Fuse and circuit breakers , Terms & characteristics

2. UNIT-02
FUSE AND CIRCUIT BREAKERS
CO-02- Understand the construction and working of
Fuse and circuit breakers

3. Meaning of Switchgear
The apparatus used for switching, controlling
and protecting the electrical circuits and
equipment is known as switchgear

4. Essential features of Switchgear
(i) Complete reliability : When fault occurs on
any part of the power system, the switchgear
must operate to isolate the faulty section
from the remainder circuit.
(ii)Absolutely certain discrimination : When
fault occurs on any section of the power
system, the switchgear must be able to
discriminate between the faulty section and
the healthy section. It should isolate the
faulty section from the system without
affecting the healthy section. This will ensure
continuity of supply.

5. (iii) Quick operation: When fault occurs on any part
of the power system, the switchgear must operate
quickly so that power system equipment .
(iv) Provision for manual control : A switchgear must
have provision for manual control. In case the
electrical (or electronics) control fails, the
necessary operation can be carried out through
manual control.
(v) Provision for instruments : There must be
provision for instruments which may be required.
These may be in the form of ammeter or voltmeter
on the unit itself or the necessary current and
voltage transformers for connecting to the main
switchboard or a separate instrument panel.

6. Switchgear Equipment
Switchgear covers a wide range of equipment concerned with
switching and interrupting currents under both normal and
abnormal conditions. It includes switches, fuses, circuit breakers,
relays and other equipment.
• Some devices is given below.
1. Switches
(i) Air switches
(ii) Isolator or disconnecting switch.
(iii) Oil switches
2. Fuses.
3. Circuit breakers.
4. Relays.

7. I. Air-break switch : It is an air switch and is
designed to open a circuit under load. In
order to quench the arc that occurs on
opening such a switch, special arcing horns
are provided.
II. Isolator or disconnecting switch : It is
essentially a knife switch and is designed to
open a circuit under no load.
III. Oil switches : The contacts of such switches
are opened under oil, usually transformer oil.

8. 2. Fuses
A fuse is a short piece of wire or thin strip
which melts when excessive current flows
through it for sufficient time. It is inserted in
series with the circuit to be protected.
When a short circuit or overload occurs, the
current through the fuse element increases
beyond its rated capacity. This raises the
temperature and the fuse element melts (or
blows out), disconnecting the circuit protected
by it.

9. 3. Circuit Breakers
A circuit breaker is an equipment which can open or
close a circuit under all conditions viz. no load, full
load and fault conditions. It is so designed that it
can be operated manually (or by remote control)
under normal conditions and automatically under
fault conditions. For the latter operation, a relay
circuit is used with a circuit breaker.
4. Relays
A relay is a device which detects the fault and
supplies information to the breaker for circuit
interruption.
When a fault occurs the relay contacts are closed
and the trip coil of the circuit breaker is energised to
open the contacts of the circuit breaker.

10. Differences between Indoor type and Outdoor
type Switchgear
(i) Indoor type.
• For voltages below 66 kV, switchgear is
generally installed indoor because of
economic considerations.
• The indoor switchgear is generally of metal-
clad type. In this type of construction, all
live parts are completely enclosed in an
earthed metal casing.
• The primary object of this practice is the
definite localization and restriction of any
fault to its place of origin.

11. (ii) Outdoor type.
• For voltages beyond 66 kV, switchgear
equipment is installed outdoor.
• It is because for such voltages, the clearances
between conductors and the space required
for switches, circuit breakers, transformers
and others equipment become so great that it
is not economical to install all such equipment
indoor.
• The circuit breakers , isolators, transformers
and bus-bars occupy considerable space on
account of large electrical clearance
associated with high voltages.

12. Fuses
A fuse is a short piece of metal,
inserted in the circuit, which melts
when excessive current flows through
it and thus breaks the circuit.

13. Desirable Characteristics of Fuse elements
The function of a fuse is to carry the
normal current without overheating but
when the current exceeds its normal
value, it rapidly heats up to melting point
and disconnects the circuit protected by
it. In order that it may perform this
function satisfactorily.

14. Desirable characteristics
(i) low melting point e.g., tin, lead.
(ii) high conductivity e.g., silver, copper.
(iii) free from deterioration due to oxidation e.g., silver.
(iv) low cost e.g., lead, tin, copper.

