1. F RO M :
D R. MO N IK A VA RD I A
A S S O C IA T E P R O F E S S O R , E E
Geetanjali Institute of Technical Studies
Department of Electrical Engineering
Power System – I
Unit 3- Overvoltages & Insulation Requirements
37. What is a Circuit Breaker?
A circuit breaker is defined as a switching device that
can be operated manually or automatically for
controlling and protecting an electrical power system.
It consists of two main contacts: a fixed contact and a
moving contact.
The contacts are normally closed and allow current to
flow through the circuit.
When a fault occurs, such as a short circuit or an
overload, the contacts are separated by a mechanism
that releases stored potential energy.
This mechanism can be spring-operated, pneumatic,
hydraulic, or magnetic.
38. How A Circuit Breakers Works (Working Principle)
Detection of Fault: The circuit breaker detects a
fault condition, such as an overload or a short
circuit in the electrical system.
Activation of Mechanism: Once a fault is detected,
the circuit breaker activates a mechanism that
triggers the release of stored potential energy.
39. How A Circuit Breakers Works (Working Principle)
Separation of Contacts: This energy then separates the
two main contacts of the circuit breaker: a fixed contact
and a moving contact.
Arc Formation: The separation of contacts creates a highly
conductive and ionized plasma known as an arc.
40. Cont...
Arc Quenching: The circuit breaker uses various
methods to extinguish or quench the arc, such as
cooling, compressing, or replacing the ionized
medium with fresh gas.
Current Interruption: This process interrupts the
flow of current, stopping any damage from the
fault condition.
41. Cont…
Resetting: Once the fault is cleared, the circuit
breaker can be manually or automatically reset,
allowing the current to flow again.
Re-establishment of Contacts: Resetting the
breaker brings the fixed and moving contacts
back together, re-establishing the flow of current
through the circuit.
42. circuit breakers are classified by their arc quenching medium
Oil circuit breaker
Air circuit breaker
SF6 circuit breaker
Vacuum circuit breaker
43. OCB – Oil Circuit Breaker – Construction and Working Principle
An Oil circuit breaker OCB is a type of circuit breaker
that uses insulating oil as a dielectric medium to
quench the arc and break the circuit safely.
The oil used is insulating oil used usually transformer
oil that has better dielectric strength than air.
The heat produced by the arc vaporizes the oil,
producing a hydrogen gas bubble surrounding the arc.
The pressure of the oil compresses the gas bubble
increasing its dielectric strength which extinguishes
the arc during the zero-crossing.
45. Construction of Oil Circuit Breaker
The construction of this kind of circuit breaker is simple.
It includes current-carrying contacts that are surrounded
in a strong and metal tank.
Here the tank is loaded with transformer oil.
The transformer oil works as an insulator & arc
extinguishing medium among the existing element & earth.
At the peak of the transformer oil, the air can be filled
within the tank that acts as a pad to manage the transferred
oil on the creation of gas in the region of the arc.
It absorbs the mechanical shock of the rising movement of
oil.
46. Working Principle
In normal operation of this circuit breaker, the contact
in the breaker will be closed as well as carries the
current.
Once the fault happens within the system, then the
contacts will move apart & an arc will be struck among
the contacts.
Because of this arc, a huge amount of heat will be
released & high temperature can be achieved to vaporize
the nearby oil to gas.
So this gas will be surrounded by the arc & its unstable
increase around it will move the oil violently.
47. Advantages
The advantages of the oil circuit breaker include the
following.
It uses less amount of oil
Dielectric strength of oil is high
Oil in the breaker will absorb the energy of arc when
decomposing.
Less space
Risk of fire can be reduced
Maintenance also reduced
48. Disadvantages
The disadvantages of the oil circuit breaker include the following.
It uses less quantity of oil so that carbonization amount will be
increased
Removing of the gases within the contact space is difficult
The dielectric strength will declines the oil quickly because of the
high quantity of carbonization.
Arcing time is high
Do not allow interruption with high-speed
Controlling of arc interruption can be done based on the length of
arc.
It can form any volatile mixture through the air.
49. Air Circuit Breaker
An Air Circuit Breaker (also known as an Air Blast Circuit
Breaker or ACB) is an automatically operated electrical switch
that uses air to protect an electrical circuit from damage
caused by excess current from an overload or short circuit.
Its primary function is to interrupt current flow after a fault is
detected.
When this happens, an arc will appear between the contacts
that have broken the circuit.
Air circuit breakers use compressed air to blow out the arc, or
alternatively, the contacts are rapidly swung into a small
sealed chamber, the escaping of the displaced air, thus
blowing out the arc.
50. Working Principle of Air Circuit Breaker
The working principle of this breaker is rather
different from those in any other types of circuit
breakers.
The main aim of all kind of circuit breaker is to
prevent the reestablishment of arcing after current
zero by creating a situation where in the contact gap
will withstand the system recovery voltage.
For interrupting arc it creates an arc voltage in excess
of the supply voltage. Arc voltage is defined as the
minimum voltage required maintaining the arc.
