1. What is the special feature of vacuum circuit breaker?

In a vacuum circuit breaker, vacuum is used as the arc quenching medium in the range of

10-6 It is superior medium than any other arc quenching medium as vacuum offers highest

insulating strength.

For example when contacts of a circuit breaker are opened in vacuum the interruption

occurs at first current zero with dielectric strength between the contacts building up at a rate

thousand times higher than obtained with other OCB’s.

2. What is circuit breaker?

A circuit breaker is a mechanical switching device used in power circuits and is one of the

most important components of the switchgear. A switch gear is a combination of a few devices

used to connect and disconnect a part of electric power supplies and systems. The switching

device is referred to as circuit breaker. The main component of circuit breaker is the contact

system which comprises a set of moving contacts a set of fixed contacts their current carrying

conductors or leads and an opening mechanism which is often spring loaded.

3. Write the classification of circuit breakers based on the medium used for arc extinction?

Air break circuit breaker

Oil circuit breaker

Minimum oil circuit breaker

Air blast circuit breaker

SF6 circuit breaker

Vacuum circuit breaker

4. What is the main problem of the circuit breaker?

When a fault occurs in protected line the relay coil will be energized, which will in turn

activate the CB. When the contacts of the breaker are separated, an arc is struck between them.

This arc delays the current interruption process and also generates enormous heat which may

cause damage to the system or to the breaker itself. This is the main problem.

5. What are demerits of MOCB (minimum oil circuit breaker)?

Short contact life

Frequent maintenance

Possibility of explosion

Larger arcing time for small currents

Prone to restricts

6. What are the advantages of oil as arc quenching medium?

The following are the advantages of oil as arc quenching medium,

The absorption of arc energy in decomposing the oil.

The good cooling properties of the gas formed.

The cooling surface presented by surrounding oil in the close proximity to the arc.

The action of as an insulator.

7. What are the hazards imposed by oil when it is used as an arc quenching medium?

The following are the hazards or chief disadvantages of using oil as arc quenching

medium,

It is easily inflammable.

It may form an explosive mixture with air.

It requires maintenance.

8. What are the advantages of MOCB over a bulk oil circuit breaker?

The following are the advantages of MOCB over a bulk oil circuit breaker,

• It requires lesser quantity of oil.

• It requires smaller space.

• There is a reduced risk of fire.

• Maintenance problem are reduced.

9. What are the disadvantages of MOCB?

The following are the disadvantages of MOCB,

The degree of carbonization is increased due to smaller quantity of oil

There is difficulty of removing the gases from the contact space in time

The dielectric strength of the oil deteriorates rapidly due to high degree of

carbonization.

10. What are the types of air blast circuit breaker?

There are three types of air blast circuit breaker as follows,

Axial-blast type.

Cross blast.

Radial-blast.

11. What are the advantages of air blast circuit breaker over oil circuit breaker?

The following are the advantages of air blast circuit breaker,

The risk of fire is diminished

The arcing time is very small due to rapid buildup of dielectric strength between

contacts

The arcing products are completely removed by the blast whereas oil deteriorates with

successive operations

12. What are the demerits of using oil as an arc quenching medium?

The demerits of using oil as an arc quenching medium,

• The air has relatively inferior arc quenching properties

• The air blast circuit breakers are very sensitive to variations in the rate of rise of

restriking voltage

• Maintenance is required for the compression plant which supplies the air blast.

13. What is meant by electro negativity of SF6 gas?

Sulfur Hexafluoride (SF6) gas has strong electronegative property i.e, free electrons that

are produced during arcing phenomenon (Free electrons are produced because of ionization of

atoms surrounding the arcing contacts) readily removed from the discharge by the formation of

negative ions which are formed by the interaction of sf6 gas molecule with the electrons. This

negative ions formed are relatively heavier and immobile compared to the free electrons. This is

called as electro negativity of SF6 gas.

14. What are the characteristic of SF6 gas?

Sulphur hexa-fluoride gas has good dielectric strength and excellent arc quenching

property. It is inert, nontoxic, non inflammable and heavy. At atmospheric pressure, its dielectric

strength is 2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal

to that of the transformer oil.

15. Write the classifications of test conducted on circuit breakers.

The tests conducted on circuit breakers may be classified into,

Type test Routine test

Reliability test

Commissioning test.

