Study of 6.

6kV
Vacuum Circuit Breaker
Project Report submitted in the partial fulfilment of the requirement for the
degree of
Bachelor of Engineering in Power Engineering

Under the guidance of

Shri. P.K. Yadav

Director
NPTI, Nagpur (WR)

Submitted by:
Ashwina Gharde
Nishtha Sharma
Sinni Pawar
Vaishali Wakde
Yogita Rachchawar

National Power Training Institute, Nagpur (WR)

RTM Nagpur University
1

(April 2016)

CERTIFICATE
This is to certify that the project entitled

STUDY OF 6.6KV VACUUM CIRCUIT BREAKER

is being submitted by Ashwina Gharde, Nishtha Sharma, Sinni Pawar,
Vaishali Wakde, Yogita Rachchawar, in partial fulfilment of B.E. Power
Engineering from National Power Training Institute, Nagpur (WR) to
RTM Nagpur University and is a record of their work carried out
under the guidance of Shri P.K. Yadav.

Shri. P.K. Yadav

Shri. S.I. Mahant

Project Guide

Course Co-ordinator

Director, NPTI(WR)

Dy. Director, NPTI(WR)

Shri. N.C. Moharil

Director, NPTI(WR)

ACKNOWLEDGEMENT
It is a great pleasure and moment of immense satisfaction for us to
express our profound gratitude and indebtedness toward our project guide
Shri P. K. Yadav(Director) whose enthusiasm was a source of great inspiration
to us. We are thankful for the able guidance and untiring attention which he
conferred on us from beginning to completion of the project. We are extremely
grateful to Shri A. G. Vinchurkar, (Principal Director), Shri N. C. Moharil
(Director) for providing an excellent academic climate in institution which has
made this endeavor possible. We also take this opportunity to express our
gratitude to Shri. S. I. Mahant (Course Co-ordinator) for his valuable
suggestion and support.

We also thank officials of Khaparkheda Power Plant, Nagpur for giving

us an opportunity to undergo project training and special thanks to
Shri. Pantavne (Dy. Executive Engineer) for his Guidance throughout the
training.

PROJECT ASSOCIATES:
Ashwina Gharde
Nishtha Sharma
Sinni Pawar
Vaishali Wakde
Yogita Rachchawar

** CONTENTS **
Sr.
No.

Topics

Page
No.

History of Circuit Breaker

Introduction

General construction

15

Description of VM-3 Vacuum Circuit Breaker

20

Interlocks

26

Ratings and Specifications

29

Maintenance

33

Troubleshooting

35

Power Circuitry

40

10

Merits and Demerits

41

11

Index of Diagrams

43

12

References

44

1. History of a Circuit Breaker:

An early form of circuit breaker was described by Thomas Edison in an 1879
patent application, although his commercial power distribution system used
fuses. Its purpose was to protect lighting circuit wiring from accidental short
ciruits and overloads. A modern miniature circuit breaker similar to the ones
now in use was patented by Brown, Boveri & Cie in 1924. Hugo Stotz, an
engineer who had sold his company to BBC, was credited as the inventor on
DRP (Deutsches Reichspatent) 458392. Stotz's invention was the forerunner of
the modern thermal-magnetic breaker commonly used in household load centers
to this day. Interconnection of multiple generator sources into an electrical grid
required development of circuit breakers with increasing voltage ratings and
increased ability to safely interrupt the increasing short circuit currents
produced by networks. Simple air-break manual switches produced hazardous
arcs when interrupting high currents; these gave way to oil-enclosed contacts,
and various forms using directed flow of pressurized air, or of pressurized oil, to
cool and interrupt the arc. By 1935, the specially constructed circuit breakers
used at the Boulder Dam project use eight series breaks and pressurized oil flow
to interrupt faults of up to 2,500 MVA, in three cycles of the AC power
frequency.

2. INTRODUCTION:
2.1 Description:
A circuit breaker is an automatically operated electrical switch designed to
protect an electrical circuit from damage caused by overload or short circuit. Its
basic function is to detect a fault condition and interrupt current flow. Unlike a
fuse, which operates once and then must be replaced, a circuit breaker can be
reset (either manually or automatically) to resume normal operation. Circuit
breakers are made in varying sizes, from small devices that protect an individual
household appliance up to large switchgear designed to protect high voltage
circuits feeding an entire city.

2.2 Operation and Arc Formation:

All circuit breaker systems have common features in their operation. Although
details vary substantially depending on the voltage class, current rating and type
of the circuit breaker.
The circuit breaker must detect a fault condition; in low voltage circuit breakers
this is usually done within the breaker enclosure. Circuit breakers for large
currents or high voltages are usually arranged with protective relay pilot
devices to sense a fault condition and to operate the trip opening mechanism.
The trip solenoid that releases the latch is usually energized by a separate
battery, although some high-voltage circuit breakers are self-contained with
current transformers, protective relays and an internal control power source.
Once a fault is detected, the circuit breaker contacts must open to interrupt the
circuit; some mechanically-stored energy (using something such as springs or
compressed air) contained within the breaker is used to separate the contacts,
although some of the energy required may be obtained from the fault current
itself. Small circuit breakers may be manually operated, larger units have soleno
ids to trip the mechanism, and electric motors to restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive
heating, and must also withstand the heat of the arc produced when interrupting
(opening) the circuit. Contacts are made of copper or copper alloys, silver alloys
6

and other highly conductive materials. Service life of the contacts is limited by
the erosion of contact material due to arcing while interrupting the current.
Miniature and molded-case circuit breakers are usually discarded when the
contacts have worn, but power circuit breakers and high-voltage circuit breakers
have replaceable contacts.

