Circuit Breakers

1. EE3601 Protection and
Switchgear

2. UNIT V CIRCUIT BREAKERS
Physics of arcing phenomenon and arc interruption - DC and AC
circuit breaking – re-striking voltage and recovery voltage - rate of rise of
recovery voltage - resistance switching - current chopping - interruption
of capacitive current - Types of circuit breakers – air blast, air break, oil,
SF6 and vacuum circuit breakers – comparison of different circuit
breakers – Rating and selection of Circuit breakers.

3. Circuit Breakers
• A circuit breaker is a piece of equipment which
can
(i) make or break a circuit either manually or by
remote control under normal conditions
(ii) break a circuit automatically under fault
conditions
(iii) make a circuit either manually or by remote
control under fault conditions

4. Circuit breaker Operating principle
• A circuit breaker essentially consists of fixed and
moving contacts, called electrodes. Under normal
operating conditions, these contacts remain
closed and will not open automatically until and
unless the system becomes faulty.
• Of course, the contacts can be opened manually
or by remote control whenever desired.
• When a fault occurs on any part of the system, the
trip coils of the circuit breaker get energised and
the moving contacts are pulled apart by some
mechanism, thus opening the circuit.

5. Circuit breaker Operating principle
• When the contacts of a circuit breaker are
separated under fault conditions, an arc is struck
between them. The current is thus able to
continue until the discharge ceases.
• The production of arc not only delays the current
interruption process but it also generates
enormous heat which may cause damage to the
system or to the circuit breaker itself.
• Therefore, the main problem in a circuit breaker is
to extinguish the arc within the shortest possible
time so that heat generated by it may not reach a
dangerous value.

6. Arc Phenomenon
• Short circuit – large current – contacts open –
heat produced – ionization – arc struck between
contacts – arc provides low resistance path –
potential across contacts is low.
• During the arcing period, the current flowing
between the contacts depends upon the arc
resistance.
• The greater the arc resistance, the smaller the
current that flows between the contacts.

9. Arc Phenomenon
• The arc resistance depends upon the following
factors :
• (i) Degree of ionisation— the arc resistance
increases with the decrease in the number of
ionised particles between the contacts.
• (ii) Length of the arc— the arc resistance
increases with the length of the arc i.e.,
separation of contacts.
• (iii) Cross-section of arc— the arc resistance
increases with the decrease in area of X-section
of the arc.

10. Principles of Arc Extinction
• Before discussing the methods of arc extinction, it
is necessary to examine the factors responsible
for the maintenance of arc between the contacts.
These are :
• (i) p.d. between the contacts
• (ii) ionised particles between contacts

11. Principles of Arc Extinction
• Taking these in turn,
• (i) When the contacts have a small separation, the p.d.
between them is sufficient to maintain the arc. One way
to extinguish the arc is to separate the contacts to such a
distance that p.d. becomes inadequate to maintain the
arc. However, this method is impracticable in high
voltage system where a separation of many metres may
be required.
• (ii) The ionised particles between the contacts tend to
maintain the arc. If the arc path is deionised, the arc
extinction will be facilitated. This may be achieved by
cooling the arc or by bodily removing the ionised
particles from the space between the contacts.

12. Methods of Arc Extinction
• There are two methods of extinguishing the arc in circuit
breakers viz.
• 1. High resistance method.
• 2. Low resistance or current zero method

13. Methods of Arc Extinction
• 1. High resistance method. In this method, arc resistance
is made to increase with time so that current is reduced
to a value insufficient to maintain the arc. Consequently,
the current is interrupted or the arc is extinguished. The
principal disadvantage of this method is that enormous
energy is dissipated in the arc. Therefore, it is employed
only in d.c. circuit breakers and low-capacity a.c. circuit
breakers.

