Fuse Circuit Breaker

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UNIT - 1

SWITCHES AND FUSES

Switches and fuses: Introduction, energy management of power system

Definition of switchgear,

Switches - isolating, load breaking and earthing

Introduction to fuse, fuse law, cut -off characteristics

Time current characteristics, fuse material, HRC fuse, and liquid fuse

pplication of fuse

!nergy demand management, also known as demand side management "D#$%, is the

modification of consumer demand for energy through &arious methods such as financial

incenti&es and education' (sually, the goal of demand side management is to encourage the

consumer to use less energy during peak hours, or to mo&e the time of energy use to off-peak

times such as nighttime and weekends' )eak demand management does not necessarily decrease

total energy consumption, but could be e*pected to reduce the need for in&estments in networks

and+or power plants'

The term D#$ was coined during the time of the ./ energy crisis and . energy crisis'

!lectricity use can &ary dramatically on short and medium time frames, and the pricing system

may not reflect the instantaneous cost as additional higher-cost "0peaking0% sources are brought

on-line' In addition, the capacity or willingness of electricity consumers to ad1ust to prices by

altering demand "elasticity of demand% may be low, particularly o&er short time frames' In many

markets, consumers "particularly retail customers% do not face real-time pricing at all, but pay

rates based on a&erage annual costs or other constructed prices'

2arious market failures rule out an ideal result' 3ne is that suppliers4 costs do not include all

damages and risks of their acti&ities' !*ternal costs are incurred by others directly or by damage

to the en&ironment, and are known as e*ternalities' Theoretically the best approach would be to

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Switch #ea% and "%otection 1&EE$'



add e*ternal costs to the direct costs of the supplier as a ta* "internali5ation of e*ternal costs%'

nother possibility "referred to as the second-best approach in the theory of ta*ation% is to

inter&ene on the demand side by some kind of rebate'

!nergy demand management acti&ities should bring the demand and supply closer to a percei&ed

optimum'

6o&ernments of many countries mandated performance of &arious programs for demand

management after the ./ energy crisis' n early e*ample is the 7ational !nergy Conser&ation

)olicy ct of .8 in the ('#', preceded by similar actions in California and 9isconsin in .:'

Switch #ea%

In an electric power system, switchgear is the combination of electrical disconnect switches,

fuses or circuit breakers used to control, protect and isolate electrical equipment' #witchgear is

used both to de-energi5e equipment to allow work to be done and to clear faults downstream'

This type of equipment is important because it is directly linked to the reliability of the electricity

supply'

The &ery earliest central power stations used simple open knife switches, mounted on insulating

panels of marble or asbestos' )ower le&els and &oltages rapidly escalated, making opening

manually operated switches too dangerous for anything other than isolation of a de-energi5ed

circuit' 3il-filled equipment allowed arc energy to be contained and safely controlled' ;y the

early <=th century, a switchgear line-up would be a metal-enclosed structure with electrically

operated switching elements, using oil circuit breakers' Today, oil-filled equipment has largely

been replaced by air-blast, &acuum, or #>? equipment, allowing large currents and power le&els

to be safely controlled by automatic equipment incorporating digital controls, protection,

metering and communications'

High &oltage switchgear was in&ented at the end of the th century for operating motors and

other electric machines' The technology has been impro&ed o&er time and can be used with

&oltages up to ,== k2'

Typically, the switchgear in substations is located on both the high &oltage and the low &oltage

side of large power transformers' The switchgear on the low &oltage side of the transformers may

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be located in a building, with medium-&oltage circuit breakers for distribution circuits, along



with metering, control, and protection equipment' >or industrial applications, a transformer and

switchgear line-up may be combined in one housing, called a uniti5ed substation or (##'

Fuse

In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a

sacrificial de&ice to pro&ide o&ercurrent protection, of either the load or source circuit' Its

essential component is a metal wire or strip that melts when too much current flows, which

interrupts the circuit in which it is connected' #hort circuit, o&erloading, mismatched loads or

de&ice failure are the prime reasons for e*cessi&e current'

fuse interrupts e*cessi&e current "blows% so that further damage by o&erheating or fire is

pre&ented' 9iring regulations often define a ma*imum fuse current rating for particular circuits'

3&ercurrent protection de&ices are essential in electrical systems to limit threats to human life

and property damage' >uses are selected to allow passage of normal current plus a marginal

percentage and to allow e*cessi&e current only for short periods' #low blow fuses are designed to

allow harmless short term higher currents but still clear on a sustained o&erload' >uses are

manufactured in a wide range of current and &oltage ratings and are widely used to protect

wiring systems and electrical equipment' #elf-resetting fuses automatically restore the circuit

after the o&erload has cleared@ these are useful, for e*ample, in aerospace or nuclear applications

where fuse replacement is impossible'

' safety de&ice that protects an electric circuit from becoming o&erloaded' >uses contain

a length of thin wire "usually of a metal alloy% that melts and breaks the circuit if too

much current flows through it' They were traditionally used to protect electronic

equipment and pre&ent fires, but ha&e largely been replaced by circuit breakers'

<' cord of readily combustible material that is lighted at one end to carry a flame along its

length to detonate an e*plosi&e at the other end'

Const%uction

fuse consists of a metal strip or wire fuse element, of small cross-section compared to the

circuit conductors, mounted between a pair of electrical terminals, and "usually% enclosed by a

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non-combustible housing' The fuse is arranged in series to carry all the current passing through



the protected circuit' The resistance of the element generates heat due to the current flow' The

si5e and construction of the element is "empirically% determined so that the heat produced for a

normal current does not cause the element to attain a high temperature' If too high a current

flows, the element rises to a higher temperature and either directly melts, or else melts a soldered

1oint within the fuse, opening the circuit'

The fuse element is made of 5inc, copper, sil&er, aluminum, or alloys to pro&ide stable and

predictable characteristics' The fuse ideally would carry its rated current indefinitely, and melt

quickly on a small e*cess' The element must not be damaged by minor harmless surges of

current, and must not o*idi5e or change its beha&ior after possibly years of ser&ice'

The fuse elements may be shaped to increase heating effect' In large fuses, current may be

di&ided between multiple strips of metal' dual-element fuse may contain a metal strip that

melts instantly on a short - circuit, and also contain a low-melting solder 1oint that responds to

long-term o&erload of low &alues compared to a short-circuit' >use elements may be supported

by steel or nichrome wires, so that no strain is placed on the element, but a spring may be

included to increase the speed of parting of the element fragments'

The fuse element may be surrounded by air, or by materials intended to speed the quenching of

the arc' #ilica sand or non-conducting liquids may be used'

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Cha%acte%istic pa%a+ete%s and Fuse aw



ma*imum current that the fuse can continuously conduct without interrupting the circuit'