2.2.1 Arc Phenomenon:

When a short circuit occurs, a heavy current flows through 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 and hence rise in temperature. The heat produced in the
medium between the contacts is sufficient to ionize the air or vaporize and
ionize the oil. This acts as a conductor and arc is formed between the contacts.
The potential difference between the contacts is small and just sufficient to
maintain the arc. The arc provides a low resistance path and consequently the
current in the circuit remains uninterrupted till the arc persists.
Now, the current flowing between the contacts depends on the resistance in the
path. This resistance depends on,
a. Degree of Ionization: The arc resistance increases with the increase in the
degree of ionized particles.
b. Length of Arc: The arc resistance increases with the increase in the length
of the arc (also the separation of the contacts).
c. Cross-section of the arc: The arc resistance increases with the decrease in
the X-section of the arc.

2.2.2 Important Characteristics:

a. Arc Voltage: It is the voltage that appears across the contacts of the circuit
breaker during the arcing period. As soon as the contacts of the circuit
breaker separate, the arc is formed. The voltage that appears across the
contacts during the arcing period is called the arc voltage. Its value is low
except for the period the fault current is at or near zero current point. At
current zero the arc voltage rises rapidly to the peak value and this value tries
to maintain the current flow in the arc.
7

b. Restriking Voltage: It is the voltage which appears across the terminals of

each pole of a circuit breaker immediately after the breaking of the circuit
i.e, at current zero.

c. Recovery Voltage: It is the normal frequency r.m.s voltage that reappears at

the poles of a circuit breaker after final arc extinction.
d. Rate of Rise of Restriking Voltage (R.R.R.V): It is the rate expressed in
volts per micro second, representative of the increase of the restriking
voltage.
e. Peak Restriking Voltage: It is the maximum instantaneous voltage attained
by the restriking voltage.

2.3 Arc Extinction:

The various methods of arc extinction are,
2.3.1 High Resistance Method:
In this method the arc resistance is mad to increase with time so that the
current reduces to a value where it's insufficient to maintain the arc. The
disadvantage of this method is that enormous amount of heat is
dissipated. Therefore, it is applied only in d.c. or low capacity a.c. circuit
breaker.
The resistance of the arc may be increased by,
i. Lengthening of the arc: Since resistance is directly proportional to its
length thus the gap between contacts is increased to increase its
length.
8

ii. Cooling of the arc: Cooling helps in the deionization of the medium
between the contacts. This increases the arc resistance. this can be
obtained by a gas blast directed along the arc.
iii. Reducing the X-section area: If the X-section is reduced then the
voltage required to maintain the arc increases. Thus resistance
increases. This can be achieved by allowing the arc to pass through a
narrow opening or by having smaller area of contacts.
iv. Splitting the arc: The resistance ca be increased by splitting the arc
into a number of smaller arcs in series. This can be done by
introducing some conducting plates in between the arcs.
2.3.2 Low Resistance Method:
This method is used for arc extinction in a.c. circuits only. In this method
the resistance is kept low until current turns zero and then arc extinguishes
naturally and is prevented from restriking in spite of the rising voltage across
the contacts.
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
contacts contain ions and electrons so that it has small di-electric strength and
can be easily broken down by the rising voltage called the restriking voltage. If
such a breakdown occurs the arc will persist for another half cycle. But, if
immediately after current zero, the dielectric strength of the medium is built up
more rapidly than the restriking voltage, the arc fails to restrike and the current
will be interrupted. This can be achieved by,
i. causing the ionized particles in the space between to recombine into
neutral molecules
ii. sweeping the ionized particles away and replacing them with unionized
particles.
Thus rapidly deionizing can be done by,
a. Lengthening of the gap: The dielectric depends on the length of the
gap, thus if the contacts are opened rapidly, higher dielectric strength of the
medium can be achieved.

b. High pressure: If pressure in the vicinity is increased the, the density

of the particles constituting the discharge also increases. This increased density
of particles because higher rate of de-ionization and consequently the dielectric
strength of the medium will increase.
c. Cooling: Natural combination of ionized particle take place more
rapidly if they are allowed to cool. Therefore, dielectric strength of the medium
can be increased by cooling.
d. Blast Effect: If the ionized particles between the contacts are swept
away and replaced by unionized particles, the dielectric strength of the medium
can be increased considerably.

2.4 Types of circuit breakers

Many different classifications of circuit breakers can be made, based on their
features such as voltage class, construction type, interrupting type, and
structural features.

2.4.1 Low-voltage circuit breakers:

Low-voltage (less than 1,000 VAC) types are common in domestic, commercial
and industrial application, and include:

a. MCB (Miniature Circuit Breaker): Rated current not more than

100 A. Trip characteristics normally not adjustable. Thermal or thermalmagnetic operation. Breakers illustrated above are in this category.
There are three main types of MCBs:
1. Type B - trips between 3 and 5 times full load current;
2. Type C - trips between 5 and 10 times full load current;
3. Type D - trips between 10 and 20 times full load current.

10

b. MCCB (Molded Case Circuit Breaker): Rated current up to 2,500 A.