51. Air Circuit Breaker Construction
The external parts of ACB mainly include the ON &
OFF button, an indicator for the position of the
main contact, an indicator for the mechanism of
energy storage, LED indicators, RST button,
controller, rated nameplate, handle for energy
storage, displays, shake, fault trip rest button,
rocker repository, etc.
53. Air Circuit Breaker Working
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 the low-voltage interruption and now tends to
replace high-voltage oil breakers.
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 the main contacts, there will not be any arcing in the
main contact. The arcing is only initiated when finally the arcing contacts are
separated. Each of the arc contacts is fitted with an arc runner which helps.
54. Applications of Air Circuit Breakers
Air Circuit Breakers are used for controlling the power station
auxiliaries and industrial plants. They offer protection to
industrial plants, electrical machines like transformers, capacitors,
and generators.
They are mainly used for the protection of plants, where there are
possibilities of fire or explosion hazards.
The air brake principle of the air breaker circuit arc is used in DC
circuits and AC circuits up to 12KV.
The air circuit breakers have high resistance power that helps in
increasing the resistance of the arc by splitting, cooling, and
lengthening.
An air circuit breaker is also used in the Electricity sharing system
and NGD about 15kV
55. SF6 Circuit Breaker
Sulphur Hexafluoride (SF6) Circuit Breaker
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.
56. Properties of Sulphur hexafluoride Circuit Breaker
Sulphur hexafluoride possesses very good insulating and arc
quenching properties. These properties are
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.
57. Construction of SF6 Circuit Breakers
SF6 circuit breakers mainly consist of two parts,
namely (a) the interrupter unit and (b) the gas
system.
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 SF6 gas reservoir. This unit
consists slide vents in the moving contacts which
permit the high-pressure gas into the main tank.
59. 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.
60. 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.
61. Advantage of SF6 circuit breaker
SF6 gas has excellent insulating, arc extinguishing and many other
properties which are the greatest advantages of SF6 circuit breakers.
The gas is non-inflammable and chemically stable. Their decomposition
products are non-explosive and hence there is no risk of fire or explosion.
Electric clearance is very much reduced because of the high dielectric
strength of SF6.
Its performance is not affected due to variations in atmospheric condition.
It gives noiseless operation, and there is no over voltage problem because
the arc is extinguished at natural current zero.
There is no reduction in dielectric strength because no carbon particles are
formed during arcing.
It requires less maintenance and no costly compressed air system is
required.
62. Disadvantages of SF6 circuit breakers
SF6 gas is suffocating to some extent. In the case of
leakage in the breaker tank, the SF6 gas being heavier
than air and hence SF6 are settled in the surroundings
and lead to the suffocation of the operating personnel.
The entrance of moisture in the SF6 breaker tank is very
harmful to the breaker, and it causes several failures.
The internal parts need cleaning during periodic
maintenance under clean and dry environment.
The special facility requires for transportation and
maintenance of quality of gas.
63. Vacuum circuit breaker
A breaker which used vacuum as an arc extinction
medium is called a vacuum circuit breaker.
In this circuit breaker, the fixed and moving contact
is enclosed in a permanently sealed vacuum
interrupter.
The arc is extinct as the contacts are separated in
high vacuum.
It is mainly used for medium voltage ranging from
11 KV to 33 KV.
64. The vacuum circuit breaker has mainly two phenomenal properties
High insulating strength: In comparison to various
other insulating media used in circuit breaker
vacuum is a superior dielectric medium. It is better
than all other media except air and SF6, which are
employed at high pressure.
When an arc is opened by moving apart the contacts
in a vacuum, an interruption occurs at the first
current zero. With the arc interruption, their
dielectric strength increases up to a rate of
thousands time as compared to other breakers.
65. Construction of Vacuum Circuit Breaker
It is very simple in construction as compared to any
other circuit breaker.
Their construction is mainly divided into three parts,
i.e., fixed contacts, moving contact and arc shield which
is placed inside the arc interrupting chamber.
The outer envelope of vacuum circuit breaker is made
up of glass because the glass envelope help in the
examination of the breaker from outside after the
operation.
The fixed and moving contacts of the breaker are placed
inside the arc shield.
67. Working Vacuum Circuit Breaker
When the fault occurs in the system, the contacts of the
breaker are moved apart and hence the arc is developed
between them.
When the current carrying contacts are pulled apart,
the temperature of their connecting parts is very high
due to which ionization occurs.
Due to the ionization, the contact space is filled with
vapour of positive ions which is discharged from the
contact material.
The density of vapour depends on the current in the
arcing.
68. Advantages of Vacuum Circuit Breaker
Vacuum circuit breaker does not require any additional
filling of oil or gas. They do not need periodic refilling.
Rapid recovery of high dielectric strength on current
interruptions that only a half cycle or less arcing occurs
after proper contact separation.
Breaker unit is compact and self-contained. It can be
installed in any required orientation.
Because of the above reasons together with the
economic advantage offered, vacuum circuit breaker
has high acceptance.
69. Disadvantage of Vaccum Circuit Breaker
Requirements of high technology for production of
vacuum interrupters.
It needs additional surge suppressors for the
interruption of low magnetizing currents in a
certain range.
Loss of vacuum due to transit damage or failure
makes the entire interrupter useless, and it cannot
be repaired on site.