16. What are the indirect methods of circuit breaker testing?

The indirect methods of circuit breaker testing

Unit test

Synthetic test

Substitution testing

Compensation testing

Capacitance testing

17. What are the advantages of synthetic testing methods?

The advantages of synthetic testing methods are,

• The breaker can be tested for desired transient recovery voltage and RRRV.

• Both test current and test voltage can be independently varied. This gives

flexibility to the test

• The method is very simple

• With this method a breaker capacity (MVA) of five times of that of the capacity

of the test plant can be tested.

18. Differentiate between a fuse and a circuit breaker.

The fuse is a device working on the electrical and thermal properties of the conducting

material, while a circuit breaker is a device working on the electromechanical principles.

• Once used a fuse has to be replaced but the circuit breaker can be reused after correction

of the fault in the system.

• Fuses offer protection against only power overloads, while circuit breaker offers

protection against power overloads and short circuits (voltage imbalances).

19. What is dielectric test of a circuit breaker?

It consists of overvoltage withstand test of power frequency lightning and impulse

voltages. Tests are done for both internal and external insulation with switch in both open and

closed conditions. There are two type of dielectric test they are,

One minute dry and

Wet power frequency test.

The main aim for conducting dielectric test is to check power frequency and impulse

voltage withstand test.

20. What is meant by type tests of circuit breakers?

These tests are carried out on first few circuit breakers to prove the rated characteristics of

the breakers. The necessary information which includes assigned ratings, drawings, reference

standards, rated operating pressure and voltage, support structure etc must he supplied to the

testing authorities More conducting these test.. These details are included in the type tests report.

After certifying the breaker by carrying out these tests, there should not be any change in design.

21. Mention the types of type tests on circuit breakers.

Type tests are classified as follows,

a) Mechanical tests

b) Tests of temperature rise, milli volt drop test

c) High voltage test

d) Basic short circuit test

i) Making test

ii) Breaking test

iii) Operating sequence tests at 10%, 30%, 60%, and 100% of rated breaking

current with specified TRV conditions.

e) Critical current tests

f) Single phase short circuit test

g) Short time current test

22. Define routine test on circuit breakers.

Before dispatch of circuit breakers, these tests are performed. Routine test is

defined as a test of every circuit breaker made to the same specifications. They include the

following tests. a) Mechanical operation tests b) Milli-volt drop test, Measurement of resistance c)

Power frequency voltage tests d) Voltage tests on auxiliary circuits, control circuits the quality of

the circuit breaker can be very well checked by these tests. Also any defect in the materials and

construction is detected.

23. Define reliability tests conducted on circuit breakers.

The newly manufactured circuit breakers are tested by type tests and routine tests. But the

conditions during these tests are not the conditions that exist at the field. At site the circuit breaker

is subjected to various stresses due to,

a) Variation in ambient temperatures

b) Extremely low and high temperatures

c) Rain moisture

d) Vibrations on account of earthquakes

e) Dust and chemical fumes, Overloads and over voltages.

24. What is meant by the term current chopping?

When low inductive currents, such as magnetiing current of transformer, shunt reactors are

interrupted by breaker, there is rapid deionization of contact space and may cause the current to

be interrupted before its neutral zero. This phenomena is called current chopping. This generally

occurs during opening small inductive currents by vacuum circuit breaker or air blast circuit

breaker. The transient voltage having high restriking voltage RRRVappears across the contacts,

unless the arc restrikes. If it restrikes a further, chop may occur or several chops may occur before

the current is finally interrupted. Circuit breaker may fail to clear the current

25.What are the basic requirements of circuit breakers?

The basic function of a circuit breaker is to isolate the faulty part of power system in case of

abnormal conditions. It should fulfill following basic requirement,

Make or break circuit either manually or by remote control under normal conditions.

Break a circuit automatically under faulty conditions.

26. What is self compensated explosion spot?

It is also called as plain explosion pot. In this type the explosion pot is fixed and the

moving contacts are enclosed in a strong shell of insulating material. The moving contact

comprises of stud which moves vertically out of fixed finger contacts and relatively close fitting

throat at the lower end of the pot.

When the contacts part, the gas generated by the arc produces a very high pressure in the

pot. The effect of this pressure combined with the effect produced by the streams of vapour

flowing turbulently into the arc tends to cause arc-extinction.

27. What is the special feature of vacuum circuit breaker?

In a vacuum circuit breaker, vacuum is used as the arc quenching medium in the

range of 10-6It is superior medium than any other arc quenching medium as vacuum offers highest

insulating strength.