Thermal or thermal-magnetic operation. Trip current may be adjustable in larger
ratings.
Low-voltage circuit breakers are also made for DC applications. Direct
current requires special breakers because the arc is continuousunlike an AC
arc, which tends to go out on each half cycle. A direct current circuit breaker
has blow-out coils that generate a magnetic field that rapidly stretches the arc.
Small circuit breakers are either installed directly in equipment, or are arranged
in a breaker panel.

2.4.2 Medium-voltage Circuit Breakers:

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled
into metal-enclosed switchgear line ups for indoor use, or may be individual
components installed outdoors in a substation. Air-break circuit breakers
replaced oil-filled units for indoor applications, but are now themselves being
replaced by vacuum circuit breakers (up to about 40.5 kV). Like the high
voltage circuit breakers described below, these are also operated by current
sensing protective relays operated through current transformers. The
characteristics of Medium-voltage breakers are given by international standards
such as IEC 62271. Medium-voltage circuit breakers nearly always use separate
current sensors and protective relays, instead of relying on built-in thermal or
magnetic overcurrent sensors.
Medium-voltage circuit breakers can be classified by the medium used to
extinguish the arc:

a. Vacuum circuit breakers: With rated current up to 6,300 A, and

higher for generator circuit breakers. These breakers interrupt the current by
creating and extinguishing the arc in a vacuum container - aka "bottle". Long
life bellows are designed to travel the 6-10 mm the contacts must part. These
are generally applied for voltages up to about 40,500 V, which corresponds
roughly to the medium-voltage range of power systems. Vacuum circuit
breakers tend to have longer life expectancies between overhaul than do air
circuit breakers.
11

In such breakers, vacuum (degree of vacuum being in the range of 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
mediums. For eg, when contacts of a breaker are open in vacuum, the
interruption occurs at first current zero with dielectric strength between the
contacts building up at a rate of 1000 times higher than that obtained with other
circuit breaker.
When contacts of a breaker are open in vacuum, an arc is produced between the
contacts by the ionization of metal vapour of contacts. However, the arc is
quickly extinguished because the metallic vapours, electrons and ions produced
during arc rapidly condense on the surface of the circuit breaker contacts
resulting in a quick recovery of dielectric strength as soon as the arc produces in

12

vacuum it is quickly extinguished due to the fast weight of recovery of

dielectric strength.
b. Air circuit breakers: Rated current up to 6,300 A and higher for
generator circuit breakers. Trip characteristics are often fully adjustable
including configurable trip thresholds and delays. Usually electronically
controlled, though some models are microprocessor controlled via an integral
electronic trip unit. Often used for main power distribution in large industrial
plant, where the breakers are arranged in draw-out enclosures for ease of
maintenance.

BULK OIL CIRCUIT BREAKER

SF6 circuit breakers extinguish the arc in a chamber filled with sulfur
hexafluoride gas. Moreover, Bulk Oil and Medium Oil are now phasing out.
Medium-voltage circuit breakers may be connected into the circuit by bolted
connections to bus bars or wires, especially in outdoor switchyards. Mediumvoltage circuit breakers in switchgear line-ups are often built with draw-out
construction, allowing breaker removal without disturbing power circuit
connections, using a motor-operated or hand-cranked mechanism to separate the
breaker from its enclosure. Some important manufacturer of VCB from 3.3 kV
to 38 kV are ABB, Eaton, Siemens, HHI (Hyundai Heavy Industry), S&C
Electric Company, Jyoti and BHEL.

2.4.3 High-voltage circuit breakers:

13

Electrical power transmission networks are protected and controlled by highvoltage breakers. The definition of high voltage varies but in power
transmission work is usually thought to be 72.5 kV or higher, according to a
recent definition by the International Electrotechnical Commission (IEC). Highvoltage breakers are nearly always solenoid-operated, with current sensing
protective relays operated through current transformers. In substations the
protective relay scheme can be complex, protecting equipment and buses from
various types of overload or ground/earth fault.
High-voltage breakers are broadly classified by the medium used to extinguish
the arc.
Bulk oil(phasing out)
Minimum oil(phasing out)
SF6
Due to environmental and cost concerns over insulating oil spills, most new
breakers use SF6 gas to quench the arc. High-voltage AC circuit breakers are
routinely available with ratings up to 765 kV. 1,200 kV breakers were launched
by Siemens in November 2011, followed by ABB in April the following year.
High-voltage direct current circuit breakers are still a field of research as of
2015. Such breakers would be useful to interconnect HVDC transmission
systems.
a. Sulfur hexafluoride (SF6) circuit breakers:
A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur
hexafluoride gas to quench the arc. They are most often used for transmissionlevel voltages and may be incorporated into compact gas-insulated switchgear.
In cold climates, supplemental heating or de-rating of the circuit breakers may
be required due to liquefaction of the SF6 gas.

14

3. GENERAL CONSTRUCTION
The 6.6kV Vacuum Circuit Breaker-type VM metal clad switchgear is of
horizontal draw out pattern suitable for easy extension of switchboard on both
directions for systems up to 12kV. The design incorporates single bus bar
system and a set of interlocks for safety of operations and is fully
compartmentalized.
A panel consists of fixed portion and moving portion of modular construction
having four high voltage chambers namely breaker chamber, bus bar chamber,
CT chamber and pressure relief chamber. Instrument panel is a separate low
voltage chamber. Moving portion comprises of wheel mounted truck fitted with
an operating mechanism, vacuum interrupters and isolating contacts. Motor
operated spring closing mechanism keeps the springs charged after every
closing operation making it ready for the next operation. Springs can also be
charged manually in case of failure of auxiliary power to the spring charging
motor.