14. Methods of Arc Extinction
• The resistance of the arc may be increased by :
• (i) Lengthening the arc. The resistance of the arc is directly proportional to its
length. The length of the arc can be increased by increasing the gap between
contacts.
• (ii) Cooling the arc. Cooling helps in the deionisation of the medium between
the contacts. This increases the arc resistance. Efficient cooling may be obtained
by a gas blast directed along the arc.
• (iii) Reducing X-section of the arc. If the area of X-section of the arc is reduced,
the voltage necessary to maintain the arc is increased. In other words, the
resistance of the arc path is increased. The cross-section of the arc can be
reduced by letting the arc pass through a narrow opening or by having smaller
area of contacts.
• (iv) Splitting the arc. The resistance of the arc can be increased by splitting the
arc into a number of smaller arcs in series. Each one of these arcs experiences
the effect of lengthening and cooling. The arc may be split by introducing some
conducting plates between the contacts.

15. Methods of Arc Extinction
• Low resistance or Current zero method. This method is employed for arc
extinction in a.c. circuits only. In this method, arc resistance is kept low until
current is zero where the arc extinguishes naturally and is prevented from
restriking inspite of the rising voltage across the contacts. The de-ionisation of
the medium can be achieved by:
• (i) lengthening of the gap. The dielectric strength of the medium is
proportional to the length of the gap between contacts. Therefore, by opening
the contacts rapidly, higher dielectric strength of the medium can be achieved.
• (ii) high pressure. If the pressure in the vicinity of the arc is increased, the
density of the particles constituting the discharge also increases. The increased
density of particles causes higher rate of de-ionisation and consequently the
dielectric strength of the medium between contacts is increased.
• (iii) cooling. Natural combination of ionised particles takes place more rapidly if
they are allowed to cool. Therefore, dielectric strength of the medium between
the contacts can be increased by cooling the arc.
• (iv) blast effect. If the ionised particles between the contacts are swept away
and replaced by unionised particles, the dielectric strength of the medium can
be increased considerably. This may be achieved by a gas blast directed along
the discharge or by forcing oil into the contact space.r a.c. circuit breakers
employ this method for arc extinction.

16. Fault clearing time of a circuit breaker

17. Arc voltage : It may be defined as the voltage that appears
across the contact during the arcing period, when the electric
current flow is maintained in the form of an arc.
The voltage drop across the arc is called arc voltage, A typical
value may be about 3 % of rated voltage

18. Restriking voltage: It may be defined as the transient voltage that
appears across the breaking contact at the instant of arc extinction.
Recovery voltage : It may be defined as the voltage that appears across
the breaker contact after the complete removal of transient oscillations
and final extinction of arc has resulted in all the poles.

19. Arc Interruption
There are two methods of arc interruption.
1. High resistance Interruption
 Cooling of arc
 Lengthening of arc
 Splitting of arc
 Constraining of arc
2. Low resistance interruption (or) current zero interruption
 Arc resistance is kept low until current zero

20. There are two theories to explain the zero current interruption
of the arc.
1. Recovery rate theory (Slepain’s Theory)
2. Energy balance theory (Cassie’s Theory)
Recovery rate theory
The arc is a column of ionised gases.
To extinguish the arc, the electrons and ions are to be removed from the
gap immediately after the current reaches a natural zero
 Ions and electrons can be removed either by recombining them into
neutral molecules or by sweeping them away by inserting insulating
medium (gas or liquid) into the gap.
The arc is interrupted if ions are removed from the gap at a rate faster
than the rate of ionisation.

21. In this method , the rate at which the gap recovers its dielectric
strength is compared with the rate at which the restriking voltage
(transient voltage) across the gap rises.
If the dielectric strength increases more rapidly than the restriking
voltage, the arc is extinguished.
If the restriking voltage rises more rapidly than the dielectric strength,
the ionization persists and breakdown of the gap occurs, resulting in
an arc for another half cycle.

22. Energy balance theory
The space between the contacts contains some ionised gas immediately
after current zero and hence, it has a finite post-zero resistance.
At the current zero moment, power is zero because restriking voltage is
zero.
When the arc is finally extinguished the power again becomes zero, the
gap is fully de-ionised and its resistance is infinitely high.