Vacuum breakers are also similar in construction like the other types of breakers, except

that the breaking medium is vacuum and the medium sealed to ensure vacuum. It has pure

oxygen-free copper main connections; stainless steel bellows and has composite weld-resistant

main contact materials. A typical contact material comprises a tungsten matrix impregnated with a

copper and antimony alloy to provide a low melting point material to ensure continuation of the

arc until nearly current zero.

28. Draw the schematic diagram of showing constructional features of air break circuit

breaker.

29. Draw simple diagram which shows development of arc in contacts.

30. Write any two merits of vacuum breakers.

• They are compact in size and have longer life.

• There are no fire hazards.

• No generation of gas during and after operation.

• Having low arc energy.

• They require less maintenance and quite in operation.

31. List the routine tests conducted on circuit breakers.

Routine test is defined as a test of every circuit breaker made to the same specifications.

They include the following tests.

a) Mechanical operation tests

b) Milli-volt drop test, Measurement of resistance

c) Power frequency voltage tests

d) Voltage tests on auxiliary circuits.

32. What are the advantages of SF6 over air blast circuit breaker?

SF6 has excellent insulating property. SF6 has high electro-negativity. It is inert non-toxic

in nature, non-inflammable and heavy. At atmospheric pressure its dielectric strength is 2.5 times

of that of air. At three times atmospheric pressure, its dielectric strength is equal that of

transformer oil.

33. Draw the schematic representation of axial blast type circuit breaker.

34. Draw the schematic which depicts the construction of SF6 breaker.

35. Mention different types of circuit breakers.

According to interrupting medium

• Air blast circuit breakers

• Oil circuit breakers

• Vacuum circuit breakers

• SF6 circuit breakers

According to operation

• Gravity opened circuit breakers.

• Gravity closed circuit breakers.

• Horizontal break circuit breakers.

36. What are the type tests carried out on circuit breakers?

Type tests are classified as follows,

a) Mechanical tests

b) Tests of temperature rise, milli volt drop test

c) High voltage test

d) Basic short circuit test.

Critical current tests

f) Single phase short circuit test

g) Short time current test.

37. What are desirable parameters of fuse element?

• Fusing Factor

• Minimum Fusing Current

• Current Rating of Fuse 

• Prospective Current in Fuse

• Melting Time of Fuse or Pre-arcing Time of Fuse

• Arcing Time of Fuse

• Operating Time of Fuse 

38. What are the main functions of isolator.

 Isolator is a knife switch, designed to open the circuit under no load condition.· It is also

called disconnector. Its prime purpose is operator safety.· It is physical and visual disconnection

of the circuit from power supply.· Isolators are usually used on both sides of the C.B in order to

repair or replace the C.B.· It is used to separate any section from rest part of the power supply.

39. What is meant by incipient fault?

It's a fault that's not always present - otherwise known as an 'intermittent fault' for

example, a TV screen may work fine when first switched on, but as it 'warms up', it goes blank -

indicating a 'dry-joint'

The incipient fault is a fault that starts as small thing and gets developed into major

failure. Like for example some partial discharge in a void in the insulation over extended period

can burn away adjacent insulation, eventually further developing in to a solid breaking.

40. Write about breaking capacity of circuit breaker.

Breaking capacity or interrupting capacity is the current that a fuse, circuit breaker, or

other electrical apparatus is able to interrupt without being destroyed or causing an electric

arc with unacceptable duration. The prospective short circuit current which can occur under short

circuit conditions should not exceed the rated breaking capacity of the apparatus. Otherwise

breaking of the current cannot be guaranteed. Breaking capacity current corresponds to a voltage,

so an electrical apparatus may have more than one breaking capacity current, according to the

voltage. Breaking current may be stated in terms of the total current or just in terms of the

alternating-current (symmetrical) component. Since the time of opening of a fuse or switch is not

coordinated with the reversal of the alternating current, in some circuits the total current may be

offset and can be larger than the alternating-current component by itself.

41. What are various ratings of circuit breaker?

• Rated AC voltage.

• Rated DC voltage.

• Continuous current rating.

• Breaking capacity rating.

• System frequency.

• Short-circuit current.

• Closing latching current.

42. State difference between a switch, an isolator and a circuit breaker.

Switch: A device for making and breaking the connection in an electric circuit.

Circuit breaker: It is a device which can make or break under normal conditions either

manually or automatically.