15

3.1 Main Breaker Chamber:

The main breaker chamber is a welded steel structure which accepts the moving
portion at the floor level. The isolating contacts are the multi finger type with
the copper fingers silver plated. The isolating contacts assemblies are mounted
on epoxy support insulators and are a self aligning type. This chamber also
includes features like secondary isolating contacts socket, guides for the moving
portion, earthing contact which mates with an earthing strip on the moving
portion and safety shutters.
The moving portion of the VCB can be kept either in the SERVICE or TEST
position inside the breaker chamber. The front door can be kept closed giving a
neat and flush appearance and making the switchgear dust and vermin proof.
Also the position of moving portion can be seen through a glass window on the
door. The control cables enter at the front compartment of this chamber or
elsewhere as per specific requirement.

16

3.2 Instrument and Relay Chamber:

The instrument and relay chamber is of folded sheet steel construction with a
hinged instrument panel suitable for flush mountings at instruments in the front
to provide better access for maintenance etc. A removable cover is provided on
the top of the instrument and relay chamber.
17

3.3 Busbar Chamber:

The bus bar chamber is of welded steel construction and houses the bus bars
which consist of multiple parallel aluminum bars or copper bars supported on
epoxy support insulators.
The bus bars are of air insulated bar type and are easy to erect needing no
special techniques of insulation for jointing. The bus bar chamber is provided
with bolted covers at the top and back. The insulated barriers provide partition
between two adjacent bus bar chambers. In the rear side a compartment is
provided for mounting additional relays and fitments.

3.4 C T Chamber:
The Cable and termination chamber is also of welded angle iron construction
with ample space for cable termination and current transformers of various
types.
Access to various CTs can be made by opening the bolted back cover which
can be removed without disturbing the HT cables.

3.4 Moving Portion:

The moving portion consists of a truck frame with four wheels on which three
vacuum interrupters and the operating mechanism are mounted. The interrupters
are mounted on epoxy support insulators and are shielded from each other by
means of insulating barriers.

3.5 Secondary Plug and Socket Arrangement:

A standard 32 pin plug and socket arrangement is provided on every panel. This
meets all the requirements of low voltage connections between the cubicle and
the truck. The plug is assembled at the end of a flexible conductor hose
provided in the instrument and relay chamber. An interlocking link is assembled
in the plug body. The socket is always mounted on the truck top. Two spring
loaded interlocking pins are assembled in the socket mounting bracket. The pins
when kept in free position allow the interlocking link of plug to push the
18

blocking levers and the plug can be inserted in to the socket. The plug is locked
over the socket by pressing down the flap over the pins on the socket body. The
interlocking pins shall then be brought to the locked positions so that the
interlocking link is on plug body is kept in its place. The pin not brought into
locked position will not allow the movement of truck further inside the cubicle
as in the free position the projection of pins will foul with the position plate in
the breaker chamber. Also when the truck is at the SERVICE or TEST position
it is not possible to disengage the plug from the socket.
The interlocking pins can be brought to free position only before the TEST
position when the truck is being pushed in. the blocking levers do not allow the
interlocking pins to be brought to locked position unless the plug with
interlocking link is engaged with socket.

19

4. Description of VM-3 Vacuum Circuit Breaker

The VM3AF vacuum circuit breakers are designed to handle all recognised
switching duties. The Breakers are extremely reliable in service, required only a
minimum of maintenance and have long life expectancy. Moreover, their Small
size and weight, quiet and low vibrations operation and the fact that they are not
affected by temperature or represent a fire risk enable the Breakers to be used in
locations subject to adverse conditions.
The three breaker poles, each with its vacuum interrupters, are mounted on a
common mechanism housing. The energy storing spring mechanism is motor
operated and can be actuated by hand also.

4.1 Construction:
The construction of vacuum breaker is shown in figure 3. The breaker polls are
fixed to the rear of the mechanism housing by 2 cast resin insulators, each end
fitted with phase barriers.
The insulated support are either aluminium castings or are made of sheet Steel,
depending upon the rated normal current and rated short circuit breaking
current.
The pole terminals and are designed for direct bus connections.
The energy storing mechanism and all the control and actuating devices are
installed in the mechanism housing. The Breakers can be operated by hand and
also electrically by means of solenoids.
The ON/OFF indicator, the spring charging indicator and the operation counter
are fitted on the front of the mechanism housing.

4.2 Breaker Pole:

The vacuum interrupter is rigidly fixed by the by the upper terminal to pole
support. The lower ceremic part of the interrupter is established against lateral
forces by a centring ring on pole support. The external forces due to switching
operations and the contact pressure are absorbed by the insulating Struts.
20

4.3 Current Part Assembly:

The current part assembly consists of upper terminal and pole support the fixed
contacts and the moving contact stem is connected by the terminal clamp to
Copper flexible.

4.4 Vacuum Interrupter:

The basic construction can be seen in figure 4. The moving contact moves in
guide. The metal bellows follows the travel of the moving contact and seals the
interrupter against the surrounding atmosphere.