23.  In between these two limits, first the power increases reaches a
maximum value, then decreases and finally reaches zero value as
shown in Fig. 5.4.
 Due to the rise of restriking voltage and associated current, energy is
generated in the space between the contacts.
 The energy appears in the form of heat. The circuit breaker is
designed to remove this generated heat as early as possible by
cooling the gap, giving a blast of air or flow of oil at high velocity
and pressure.
 If the rate of removal of heat is faster than the rate of heat generation
the arc is extinguished. If the rate of heat generation is more than the
rate of heat dissipation, the space breaks down again resulting in an
arc for another half cycle.

24. Expression for Restriking Voltage and RRRV

27. The maximum value of restriking voltage occurs at t=π/ωn
The maximum value of restriking voltage =2 Vm
The rate of rise of restriking voltage,
RRRV=
The maximum value of RRRV occurs at t=π/2ωn
The maximum value of RRRV =ωnVm

28. Resistance Switching
 To reduce the restriking voltage, RRRV and severity of the
transient oscillations, a resistance is connected across the contacts
of the circuit breaker
This is known as resistance switching. The resistance is in
parallel with the arc.
A part of the arc current flows through this resistance resulting in
a decrease in the arc current and increase in the deionization of the
arc path and resistance of the arc.
This process continues and the current through the shunt
resistance increases and arc current decreases. Due to the decrease
in the arc current, restriking voltage and RRRV are reduced

29. The analysis of resistance switching can be made to find out the
critical value of the shunt resistance to obtain complete damping of
transient oscillations. Figure shows the equivalent electrical circuit
for such an analysis.

35. Problem:
In a 220 kV system, the reactance and capacitance up to the location
of circuit breaker is 8Ω and 0.025 μF, respectively. A resistance of
600 ohms is connected across the contacts of the circuit breaker.
Determine the following:
1. Natural frequency of oscillation
2. Damped frequency of oscillation
3. Critical value of resistance which will give no transient oscillation
4. The value of resistance which will give damped frequency of
oscillation, one-fourth of the natural frequency of oscillation.

36. Current Chopping
• When low inductive current is being interrupted and the arc
quenching force of the circuit breaker is more than necessary to
interrupt a low magnitude of current, the current will be interrupted
before its natural zero instant.
• In such a situation, the energy stored in the magnetic field appears in
the form of high voltage across the stray capacitance, which will
cause restriking of the arc.
• The energy stored in the magnetic field is ½ Li2, if i is the
instantaneous value of the current which is interrupted. This will
appear in the form of electrostatic energy equal to ½ cv2. As these
two energies are equal, they can be related as follows.

38. •If the value of v is more than the withstanding capacity of
the gap between the contacts, the arc appears again.
•Since the quenching force is more, the current is again
chopped.
•The phenomenon continues till the value of v becomes
less than the withstanding capacity of the gap.

39. A circuit breaker interrupts the magnetising current of a 100
MVA transformer at 220 kV. The magnetising current of the
transformer is 5% of the full load current. Determine the
maximum voltage which may appear across the gap of the
breaker when the magnetising current is interrupted at 53% of its
peak value. The stray capacitance is 2500 μF. The inductance is
30H.

40. Interruption of capacitive current
• The interruption of capacitive current produces high voltage
transients across the gap of the circuit breaker.
• This occurs when an unloaded long transmission line or a
capacitor bank is switched off.
• The figure shows an equivalent electrical circuit of a simple
power system. C represents stray capacitance of the circuit
breaker. CL represents line capacitance. The value of CL is
much more than

42. • At the instant M, the capacitance current is zero and the
system voltage is maximum. If an interruption occurs, the
capacitors CL remains charged at the maximum value of the
system voltage. After instant M, the voltage across the breaker
gap is the difference of Vc and VCL. At instant N, i.e. half-cycle
from A, the voltage across the gap is twice the maximum value
of Vc. At this moment, the breaker may restrike.
• If the arc restrikes, the voltage across the gap becomes
practically zero. Thus, the voltage across the gap falls from
2Vcmax to zero. A severe high frequency oscillation occurs.