Isolator: It is a device which is used to disconnect a part of the system for maintenance

purpose and repairs. If two isolators are connected on either side of circuit breakers then circuit

breakers must be opened before isolators.

43. Define switchgear.

A switchgear has to perform the function of carrying, making and breaking the normal

load current like a switch and it has to perform the function of clearing the fault in addition to that

it also has provision of metering and regulating the various parameters of electrical power system.

Thus the switchgear includes circuit breaker, current transformer, voltage transformer, protection

relay, measuring instrument, electrical switch, electrical fuse, miniature circuit breaker, lightening

arrestor or surge arrestor, electrical isolator and other associated equipment.

44. What is an arc in circuit breaker?

During opening of current carrying contacts in a circuit breaker the medium in between

opening contacts become highly ionized through which the interrupting current gets low resistive

path and continues to flow through this path even the contacts are physically separated. During

the flowing of electric current from one contact to other the path becomes so heated that it glows.

This is called arc.

45. Write short notes on arc interruption.

In high interruption method we can increase the electrical resistance many times to such a

high value that it forces the current to reach to zero and thus restricting the possibility of arc being

re struck. Proper steps must be taken in order to ensure that the rate at which the resistance is

increased or decreased is not abnormal because it may lead to generation of harmful

induced voltages in the system. The arc resistance can be increased by various methods like

lengthening or cooling of the arc etc.

46. Write about basic operation of circuit breaker.

When separation of contacts has just started, distance between the current contacts is small

as a result the voltage gradient between contacts becomes high. This high voltage gradient

between the contacts ionized the oil and consequently initiates arcing between the contacts. This

arc will produce a large amount of heat in surrounding oil and vaporizes the oil and decomposes

the oil in mostly hydrogen and a small amount of methane, ethylene and acetylene. The hydrogen

gas cannot remain in molecular form and its is broken into its atomic form releasing lot of heat.

The arc temperature may reach up to 5000° K. Due to this high temperature the gas is liberated

surround the arc very rapidly and forms an excessively fast growing gas bubble around the arc. It

is found that the mixture of gases occupies a volume about one thousand times that of the oil

decomposed. From this figure we can assume how fast the gas bubble around the arc will grow in

size. If this growing gas bubble around the arc is compressed by any means then rate of de –

ionization process of ionized gaseous media in between the contacts will accelerate which rapidly

increase the dielectric strength between the contacts and consequently the arc will be quenched at

zero crossing of the current cycle. This is the basic operation of oil circuit breaker. 

47. Mention the types of oil circuit breaker.

There are mainly two types of oil circuit breakers available,

• Bulk Oil Circuit Breaker or BOCB: is used as arc quenching media as well as

insulating media between current carrying contacts and earthed parts of the

breaker. The oil used here is same as transformer insulating oil.

• Minimum Oil Circuit Breaker or MOCB.

48. Write about Minimum oil circuit breaker.

These types of circuit breakers utilize oil as the interrupting media. However, unlike bulk

oil circuit breaker, a minimum oil circuit breaker places the interrupting unit in insulating

chamber at live potential. The insulating oil is available only in interrupting chamber. The

features of designing MOCB are to reduce requirement of oil, and hence these breaker are

called minimum oil circuit breaker.

49. Write about the construction of bulk oil circuit breaker.

The basic construction of bulk oil circuit breaker is quite simple. Here all moving contacts

and fixed contacts are immerged in oil inside closed iron vessel or iron tank. Whenever the

current carrying contacts are being open within the oil the arc is produced in between the

separated contacts. An air cushion is necessary between the oil surface and tank roof to

accommodate the displaced oil when gas forms around the arc. That is why the oil tank is not

totally filled up with oil it is filled up to certain level above which the air is tight in the tank. 

50. Write short notes on arc quenching in bulk-oil circuit breaker.

The arc quenching in bulk oil circuit breaker takes place when electric current passes

through zero crossing. If we go through the arc quenching phenomenon more thoroughly we will

find many other factors effects the arc quenching in bulk oil circuit breaker. As the gas bubble is

enclosed by the oil inside the totally air tight vessel, the oil surround it will apply high pressure on

the bubble, which results highly compressed gas around the arc. As the pressure is increased the

de – ionization of gas increases which helps the arc quenching. The cooling effect of hydrogen

gas also helps in arc quenching in oil circuit breaker.