Speed: The speed at which a fuse blows depends on how much current flows through it and the

material of which the fuse is made' The operating time is not a fi*ed inter&al, but decreases as the

current increases' >uses ha&e different characteristics of operating time compared to current,

characteri5ed as fast-blow, slow-blow, or time-delay, according to time required to respond to an

o&ercurrent condition' standard fuse may require twice its rated current to open in one second,

a fast-blow fuse may require twice its rated current to blow in =' seconds, and a slow-blow fuse

may require twice its rated current for tens of seconds to blow'

>use selection depends on the load4s characteristics' #emiconductor de&ices may use a fast or

ultrafast fuse as semiconductor de&ices heat rapidly when e*cess current flows' The fastest

blowing fuses are designed for the most sensiti&e electrical equipment, where e&en a short

e*posure to an o&erload current could be &ery damaging' 7ormal fast-blow fuses are the most

general purpose fuses' The time delay fuse "also known as anti-surge, or slow-blow% are designed

to allow a current which is abo&e the rated &alue of the fuse to flow for a short period of time

without the fuse blowing' These types of fuse are used on equipment such as motors, which can

draw larger than normal currents for up to se&eral seconds while coming up to speed'

The I't aue: The amount of energy spent by the fuse element to clear the electrical fault' This

term is normally used in short circuit conditions and the &alues are used to perform co-ordination

studies in electrical networks' I<t parameters are pro&ided by charts in manufacturer data sheets

for each fuse family' >or coordination of fuse operation with upstream or downstream de&ices,

both melting I<t and clearing I<t are specified' The melting I<t, is proportional to the amount of

energy required to begin melting the fuse element' The clearing I<t is proportional to the total

energy let through by the fuse when clearing a fault' The energy is mainly dependent on current

and time for fuses as well as the a&ailable fault le&el and system &oltage' #ince the I<t rating of

the fuse is proportional to the energy it lets through, it is a measure of the thermal damage and

magnetic forces that will be produced by a fault'

!%ea.in# capacit/: The breaking capacity is the ma*imum current that can safely be interrupted

by the fuse' 6enerally, this should be higher than the prospecti&e short circuit current' $iniature

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fuses may ha&e an interrupting rating only = times their rated current' #ome fuses are

designated High Rupture Capacity "HRC% and are usually filled with sand or a similar material'



>uses for small, low-&oltage, usually residential, wiring systems are commonly rated, in 7orth

merican practice, to interrupt =,=== amperes' >uses for larger power systems must ha&e higher

interrupting ratings, with some low-&oltage current-limiting high interrupting fuses rated for

/==,=== amperes' >uses for high-&oltage equipment, up to :,=== &olts, are rated by the total

apparent power "mega&olt-amperes, $2% of the fault le&el on the circuit'

0ated ota#e: 2oltage rating of the fuse must be greater than or equal to what would become

the open circuit &oltage' >or e*ample, a glass tube fuse rated at /< &olts would not reliably

interrupt current from a &oltage source of <= or </= 2' If a /< 2 fuse attempts to interrupt the

<= or </= 2 source, an arc may result' )lasma inside that glass tube fuse may continue to

conduct current until current e&entually so diminishes that plasma re&erts to an insulating gas'

Rated &oltage should be larger than the ma*imum &oltage source it would ha&e to disconnect'

Rated &oltage remains same for any one fuse, e&en when similar fuses are connected in series'

Connecting fuses in series does not increase the rated &oltage of the combination "nor of any one

fuse%'

$edium-&oltage fuses rated for a few thousand &olts are ne&er used on low &oltage circuits,

because of their cost and because they cannot properly clear the circuit when operating at &ery

low &oltages'

ota#e d%op: &oltage drop across the fuse is usually pro&ided by its manufacturer' Resistance

may change when a fuse becomes hot due to energy dissipation while conducting higher

currents' This resulting &oltage drop should be taken into account, particularly when using a fuse

in low-&oltage applications' 2oltage drop often is not significant in more traditional wire type

fuses, but can be significant in other technologies such as resettable fuse "))TC% type fuses'

Te+pe%atu%e de %atin#: mbient temperature will change a fuse4s operational parameters'

fuse rated for at <: AC may conduct up to =B or <=B more current at = AC and may open

at 8=B of its rated &alue at == AC' 3perating &alues will &ary with each fuse family and are

pro&ided in manufacturer data sheets'

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Fuse 2ate%ias

>uses come in a &ast array of si5es and styles to ser&e in many applications, manufactured in



standardi5ed package layouts to make them easily interchangeable' >use bodies may be made of

ceramic, glass, plastic@ fiberglass, molded mica laminates, or molded compressed fiber depending

on application and &oltage class'

2utipe fuse hode%s

Cartridge "ferrule% fuses ha&e a cylindrical body terminated with metal end caps' #ome cartridgefuses are manufactured with end caps of different si5es to pre&ent accidental insertion of the

wrong fuse rating in a holder, gi&ing them a bottle shape'

>uses for low &oltage power circuits may ha&e bolted blade or tag terminals which are secured

by screws to a fuse holder' #ome blade-type terminals are held by spring clips' ;lade type fuses

often require the use of a special purpose e*tractor tool to remo&e them from the fuse holder'

Renewable fuses ha&e replaceable fuse elements, allowing the fuse body and terminals to be

reused if not damaged after a fuse operation'

>uses designed for soldering to a printed circuit board ha&e radial or a*ial wire leads' #urface

mount fuses ha&e solder pads instead of leads'

High-&oltage fuses of the e*pulsion type ha&e fiber or glass-reinforced plastic tubes and an open

end, and can ha&e the fuse element replaced'

#emi-enclosed fuses are fuse wire carriers in which the fusible wire itself can be replaced' The

e*act fusing current is not as well controlled as an enclosed fuse, and it is e*tremely important to

use the correct diameter and material when replacing the fuse wire, and for these reasons these

fuses are slowly falling from fa&or' These are still used in consumer units in some parts of the

world, but are becoming less common' 9hile glass fuses ha&e the ad&antage of a fuse element

&isible for inspection purposes, they ha&e a low breaking capacity which generally restricts them

to applications of : or less at <:= 2C' Ceramic fuses ha&e the ad&antage of a higher

breaking capacity, facilitating their use in circuits with higher current and &oltage' >illing a fuse

body with sand pro&ides additional cooling of the arc and increases the breaking capacity of the

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fuse' $edium-&oltage fuses may ha&e liquid-filled en&elopes to assist in the e*tinguishing of the

arc' #ome types of distribution switchgear use fuse links immersed in the oil that fills the

equipment'



>use packages may include a re1ection feature such as a pin, slot, or tab, which pre&ents

interchange of otherwise similar appearing fuses' >or e*ample, fuse holders for 7orth merican

class RE fuses ha&e a pin that pre&ents installation of similar-appearing class H fuses, which

ha&e a much lower breaking capacity and a solid blade terminal that lacks the slot of the RE

type'