15. Types of Fuses
(1) Low voltages fuses
(i) Semi-enclosed rewireable fuse
(ii) High rupturing capacity (H.R.C.) cartridge fuse.
(iii) H.R.C. fuse with tripping device.
(2) High voltage fuses
(i) Cartridge type
(ii) Liquid type.
(iii) Metal clad fuses.

16. Semi-enclosed rewireable fuse
or Kit-Kat Fuses High rupturing capacity (H.R.C.)
H.R.C. fuse with tripping device.

17. Cartridge type fuse for LV Cartridge type fuse for HV
Liquid type.
Metal clad fuses

18. Fuse Element Materials
• The most commonly used materials for fuse
element are lead, tin, copper, zinc and silver.
• For small currents up to 10 A, tin or an alloy of
lead and tin (lead 37%, tin 63%) is used for
making the fuse element.
• For larger currents, copper or silver is employed.
It is a usual practice to tin the copper to protect it
from oxidation.
• Zinc (in strip form only) is good if a fuse with
considerable time-lag is required i.e., one which
does not melt very quickly with a small overload.

19. The present trend is to use silver despite its high cost due to the following
reasons :
(i) It is comparatively free from oxidation.
(ii) It does not deteriorate when used in dry air.
(iii) The coefficient of expansion of silver is so small that no critical fatigue
occurs. Therefore, the fuse element can carry the rated current continuously
for a long time.
(iv) The conductivity of silver is very high. Therefore, for a given rating of fuse
element, the mass of silver metal required is smaller than that of other
materials. This minimizes the problem of clearing the mass of vapourised
material set free on fusion and thus permits fast operating speed.
(v) Due to comparatively low specific heat, silver fusible elements can be raised
from normal temperature to vapourisation quicker than other fusible
elements. Moreover, the resistance of silver increases abruptly as the melting
temperature is reached, thus making the transition from melting to
vapourisation almost instantaneous. Consequently, operation becomes very
much faster at higher currents.
(vi) Silver vapourises at a temperature much lower than the one at which its
vapour will readily ionize. Therefore, when an arc is formed through the
vapourised portion of the element, the arc path has high resistance. As a
result, short-circuit current is quickly interrupted.

20. Important Terms of Fuse
(i) Current rating of fuse element.
It is the current which the fuse element can normally
carry without overheating or melting.
It depends upon the temperature rise of the contacts
of the fuse holder, fuse material and the surroundings
of the fuse.
(ii) Fusing current.
It is the minimum current at which the fuse element
melts and thus disconnects the circuit protected by it.
Obviously, its value will be more than the current
rating of the fuse element.

22. iii) Fusing factor.
It is the ratio of minimum fusing current to the
current rating of the fuse element i.e.

23. (iv) Prospective Current
The r.m.s.value of the first loop of fault current
is known as prospective current.
Therefore, prospective current can be
defined as under: It is the r.m.s. value of the
first loop of the fault current obtained if the
fuse is replaced by an ordinary conductor of
negligible resistance.

24. (v) Cut-off current.
It is the maximum value of fault current actually
reached before the fuse melts.
(vi) Pre-arcing time.
It is the time between the commencement of fault
and the instant when cut off occurs.
When a fault occurs, the fault current rises rapidly
and generates heat in the fuse element. As the fault
current reaches the cut off value, the fuse element
melts and an arc in initiated. The time from the start
of the fault to the instant the arc is initiated is
known as pre-arcing time. The pre-arcing time is
generally small : a typical value being 0·001second

25. (vii) Arcing time.
This is the time between the end of pre-arcing time and
the instant when the arc is extinguished.
(viii) Total operating time.
It is the sum of pre-arcing and arcing times.
It may be noted that operating time of a fuse is generally quite low (say 0·002
sec.) as compared to a circuit breaker (say 0·2 sec or so). This is an added
advantage of a fuse over a circuit breaker. A fuse in series with a circuit breaker
of low-breaking capacity is a useful and economical arrangement to provide
adequate short-circuit protection. It is because the fuse will blow under fault
conditions before the circuit breaker has the time to operate.

26. (ix) Breaking capacity.
It is the r.m.s. value of a.c. component of maximum
prospective current that a fuse can deal with at rated
service voltage.

27. HRC fuse
Construction of HRC fuse:
HRC fuse mainly consists of heat resisting
ceramic body. The current carrying element is
compactly surrounded by the filling powder.
Filling material acts as an arc quenching and
cooling medium when the fuse element blows off
due to excessive heat generated under abnormal
conditions.