4.5 Arc Quenching Principle:

When the contact separate, the current to be interrupted initiates the metal
vapour arc discharge and flows through the plasma until next current zero. The
arc is then extinguished and the conductive metal vapour condenses on the
metal surfaces within a matter of microseconds. As a result, the dielectric
strength in the break builds up very rapidly.
The contacts are so designed that the self-generated field causes the arc to
travel. This prevents the local overheating of the contacts when interrupting
large currents.
The metal vapour arc discharge can only be maintained if a certain minimum
current flows. A current that does not attain this level is chopped prior to current
zero. This chopping current is kept to a minimum in order to prevent unduly
high over voltage build up with inductive circuits are switched. The use of
special contact material ensures that the current chopping is limited to low
value.
The Rapid build of the dielectric strength in the break enables sucks to be safely
extinguished even if the contact separation occurs immediately prior to the
current zero.
The arc drawn in the vacuum breaker is not cooled. The metal vapour plasma is
highly conductive and the resulting arc voltage only attains values between 20
to 200 volts. For this reason and because of the short arcing times the arc energy
21

developed in the break is very small. This also account for the long electrical
life expectancy of the vacuum interrupter. Owing to the high vacuum (<109 bar)
in the interrupter contact clearances of only about 6 to 20 mm depending upon
the rated voltage are needed in order to attain a high dielectric strength.

4.6 Switching Operation:

When closing command is initiated the charged closing spring actuates the
moving contact through breaker shaft, lever, insulated coupler and drive link.
The forces that occur when the movement of the insulated coupler is converted
into the vertical motion of the moving contacts are absorbed by the drive link,
which pivots on the pole Support and adaptor. During closing, the tripping
Springs and the contact pressure springs are charged and latched by Pawl.
The recharging of the closing spring takes place automatically immediately after
closing if the supply of motor is on, otherwise hand charging can be done with
the help of manual charging handle. In the closed condition the contact pressure
springs and atmospheric pressure, maintained the necessary contact pressure.
The contact pressure springs automatically compensates for arc erosion which is
very small
When at ripping command is given, the energy stored in the tripping and contact
pressure springs is released by the Pawl. The opening sequence is similar to
closing. The Residue forces of the tripping springs arrests the moving contacts
in the open position .

22

4.7 Operating Mechanism:

The motor operated spring closing mechanism of vacuum circuit breaker is of
modular design. It contain separate models for all of the functions required for
switching operation. As well as for indication and controls. Each module can be
dismantled and install easily without it being necessary to carry out any
adjustments

4.8 Mode of Operation:

The operating mechanism is of stored energy types that is the charging of
spring is not followed automatically by the closing opening or opening breaker
contacts. When the Stored energy mechanism is charge, the instant of operation
can be chosen as desired. The mechanical energy for carrying out and openclose - open sequence for auto reclosing duty is stored in the closing and
tripping springs.

23

a. Charging of the Closing Spring with Circuit Breaker in Open

Condition:
It shows the operating mechanism linkage position prior to closing spring
Charging when the circuit breaker is an open conditions. The charging shaft is
supported in the charging mechanism but is not couple mechanically with the
charging mechanism. Fitted to it are the crank at 1 end the camp together with
liver at the other
When the charging mechanism is actuated by hand or by Motor, the flange
turns until the driver locate in the cut away part of CAM disc thus causing the
spring charging shaft to follow. The crank charges the closing springs. when
this has been full tensioned, the crank actuates the linkage for the closing
spring charged indicator via and also the limit switches for interrupting the
motor supply. When the closing spring is charged, cam Disk follows idly that is
it is bought in 2 position for closing. figure 6 shows the final position after the
closing spring is fully charged and the breaker is an open condition.

b. Closing Operation:
After the completion of charging operation of the closing springs as explained
in (A) the breaker is ready for closing. If the breaker is to be closed locally the
spring is released by pressing ON button fig 6. In case of remote operation the
closing Solenoid unlatched the closing spring.
As the closing spring is under discharge process, the charging shaft is turned by
crank. The cam disc at the other end of the charging shaft actuates the driver
lever with the results that break shaft is turned by lever via coupling rod. At the
same time, lever, fixed on the breaker Shaft operate the three insulated coupler
for the breaker poles. Lever charges the tripping springs during closing and the
breaker is latch in the closed position by Lever with Pawl roller and bipawl
The crank on the charging shaft moves linkage and thus the closing spring
charged indication is cancelled by the limit switches in the motor supply
circuit. Manual recharging of the closing spring is also possible immediately

24

c. Opening Operations:
If the breaker is to be trip locally, the spring is released by pressing the OFF
button. In the case of an electrical command being given, the tripping solenoid
unlatches the tripping spring . The tripping spring turn the breaker shaft via
lever the sequence being similar to that for closing

d. Manual Operation for Spring Charging:

Normally the closing spring is charged by an electrical Motor. A built in feature
also facilitates manual charging of spring if the power supply to Motors fails.
This is achieved by inserting the hand crank in the opening and turning in clock
wise until the indicators show the closing sprint charged.
The hand crank couples with charging mechanism via a trip -free facility the
operator is thus not expose to any risk if the control supply recover during the
manual spring charging operation.