>use wire Cu 9ire

rating "% diameter

"mm%

/ =':

: ='<=

= ='/:

: =':=

<= ='?=

<: ='.:

/= ='8:

: '<:

?= ':/

8= '8

== <'=

H0C Fuse

It is a high rupturing capacity cartridge type of fuse' It is one of the simplest form of fuse which

is used for distribution purposes' The low and uncertain breaking capacity of semi closed fuses is

o&ercome in HRC >uses'

Const%uction: The body of this fuse is of heat resisting ceramic with metal end caps and is of

cylindrical shape' ;etween end caps, the fi*ed elements are mounted, which are welded to the

end caps' The fuse element is generally sil&er, attached between the fi*ed elements' The body



space surrounding the fuse is completely filled with quart5 sand, plaster of paris or marble dust'

The filling powder material is selected such that its chemical reaction with sil&er &apour forms

&ery high resistance substance'



4pe%ation

The &arious steps in the operation of the HRC >use are

' 3ccurrence of fault or short circuit

<' Increase in current through fuse element to high &alue

/' $elting of sil&er element

' 2apori5ation of the sil&er element

:' >usion of the sil&er &apour and formation of high resistance substance

?' !*tinction of arc

The electrical phenomena associated with the operation of the HRC >use are



' >ormation of high resistance substance due to chemical reaction of sil&er &apour with

filling powder

<' s current is cut off, the high resistance gets con&erted to an insulator like glass beads /'

Creation of transient &oltage at the instant of breaking fault current The physical



phenomena

include the rise in temperature and generation of high internal pressure on the interruption of

fault current'

Cut off cha%acte%istics

Appications of H0C fuse

The main applications of HRC >use are to protect the low &oltage distribution system against the

o&erload and short circuit conditions' >or the back up protection to circuit breakers' )rotection of meshed feeders with the steady load'



UNIT 7 '

"0INCI"8ES 4F CI0CUIT !0EA9E0S



)rinciples of circuit breakers: Introduction, requirement of a circuit breakers

Difference between an isolator and circuit breaker

basic principle of operation of a circuit breaker, phenomena of arc, properties of arc,initiation and maintenance of arc,

arc interruption theories - slepianFs theory and energy balance theory,

Re striking &oltage, reco&ery &oltage, Rate of rise of Re striking &oltage,

DC circuit breaking, C circuit breaking, current chopping, capacitance switching,resistance switching

Rating of Circuit breakers'

Int%oduction

9here fuses are unsuitable or inadequate, protecti&e relays and circuit breakers are used in

combination to detect and isolate faults' Circuit breakers are the main making and breaking

de&ices in an electrical circuit to allow or disallow flow of power from source to the load' These

carry the load currents continuously and are e*pected to be switched 37 with loads "making

capacity%' These should also be capable of breaking a li&e circuit under normal switching 3>>

conditions as well as under fault conditions carrying the e*pected fault current until completely

isolating the fault side "rupturing+breaking capacity%' (nder fault conditions, the breakers should

be able to open by instructions from monitoring de&ices like relays' The relay contacts are used

in the making and breaking control circuits of a circuit breaker, to pre&ent breakers getting

closed or to trip breaker under fault conditions as well as for some other interlocks'

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"u%pose of ci%cuit %ea.e%s ;switch#ea%<

The main purpose of a circuit breaker is toG

#witch load currents



$ake onto a fault

;reak normal and fault currents

Carry fault current without blowing itself open "or up% i'e' no distortion due to magnetic

forces under fault conditions'

The i+po%tant cha%acte%istics f%o+ a p%otection point of iew a%e:

The speed with which the main current is opened after a tripping impulse is recei&ed

The capacity of the circuit that the main contacts are capable of interrupting'

The first characteristic is referred to as the ‘tripping time’ and is e*pressed in cycles' $odern

high-speed circuit breakers ha&e tripping times between three and eight cycles' The tripping or

total clearing or break time is made up as followsG

3pening timeG The time between instant of application of tripping power to the instant of

separation of the main contacts'

= rcing timeG The time between the instant of separation of the main circuit breaker contacts to

the instant of arc e*tinction of short-circuit current'

Total break or clearing time

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The second characteristic is referred to as Jrupturing capacityF and is e*pressed in $2' The

selection of the breaking capacity depends on the actual fault conditions e*pected in the system

and the possible future increase in the fault le&el of the main source of supply' In the earlier

chapters we ha&e studied simple e*amples of calculating the fault currents e*pected in a

system' These simple calculations are applied with standard ratings of transformers, etc', to

select the appro*imate rupturing capacity duty for the circuit breakers'

0e>ui%e+ent of ci%cuit

%ea.e%s Int%oduction

s already seen in the last chapter, whene&er any fault occurs in the power system then that part

of the system must be isolated from the remaining healthy part of the system' This function is

accomplished by circuit breakers' Thus a circuit breaker will make or break a circuit either

manually or automatically under different conditions such as no load, full load or short circuit'

Thus it pro&es to be an effecti&e de&ice for switching and protection of different parts of a power

system' In earlier days fuse was included in' the protecti&e system' ;ut due to some limitations

they are not used in practice now a day' The main difference between a fuse and circuit breaker

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is that under fault condition the fuse melts and it is to be replaced whereas the circuit breaker Gan

close or break the circuit without replacement'

Requirements of Circuit ;reakerG The power associated with the circuit breakers is large and it

forms the link between the consumers and suppliers' The necessary requirements of circuit



breakers are as follows, ' The normal working current and the short circuit current must be

safely interrupted by the circuit breaker' <' The faulty section of the system must be isolated by

circuit breaker as quickly as possible keeping minimum delay' / It should not operate with flow

of o&ercurrent during healthy conditions' ' The faulty circuit only must be isolated without

affecting the healthy one'

!asic p%incipe of ope%ation of a ci%cuit %ea.e%

The >ig' #hows the elementary diagram of a circuit breaker' It consists of two contacts a fi*ed

contact and a mo&ing contact' handle is attached at the end of the mo&ing contact' It can be

operated manually or automatically' The automatic operation needs a separate mechanism which

consists of a trip coil' The trip coil is energi5ed by secondary of current transformer' The

terminals of the circuit breaker are bought to the supply'

!asic action of ci%cuit %ea.e%

(nder normal working conditions the e'm'f produced in the secondary winding of the

transformer is insufficient to energi5e the trip coil completely for its operation' Thus the contacts

remain in closed position carrying the normal working current' The contacts can be opened

manually also by the handle' (nder abnormal or faulty conditions high current in the primary

winding of the current transformer induces sufficiently high e'm'f in the secondary winding so