51. Draw the schematic representation of single pole air blast circuit breaker.

52. Draw the schematic arrangement of double extinction

53. What is the working principle in arc quenching in MOCB?

In a minimum oil circuit breaker, the arc drawn across the current carrying contacts is

contained inside the arcing chamber. Hence the hydrogen bubble formed by the vaporized oil is

trapped inside the chamber. As the contacts continue to move, after its certain travel an exit vent

becomes available for exhausting the trapped hydrogen gas. There are two different types of

arcing chamber is available in terms of venting are provided in the arcing chambers. One is axial

venting and other is radial venting. In axial venting, gases (mostly Hydrogen), produced due to

vaporization of oil and decomposition of oil during arc, will sweep the arc in axial or longitudinal

direction.

54. What are the disadvantages of air blast circuit breakers?

There are some disadvantages of air blast circuit breaker.

1. In order to have frequent operations, it is necessary to have sufficiently high capacity air

compressor. Frequent maintenance of compressor, associated air pipes and automatic

control equipments is also required.

2. Due to high speed current interruption there is always a chance of high rate of rise of re-

striking voltage and current chopping.

3. There also a chance of air pressure leakage from air pipes junctions.

55. Mention the types of air circuit breakers.

It can be sub divided further into three different categories.

1. Axial Blast ACB.

2. Axial Blast ACB with side moving contact.

3. Cross Blast ACB.

56. Write short notes on axial blast air circuit breaker.

In axial blast ACB the moving contact is in contact with fixed contact with the help of a

spring pressure as shown in the figure. There is a nozzle orifice in the fixed contact which is

blocked by tip of the moving contact at normal closed condition of the breaker. When fault

occurs, the high pressure air is introduced into the arcing chamber. The air pressure will counter

the spring pressure and deforms the spring hence the moving contact is withdrawn from the fixed

contact and nozzle hole becomes open. At the same time the high pressure air starts flowing along

the arc through the fixed contact nozzle orifice. This axial flow of air along the arc through the

nozzle orifice will make the arc lengthen and colder hence arc voltage become much higher than

system voltage that means system voltage is insufficient to sustain the arc consequently the arc is

quenched.

57. Write short notes on cross blast air circuit breaker.

The working principle of cross blast air circuit breaker is quite simple. In this system of air

blast circuit breaker the blast pipe is fixed in perpendicular to the movement of moving contact in

the arcing chamber and on the opposite side of the arcing chamber one exhaust chamber is also

fitted at the same alignment of blast pipe, so that the air comes from blast pipe can straightly enter

into exhaust chamber through the contact gap of the breaker. The exhaust chamber is spit with arc

splitters. When moving contact is withdrawn from fixed contact, an arc is established in between

the contact, and at the same time high pressure air coming from blast pipe will pass through the

contact gap and will forcefully take the arc into exhaust chamber where the arc is split with the

help of arc splitters and ultimately arc is quenched.

58. What are the electrical properties of SF6?

SF6 gas is highly electronegative. Due to high electro negativity, it absorbs free electrons

which produced due to arcing between contacts of circuit breaker. Combination of free electrons

with molecules produces heavy and big ions, which have very low mobility. Because of

absorption of free electrons and low mobility of ions. Sulphur hexafluoride has very excellent

dielectric property. Dielectric strength of sulfur hexafluoride gas is about 2.5 times more than that

of air.

59. Mention the disadvantages of SF6 circuit breaker.

The following are the advantages of SF6 circuit breaker,

The SF6 gas is identified as a greenhouse gas, safety regulation are being

introduced in many countries in order to prevent its release into atmosphere.

Puffer type design of SF6 CB needs a high mechanical energy which is almost five

times greater than that of oil circuit breaker.

60. What are the types of SF6 circuit breaker.

There are mainly three types of SF6 CB depending upon the voltage level of application-

1. Single interrupter SF6 CB applied for up to 245 KV(220 KV) system.

2. Two interrupter SF6 CB applied for up to 420 KV(400 KV) system.

3. Four interrupter SF6 CB applied for up to 800 KV(715 KV) system.

61. Mention the advantages of vacuum circuit breaker.

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

thanSF6 Circuit breaker. Beside of that contraction of VCB is much user friendly. Replacement of

vacuum interrupter (VI) is much convenient.