43. • At instant P, the system voltage reaches its positive maximum
shown by the point T in the figure, and at this moment the
voltage across the gap becomes 4emax. The capacitive current
reaches zero again and there may be an interruption
• In this way, the voltage across the gap goes on increasing. But
in practice, it is limited to 4 times the peak value of the system
voltage. Thus, it is seen that there is a problem of high
transient of high transient voltage while interrupting a
capacitive current.

44. Classification of Circuit Breakers
• Classification based on Voltage
– Low voltage Circuit Breaker (less than 1 kV)
– Medium voltage Circuit Breaker ( 1 kV to 52 kV)
– High Voltage Circuit Breaker (66 kV to 220 kV)
– Extra High voltage circuit breaker ( 300 kV to 765 kV
– Ultra High voltage Circuit breaker ( above 765 kV)
 Classification based on Location
 Indoor type
 Outdoor type
• Classification based on medium used for arc quenching
– Air break circuit breaker
– Oil circuit breaker
– Air blast circuit breaker
– Sulphur hexafluoride SF6 circuit breaker
– Vaccum circuit breaker

45. Oil circuit breakers
Types of oil circuit breakers
• Bulk oil circuit breakers
Plain break oil circuit breakers
Arc control oil circuit breakers
• Low oil circuit breakers

46. Bulk Oil Circuit Breaker

47. Plain break oil circuit breakers

48. Disadvantages of Plain break oil circuit breakers
• There is no special control over the arc other than
the increase in length by separating the moving
contacts.
• These breakers have long and inconsistent arcing
times
• These breakers do not permit high speed
interruption.

49. Arc control oil circuit breakers
• Self blast oil circuit breakers
Plain explosion pot
Cross jet explosion pot
Self-compensated explosion pot
• Forced blast oil circuit breakers

50. Plain explosion pot

51. Cross jet explosion pot

52. Self-compensated explosion pot

53. Low oil (or) minimum oil circuit breakers

54. Practical View of oil CB

56. Air break CB
ACBs are still preferable choice up to voltage 15 KV

57. Air Blast CB

58. Axial blast air circuit breaker

59. Cross blast air circuit breaker

60. Advantages of Air blast circuit breaker
• There is no chance of fire hazard caused by oil.
• The breaking speed of circuit breaker is much higher during
operation of air blast circuit breaker.
• Arc quenching is much faster during operation of air blast circuit
breaker.
• The duration of arc is same for all values of small as well as high
currents interruptions.
• As the duration of arc is smaller, so lesser amount of heat realized
from arc to current carrying contacts hence the service life of the
contacts becomes longer.
• The stability of the system can be well maintained as it depends on
the speed of operation of circuit breaker.
• Requires much less maintenance compared to oil circuit breaker.

62. SF6 CB
sulphur hexafluoride gas (SF6) gas at a pressure of about 2.8 kg/cm2

63. Advantages of SF6 circuit breaker
• 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 breakers have low maintenance cost, light foundation
requirements and minimum auxiliary equipment.
• Since SF6 breakers are totally enclosed and sealed from atmosphere,
they are particularly suitable where explosion hazard exists e.g., coal
mines.

67. Vacuum CB

68. Cross sectional view of vaccum CB

69. • Service life of vacuum circuit breaker is much longer than other
types of circuit breakers.
• There is no chance of fire hazard as oil circuit breaker.
• It is much environment friendly than SF6 Circuit breaker.
• Beside of that contraction of VCB is much user friendly.
• Replacement of vacuum interrupter (VI) is much convenient.