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that the trip coil is energi5ed' This will start opening motion of the contacts' This action will not

be instantaneous as there is always a time lag between the energi5ation of the trip circuit and the

actual opening of the contacts' The contacts are mo&ed towards right away from fi*ed contact' s

we ha&e seen already the separation of contacts will not lead to breaking or interruption of circuit

as an arc is struck between the contacts' The production of arc delays the current interruption and



in addition to this it produces large amount of heat which may damage the system or the breaker'

Thus it becomes necessary to e*tinguish the arc as early as possible in minimum time, so that

heat produced will lie within the allowable limit' This will also ensure that the mechanical

stresses produced on the parts of circuit breaker are less the time inter&al which is passed in

between the ener&ation of the trip coil to the instant of contact separation is called the opening

time' It is dependent on fault current le&el' The time inter&al from the contact separation to the

e*tinction of arc is called arcing time It depends not only on fault current but also on a&ailability

of &oltage for maintenance of arc and mechanism used for e*tinction of arc'

"heno+ena of a%c, p%ope%ties of a%c, initiation and +aintenance of a%c

Fo%+ation of an A%c: (nder faulty conditions hea&y current flows through the contacts of the

circuit breaker before they are opened' s soon as the contacts start separating, the area of

contact decreases which will increase the current density and consequently rise in the

temperature' The medium between the contacts of circuit breaker may be air or oil' The heat

which is produced in the medium is sufficient enough to ioni5e air or oil which will act as

conductor' Thus an arc is struck between the contacts' The p'd' between the contacts is sufficient

to maintain the arc' #o long as the arc is remaining between the contacts the circuit is said to be

uninterrupted' The current flowing between the contacts depends on the arc resistance' 9ith

increase in arc resistance the current flowing will be smaller' The arc resistance depends on

following factors,

a% Degree of ioni5ationG If there is less number of ioni5ed particles between the contacts

then the arc resistance increases'

b% Kength of arcG The arc resistance is a function of length of arc which is nothing but

separation between the contacts' $ore the length more is the arc resistance'

c% Cross-section of arcG If the area of cross-section of the arc is less then arc resistance is large'

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Initiation of rc There must be some electrons for initiation of an arc when fault occurs

circuit breaker contacts start separating from each other and the electrons are emitted

which are produced by following methods' ;y high &oltage gradient at the cathode,

resulting in field emission by increase of temperature resulting in thermionic emission' ;y

High 2oltage 6radient s the mo&ing contacts start separating from each other, the area

of contact and pressure between the separating contacts decreases' high fault current



causes potential drop "of the order %between the contacts which will remo&e the electrons

from cathode surface' This process is called field emission'

;y Increase of Temperature 9ith the separation of contacts there is decrease in contact

area which will increase the current density and consequently the temperature of the

surface as seen before which will cause emission of electrons which is called thermal

electron emission' In most of the circuit breakers the contacts are made up of copper

which is ha&ing less thermionic emission'

$aintenance of an rc In the pre&ious section we ha&e seen the initiation of the arc by

field emission emission' The electrons while tra&elling towards anode collide with

another electron to dislodge them and thus the arc is maintained' The ioni5ing is lactated

by,

i% High temperature of the medium around the contacts due to high current densities'

Thus the

kinetic energy gained by mo&ing electrons is increased'

ii% ii% The increase in kinetic energy of mo&ing electrons due to the &oltage gradient which

dislodge more electrons from neutral molecules' iii% The separation of contacts of circuit

breaker increases

the length of path which will increase number of neutral molecules' This will decrease

the

density of gas which will increase free path mo&ement of the electrons'

A%c E?tinction It is essential that arc should be e*tinguished as early as possible' There

are two methods of e*tinguishing the arc in circuit breakers which are namely,

a% High resistance method b% Kow resistance or current 5ero method

Hi#h 0esistance 2ethod In high resistance method the arc resistance is increased with

time' This will reduce the current to such a &alue which will be insufficient to maintain

the arc thus the current is interrupted and the arc is e*tinguished' This method is

employed in only d'c circuit'

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"a#e '1


The resistance of the arc may he increased by lengthening the arc, cooling the arc, reducing the



cross-section of the arc and splitting the arc' These methods will be discussed in detail later in

this chapter'

8ow 0esistance 2ethod The low resistance or current 5ero method is employed for arc

e*tinction in ac' circuits' In this method arc resistance is kept low until current 5ero where

e*tinction of arc takes place naturally and is pre&ented from restriking' This method is employed

in many of the modern a'c' circuit breakers'

8ow 0esistance o% @e%o "oint E?tinction

This method is used in ac' arc interruption' -I he current becomes 5ero two tires in a cycle' #o at

each current 5ero point the arc &anishes for small instant and again it appears' ;ut in au*illary

circuit breakers the arc is interrupted at a current 5ero point' The space between the contacts is

ioni5ed quickly if there is fresh unioni5ed medium such as oil or fresh air or #>, gas between the

contacts at current 5ero point' This will make dielectric strength of the contact space to increasesuch that arc will be interrupted and discontinued after current 5ero' This action produces high

&oltage across the contacts which are sufficient to reestablish the arc' Thus the dielectric strength

must be building more than the restricting &oltage for faithful interruption of arc' Then the arc is

e*tinguished at ne*t current 5ero' 9hile designing the circuit breakers the care is taken so as to

remo&e the hot gases from the contact space immediately after the arc' #o that it can be filled by

fresh dielectric medium ha&ing high dielectric strength' In summary we can say that the arc

e*tinction process is di&ided in thee parts, a% rcing phase b% Current 5ero phase c% )ost arc

phase In arcing phase, the temperature of the contact space is increased due to the arc' The heat

produced must be remo&ed quickly by pro&iding radial and a*ial flow to gases' The arc can not

be broken abruptly but its diameter can be reduced by the passage of gas o&er the arc' 9hen a*'

Current wa&e is near its 5ero, the diameter of the arc is &ery less and consequently arc is

e*tinguished' This is nothing but current .ero phase' 7ow in order to a&oid the reestablishment

of arc, the contact space must be filled with dielectric medium ha&ing high dielectric strength'

This is post arc phase in which hot gases are remo&ed and fresh dielectric medium is introduced'

rc Interruption Theories There are two main theories e*plaining current 5ero interruption of arc

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I< 0ecoe%/ 0ate Theo%/ o% Sepians Theo%/



'< Ene%#/ aance theo%/ o% Cassies Theo%/

Sepians Theo%/ #lepian described the process as a race between the dielectric strength and

restriking &oltage' fter e&ery current 5ero, there is a column of residual ioni5ed gas' This may

cause arc to strike again by de&eloping necessary restriking &oltage and this &oltage stress is

sufficient to detach electrons out of their atomic orbits which releases great heat' #i in this theory

rate at which positi&e ions and electrons recombine to form neutral molecules is compared with

rate of rise of restriking &oltage' Due to recombination dielectric strength of gap gets reco&ered'