28. Working:
Under normal conditions, the fuse element is at
a temperature below its melting point.
Therefore, it carries the normal current without
overheating.
When a fault occurs, the current increases and
the heat produced is sufficient to melt these
elements. Fuse element melts before the fault
current reaches its first peak value. Vaporized
metal /fuse element chemically reacts with
filling powder and results in the formation of
high resistance substance that helps in
quenching the arc.

29. Advantages / Merits
1. When compared to other circuit interrupters of same capacity
HRC fuses are the cheaper one.
2. Simple and easy to install.
3. No maintenance required.
4. High breaking capacity.
5. They are consistent in performance.
6. Their inverse time characteristic makes them much suited for
overload protection.
7. They are capable of clearing high as well as low currents.
8. Quick operation.
Disadvantages / Demerits
1. These fuses once blown out cannot be reused.
2. Heat produced by the arc may affect the associated switches.
3. Causes over heating of adjacent contacts.
4. The current-time characteristic of a fuse cannot always be
correlated with that of the protective device.

30. Application of HRC Fuses:
1. Protection of radial lines.
2. Transformer protection.
3. It is also used of capacitor protection.
4. It is also provide protection of underground
distribution system.
5. It is used in meter board in residential
application.

31. Arc formation process
• When a short-circuit occurs, a heavy current flows through the contacts of the
circuit breaker before they are opened by the protective system.
• At the instant when the contacts begin to separate the contact area decreases
rapidly and large fault current causes increased current density and hence rise in
temperature.
• The heat produced in the medium between contacts (usually the medium is oil
or air) is sufficient to ionise the air or vapourise and ionise the oil. The ionised air
or vapour ,acts as conductor and an arc is struck between the contacts.
• The potential difference between the contacts is quite small and is just sufficient
to maintain the arc. The arc provides a low resistance path and consequently the
current in the circuit remains uninterrupted so long as the arc persists.

32. 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.
The arc resistance depends upon the following factors:
Degree of ionization - the arc resistance increases with
the decrease in the number of ionised particles
between the contacts.
Length of the arc - the arc resistance increases with the
length of the arc i.e. separation of contacts.
Cross section of arc - the arc resistance increase with
the decrease in the area of cross section of the arc.

33. Methods of arc extinction (HT & LT method)
There are two methods of extinguishing the arc
in circuit breakers.
1. High resistance method
2. Low resistance or current zero method.

34. 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 Land low-capacity a.c. circuit breakers.

35. The resistance of the arc may be increased by
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.
Cooling the arc - Cooling helps in medium between the contacts.
This increases the arc may be obtained by a gas resistance.
Efficient cooling blast directed along the arc.
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.
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.

36. 2. Low Resistance or Current zero Method
• This method is employed for arc extinction in AC Circuits only. In
this method, arc resistance is kept low until current zero where
the arc extinguishes naturally and is prevented from restriking
inspite of the rising voltage across the contacts.
• All modern high power AC Circuit Breakers employ this method
for arc extinction. In an a.c. system, current drops to zero after
every half-cycle. At every current zero, the arc extinguishes for a
brief moment.
• Now the medium between the contacts contains ions and
electrons so that it has small dielectric strength and can be easily
broken down by the rising contact voltage known as restriking
voltage. If such a break-down does occur, the arc will persist for
another half-cycle. If immediately after current zero, the
dielectric strength of the medium between contacts is built up
more rapidly than the voltage across the contacts, the arc fails to
restrike and the current will be interrupted.

37. The rapid increase of dielectric strength of the
medium near current zero can be achieved by-
• causing the ionised particles in the space
between contacts to recombine into neutral
molecules.
• sweeping the ionised particles away and
replacing them by unionised
particles. Therefore, the real problem in AC arc
interruption is to rapidly deionise the medium
between contacts as soon as
the current becomes zero so that the rising
contact voltage or restriking voltage cannot
breakdown the space between contacts.

38. The deionisation 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 deionisation 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 un-ionised particles, the dielectric strength of the
medium can be increased consider-ably. This may be achieved by a gas
blast directed along the discharge or by forcing oil into the contact space.

39. Circuit Breaker Ratings
A circuit breaker may be called upon to operate under all
conditions. However, major duties are imposed on the circuit
breaker when there is a fault on the system in which it is
connected.
Under fault conditions, a circuit breaker is required to perform
the following three duties :
1. It must be capable of opening the faulty circuit and breaking
the fault current.
2. It must be capable of being closed on to a fault.
3. It must be capable of carrying fault current for a short time
while another circuit breaker (in series) is clearing the fault.
The rating of a circuit breaker includes
I. Breaking capacity or Rated short circuit breaking current.
II. Making capacity or Rated short circuit making current.
III. Short-time capacity or Rated short time current.