70. Vaccum VI Interrupter

71. References:
http://www.openelectrical.org/wiki/index.php
?title=Circuit_breakers

72. Miniature Circuit Breaker (MCB)
• A MCB is an electromechanical device which
makes and breaks the circuit in normal operation
and disconnects the circuit under the abnormal
condition when current exceeds a preset value.
• MCB is a high fault capacity current limiting, trip
free, automatic switching device with thermal
and magnetic operation to provide protection
against overload and short circuit.

73. Miniature Circuit Breaker (MCB)

74. Miniature Circuit Breaker (MCB)

75. Miniature Circuit Breaker (MCB)

76. Features of Miniature Circuit Breaker (MCB)
• Its operation is very fast and opens in less than one
millisecond
• No tripping circuit is necessary and the operation is
automatic
• Provides protection against overload and short circuit
without noise, smoke or flame.
• It can be reset very quickly after correcting the fault, just
by switching a button. No rewiring is required.
• It can not be reclosed if fault persists.
• The mechanical life is upto or more than one lakh
operating cycle.

77. Features of Miniature Circuit Breaker (MCB)
• Generally MCBs are rated for a.c. voltage of 240 V for
single phase, 415 V for three phase or 220 V d.c.
• The current rating available is form 0.5 A to 63 A.
• It is available as single pole (SP), Double Pole (DP),
Tripple Pole (TP) with short circuit breaking capacity form
1 kA to 10 kA with a rated frequency.

78. Moulded Case Circuit Breaker (MCCB)
• It is used for circuits having current ranges form 63 A to
3000 A.
• Its working is based on thermal mechanism. It has a
bimettalic contact which expands and contracts when
there are changes in temperature.
• Under normal condition, the contacts are closed allowing
current to pass.
• Under overload or short circuit condition, current
exceeds its safe value. Due to this, heat is generated and
the contacts are opened to interrupt the circuit .

79. Moulded Case Circuit Breaker (MCCB)
• Due to the interruption of high current, there
is arc formation. Hence in MCCB there are arc
extinguishers which suppress the arc.
• There is a disconnection switch, with the help
of which, the MCCB can be operated
manually.

80. Moulded Case Circuit Breaker (MCCB)
• Practically it has adjustable trip settings and
hence it can be used for high current
application.
• It can be easily reset after the fault
rectification. Thus it provides operational
safety and convenience.
• All the operating parts of MCCB are covered
within a plastic moulded housing made in two
halves.

81. Moulded Case Circuit Breaker (MCCB)
• The two halves are joined together to form
the whole structure.
• The basic difference between MCB and MCCB
is the current rating.
• Hence MCCBs are used for industrial and
commercial applications such as main feeder
protection, generator and motor protection,
capacitor bank protection, welding
applications and applications which require
adjustable trip setting.

82. Moulded Case Circuit Breaker (MCCB)

83. Comparison of MCB and Fuse
S.No. Fuse MCB
1 The operation of fuse is highly
dependent on selection of its proper
rating. If fuse wire is not selected
properly then it results in non operation
of fuse even in case of short circuit
MCB instantly disconnects the supply
automatically in the event of short circuit
or overload. It thus eliminates the risk of
fire and prevents damage to wiring
system
2 If the fuse wire after operation is
replaced with a newer one but go loose
then it may be dangerous. Also to
replace a blown fuse in between current
carrying points is dangerous specially in
dark.
Restarting power supply after tripping
due to overload or short is easy
3 During replacement of fuse wire, the
exact size of fuse wire may not be
available. Also for replacement a kit of
hand tools has to be kept ready
No maintenance and repairs is required
for MCB. The distribution system
employing MCB provides satisfactory
operation and lasts for years.
4 The board employing fuse is not
compact
The board employing MCBs give beautiful
look as it is compact and elegant

84. Rating of circuit breaker
A circuit breaker has to perform the following major duties under short-
circuit conditions.
1. To open the contacts to clear the fault
2. To close the contacts onto a fault
3. To carry fault current for a short time while another circuit breaker is
clearing the fault
Therefore, in addition to the rated voltage, current and frequency, circuit
breakers have the following important ratings
(i) Breaking Capacity
(ii) Making Capacity
(iii) Short-time Capacity

85. Breaking Capacity
• It is current (r.m.s.) that a circuit breaker is
capable of breaking at given recovery voltage
and under specified conditions (e.g., power
factor, RRRV).