#o rate of reco&ery of dielectric strength is compared with rate of rise of restriking &oltage' If the

restriking &oltage rises more rapidly than the dielectric strength, gap space breaks down and arc

strikes again and persists' In the >ig' a% Rate of dielectric strength is more than restriking &oltage'

b% Rate of dielectric strength is less ------ -= than rate of rise of restriking &oltage' The

assumption made while de&eloping this theory is that the restriking &oltage and rise of dielectric

strength are comparable quantities which is not quite correct the second drawback is that thetheory does not consider the energy relations in the arc e*tinction' The arcing phase is not

co&ered by this theory so it is incomplete'

Cassies Theo%/ lternati&e e*planation of abo&e process s afforded by Cassie4s theory or also

called !nergy balance theory' Cassie suggested that the reestablishment of arc or interruptions of

an arc both are energy balance process' If the energy input to an arc continues to increase, the arc

restrikes and if not, arc gets interrupted' Theory makes the following assumptions

a% rc consists of a cylindrical column ha&ing uniform temperature at its cross section' The

energy distributed in the column is uniform

b% The temperature remains constant'

Dept of EEE, S!IT "a#e '3


c% The cross section of the arc ad1usts itself to accommodate the arc current'



d% )ower dissipation is proportional to cross sectional area of arc column interruption theories -

slepianFs theory and energy balance theory'

;reakdown occurs if power fed to the arc s more than power loss' The theory is true for high

currents' Immediately after current 5ero, contact space contains ioni5ed gas and therefore has a

finite post 5ero resistance' 7ow there is rising restriking &oltage' This rising res' triking &oltage

causes a current to flow between the contacts' Due to this current flow, power gets dissipated as

heat in the contact space of circuit breaker' Initially when restriking &oltage is 5ero,

automatically current and hence power is 5ero' It is again 5ero when the space has become fully

deioni5e and resistance between the contacts is infinitely high' In between these two e*treme

limits, power dissipated rises to a ma*imum' If the heat so generated e*ceeds the rate at which

heat can be remo&ed from contact space, ioni5ation will persist and breakdown will occur, gi&ing

an arc for another half cycle'

0e -st%i.in# ota#e, %ecoe%/ ota#e, 0ate of %ise of 0e st%i.in# ota#e

T%ansient 0ecoe%/ ota#e The transient reco&ery while has effect on the beha&ior of circuit

breaker' This &oltage appears between the contacts immediately after final arc interruption' This

causes high dielectric stress between the contacts' If this dielectric strength of the medium

between the contacts does not build up faster than the rate of rise of the transient reco&ery

&oltage then the breakdown takes place which will cause restriking of arc Thus it is &ery

important that the dielectric strength of the contact space must build &ery rapidly that rate of rise



of transient reco&ery &oltage so that the Interruption of current by the circuit breaker takes place

successfully' The rate of rise of this transient &oltage depends on the circuit parameters and the



type of the switching duty in&oked' The rate of building up of the dielectric strength depends on

the effecti&e design of the interrupter and the circuit breaker' If it is desired to break the

capaciti&e currents while opening the capacitor banks, there may appear a high &oltage across the

contacts which can cause re ignition of the arc after initial arc e*tinction' Thus if contact space

breaks down within a period of one fourth of a cycle from initial arc e*tinction the phenomenon

is called Reigniting' $o&ing contacts of circuit breakers mo&e a &ery small distance from the

fi*ed contacts then reigniting may occur without o&er&oltage' ;ut the arc gets e*tinguished in

the ne*t current 5ero by which time mo&ing contacts should be mo&ed by sufficient distance

from fi*ed contacts' Thus the re ignition is in a way not harmful as it will not lead to any

o&er&oltage beyond permissible limits' If the breakdown occurs after one fourth of a cycle, the

phenomenon is called Restrike' In restriking, high &oltage appear across the circuit breaker

contacts during capaciti&e current breaking' In restrikes, &oltage will go on increasing which may

lead to damage of circuit breaker' Thus the circuit breakers used for capacitors should be free

from Restrike I'e' they4 should ha&e adequate rating'

Effect of Diffe%ent "a%a+ete%s on T%ansient 0ecoe%/ ota#e ;T0< s seen from the

pre&ious section, after the final current, 5ero high frequency transient &oltage appears across the

circuit breaker poles which is superimposed on power frequency system &oltage and tries to

restrike the arc' This &oltage may last for a few tens or hundreds of microseconds' If the shape of

this TR2 is seen on the oscilloscope then it can be seen that it may be oscillatory, non-oscillatory

or a combination of two depending upon the characteristics of the circuit and the circuit breaker'

The wa&eform is as shown in the >ig'



Transient &oltage #hape of transient reco&ery &oltage This &oltage has a power frequency

component and an oscillatory transient component' The oscillatory component is due to

inductance and capacitance in the circuit' The power frequency component is due to the system



&oltage' This is shown in the >ig'

Lero power factor If we consider 5ero power factor currents, the peak &oltage ! is impressed on

the circuit breaker contacts at the current 5ero instant This instantaneous &oltage gi&es more

transient and pro&ides high rate of rise of TR2' Hence if the p'f' is low then interrupting of such

current is difficult'

0ecoe%/ ota#e s seen pre&iously it is the &oltage ha&ing normal power frequency which

appears after the transient &oltage'

Effect of 0eactance D%op on 0ecoe%/ ota#e Home fault is taking place let us consider that

the &oltage appearing across circuit breaker is 2' s the fault current increases, the &oltage drop

in reactance also increases' fter fault clearing the &oltage appearing say 2< is slightly less than

2,' The system takes some time to regain the original &alue'

ffect of rmature Reaction on Reco&ery 2oltage $e short circuit currents are at lagging power

factor' These lagging p'f' currents ha&e a demagneti5ing armature reaction in alternators'Thus the

induced end of alternators decreases To regain the original &alue this emf takes some time' Thus

the frequency component of reco&ery &oltage is less than the normal &alue of system &oltage'

Dept of EEE, S!IT "a#e '$


DC ci%cuit %ea.in#, AC ci%cuit %ea.in#

D'C' Circuit ;reaking The breaking in case of d'c' can be e*plained as follows' >or this, we will

consider a circuit which will consist of generator with &oltage !, resistance R' inductor K and the



circuit breaker as shown in the >ig'

The &oltage-current relationship can be represented as shown in the graph it could be seen that

cur&e ; represents the &oltage ! - iR, i is nothing but current at any instant' The cur&e MN

represents the &oltage-current characteristics of the arc for decreasing currents'

2oltage-current relationship

9hen the circuit breaker starts opening it carries the load current I' In the graph shown the

current is shown to be reduced respecti&ely' #ection represents &oltage drop i/R whereas qs

represent arc &oltage which is greater than a&ailable &oltage' The arc becomes unstable and the

difference in &oltage is supplied by inductance K across which the &oltage is K' >or decreasing