40. • Breaking capacity :
This is the maximum short circuit current which
a circuit breaker can withstand before it, finally
cleared by opening its contacts.
• It is a common practice to express the breaking
capacity in MVA by taking into account the
rated breaking current and rated service
voltage. Thus, if I is the rated breaking current
in amperes and V is the rated service line
voltage in volts, then for a 3-phase circuit.

41. • Making capacity
The peak value of current during the first cycle
of current wave after the closure of circuit
breaker is known as making capacity.

42. • Short-time capacity
It is the period for which the circuit breaker is able
to carry fault current while remaining closed.
• The short-time rating of a circuit breaker depends
upon its ability to withstand-
(a) the electromagnetic force effects and
(b) the temperature rise.
The oil circuit breakers have a specified limit of 3
seconds when the ratio of symmetrical breaking
current to the rated normal current does not
exceed 40. However, if this ratio is more than 40,
then the specified limit is 1 second.

43. • Circuit Breaker Terminology.
• Arc Voltage. It is the voltage that appears
across the contacts of the circuit breaker during
the arcing period.

44. • Pre-arcing time. It is the time between the
commencement of fault and the instant when cut off
occurs.
• Arcing time. This is the time between the end of pre-
arcing time and the instant when the arc is
extinguished.
• Prospective Current. It is the r.m.s. value of the first
loop of the fault current obtained if the use is
replaced by an ordinary conductor of eligible
resistance.
• Cut-off current. It is the maximum value of fault
current actually reached before the CB opens its
contacts.

45. • Total operating time.
It is the sum of pre-arcing and arcing times.
• Breaking capacity.
It is the r.m.s. value of a.c. component
of maximum prospective current that a CB can
deal with at rated service voltage

46. • Restriking voltage
It is the transient voltage that appears across
the contacts at or near current zero during arcing
period.
• Recovery voltage.
It is the normal frequency (50 Hz) r.m.s.
voltage that appears across the contacts of the
circuit breaker after final arc extinction. It is
approximately equal to the system voltage.

47. • RRRV (Rate Of Rise Of The Restriking Voltage):
The RRRV (rate of rise of the restriking voltage)
is defined as the slope of the steepest tangent
to the restriking voltage curve. It is expressed in
volts per micro-second.
• TRV (Transient Recovery Voltage) :TRV is the
voltage difference observed between the
breaker terminals immediately after the current
interruption of the breaker.
• Total Break Time Fault clearing time is the sum
of "relay time" and "circuit breaker time".
Circuit breaker time is also called "total break
time“

48. • Classification of Circuit Breakers
There are several ways of classifying the circuit breakers.
However, the most general way of classification is on the
basis of medium used for arc extinction.
(i) Oil circuit breakers:
which employ some insulating oil (e.g., transformer oil) for arc extinction.
(ii) Air-blast circuit breakers:
In which high pressure air-blast is used for extinguishing the arc.
(iii) Sulphur hexafluoride circuit breakers :
In which sulphur hexafluoride (SF6) gas is used for arc extinction.
(iv)Vacuum circuit breakers :
In which vacuum is used for arc extinction.
Each type of circuit breaker has its own advantages and disadvantages. In
the following sections, we shall discuss the construction and working of
these circuit breakers with special emphasis on the way the arc extinction is
facilitated.

49. Working of Circuit Breaker by Trip Circuit Mechanism
When a short circuit occurs at point F on the transmission line, the current
flowing in the line increases to an enormous value. This results in a heavy
current flow through the relay coil, causing the relay to operate by closing
its contacts. This in turn closes the trip circuit of the breaker, making the
circuit breaker open and isolating the faulty section from the rest of the
system. In this way, the relay ensures the safety of the circuit equipment
from damage and normal working of the healthy portion of the system.

50. Plain Break Oil CB

52. Principle of Working
A plain-break oil circuit breaker involves the
simple process of separating the contacts
under the whole of the oil in the tank. There is
no special system for arc control other than the
increase in length caused by the separation of
contacts. The arc extinction occurs when a
certain critical gap between the contacts is
reached.