62. What is the principle behind voltage earth leakage circuit breaker?

The working principle of voltage ELCB is quite simple. One terminal of the relay coil is

connected to the metal body of the equipment to be protected against earth leakage and other

terminal is connected to the earth directly. If any insulation failure occurs or live phase wire

touches the metal body, of the equipment, there must be a voltage difference appears across the

terminal of the coil connected to the equipment body and earth. This voltage difference produces

a current to flow the relay coil.

63. Draw the schematic representation of basic action of circuit breaker.

64. Draw the schematic representation of cross blast type circuit breaker.

65. Write short notes on importance of dielectric strength of vacuum.

For a given contact gap, vacuum provides, about eight times more dielectric strength than

air and four times more dielectric strength than SF6 gas at one bar. As the dielectric strength is so

high, the contact gap of vacuum circuit breaker can be maintained very small. In this small

contact gap, arc quenching is safely possible due to high dielectric strength and also vacuum has

the fast recovery strength after full arc interruption to its full dielectric value at current zero. This

makes, vacuum switchgear, most suitable for capacitor switching.

66. What are the basic requirements of a circuit breaker?

The circuit breaker is desired to be at open position as fast as possible. It is because of

limiting contacts erosion and to interrupt faulty current as rapidly as possible. But total travel

distance of the moving contact is not determined only by necessity of interruption of faulty

current, but rather the contacts gap needed to withstand the normal dielectric stresses and

lightning impulse voltage appears across the contacts when the CB is at open position.

67. What are the basic requirements of a circuit breaker during closing operation?

During closing operation of circuit breaker the followings are required,

1. The moving contact must travel towards fixed contact at sufficient speed to prevent pre-

arcing phenomenon. As the contact gap reduces, arcing may start before contacts are

closed finally.

2. During closing of contacts, the medium between contacts is replaced, hence sufficient

mechanical power to be supplied during this circuit breaker operation to compress

dielectric medium in the arcing chamber.

3. After hitting fixed contact, the moving contact may bounce back, due to repulsive force

which is not at all desirable. Hence sufficient mechanical energy to be supplied to

overcome repulsive force due to closing operation on fault.

4. In spring - spring mechanism, generally tripping or opening spring is charged during

closing operation. Hence sufficient mechanical energy also to be supplied to charge the

opening spring.

68. What are the provisions provided to metal enclosed switchgear to operate efficiently?

To ensure safe and proper sequence of operation, the following provisions to be provided,

1. The circuit breaker cannot be removed or drawn out from its compartment unless it is

in OFF condition.

2. Not only that, it should be impossible to isolate the circuit breaker from bus bar unless it is

in 

3. The operation of the CB is impossible unless it is in service position, isolated position,

earthed position, and removed from the compartment.

4. It is impossible to switch ON the circuit breaker in its service position unless auxiliary

circuit is connected with it.

69. What are the types of metal enclosed indoor switchgear?

There are mainly two types of metal enclosed indoor switchgear-

Vertical isolation and horizontal draw out,

Horizontal isolation and horizontal draw out.

This type of switchgear is designed and constructed in such a way, that, normal operation,

maintenance, inspection can be carried out very easily and safely. All the components can

easily be replaced.

70. Write short notes on construction of metal enclosed indoor switchgear.

Indoor switchgear housing has three chambers, namely circuit breaker chamber (Front),

bus bar chamber (Top), and current transformer cum cable connection chamber (Rear). The

metering and really panel consists of a separate low voltage chamber may or may not be fixed

with the main gear housing. The withdrawal partition comprises a wheel mounted truck with an

operating mechanism, interrupters and isolating units. The spring charging mechanism may be

either hand operated or motor operated. For motor operated spring charging mechanism, a manual

charging provision should be provide to charge the spring during failure of spring charging motor

or absent of auxiliary supply.

71. What are the features of low voltage air circuit breaker?

Low voltage air circuit breaker is preferable because of the following features,

Simplicity

Efficient performance

High normal current rating up to 600A

High fault withstanding capacity up to 63 kA

72. Which terms represents the rating of circuit breaker?

The rating of a circuit breaker includes,

1. Rated short circuit breaking current.

2. Rated short circuit making current.

3. Rated operating sequence of circuit breaker.

4. Rated short time current.

73. Draw the schematic diagram of arc control device.

74. Draw the schematic diagram of air break switch gear.

75. Write short notes on rated short circuit breaking current.

This is the maximum short circuit current which a circuit breaker can withstand before it.