&alues of t currents this &oltage is negati&e and according to Ken54s law it tries to maintain the

arc'

Dept of EEE, S!IT "a#e '(


The &oltage across inductance K is seen to be positi&e in the region of currents i, mid i< since the

arc characteristics lies below the cur&e ;' The arc current in this region tries to increase so

interruption of current is not possible in this region' fterwards the arc is lengthened with

Increase in contact separation which will raise the arc &oltage abo&e the cur&e ;' The operation



in case of d'c' circuit breakers is said to be ideal if the characteristics of the arc &oltage are abo&e

the cur&e ; e&en in the region of currents i and i<' This is shown in the fig''

>ig' rc &oltage characteristics

It can be seen that arc &oltage is greater than ! - lit and the balance between the &oltages is

supplied by the &oltage across the inductance el, which is proportional to d i rate of change of current dI'

Thus the function of the circuit breaker is to raise the arc characteristics without affecting its

stability' This is done by reducing the arcing time which is the time from contact separation to

final e*tinction of arc' ;ut it will increase e*tinction &oltage' Hence compromise between arcing

time and arc e*tinction &oltage is made'

ABCB Ci%cuit !%ea.in# There is a difference between breaking in case of d c' and ac' circuits' In

ac' circuits the current passes through 5ero twice in one complete cycle' 9hen the currents are

reduced to 5ero the beakers are operated to cut-off the current' This will a&oid the striking of the

arc' ;ut this conditions is difficult to achie&e and &ery much e*pensi&e' The restriking of arc

when current is interrupted is dependent on the &oltage between the contact gap at that instant

which will in turn depend on power factor' Higher the power factor, lesser is the &oltage

appearing across the gap than its peak &alue'

Dept of EEE, S!IT "a#e ')


Cu%%ent choppin#, capacitance switchin#, %esistance switchin#



In power systems capacitor banks are used in the network which supplies reacti&e power at

leading power factors there are &arious aspects like long transmission where it is required

interrupt the capaciti&e current which is difficult' To understand this difficulty let us consider a

simple circuit shown in the >ig

The &alue of load capacitance CK is greater than C' The &oltage across a capacitor cannot change

instantaneously' The currents supplied to the capacitor are generally small and interruption of

such currents take place at first current 5ero' lso at the beginning, the rate of rise of reco&ery

&oltage is low and increases slowly' 9hene&er such circuit is opened a charge is trapped in the

capacitance Ct The &oltage across the load capacitance will hold the same &alue when circuit

was opened' This &oltage is making but peak of supply &oltage as power factor angle is nearly

=A leading' fter opening the circuit the &oltage 2c across the capacitance C oscillates andapproaches a new steady &alue' ;ut due to small &alue of capacitance C' the &alue attained is

close to the supply &oltage' The reco&ery &oltage Cr is nothing but difference between and CK'

Its initial &alue is 5ero as the circuit breaker will be closed and increases slowly in the beginning'

9hen 2c re&erses after half cycle, the reco&ery &oltage is about twice the normal peak &alue'

Therefore it is possible that at this instant arc may restrike as the electrical strength between the

circuit breaker contacts is not sufficient' The circuit will be reclosed and et oscillates at a high

frequency' The supply &oltage at this instant will be at its negati&e peak@ therefore a high

frequency oscillation takes place' t the instant of rest rucking the arc, the reco&ery &oltage 2, is

5ero' The &oltage across the load capacitance reaches - times the peak &alue of normal supply

&oltage' The reco&ery &oltage then starts increasing' If again restriking of arc takes place, a high

frequency of oscillation of CK takes place' #uch se&eral repetitions of the restriking cycle will

increase the &oltage across load capacitance to a dangerously high &alue' In practice this &oltage

is limited to times the normal peak of the &oltage'

Dept of EEE, S!IT "a#e '*




0esistance switchin#

Resistance #witching It can be seen from pre&ious sections that the interruption of lowinducti&ecurrents, interruption of capaciti&e currents g9e rise to se&ere &oltage oscillations'

These e*cessi&e &oltage surges during circuit interruption can be pre&ented by the use of shunt

resistance R across the circuit breaker contacts' This process is known as Resistance #witching'

9hen the rtsistance is connected across the arc, a part of the arc current flows through the

resistance' This will lead to decrease in arc current and increase in rate of deioni5ation of the are

path and resistance of arc' This will increase current through shunt resistance ' This process

continues until the current through the arc is di&erted through the resistance either !*ternal ' ---

resistance completely or in ma1or part' If C irt the small &alue of the current remains in the arc

then the path ' becomes so unstable that it is >*ed $o&ed switch easily e*tinguished' contact

contact ' The resistance may be automatically switched in and arc current can be transferred' The

time required for this action is &ery small s shown in'' >ig the arc first appears across points

and ; which is then transferred across and C' The shunt resistance also ensures the effecti&e

damping of the high frequency re-striking >ig' &oltage transients' This is shown in the >ig'

Dept of EEE, S!IT "a#e 3&




!ehaio% unde% faut conditions

;efore the instant of short-circuit, load current will be flowing through the switch and this can be

regarded as 5ero when compared to the le&el of fault current that would flow



1BA%c

The arc has three partsG ' Cathode end ;7e<: There is appro*imately /=O:= 2 drop due to

emission of electrons'

'B A%c cou+n: Ioni5ed gas, which has a diameter proportional to current' Temperature can

be in the range of ?===O<: === AC'



3B Anode end ;e<: 2olt drops =O<= 2'

9hen short-circuit occurs, fault current flows, corresponding to the network parameters' The

breaker trips and the current are interrupted at the ne*t natural current 5ero' The network reacts

by transient oscillations, which gi&es rise to the transient reco&ery &oltage "TR2% across the

circuit breaker main contacts'

ll breaking principles in&ol&e the separation of contacts, which initially are bridged by a hot,

highly conducti&e arcing column' fter interruption at current 5ero, the arcing 5one has to be

cooled to such an e*tent that the TR2 is o&ercome and it cannot cause a &oltage breakdown

across the open gap' Three critical phases are distinguished during arc interruption, each

characteri5ed by its own physical processes and interaction between system and breaker'

Hi#h cu%%ent phase

This consists of highly conducti&e plasma at a &ery high temperature corresponding to a low

mass density and an e*tremely high flow &elocity' )roper contact design pre&ents the e*istence

of metal &apor in the critical arc region'

The%+a phase

;efore current 5ero, the diameter of the plasma column decreases &ery rapidly with the decaying

current but remains e*istent as an e*tremely thin filament during the passage through current

5ero' This thermal phase is characteri5ed by a race between the cooling of the rest of the plasma

and the reheating caused by the rapidly rising &oltage' Due to the temperature and &elocity

difference between the cool, relati&ely slow a*ial flow of the surrounding gas and the rapid flow

in the hot plasma core, &igorous turbulence occurs downstream of the throat, resulting in