53. Construction
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. The
air cushion provides sufficient room to allow for
the reception of the arc gases without the
generation of unsafe pressure in the dome of
the circuit breaker. It also absorbs the
mechanical shock of the upward oil movement.

54. Working
under normal operating conditions, the fixed and moving contacts
remain closed and the breaker carries the normal circuit current.
When a fault occurs, the moving contacts are pulled down by the
protective system and an arc is struck which vaporizes the oil
mainly into hydrogen gas.
• The arc extinction is facilitated by the following processes :
1. The hydrogen gas bubble generated around the arc cools the arc
column and aids the deionization of the medium between the
contacts.
2. The gas sets up turbulence in the oil and helps in eliminating the
arcing products from the arc path.
3. As the arc lengthens due to the separating contacts, the dielectric
strength of the medium is increased. The result of these actions is
that at some critical gap length, the arc is extinguished and
the circuit current is interrupted.

55. Advantages of plain break oil circuit breakers
1. Oil produces hydrogen during arcing. The hydrogen helps
extinguish the arc.
2. The oil provides insulation for the live exposed contacts
from the earthed portions of the container.
3. Oil provides insulation between the contacts after the arc
has been extinguished.
Disadvantages of plain break oil circuit breakers
1. There is no special control over the arc other than the
increase in length by separating the moving contacts.
Therefore, for successful interruption, long arc length is
necessary.
2. These breakers have long and inconsistent arcing times.
3. These breakers do not permit high speed interruption.

56. Applications Plain Break Oil CB
1. Plain-break oil circuit breakers are used only
for low-voltage applications.
2. It is a usual practice to use such breakers for
low capacity installations for voltages not
exceeding 11 kV.

57. ACB- Air Circuit Breaker (Axial blast)

59. Construction :
• Fig .shows the essential components of a typical axial blast air circuit
breaker. The fixed and moving contacts are held in the closed position
by spring pressure under normal conditions. The air reservoir is
connected to the arcing chamber through an air valve.
• This valve remains closed under normal conditions but opens
automatically by the tripping impulse when a fault occurs on the
system . When a fault occurs, the tripping impulse causes opening of
the air valve which connects the circuit breaker reservoir to the arcing
chamber.
• The high pressure air entering the arcing chamber pushes away the
moving contact against spring pressure. The moving contact is
separated and an arc is struck. At the same time, high pressure air
blast flows along the arc and takes away the ionised gases along with
it. Consequently, the arc is extinguished and current flow is
interrupted.
• In such circuit breakers, the contact separation required for
interruption is generally small (1·75 cm or so). Such a small gap may
constitute inadequate clearance for the normal service voltage.
Therefore, an isolating switch is incorporated as a part of this type of
circuit breaker. This switch opens immediately after fault interruption
to provide the necessary clearance for insulation

60. Working :
The fixed and moving contacts are held in closed
position by spring pressure under normal
condition. When a fault occurs, the tripping
impulse causes opening of the air valve, which
connects the C.B. reservoir to the arcing chamber
pushes away the moving contacts against spring
pressure. The moving contact is separated and
arc is struck. At the same time, high pressure air
blast flows along the arc and arc gets extinguish.

61. ACB- Air Circuit Breaker (cross blast)

62. Construction:
In this type of circuit breaker, an air-blast is directed at
right angles to the arc. The cross-blast lengthens and forces
the arc into a suitable chute for arc extinction. Fig. shows
the essential parts of a typical cross-blast air circuit breaker.
working :
When the moving contact is withdrawn, an arc is struck
between the fixed and moving contacts. The high pressure
cross-blast forces the arc into a chute consisting of arc
splitters and baffles. The splitters serve to increase the
length of the arc and baffles give improved cooling. The
result is that arc is extinguished and flow of current is
interrupted. Since blast pressure is same for all currents,
the inefficiency at low currents is eliminated. The final gap
for interruption is great enough to give normal insulation
clearance so that a series isolating switch is not necessary.

63. Advantages air-blast circuit breakers
(i) The risk of fire is eliminated.
(ii) The arcing products are completely removed by the blast
whereas the oil deteriorates with successive operations; the
expense of regular oil replacement is avoided.
(iii) The growth of dielectric strength is so rapid that final contact
gap needed for arc extinction is very small. This reduces the
size of the device.
(iv) The arcing time is very small due to the rapid build up of
dielectric strength between contacts. Therefore, the arc
energy is only a fraction of that in oil circuit breakers, thus
resulting in less burning of contacts.
(v) Due to lesser arc energy, air-blast circuit breakers are very
suitable for conditions where frequent operation is required.
(vi) The energy supplied for arc extinction is obtained from high
pressure air and is independent of the current to be
interrupted.