Finally cleared by opening its contacts. When a short circuit flows through a circuit breaker, there

would be thermal and mechanical stresses in the electric current carrying parts of the breaker. If

the contact area and cross-section of the conducting parts of the circuit breaker are not sufficiently

large, there may be a chance of permanent damage in insulation as well as conducting parts of the

CB.

76. Write short notes on Rated short circuit making current.

The short circuit making capacity of circuit breaker is expressed in peak value not in rms

value like breaking capacity. Theoretically at the instant of fault occurrence in a system, the fault

current can rise to twice of its symmetrical fault level. At the instant of switching on a circuit

breaker in faulty condition, of system, the short circuit portion of the system connected to the

source. The first cycle of the electric current during a circuit is closed by circuit breaker, has

maximum amplitude. This is about twice of the amplitude of symmetrical fault current waveform.

77. Write short notes on Rated Operating Sequence.

This is mechanical duty requirement of circuit breaker operating mechanism. The

sequence of rated operating duty of a circuit breaker has been specified as

O – t – CO - t' - CO

where O indicates opening operation of CB. CO represents closing operation immediately

followed by an opening operation without any intentional time delay. t' is time between two

operations which is necessary to restore the initial conditions and / or to prevent undue heating of

conducting parts of circuit breaker. t = 0.3 sec for circuit breaker intended for first auto re closing

duty, if not otherwise specified.

78. Write short notes on Rated Short Time Current.

The circuit breakers do not clear the short circuit current as soon as any fault occurs in the

system. There always some intentional and an intentional time delays present between the instant

of occurrence of fault and instant of clearing the fault by CB. This delays are because of time of

operation of protection relays, time of operation of circuit breaker and also there may be some

intentional time delay imposed in relay for proper coordination of power system protection. Even

a circuit breaker fails to trip, the fault will be cleared by next higher positioned circuit breaker. In

this case the fault clearing time is longer. Hence, after fault, a circuit breaker has to carry the short

circuit for certain time. The summation of all time delays should not be more than 3 seconds,

hence a circuit breaker should be capable of carrying a maximum faulty current for at least this

short period of time.

79. What are the major effects caused by short circuit current inside a circuit breaker.

The short circuit current may have two major affects inside a circuit breaker.

1. Because of the high electric current, there may be high thermal stress in the insulation

and conducting parts of CB.

2. The high short circuit current, produces significant mechanical stresses in different

current carrying parts of the circuit breaker.

80. Write short notes on Rated Voltage of Circuit Breaker.

Rated voltage of circuit breaker depends upon its insulation system. For below 400 KV

system, the circuit breaker is designed to withstand 10% above the normal system voltage. For

above or equal 400 KV system the insulation of circuit breaker should be capable of withstanding

5% above the normal system voltage. That means, rated voltage of circuit breaker corresponds to

the highest system voltage. This is because during no load or small load condition the voltage

level of power system is allowed rise up to highest voltage rating of the system.

81. Write about Sustained Low Frequency Test.

This test is generally done at power frequency (In India it is 50 Hz & in America it is 60

Hz). This is most commonly used high voltage test, carried on H.V. equipment. This test i.e.

sustained low frequency test are carried out on a specimen of insulating material to determine and

ensure, dielectric strength, dielectric losses of the insulating material. This test is also carried out

on high voltage equipment and high voltage electrical insulators to ensure the dielectric strength

and losses of these equipment and insulators.

82. Write about High Voltage D. C. Test.

High voltage DC test is normally applicable to the those equipments which are used in

high voltage DC transmission system. But this test is also applicable for high voltage AC

equipments, when high voltage AC testing is not possible due to unavoidable condition. For

example mainly at site, after installation of equipments it is quite difficult to arrange for high

voltage alternating power as high voltage transformer may not be available at site. Hence, high

voltage test with alternating power is not possible at site after installation of equipment. At that

situation high voltage DC test is most suitable.

83. Write about High Frequency Test.

The insulators used at high voltage transmission system, may be subjected to breakdown

or flash-over during high frequency disturbances. The high frequency disturbances occurs in the

HV system due to switching operations or any other external causes. High frequency in power

may cause failure of insulators even at comparatively low voltage due to high dielectric loss and

heating. So the insulation of all high voltage equipment must ensure the high frequency voltage

withstanding capacity during its normal life span. Mainly sudden interruption of line current

during switching and open circuit fault, gives rise to the frequency of voltage wave form in the

system.