Dept of EEE, S!IT "a#e 3'


effecti&e cooling of the arc' This turbulence is the dominant mechanism, which determines

thermal re-ignition or interruption'

Dieect%ic phase

fter successful thermal interruption, the hot plasma is replaced by a residual column of hot, but

no longer electrically conducting medium' Howe&er, due to marginal ion-conducti&ity, local



distortion of the electrical field distribution is caused by the TR2 appearing across the open

break' This effect strongly influences the dielectric strength of the break and has to be taken into

account when designing the geometry of the contact arrangement'

Introduction, requirement of a circuit breakers, difference between an isolator and circuit breaker,

basic principle of operation of a circuit breaker, phenomena of arc, properties of arc, initiation

and maintenance of arc, arc interruption theories - slepianFs theory and energy balance theory, Re

striking &oltage, reco&ery &oltage, Rate of rise of Re striking &oltage, DC circuit breaking, C

circuit breaking, current chopping, capacitance switching, resistance switching, Rating of Circuit

breakers'



CI0CUITS !0EA9E0S

Ai% Ci%cuit %ea.e%s O ir break and ir blast Circuit breakers

oil Circuit breakers O #ingle break, double break, minimum 3C;

#>? breaker - )reparation of #>? gas



)uffer and non )uffer type of #>? breakers

2acuum circuit breakers - principle of operation and constructional details

d&antages and disad&antages of different types of Circuit breakers



T/pes of ci%cuit %ea.e%s

The types of breakers basically refer to the medium in which the breaker opens and closes' The

medium could be oil, air, &acuum or #>?' The further classification is single break and double

break' In a single break type only the busbar end is isolated but in a double break type, both

busbar "source% and cable "load% ends are broken' Howe&er, the double break is the mostcommon and accepted type in modern installations'



A%c cont%o deice: breaker consists of mo&ing and fi*ed contact, and during the breaker

operation, the contacts are broken and the arc created during such separation needs to be

controlled' The arc control de&ices, otherwise known as tabulator or e*plosion pot achie&es thisG

' Turbulence caused by arc bubble'

<' $agnetic forces tend to force main contacts apart and mo&ement causes oil to be sucked

in through ports and squirted past gap'

/' 9hen arc e*tinguished "at current 5ero%, ioni5ed gases get swept away and pre&ents

prestriking of the arc



Ai% %ea. switch#ea%

Interrupting contacts situated in air instead of any other artificial medium rc is chopped into anumber of small arcs by the rc-#hute as it rises due to heat and magnetic forces' The air circuit breakers are normally employed for /8=P8= 2 distribution'

1&EE$

Ai% %ea. switch#ea%

These types of circuit breakers are used in earlier days for the &oltage ranges of k& to ==k2'

t the bottom there is a tank which is called air reser&oir with the &al&es' 3n this reser&oir there

are three hollow insulator columns' 3n the top of each insulator column there is double arc

e*tinguishing chamber' The current carrying parts are connected to the arc e*tinction chambers in

series' The assembly of entire arc e*tinction chamber is mounted on insulators as there e*ists

large &oltage between the conductors and air reser&oir' The double arc e*tinction chamber is

shown separately in the >ig below' It can be seen that for each circuit breaker pole there are si*

break as there are three double arc e*tinction poles in series' !ach arc e*tinction chamber consists of two fi*ed and two mo&ing contacts' These contacts can mo&e a*ially so as to open or

close' The position depends on air pressure and spring pressure' The opening rod is operated by

when it gets control signal "may be electrical or prtearnatic%' This will lead to flow of high

pressure air by opening the &al&e' The high pressure air enters the double arc e*tinction chamber

rapidly' Due to the flow of air the pressure on mo&ing contacts increases than spring pressure and

contacts open The contacts tra&el through a small distance against the spring pressure' Due to the



motion of mo&ing contacts the port for outgoing air is closed and the whole arc e*tinction

chamber is filled with high pressure air' ;ut during the arcing period the air passes through the

openings shown and takes away ioni5ed air of arc' In case of making operation the &al&e is

turned which connects hdlow column of insulator and the reser&oir' The air is passed to the

atmosphere due to which pressure of air in the chamber is dropped to atmospheric pressure and

closing of mo&ing contacts is achie&ed against spring pressure'

Wo%.in#: n au*iliary compressed air system is required by this type of circuit breaker' This

will supply air to the air reser&ior of the breaker' During the opening operation, the air is allowed

to enter in the e*tinction chamber which push', away mo&ing contacts' The contacts are



separated and the blast of air will take ioni5ed gases with it and helps in e*tinguishing the arc'

Adanta#es: The &anous ad&antages of air blast circuit breakers are, i% 7o fire ha5ards are

possible with this type of circuit breaker' ii% The nigh speed operation is achie&ed' iii% The time

for which arc persists is short' Thus the arc gets e*tinguished early' i&% s arc duration is short

and consistent, the amount of heat released is less and the contdct points are burnt to a less

e*tent' #o life of circuit breaker is increased' &% The e*tinguishing medium in this type of circuit

breaker is compressed air which is supplied fresh at each operation' The arc energy at each

operation is less than that compared with oil circuit breaker' #o air blast circuit breaker is most

suitable where frequent operation is required' &i% This type of circuit breaker is almost

maintenance free' &ii% It pro&ides facility of high speed reclosure' &iii% The stability of the system

can be well maintained'

Disadanta#es: The &arious disad&antages of air blast circuit breakers are, i% If air blast circuit

breaker is to be used for frequent operation it is necessary to ha&e a compressor with sufficientcapacity of high prewure air' ii% The maintenance of compressor and other reated equipments is

required' iii% There is possibility of air leakages at the pipe fittings' i&% It is &ery sensiti&e to

restriking &oltage' Thus current chopping may occur which may be a&oided by employing

resistance switching'

Appications GThe 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 /< k2 to == k2 with breaking capacities upto .==

$2' This will require only one or two cycles' There are two ma1or types - cross blast and a2ial

blast'



Ai% !%ea.; ci%cuit %ea.e% <

In air circuit breakers the atmospheric pressure air s used as an arc e*tinguishing medium' the

principle of high resistance interruption is employed for such type of breakers' The length of the

arc is increased using arc runners which will increase its resistance in such a way that the &oltage

drop across the arc becomes mom than the supply &oltage and the arc will he e*tinguished This

type of circuit breaker is employed in both ac and d'c' type of circuits upto < k2' These arenormally indoor type and installed on &ertical panels' The lengthening of arc is done with the

help of mangetic fields' #ome typical ratings of this type of circuit breaker are ?=2 - /'/ k2

with current range == - /:=/ or ?'? k2 with current range =/-<== etc'