64. Disadvantages air-blast circuit breaker
(i) The air has relatively inferior arc extinguishing
properties.
(ii)The air-blast circuit breakers are very sensitive to
the variations in the rate of rise of restriking
voltage.
(iii) Considerable maintenance is required for the
compressor plant which supplies the air-blast.

65. Applications air-blast circuit breaker
The air blast circuit breakers are finding wide
applications in high voltage installations.
Majority of the circuit breakers for voltages
beyond 110 kV are of this type.

66. Puffer Type SF6 CB

67. • Construction :
• The moving cylinder is coupled with the movable
conductor against the fixed piston, and there is a relative
movement between the moving cylinder and the fixed
piston.
• The gas is compressed in the cavity.
• This trapped gas is released through the nozzle, during arc
extinction process.
• During the travel, of the moving contact and the movable
cylinder, the gas puffs over the arc and reduces the arc
diameter by axial convection and radial dissipation.
• At current zero, the arc diameter becomes too small and
the arc gets extinguished
• The puffing action continues for some time, even after
the arc extinction, and the contact space is filled with
cool, fresh gas.

68. • Working :
• SF6 is a Gas type circuit breaker since SF6 gas is used in
it for extinguishing the arc. The quenching of arc in SF6
circuit breaker takes place due to the puffer
phenomenon taking place inside this circuit breaker
Puffer means quick & short blast of gas on the arc. In
SF6 C.B blast the SF6 gas is made on the arc through
puffer action & the arc gets quenched
• As shown in fig above, the fixed & moving contacts are
kept in insulating nozzle section. The whole assembly
i.e insulating nozzle, puffer action cylinder with moving
contact ,a fixed piston is provided to it through the
other end of moving contact. The chamber is filled with
SF6gas. Under normal working condition, the contacts
are closed but on the fault , the contacts open so
moving contacts move right & SF6 gas move left side.

69. • An arc is struck between the moving contact &
fixed contact. The travel of movable cylinder
causes increase in pressure on SF6 gas & this
high pressure SF6 gas travels towards arc i.e it
puffs over the arc or in simple we can say it
blast over the arc
• so when this puffer action is completed the arc
gets extinguished. This type of C.B is called
simple SF6 C.B. but it has limitation of refilling
the SF6 which is has by disconnecting from its
circuit for this we introduced Double pressure
dead type SF6.

70. Non Puffer Type SF6 CB

72. Construction.
Fig. shows the parts of a typical SF6 circuit breaker.
It consists of fixed and moving contacts enclosed
in a chamber (called arc interruption chamber)
containing SF6 gas. This chamber is connected to
SF6 gas reservoir. When the contacts of breaker
are opened, the valve mechanism permits a high
pressure SF6 gas from the reservoir to flow
towards the arc interruption chamber.

73. • The fixed contact is a hollow cylindrical current
carrying contact fitted with an arc horn. The
moving contact is also a hollow cylinder with
rectangular holes in the sides to permit the
SF6 gas to let out through these holes after
flowing along and across the arc.
• The tips of fixed contact, moving contact and
arcing horn are coated with copper-tungsten
arc resistant material.

74. Working.
• When the breaker operates, the moving contact is
pulled apart and an arc is struck between the
contacts.
• The movement of the moving contact is
synchronised with the opening of a valve which
permits SF6 gas at 14 kg/cm2 pressure from the
reservoir to the arc interruption chamber. The
high pressure flow of SF6 rapidly absorbs the free
electrons in the arc path.
• The result is that the medium between the
contacts quickly builds up high dielectric strength
and causes the extinction of the arc . After the
breaker operation (i.e., after arc extinction), the
valve is closed by the action of a set of springs.

75. Advantages:
(i) Due to the superior arc quenching property of SF6,
such circuit breakers have very short arcing time.
(ii) Since the dielectric strength of SF6 gas is 2 to 3
times that of air, such breakers can interrupt much
larger currents.
(iii) The SF6 circuit breaker gives noiseless operation
due to its closed gas circuit and no exhaust to
atmosphere unlike the air blast circuit breaker.
iv) The closed gas enclosure keeps the interior dry so
that there is no moisture problem.

