance
Suitable for frequent operation
16. Disadvantages
It is very sensitive to restriking voltage.
Maintenance of compressor is required.
Problem of current chopping
Produces high level of noise when air is discharged to
an open atmosphere
18. Advantages:
Arc energy is absorbed in decomposing of oil
The gas formed which is mainly hydrogen, has high diffusion
rate and high heat absorption, thus provides good cooling
properties
The oil has high dielectric strength and provides insulation
between the contacts after the arc has been finally extinguished
and there has been time for the oil to flow into the gap between
contacts
Cooling oil presents the cooling surface in close proximity to the
arc
The oil used (such as transformer oil) is a very good insulator
and allows smaller clearance between live conductors and earth
components
19. Disadvantages:
Oil may be flammable and can cause fire hazards, if a
defective oil circuit breaker should fail under pressure
and cause an explosion
There is a risk of formation of explosive mixture with
the air
Due to the decomposition of the oil in the arc, the oil
becomes polluted by carbon particles, which reduces
its dielectric strength. Hence periodical maintenance
and replacements are required
21. Self generated pressure oil circuit
breaker
Arc is utilized to generate a high pressure in a chamber
known as explosion pot( pressure chamber or arc
controlling device)
The pot is made up of insulating material and is placed
in the tank
29. SF6 circuit breakers Advantages:
Due to the superior arc quenching property of SF6, such circuit
breakers have very short arcing time.
Since the dielectric strength of SF6 gas is 2 to 3 times that of air,
such breakers can interrupt much larger currents.
The SF6 circuit breaker gives noiseless operation due to its
closed gas circuit and no exhaust to atmosphere, unlike the air
blast circuit breaker.
The closed gas enclosure keeps the interior dry so that there is no
moisture problem.
There is no risk of fire in such breakers because SF6 gas is non-
inflammable.
There are no carbon deposits so that tracking and insulation
problems are eliminated.
The SF6 Circuit breakers have a low maintenance cost, light
foundation requirements and minimum auxiliary equipment.
Since SF6 breakers are totally enclosed and sealed from the
atmosphere, they are particularly suitable where explosion
hazard exists e.g., coal mines.
30. SF6 circuit breakers
Disadvantages:
SF6 breakers are costly due to the high cost of SF6.
Since SF6 gas has to be reconditioned after every
operation of the breaker, additional equipment is
required for this purpose.
Problem of perfect sealing, there may be a leakage of
SF6 gas because of imperfect joints
internal parts should be cleaned thoroughly during
periodic maintenance under clean and dry
environment
32. In a vacuum circuit breaker, the vacuum of the order of
10-5 to 10-7 torr is used as an arc quenching medium. (1
torr = 1mm of Hg). Basically, vacuum is a pressure
below atmospheric pressure where no gas is present.
These breakers have highest insulating strength than
any other medium. Vacuum circuit breaker does the
interruption of current at the first current zero. It
means it interrupts the current in the half cycle.
33. Construction
It has an outer envelope which is made up of glass joined with
end caps. This glass shield is used to examine the breaker after
operation, if the colour becomes milky white then it indicates
that the breaker is losing its vacuum.
Two contacts named as fixed contact and moving contact are
placed as shown in the figure. The main arcing process is done in
between these two contacts.
The moving contact is connected with metallic bellows which
are made up of stainless steel. These bellows are used to move
the moving member up and down and a spring mechanism is
connected with the bellows to operate them.
Sputter shield made up of stainless steel is present to prevent
metal vapour from reaching outer envelope.
The ceramic envelope is also present to insulate the chamber
34. Working of vacuum circuit breaker
Initially, the fixed contact and moving contact both are connected to
each other and the current flows through them. But as soon as any fault
occurs in the system, the moving contact start separating from the
fixed contact and arc phenomenon occurs between them.
Vacuum circuit breakers have a different way of arc generation. The arc
is generated due to vaporisation of metal vapours from the contact
surface.
A contact surface has large number of microscopic projections and
when current reaches to these projections.
Now, we know that, after disconnecting the contacts the last point of
current will be the contact surface which results in high current density
at these points and due to which resistive heating occurs.
Due to excessive heating, the metal ions starts vaporising from the
contact surface
When these ions come in between of two contacts, then they provide a
path to the current and current starts flowing which we see as an arc.
This is how arc generates in these breakers.