MANUAL OPERATING MECHANISM

25

5. Interlocks
The various interlocks provided on the breaker ensure that:
a. The VCB truck cannot be racked in or out unless circuit breaker is in
open condition.
b. The VCB truck cannot be read racked in unless secondary plug and
socket are engaged.
c. The circuit breaker closing operation is not possible unless secondary
plug and socket are engaged.
d. The secondary plug and socket cannot be disengaged when the VCB
truck is not in service test, or any intermediate position between these
two positions.
e. The circuit breaker closing operation is not possible unless the truck is in
Service or Test position.
f. The interlock mechanism cannot be operated unless the circuit breaker is
an Open condition.
g. Inter changeability of trucks of different current ratings is not possible.
5.1 Interlock features:
As explain in general description, of system of full proof interlocks has been
provided to ensure safety of operation. The interlocking features are described
below:
a. Main Interlock:
This consists of interlock shaft assembly and Cam. The cams mounted on either
side of the shaft prevents the movement of the breaker truck unless the
operating sequence is in order. Also the cam moves the breaker trucks by
approximately 35 mm in the last stage of plugging in or in the beginning of
withdrawal. The cams in the logged vertical position at as locks for the breaker
trolley in the service position and prevent throwing out of truck during short
circuit. Rotation of the interlock shaft also actuates the interlocking linkage
which allow the circuit breaker closing operation only at specific locations of
the truck in the fixed portion. During the withdrawal of truck from fixing
26

housing, interlocking feature takes care that this is possible only when the
circuit breaker is an open condition.
b. Truck Stops:
To ensure the right matching of moving and fix positions, that is the rating
wise/function wise, truck stops are provided on the moving and fixed portions.
The projected portion of the stop on the moving potion fouls with the stop on
fixed portion on attempt of inserting a wrong moving portion.
5.2 Operation of interlocks:
a. Isolation of breaker from service position:
For isolating circuit breaker from the service position it must first be tripped.
Inside the handle in to the hole of interlock shaft assembly. Move the handle
upward it comes to Free position. During the process the truck with VCB will
get pushed out disengaging Isolating contacts. The truck can now be further
pulled out manually. The truck will get stopped at Test position. In this
position the shutters with close there by physically separating the breaker
chamber from the other high voltage compartments. Remove the handle and
close the door if the truck is to be left in this position.
At this position no load operation of the breaker can be carried out. For this
rotate the inter lock shaft to Locked condition
For withdrawal of truck out of the cubicle from Test position check
Wheel covers are slided out.
Interlock is in Locked position.
Closing the tripping springs are free.
Secondary plug and socket is disconnected. Breaker will be
in-operative without secondary connection and interlock
shaft indicating locked position. For operation of breaker
outside the cubicle use jumper connections.

27

b. To insert the truck:

The truck can be pushed in only with the breaker in tripped position. Make sure
that the interlock is in lock position. Open the front door and slide out the
wheel covers. Bring the truck in front of the breaker chamber. The wheels
initially get aligned by welded guide strips at the floor level. Push the truck till
the interlock cam is stopped at the stop pin of the breaker chamber. This is the
test position of the truck inside the housing. Now rotate the interlock shafts to
free position. Engage the secondary plug and socket.
Push the truck further inside till the interlock cam is stopped by interlock block.
During this movement the truck is guided by the guide rollers and the shutters
get lifted up to open the entry of isolating contacts of HV chamber.
Now on rotating the interlock handle to the locked position, the truck will get
moved further into service position now the breakers can be closed.

28

6. RATINGS AND SPECIFICATIONS

1
2

3
4

5
6
7
8
9
10
11
12

Time required to charge the closing spring

Does the provision exist for immediate
recharging of closing spring after closing of
breaker
Whether o-co operating is possible with one
charging of spring
Mechanical indicator
Breaker service/ test
Breaker open/close
Closing spring charged/discharged
Whether operating counter is provided
Auxiliary contacts
Rated voltage
Continuous current
Making current
Breaking current
Maximum number of (NO+NC) contacts
mounted on the truck
Maximum number of NO+NC mounted on
panel (Actuated in service position only)

13
14

Whether the contacts are changeover type

Whether (NO+NC) contacts actuated with
spring charging mechanism provided

Whether contacts are silver plated

Thickness of silver plating and contact load

on each finger
Type of operating mechanism
Closing

29

15 second
Yes

Yes

No
Yes
Yes
Yes
On moving position
220 V
15 A
15
10 resistance
1 inductance
6NO+6NC
VAJC relay to provided
having 6NO+6NC
contacts
No
Yes
(4 changeover contacts)

Vacuum interrupter
design is proprietary of
BHEL Bangalore
Not applicable for V
interrupter
Motor charged spring

Tripping
4
5

Whether fixed trip or trip free

Antipumping feature trip free

Does provision exist for operating breaker

During control
Supply failure
Closing
Tripping

Whether slow operating of breaker is

possible for maintenance
Rated voltage of closing/trip coil variation in
voltage permissible
Variation in voltage permissive
Closing
Tripping
Power required at rated supply voltage
Closing coil
Trip coil
Whether manual trip/close is provided for
emergency operation
Spring charging motor
Voltage
Watts
RPM
Class of insulation

8
9

10

11
12

30

close
Spring operated
Trip free
Yes antipumping plug in
relay provided
Manual closing
Push
Button
Manual emergency trip
push
Button accessible with
door
Yes
220 V DC

85-100%
No entry 70-110%
200 W
200 W
Yes

220 V DC
600 W appox.
8000
E

1. Breaking capacity
2. Making capacity
3. Short- time capacity

6.1 Breaking Current Capacity:

The breaking current capacity of a circuit-breaker is a term used to express the
highest current that the circuit breaker is capable of breaking at a stated
recovery voltage and a stated reference restriking voltage under prescribed
conditions of use and behavior. If the current is symmetrical it is referred to as
symmetrical breaking capacity, whereas if the current is asymmetrical the
breaking capacity is referred to as asymmetrical breaking capacity.
The breaking capacity of a circuit breaker is of two two types
Symmetrical breaking capacity
Asymmetrical breaking capacity
Symmetrical breaking current capacity: It is the rms value of the ac
component of the fault current that the circuit breaker is capable of breaking
under specified condition of recovery voltage.
Asymmetrical breaking current 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.
6.2 Making Current Capacity:
The possibility of a circuit-breaker breaker being closed on a dead short must be
taken account of. The capacity of a circuit breaker to make current depends
upon its ability to withstand the effect of. The capacity of a circuit breaker to
make current depends upon its ability to withstand the effect of electromagnetic
forces. The maximum force in any phase is a function of the square of the peak
making capacity.