84. What is process behind Impulse Testing?

There may be great influence of surge or lighting on the transmission lines. These

phenomena can breakdown transmission line insulator and it may also attack, the electrical power

transformer connected at the end of the transmission lines. Surge test or impulse tests are very

high or extra high voltage tests, carried out for investing the influences of surges or lightning on

the transmission equipment.

Normally direct lightening strokes on transmission line is very rare. But when a charged

cloud comes closer to the transmission line, the line is oppositely charged due to the electrical

charge inside the cloud. When this charged cloud is suddenly discharged due to lightning stroke

nearby, the induced charge of the line no longer bound but travel through the line with velocity of

light. So it is understood that even when the lightnings do not strike the transmission conductor,

directly, still there will be a transient over voltage disturbance.

85. Draw the schematic which depicts the construction of vacuum circuit breaker.

86. Draw the equivalent electric circuit of double break oil circuit breaker.

87. Draw the operating characteristics of forced blast oil break circuit breakers.

88. Draw the simple diagram of HVDC circuit breaker.

89. Draw the current waveform which depicts rated short circuit breaking current.

90. Draw the testing circuit of Brown-Boveri’s synthetic testing type.

91. Mention the criteria’s based on which circuit breaker being classified.

The circuit breakers are classified by various ways.The different criteria for classification

of circuit breaker are as follows,

Interrupting medium.

Way of operation.

Method of control.

Tank construction.

92.Mention the applications of air blast circuit brekers.

The air blast circuit breaker has their application in high rating capacities.The air blast

circuit breakers are preferred for arc furnace duty and traction system because they are suitable

for repeated duty.These type of circuit breakers are finding their best application in systems

operating in range of 132 Kv with breaking capacities upto 7000MVA.

93.Draw the schematic diagram of double pressure type SF6 circuit breaker.

94. What are the factors which affecting the performance of plain break oil circuit

breakers?:

The critical length of the break.

The head of oil above the contacts.

The clearance between the live contacts and earthed pressurized tanks.

The speed of the contact movement .In order to extinguish the arc earlier, the speed be

high therefore double break circuit breakers are preferred.

95 . Write short notes on plain explosion pot.

It is a rigid cylinder of insulating material and encloses the fixed and moving contacts. The

moving contact is a cylindrical rod passing through (called throat) at the bottom. When a fault

occurs, the contacts get separated and an arc is struck between them. The principal limitation of

this type of pot is that it cannot be used for very low or for very high fault currents. With low fault

currents, the pressure developed is small, thereby increasing the arcing time. On the other hand,

with high fault currents, the gas is produced so rapidly that explosion pot is liable to burst due to

high pressure. For this reason, plain explosion pot operates well on moderate short-circuit currents

only where the rate of gas evolution is moderate.

96. Write short notes on cross jet explosion pot.

This type of pot is just a modification of plain explosion pot. It is made of insulating

material and has channels on one side which act as arc splitters. The arc splitters help in

increasing the arc length, thus facilitating arc extinction. When a fault occurs, the moving contact

of the circuit breaker begins to separate. As the moving contact is withdrawn, the arc is initially

struck in the top of the pot. The gas generated by the arc exerts pressure on the oil in the back

passage. When the moving contact uncovers the arc splitter ducts, fresh oil is forced across the arc

path. The arc is, therefore, driven sideways into the “arc splitters” which increase the arc length,

causing arc extinction.

97. Write about self compensated explosion pot.

This type of pot is essentially a combination of plain explosion pot and cross jet explosion pot.

Therefore, it can interrupt low as well as heavy short circuit currents with reasonable accuracy.

98. What are the important components common to most of the circuit breakers.

The following are some important components common to most of the circuit breakers

(i) Bushings.

(ii) Circuit breaker contacts.

(iii) Instrument transformers.

(iv) Bus-bars and conductors.

99. Which criteria will be taken into consideration in 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,

o The maximum fault current which is to be interrupted by the breaker.

o The maximum current to be carried momentarily

100. Define inter-turn faults in a generator.

The coils used in the alternators are generally multi-turn coils. So short circuit between the

turns of one coil may occur which is called inter turn fault. This fault occurs due to current surges

with high value of L (di/dt) voltage across the turns. But if the coils used are single turn then this

fault cannot occur. Hence for the large machines of the order of 50MVA and more, it is a normal

practice to use single turn coils. But in some countries multi-turn coils are very commonly used

where protection against inter-turn faults is must.