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A circuit breaker is a piece of equipment which
can Make or break a circuit either manually or by
remote control under normal conditions.
Break a circuit automatically under fault condition
Make a circuit either manually or by remote under
CB consists of fixed and moving contacts called
Under normal operating condition these contacts
remain closed and will not open automatically
unless the system becomes faulty .These contacts
can be opened manually or by remote control.
When a fault occurs in a circuit the trip coils of the
circuit breaker get energised and the moving
contacts are pulled apart by some mechanism
,thus opening the circuit.
When contacts of circuit breaker starts separating the
contact resistance starts increasing. This increases the (I
square r) loss which is heat produced .
This heat increases the energy of electrons in the contact
areas and the ionized particles tries to maintain the
current when contacts are separated.This flow of
charged particles form one contact to other is called
an arc .
The medium surrounding the arc also contains ions .
Due to this charged particles the arc continues even if the
breakers contacts are separated.
The voltage (potential gradient) across the arc is less and
so it continues even for low voltages.
DC arcs are to be interrupted by increasing the
resistance interruption method in which resistance of the
arc is increased so that the arc voltage can no longer
maintain the current and the arc is extinguished.
Size of DC circuit breaker increases as the voltage level
AC arcs current reduces to zero in each cycle (2 times)
If the circuit breaker contacts are opened at time when
the current passed through zero and dielectric strength of
the medium is build up rapidly so that arc cannot strike
again then arc can be extinguished successfully.
Size of AC circuit breaker can be small compared to
same voltage DC circuit breaker.
Separate the contacts of circuit breaker such that
the arc resistance increases to a very high value.
The pd between the contacts is unable to
maintain the arc current. For high voltage circuit
breakers this method is impracticable since a
separation of many meters will be required.(High
The ionized particles between the contacts tend
to maintain the arc. If the arc path is deionized
,the arc extinction is facilitated .This may be
achieved by cooling the arc or by bodily
removing the ionized particles from the space
between the contacts.(Low Resistance Method)
Arc Voltage – It is the voltage that appears across the
contacts of the circuit breakers during the arcing
period as the contacts are opened.
Recovery voltage – It is the normal frequency voltage
that appears across the contacts of circuit breaker
after final arc extinction.
Rate of rise of restriking voltage – (RRRV) It is the rate
of increase of restriking voltage .RRRV depends upon
1) recovery voltage and 2) Natural frequency of
It is the transient
voltage that appears
across the contacts at
or near current zero
during arcing period.
If dielectric strength rise
is greater than the rise
of restriking voltage
then the arc will not
It is the phenomena of current interruption before
natural current zero is reached. It occurs in air blast
circuit breaker because they retain same
extinguishing power irrespective of the magnitude
of current to be interrupted.
When interrupting low inductive current
e.g.magnetising current of transformer, a rapid
deionizing effect causes current , to fall below its
zero value before natural current zero is called
The switching Resistor (R) is connected in parallel with the
CB contacts. Current chopping produces high voltage
oscillations which can be prevented by this method.
During arc interruption CB contacts separate first and after
arc gets extinguished ‘S’ opens depending upon the
time delay provided to it.
When the fault occurs the CB contacts open and arc is
struck between them. Since R is in parallel with Cb
contacts ,a part of arc current flows through this
resistance so arc current decreases and deionization
rate increases. The arc resistance also increases so
current through R increases. This continue till the arc
current is insufficient to maintain the arc.
Breaking capacity – It is the current (r.m.s.) that a Cb
is capable of breaking at given recovery voltage and
under specified conditions.
Making Capacity – The peak value of current
(including DC component) during the first cycle of
current wave after closure of circuit breaker is known
as making capacity. Making capacity = 2.55 ×
symmetrical breaking capacity
Short time rating – It is the period for which the CB is
able to carry fault current while remaining closed.
Normal current rating – It is the r.m.s. value of current
which the CB is capable of carrying continuously at its
rated frequency under rated specified conditions.
1.In this Circuit Breaker the arc is elongated using arc
runners and arc splitters so as to increase the
resistance of the arc.
3.This increases the voltage required to maintain the
arc and if the available voltage cannot sustain the
arc ,the arc gets extinguished.
2.At current zero ,the recovery voltage across the
contacts becomes less than the arc voltage and
the arc gets extinguished.
4. The energy in the system inductance at current
zero is zero .Hence arc interruption is easier.
1 Main contacts
2 Arcing Contacts
3.Arc Rising in the direction of Arrow
4.Arc getting Split
5.Arc Splitter Plates
6.Current Carrying Terminals
Air Circuit Breakers
1.Used For low voltage levels and current levels
2.As voltage level increases, the size of breaker
becomes large so not convenient for higher voltage
and current levels.