31

6.3 Short Time Capacity:

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 is cleaning the fault. The rated short-time
current is the rms value of the current that the circuit breaker can carry safety
for a specified short period.

6.4 Rating of Circuit Breaker:

Rated voltage
Rated frequency
Rated symmetrical breaking current
Rated asymmetrical breaking current
Rated making current
Rated short time current (1sec)
Rated operating duty

32

17.8kV
50Hz
20.4kA
25.5kA
52kA
20.4sec
0-3mm-co-3mm-co.

6. Maintenance of Vacuum Circuit Breaker

A vacuum circuit breaker needs less maintenance as compare to other
breakers and no inflammable substance that supports fire compared with that of
oil in the oil circuit breaker.
The Table shows the types of Maintenance needed for vacuum circuit breaker.

Sr.No
1.

2.

Classification
General

Vacuum

Check point

Method of Inspection

Cleaning for every

point or every
component

Clean the circuit breaker to remove the dust

and dirt. The envelope of the vacuum
interrupter and insulating materials should
wiped with a clean cloth.

Contact wear

Check the contact for wear replace if the

contact piece has worn out.

Judgement of
vacuum

Withstand test at 22kv ac for one minute.

Lubrication of each Apply small amount of oil to rotary

component
sections, sliding surfaces,pins,coupling
section etc. use grease for the operating
shaft and bearings and machine oil for the
parts operating shaft and bearing and
machine oil for the parts.
3.

Opening
mechanism

Coupling section
Springs

Check for rust and wear

Visual check for rust and deformation

Clamping section

Check for loosened bolts and nuts broken

springs washers and snap pins, their
incoming off, etc.

33

Control circuit

Check for loosened connections, rust

damaged lead wires.

Control Switches

Check whether the contacts are damaged

by arcing.

6.1 Checks for Maintenance:

Visual inspection of the breaker shall be connected systematically and
checks shall be made for any loose or damaged parts.
Perform slow closing operation as suitable for better operation of the
breaker spring.
Contact erosion on the interrupter shall be checked by locating the point
below the Circuit Breaker in Closed Position.
Check the vacuum of the interrupter.
Mechanism shall be lubricated with grease/oil as per the instruction
manual.
Electrical operation of the breaker shall be checked either by using test
cabinet or by putting the truck in test position of the metal clad enclosure.

34

7. Trouble Shooting
Sr.no Trouble
1.
Motor not
running

2.

Reasons
a)Improper wiring

Remedial action
a) Check motor circuit wiring as per
schematics drawing and tighten. The
loose connection found.

b) Supply not available

b) Check incoming supply to the

motor at rectifier terminal. If not
available then restore incoming
supply.

c) Failure of motor
cutoff switch

c) Check limit switch operation if

found defective then replace limit
switch.

d) Failure of Rectifier

d) Check supply at incoming terminal

if available then check at outgoing
terminal. If supply is not available
replace rectifier.

e) Failure of motor

e) Check supply at motor terminal if

available and motor is not running,
replace the motor.

f) Gearbox jam
Motor cutoff not taking
place

f) Even after replacing motor, motor is

not running it indicates gearbox is
jammed / drawing more current
replace gear box.

Motor keeps on
Trip latch setting
running after
disturbed
fully charging the
springs .

Check the opening arrangement of

motor cut off switch . if not
functioning properly replace the lever
fitted on the LHS top of the gear box.
Adjust the setting of stud provided on
tripping mechanism to restore the
latching by rotating 1 & 2 turn
35

3.

maximum check the tripping operation

at minimum operating voltage 5 times
,if unable to do setting contact BHEL.

Trip latch does

not hold the
vacuum circuit
breaker in closed
position

7.1 Inspections, Checks and Test Without Control Power

Vacuum circuit breaker are normally shipped with their primary contact
open and their springs discharged however, it is critical to first verify the
discharged condition of the springs loaded mechanism after the de- energizing
contact power.

a. Spring Discharged Check:

Perform the spring discharge check before removing the circuit breaker
from the pallet or removing from the switchgear.
The spring discharged check consist of simply performing the following
tasks in the order given. This check assures that both the opening and closing
springs are fully discharged.
1.
2.
3.
4.
5.