76. (v) There is no risk of fire in such breakers
because SF6 gas is non-inflammable.
(vi) There are no carbon deposits so that tracking
and insulation problems are eliminated.
(vii) The SF6 breakers have low maintenance cost,
light foundation requirements and minimum
auxiliary equipment.
(viii) Since SF6 breakers are totally enclosed and
sealed from atmosphere, they are particularly
suitable where explosion hazard exists e.g.,
coal mines.

77. Disadvantages
(i) SF6 breakers are costly due to the high cost of SF6.
(ii)Since SF6 gas has to be reconditioned after every
operation of the breaker, additional equipment is
required for this purpose.

78. Applications.
A typical SF6 circuit breaker is uses for the
voltage range 50kV to 230kV.
Power Ratings 10MVA to 20MVA and
interrupting time less than 3 cycles.

79. Vacuum Circuit Breakers (VCB)

81. Vacuum Circuit Breakers (VCB)
In such breakers, vacuum (degree of vacuum
being in the range from 10−7 to 10−5 torr) is used
as the arc quenching medium. Since vacuum
offers the highest insulating strength, it has far
superior arc quenching properties than any other
medium. For example, when contacts of a breaker
are opened in vacuum, the interruption occurs at
first current zero with dielectric strength between
the contacts building up at a rate thousands of
times higher than that obtained with other circuit
breakers.

82. Working Principle.
• When the contacts of the breaker are opened in
vacuum (10−7 to 10−5 torr), an arc is produced
between the contacts by the ionisation of metal
vapours of contacts.
• However, the arc is quickly extinguished because
the metallic vapours, electrons and ions produced
during arc rapidly condense on the surfaces of the
circuit breaker contacts, resulting in quick recovery
of dielectric strength.
• As soon as the arc is produced in vacuum, it is
quickly extinguished due to the fast rate of recovery
of dielectric strength in vacuum. the arc extinction in
a vacuum breaker occurs with a short contact
separation (say 0·625 cm).

83. Construction :
Fig. shows the parts of a typical vacuum circuit
breaker. 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 leak. A glass vessel
or ceramic vessel is used as the outer insulating
body. The arc shield prevents the deterioration of
the internal dielectric strength by preventing
metallic vapours falling on the inside surface of the
outer insulating cover.

84. Working.
When the breaker operates, the moving contact
separates from the fixed contact and an arc is
struck between the contacts. The production of
arc is due to the ionisation of metal ions and
depends very much upon the material of contacts.
The arc is quickly extinguished because the
metallic vapours, electrons and ions produced
during arc are diffused in a short time and seized
by the surfaces of moving and fixed members and
shields. Since vacuum has very fast rate of
recovery of dielectric strength,

85. Advantages.
1. They are compact, reliable and have longer life.
2. There are no fire hazards.
3. There is no generation of gas during and after
operation.
4. They can interrupt heavy fault current perfectly
just before the contacts reach the definite open
position.
5. They require little maintenance and are quiet in
operation.
6. They can successfully withstand lightning surges.
7. They have low arc energy.
8. They have low inertia and hence require smaller
power for control mechanism.

86. Applications.
Vacuum circuit breakers are being employed for
outdoor applications ranging from 22 kV to 66 kV.

87. • Features of Intelligent Circuit Breaker
1. It has Easy User setting facility.
2. Over load alarm function.
3. Display function.
4. Self diagnosis function.
5. Fault memory storage.
6. LED indication of Trip condition.

88. Maintenance Schedule of OCB,ACB, SF6 and VCB Circuit Breakers
• For maintenance of circuit breaker , it must be
first switched off and then isolated from both
sides by opening concerned electrical isolator.
• Daily Maintenance Schedule for all CB’s-
Physical appearance Doors and covers to be
bolted. Check for noise, smell or damage.

89. Oil Circuit Breaker Maintenance
• For oil circuit breaker, we should check contact
burning. If burning is very light, remove the burn
beads and polish the surface.
• check & Clean the extinguishing chamber.
• Half yearly-The proper adjustment of auxiliary
switch by ensuring correct NO NC contacts at
breaker OFF and ON condition must be checked
• The spring charging motor and mechanism should
also be cleaned and associated bearing should
also be lubricated half yearly.

90. Maintenance of Air Blast Circuit Breaker
Yearly & half Yearly
• In air circuit breaker, the air leakage should be
checked monthly.
• Yearly, dew point of the operating air at the
outlet of the air dryer should be measured with
the help of Dew Point Meter or Hygro Meters.