3.Air is used as medium to extinguish the arc which
have inferior extinguishing properties compared to
SF6 or Vacuum circuit breakers
4.Operating control is manual as well as automatic.
5.It is used up to 6.6kV with a breaking capacity of
6.Suitable for repeated operation because medium of
arc extinction is air.
In this breaker, a high pressure air blast is used
as an arc quenching medium.
The contacts are opened and a flow of air blast
is maintained by opening the blast valve.
The air blast cools the arc and takes away the
arcing products to atmosphere .
This rapidly increases the dielectric strength of
the medium between the contacts and the arc
is extinguished and the flow of current is
High speed of operation
Short arcing time
High speed reclosing
Less weigh as compared to oil circuit breakers
Very less maintenance
No possibility of explosion
Cost is more
For complete compress air installation is
These breakers are more sensitive to RRRV.
For operation and maintenance ,highly
skilled persons are required
Air is admitted in the arc extinction chamber it pushes
the moving contact. This air blast takes away the
ionized gases along with it. Afterwards the arc gets
extinguished. High pressure air has higher dielectric
The design is such that the air expands into the low
pressure (atmospheric pressure zone).The air at high
speed removes heat from the arc, thus arc is
quenched. Diameter of arc is reduced.
When two contactsof thiscircuit breaker areseparated in
vacuum an arc isstruck and hot spotsareformed on thesurface
of thecontacts.Thesehot spotsproducemetal vapor and plasma.
Theamount of vapor in plasmadependson how rapidly the
vapor isemitted from contact surfacewhich dependson thearc
current. Thecurrent isof alternating nature, it passthrough zero
several times, so therateof vapor emission also becomeszero,
and thevapor already emitted getscondensed . During this
processthedielectric strength buildsup rapidly and the
restriking of arc isprevented.
It consists of fixed contact, moving contact and arc
shield mounted inside a vacuum chamber. The
movable member is connected to the control
mechanism by stainless steel bellows. This enables the
permanent sealing of the vacuum chamber so as to
eliminate the possibility of leakage,.
A glass vessel or ceramic vessel is used as outer
insulating body. The arc shield prevents the
deterioration of the internal dielectric strength by
preventing the deterioration of the internal dielectric
Outdoor application where maintenance required is
minimum.In the high voltage system from 22 KV to
66kV power Circuits.
When two contacts of circuit breaker are separated in
vacuum arc is struck and hot spots are formed on
the surface of the contacts. These hot spots
produce metal vapour and plasma.
At current zero the rate of vapour emission becomes
The vapour already emitted gets condensed .
During this process the dielectric strength builds up and
the restriking of arc is prevented.
Compact in size
Reliable and long life
Heavy fault can be interrupted effectively
No gas is generated after arc extinction
Operation is not noisy
Arc energy is low
No risk of fire
Vacuum has to be maintained at desired
This breaker makes use of oil for quenching the arc.
The circuit breaker which uses more oil or which is
bulky is called bulk oil circuit breaker.
The construction is simple and it consists of fixed and
moving contacts enclosed in a strong weather –
tight earthed tank containing oil up to a certain
level and an air cushion above the oil level.
These breakers are used up to 11 KV with an
interrupting capacity of 250MVA.
Under normal working conditions, thefixed and moving contactsare
closed. On theoccurrenceof fault, themoving contactscomedown
and an arc isstruck between thecontacts. Theoil between the
contactsgetsdecomposed and hydrogen gasbubbleisformed
around thecontacts. Thehydrogen gascoolsthearc and the
turbulenceeffect causethelengthening of arc. Thedeionization of
medium between contactstakesplaceand at somecritical length of
gap between thecontacts,thearc isextinguished.
Thehydrogen gasbubbleproducesavery high pressurein theoil. The
tank isthereforemadestrong to withstand alargepressure. Theoil
movesupwardswhen hydrogen bubbleisformed. Theair ispresent
between theoil level and tank top and actsascushion and absorbs
mechanical shock produced dueto upward oil movement.
oil has high dielectric strength
Oil absorbs arc energy while
Good cooling property of the gas formed
due to decomposition
It acts as an insulator between the live
parts and earth
Long arcing time
Do not permit high speed of interruption
Arc interruption control can be obtained
only by increasing the length of arc
Chance of fire and explosion.
When the contacts are
separated in oil arc is
formed. The heat of arc
decomposes oil and
gases are formed. These
gases expand due to
heating of the arc. The
gas flowing near the
contact zone cause
cooling and splitting of
the arc and the arc gets
In this breaker, the supporting ,current interruption and
top chamber are made of porcelain .Hence,
clearance between live parts is small and requires
less quantity of oil, hence the breaker is called
Minimum Oil Circuit Breaker. The chambers are
completely filled with oil. The oil from upper
chamber does not come to lower.