Press red open push button

Press black close push button
Again press red open push button
Verify springs condition indicator show discharged
Verify main contact status indicator shows open

b. Manual Spring Charging Check

1. Insert the manual spring charging crank into the manual change handle
socket.
2. Turn the crank clockwise until the spring conductor indicator shows the
closing spring charged.
3. Repeat the spring discharged check.
36

4. Verify that the spring are discharged and the circuit breaker primary
contact open by indicator positions.

c. Automatic Spring Charging Test

The automatic spring charging features of the circuit breaker must be
checked. Control power is required for automatic spring charging to take place
1. Open the control power circuit by opening the controls power
disconnect device.
2. Energize (close) the control power circuit disconnect.
3. Use the close & open controls to first close & then open the circuit
breaker contacts. Verify the contacts position visually by observing
the open/closed indicator on the circuit breaker.
4. De-energize control power by repeating step-1.
Disconnect the plug jumper from the switchgear first and next from
the circuit breaker.
5. Perform the spring discharged check open. Verify that the closing
spring is discharged and the primary contacts of the circuit breaker are
open.

d. Final Mechanical Inspection without Control Power

1. Make a final mechanical inspection of the circuit breaker verify
that the contacts are in the open position and the closing spring is
discharged.
2. Verify mechanical condition of springs.
3. Check for loose hardware.

7.2 Tests Performed:

a. Checking the Contact Erosion:
Checking the contact erosion in the vacuum interrupter can be done in simple
manner. After taking out the VCB mounted truck from the switchboard, check
the position of the white mark A on the interrupter moving contact. Wear is
37

still within the permissible limits as long as this mark can be seen when the
contacts are in closed position. In the event, the mark is not visible the
interrupter needs a replacement.

b. Checking the Vacuum:

Before putting the breaker in service, or an interrupter is suspected of leaking
because of mechanical damage, the vacuum may be checked as follows:
Open and isolate the breaker and detach the insulated coupler from lever.
The atmospheric pressure will force the moving contact of a hermetically
sealed interrupter into close position. A vacuum interrupter may be
assumed to be intact if it shows the following characteristics:
An appreciable closing force has to be overcome when lever is moved to
open position by hand. When the lever is released, it must automatically
return to the close position and the contacts must be heard to close.
After checking the vacuum, the lever should be refitted to the insulated
coupler.

38

c. Lubrication of Operating Mechanism:

The main points to be lubricated with centoplex 24 DL or Molykote.
All the points not marked (bearings, articulated joints and auxiliary switch)
should be treated with Molykote spray. To lubricate the mechanism points
detach its cover. Lubricate all the appropriate points, starting at the top left and
working through systematically. Parts that are not rigidly fixed should be moved
slightly to and fro to let the oil penetrate.
After lubrication, the breaker shall be operated several times to test it.
Articulated joints and bearings that cannot be dismantled should not be cleaned
with a cleaning agent prior to being oiled.

39

9. Power Circuitry

40

10. Merits of Vacuum Circuit Breakers

The vacuum switchgear has been successfully developed and is gaining rapid
popularity. The vacuum switches are likely to be popular for wide range of
applications. These switches devices have several merits such as :
1. VCB is self-contained and does not need filling of gas or oil. They do not need
auxiliary air system, oil handling system, etc. No need for periodic refilling.
2. No emission of gases, pollution free.
3. Modest maintenance of the breaker, no maintenance of interrupters. Hence
economical over long period.
4. Breakers forms a unit which can be installed at any required orientation.
Breaker unit is compact and self contained.
5. Non-explosive
6. Silent operation.
7. Large number of operation on load, or short circuit. Suitable for repeated duty.
8. Long life of the order of several hundred operations on rated normal current.
9. Constant dielectric. There are no gas decomposition products in vacuum and the
hermetically sealed vacuum interrupter keeps out all environmental effect.
10.Constant contact resistance. In vacuum, the contacts cannot be oxidized, a fact
which ensures that their very small resistance is maintained through their life.
11.High total current switched. Since contact piece erosion is small, rated normal
interrupted current is up to 30.000 times; and rated short circuit breaking current
is on the average of a hundred times.
The above reasons, together with the economic advantages offered, have
boosted acceptance of the vacuum arc quenching principle.

41

Demerits of Vacuum Circuit Breakers

1. The vacuum interrupter is more expensive than the interrupter devices in other
types of interrupters and its cost is affected by production volume. It is
uneconomical to manufacturer vacuum interrupters in small quantities.
2. Rated voltage of single interrupter is limited until very recently to about
36/3 = 20 KV above 36 KV, two interrupters are required to be connected in
series. This makes the breaker uneconomical for voltage rated above 36 KV.
3. Vacuum interrupters required high technology for production.
4. In the event of loss of vacuum, due to transient damage or failure, the entire
interrupter is rendered useless. It cannot be required at site.
5. For interrupter low magnetizing currents, in certain range, additional surge
suppressors are required in parallel with phase of a VCB.

42

11. Index of Diagrams

Page No.

Name of Diagram

Circuit Breaker

Graph of Restriking Voltage

12

Vacuum Interrupter

13

Bulk Oil Circuit Breaker

14

SF6 Circuit Breaker

15

Side View of Vacuum Circuit Breaker

16

Sectional View of Vacuum Interrupter

17

Operating Mechanism - - Closing spring charged and

Breaker in Open Position

23

Operating Mechanism - Closing spring charged and

Breaker Closed

25

Manual Operating Mechanism

38

Contact Erosion Test

40

Power Circuitry

43

12. References:
1. Operation and Maintenance Manual Indoor Metalclad Vacuum Circuit
Breaker
2. Power System Engineering Soni, Gupta, Chakrobarty & Bhatnagar
3. Switchgear protection and Power Systems - S. Rao
4. BHEL Manuals
5. NPTI Manuals
6. Wikipedia.org

44