86. Breaking Capacity
The breaking capacity of a circuit breaker is of two types
(i) Symmetrical breaking capacity
(ii) Asymmetrical breaking capacity
Symmetrical breaking capacity
It is the RMS value of the ac component of the fault current that
the circuit breaker is capable of breaking under specified
conditions of recovery voltage.
Asymmetrical breaking capacity
It is the RMS value of the total current comprising of both ac and
dc components of the fault current that the circuit breaker can
break under specified conditions of recovery voltage.

88. Breaking capacity:
The breaking capacity of a circuit breaker is generally expressed in
MVA. For a three-phase circuit breaker, it is given by
Breaking capacity = √3 x rated voltage in kV x rated current in kA.
The breaking capacity will be symmetrical if the rated current in the
above expression is symmetrical.

89. Making Capacity
• The peak value of current (including d.c.
component) during the first cycle of current wave
after the closure of circuit breaker is known as
making capacity.
Making current = √2 x 1.8 x symmetrical breaking current.
Making capacity = √2 x 1.8 x symmetrical breaking capacity

90. Short-time rating
• It is the period for which the circuit breaker is able
to carry fault current while remaining closed.
• The short-time current rating is based on thermal
and mechanical limitations.
• The circuit breaker must be capable of carrying
short-circuit current for a short period while
another circuit breaker (in series) is clearing the
fault.

91. Short-time rating
• The short-time rating of a circuit breaker depends
upon its ability to withstand
(a) the electro-magnetic force effects
(b) the temperature rise.

92. Normal current rating
• It is the r.m.s. value of current which the circuit
breaker is capable of carrying continuously at its
rated frequency under specified conditions. The
only limitation in this case is the temperature rise
of current-carrying parts.

93. Rated operating duty:
The operating duty of a circuit breaker prescribes its operations
which can be performed at stated time intervals
For the circuit breakers which are not meant for auto reclosing, there
are two alternative operating duties as given below:
(i) O-t-CO-t’-CO
(ii) O-t”-CO
Where O denotes opening operation, CO denotes closing operation
followed by opening without any intentional time lag, and t, t’ and t”
are time intervals between successive operations

94. Selection of Circuit Breaker
• The following parameters are required to be
known for selecting proper rating circuit breaker
at a given location on a power system:
• (a) the maximum fault current which is to be
interrupted by the breaker
• (b) the maximum current to be carried
momentarily.
• The fault current consists of both a.c. and d.c.
components and its correct calculation is very
complex. A simplified method is recommended by
IEEE committee is given below.

95. Selection of Circuit Breaker
• To determine firstly the required interruption
capacity of circuit breaker the highest value of
initial r.m.s alternating current for any type and
location of fault.
• It can be considered as three phase fault as it
carries maximum fault current except in some
cases.
• This current can be obtained by using sub-
transient reactance for generators and transient
reactance for synchronous motors while induction
motors are to be neglected.

96. Selection of Circuit Breaker
• Following multiplying factors can be applied to
take into account the d.c. components of currents
and decrements of both a.c. and d.c. components.
8 cycle or slow breaker 1.0
5 cycle breaker 1.1
3 cycle breaker 1.2
2 cycle breaker 1.4
before applying multiplying factor, all the given
factors are increased by 0.1 for the breakers on the
generator bus where 3 phase short circuit KVA
exceed 5,00,000.

97. Selection of Circuit Breaker

100. HVDC Breaking
The additional circuit creates artificial current zeros which are utilised
for arc interruption
Cp and Lp are connected in parallel to produce artificial current zero
after the separation of the contacts in the main circuit breaker MCB.