The fixed contact is enclosed in the quenching
chamber. Moving contact makes sliding contact
with the lower fixed contact. The operating rod is
operated by operating mechanism, the three poles
operate simultaneously.The voltage ratings are from
3.6kV to 420 KV.
Requires less quantity of oil
Requires smaller space
Maintenance is less
Cost per breaking capacity in MVA is less
Suitable for both manual and automatic
Possibility of fire and explosion.
Difficult to remove gases from the space
Oil deteriorates rapidly due to
Smaller quantity of oil, so carbonization
Electronegative –It has the ability of an atom to attract
and hold electrons. Such gas have high dielectric
stremngth.SF6 is electronegative .It forms negative
ions ,Negative ions are heavy and immobile so they do
not flow easily .Hence SF6 gas has high dielectric
Rate of rise of dielectric strength is very high.
Can be liquefied and stored in steel tanks
Dielectric strength increase linearly with pressure.
Gas is inert. Therefore contacts will not get eroded.
Gas is non –inflammable , Colorless ,odorless, Non-toxic
Thermally stable up to 55 degrees
Under normal operating conditions the contacts are closed .
On occurance of fault contacts are opened. The movable
contact moves away from the fixed contact.
The arc is struck between the fixed and moving contacts.
High pressure SF6 gas now flows over the arc and it absorbs
the free electrons from the arc.
This builds up the dielectric strength between the gap very
fast and the arc is extinguished
Silent operation, compact
Vary short arcing time
No risk of fire
No reduction in dielectric
strength due to operation
No moisture problem
No current chopping
Can interrupt larger
Suitable for explosive
environment due to
totally enclosed body
Requires conditioning of SF6
gas from time to time
SF6 gas is suffocating ,so its
leakage can cause
suffovcation of the persons
in surrounding areas.
Isolator(disconnecting switch) operates under no
load condition. It does not have any current
breaking capacity or current making capacity.
Isolator is not even used for breaking load currents.
Isolators are used in addition to circuit breakers ,and
are provided on each side of every circuit breaker
to provide isolation and enable maintenance.
Sequence of operation
While opening –Open circuit breaker first and then
While closing –Close isolators first and then close
Different types of controls are required for successful operation
of circuit breakers.
1.Relays – These are required to give a trip signal to circuit
breaker in case of fault condition. Different types of relays are
available like over current, over voltage ,under voltage, loss
of excitation,reverse power etc.
2.Sensor equipments are required to check the condition of
circuit breakers arc extinguishing medium .
The controls are pressure sensors to sense the pressure of air in
case of air blast circuit breakers .
In case of Sulfur hexafluoride circuit breakers also the pressure
sensors are required.
In case of vacuum circuit breakers also sensors are required to
check the vacuum level in the breaker.
The faults in power system may be temporary or
The temporary faults like touching of overhead
conductors due to wind are cleared automatically
after some time .
In case of temporary faults auto reclosing helps to
maintain the system supply .
The breaker opens on a fault.
After some time delay ,the breaker is closed .
If the fault is temporary and it is removed ,then the
breaker will close successfully
If the fault still persists then the breaker opens.
Sectionalizers are used in the bus bars so that fault on any
section of bus bar will not cause complete shut down.
Advantages of sectionalizers
1.If fault occurs on any section of bus bars then that section
is isolated from other sections without affecting the
2.Fault current is much lower than in case of un
sectionalized system (as the fault is fed from only one
3.Repairing and maintenance on one section can be
carried out by de energizing that section only
eliminating the possibility of complete shut down.
The breaker in the bus bars is acting as sectionalizing
breaker in the shown figure.
Fuse is a device used in circuit for protecting electrical
equipments against overloads and /or short circuits.
Fuse element or fuse wire is that part of the fuse device
which melts when an excessive current flows in the
circuit and thus isolates the faulty device from the
Desirable qualities of fuse elements
1.Low melting point
2.Low ohmic losses
4.Free from deterioration due to oxidation
It consists of a heat resisting ceramic body having
metal end caps. A fusible wire is welded to it. The
space within the body, surrounding the element is
completely packed with a powder, usually quartz,
which acts as an arc extinguishing medium.
Under fault condition, the current flowing through the
element produces very large amount heat and the
fuse element melts before the fault current reaches
first peak value. The heat produced in the process
vaporizes the melted fuse element. The chemical
reaction between the silver vapor and filling power
results in the formation of high resistance which
helps quenching the arc.