91. SF6 Circuit Breaker Maintenance
Yearly & half Yearly
• SF6 circuit breaker must be checked for SF6 gas
leakage.
• Dew point of SF6 should be checked with the help
of dew point meter or hygro meters in every 3 to
4 years interval.

92. • Maintenance of Vacuum Circuit Breaker
Yearly & half Yearly
• Using a clean, dry cloth, remove all dirt and moisture
from insulating parts.
• Inspect the entire circuit breaker and operating
mechanism for loose hardware and worn or broken
parts. Check all wiring for loose connections and
damaged insulation. Inspect all bearings and contact
surfaces for damage or excessive wear.
• All ball and roller bearings used in these circuit
breakers are sealed; they do not require lubrication.

93. Comparisons between fuse and circuit breaker.
Particular FUSE CIRCUIT BREAKER
Working Principle Fuse works on the electrical and
thermal properties of the
conducting materials.
Circuit breaker works on the
Electromagnetism and switching
principle.
Reusability Fuses can be used only once. Circuit breakers can be used a
number of times.
Status indication It does not give any indication. It gives an indication of the
status
Auxiliary contact No auxiliary contact is required. They are available with
auxiliary contact.
Switching Action Fuse cannot be used as an ON/OFF
switch.
The Circuit breaker is used e as
ON/OFF switches.
Breaking capacity Breaking capacity of the fuse is low
as compared to the circuit breaker.
Breaking capacity is high.
Operating time Operating time of fuse is very less
(0.002 seconds)
Operating time is comparatively
more than that of the fuse.
(0.02 – 0.05 seconds)

94. MODEL QUESTION BANK
Cognitive Level: Remember
1. List different Switchgear equipment
2. List Differences between Indoor type and Outdoor type
Switchgear
3. List Desirable Characteristics of Fuse elements
4. List Fuse Element Materials
5. State Merits and Demerits of HRC fuse
6. List any 3 Application of HRC fuse
7. List the different methods of arc extinction
8. List different circuit breakers
9. List the Features of Intelligent Circuit Breaker
10. State Merits and Demerits of Plain oil OCB

95. 11. State Merits and Demerits of Air circuit breaker
12. State Merits and Demerits of SF6 CB
13.State Merits and Demerits of Vacuum Circuit breaker
14. List the Applications of OCB
15. List the Applications of ACB
16. List the Applications of SF6 CB
17. List the Applications of VCB
18. Lists the steps in maintenance of OCB
19. Lists the steps in maintenance of SF6 CB
20. Lists the steps in maintenance of ACB
21. Define breaking capacity, making capacity and Sort
time rating
22. Lists the steps in maintenance of VCB

96. Cognitive Level: Understanding
23. Explain Essential features of Switchgear
24. Explain Switchgear equipment
25. Explain terms: Terms: Current Rating of Fuse
element, Fusing current, Fusing factor, Prospective
current, cut off current , Pre Arcing Time, Arcing
Time, Breaking Capacity, Total Operating Time.
26. Explain construction and working of HRC fuse
27. Explain the different methods of arc extinction
28. Explain Arc Extinction phenomena in CB by high
resistance method
29. Illustrate the working of Circuit Breaker by Trip
Circuit Mechanism
30. Explain circuit breaker rating

97. 31. Explain the terminologies – Arc-Voltage, arching Time,
Pre –Arching Time, Prospective
Current, T R V, Recovery Voltage, R RR V, Total Break Time
32. Explain Working principle of Circuit Breakers
33. Explain construction and working of Plain oil OCB ACB
34. Explain construction and working of Axial blast ACB
35. Explain construction and working of cross blast ACB
36. Explain construction and working of Puffer Type SF6 CB
37. Explain construction and working of Non Puffer Type
SF6 CB
38. Explain construction and working of VACUUM CB
39. Differentiate between fuse and circuit breaker

98. Cognitive Level: Application/Analyze
1. Compare fuse and circuit breaker.
2. Explain Working principle of Circuit Breakers
3. Explain Essential features of Switchgear
4. Explain Switchgear equipment
5. Explain terms: Terms: Current Rating of Fuse
element, Fusing current, Fusing factor, Prospective
current, cut off current , Pre Arcing Time, Arcing
Time, Breaking Capacity, Total Operating Time.
6. Explain construction and working of HRC fuse
7. Explain the different methods of arc extinction
8. Explain breaking capacity of CB
9. Explain making capacity of CB
10. Explain Short time rating of CB