Const%uction The >ig' shows the constructional details of air break circuit breaker'

It consists of two sets of contacts I% $ain contacts <% rcing contacts

During the normal operation the main contacts are closed' They are ha&ing low resistance with

sil&er plating' The arcing contactG are &ery hard, heat resistant' They are made up of copper alloy'

rc runners are pro&ided at the one end of arcing contact' 3n the upper side arc splitter plates are

pro&ided'

9orking s seen from the >ig the contacts remain in closed position during normal condition'

9hene&er fault occurs, the tripping signal makes the circuit Current breaker contacts to open'

The arc is drawn in between the contacts 9hen e&er the arc is struck between the contacts, the

surrounding air gets ioni5ed' The arc i: then cooled to reduce the diameter of arc core' 9hile

separating the main contacts are separated first' The current is then shifted to arcing contains'



Kater on the arcing contacts also start separating and arc between them is forced upwards by the

electromagnetic forces and thermal action' The arc tra&els through the arc runners' >urther it

mo&es upwards and split by arc splitter plates' Due to all this finally the arc gets e*tinguished as

the resistance of the arc is increased' Due to lengthening and cooling, arc resistance increases

which will reduce the fault current and will not allow reaching at high &alue' The current 5ero

points in the ac' wa&e will help the arc e*tinction with increase in arc resistance the drop across

it will go on increasing' 9hene&er arc lea&es the contacts it is passed through arc runners with

the help of blow out coils which pro&ide a magnetic field due to which it will e*perience a force

as gi&en by electromagnetic theory "> Q /+%' This force wilts assist in mo&ing the arc upwards'

The magnetic field produced is insufficient to e*tinguish the arc' >or systems ha&ing low

inductances arc gets e*tinguished before reaching e*tremity of runners because lengthening of

arc will increase the &oltage drop which is insufficient to maintain the arc'

Fi# wo%.in# of ai% %ea.s ci%cuit %ea.e%

>or high inductance circuits if it is not e*tinguished while tra&elling through arc runners then it is

passed through arc splitters where it is cooled' This will make the effecti&e deioni5ed by

remo&ing the heat from arc'

Appications: this type of circuit breakers are commonly employed for industrial switchgear,

au*iliary switch gear in generating stations'



Suphu% He?afuo%ide ;SF$< Ci%cuit !%ea.e% )ure sulphur he*afluride gas is inert and

thermally stable' It is ha&ing good dielectric and arc e*tinguishing properties' It is also an

electronegati&e gas and has strong tendency to absorb free electrons' #>, gas remains in gaseous

state up to a temperature of r C' Its density is about fi&e times that of air and the free heat

con&ection is '? times as much as that of air' lso being inert it is non-in flammable, non-

poisonous and odour less' The contacts of the breaker arc opened in a high pressure flow of #>?,gas and an arc is struck between them The conducting electrons front the arc are captured by the

gas to form relati&ely immobile negati&e ions' The loss of this conducting electrons de&elopes

enough strength of insulation which will e*tinguish the arc' Thus #>, circuit breakers are found

to be &ery effecti&e for high power and high &oltage ser&ice and widely used in electrical

equipment' 3nly the care to be taken is that some by-products are produced due to breakdown of

gas which are ha5ard to the health of the personnel it should be properly disposed' #e&eral types

of #>, circuit breakers are designed by &arious manufacturers in the world during the recent years

which are rated for &oltages from /'? to .?= k2' The property of this gas is that the gas

liquifies at certain low temperatures' The liquification temperature can be increased with pressure

this gas is commercially manufactured in many countries and now used e*tensi&ely, in

electrical industry' The gas is prepared by burning coarsely crushed roll sulphur in fluorine gas in

a steel bo*' The bo* must be pro&ided with staggered hori5ontal shel&es each containing about

kg of sulphur' The steel bo* is gas tight' fter the chemical reaction taking place in the bo*, the

#>? gas obtained contains impurities in the form of fluorides such as #<=, #> etc' Thus it

must be purified before it is supplied' The manufacturing of this gas at large scale reduces itscost' The dielectric strength of #>? gas at any pressure is more than that of air' 9hen the gas

comes in contact with the electric arc for long period, the decomposition effects are small and

dielectric strength is not considerably reduced and the metallic fluorides that are formed are good

insulators and are not harmful to the breaker'

#ulphur-he*a flouride "#>?% is an inert insulating gas, which is becoming increasingly popular in

modern switchgear designs both as an insulating as well as an arc-quenching medium' 6as

insulated switchgear "6I#% is a combination of breaker, isolator, CT, )T, etc', and are used to

replace outdoor substations operating at the higher &oltage le&els, namely ?? k2 and abo&e' >or

medium- and low-&oltage installations, the #>? circuit breaker remains constructionally the same

as that for oil and air circuit breakers mentioned abo&e, e*cept for the arc interrupting chamber

which is of a special design, filled with #>?' To interrupt an arc drawn when contacts of the

circuit breaker separate, a gas flow is required to cool the arcing 5one at current interruption "i'e'

current 5ero%' This can be achie&ed by a gas flow generated with a piston "known as the JpufferF



principle%, or by heating the gas of constant &olume with the arcFs energy' The resulting gas

e*pansion is directed through no55les to pro&ide the required gas flow' The pressure of the #>?

gas is generally maintained abo&e atmospheric@ so good sealing of the gas chambers is &itally

important' Keaks will cause loss of insulating medium and clearances are not designed for use in

air' Sufu% he?afuo%ide ;SF$<

#ulfur he*afluoride "#>?% is an insulating gas used in circuit breakers in two ways' In 0puffer0

designs, it4s blown across contacts as they open to displace the arcing gas' In 0blast0 designs, is

used at high pressures to open contacts as it simultaneously e*tinguishes the arc'#>? breakers are

rated for the highest &oltage of all breaker designs

'

2acuum breakers enclose the contacts within a &acuum chamber, so when the arc of metallic

&apor forms it is magnetically controlled and thereby e*tinguished at current 5ero' 2acuum

breakers are rated up to /': k2'

Saient featu%es:

• #imple and compact design'

• Kine to ground clearances as per customer specification'

• #elf aligning contacts for easy re-assembly'

• Inspection + maintenance of pole unit possible without

dismantling the breaker'

• #eparate main and arcing contacts thus eliminating the

possibility of erosion of the main contacts'



• #ingle break up to <: k2 le&el'

• Consistent operating characteristics as the closing spring is

in rela*ed condition'

• #tainless steel latches +catches for high reliability'

• Corrosion resistant materials for construction'

• $aintenance free operation of the pole unit for :-<=

years under normal conditions'

• !asy erection'

• 7o site ad1ustments'

• !asy access to all parts of operating mechanism through

front+back opening panels'

• Kow operating noise le&els'

• uto drain &al&e for unmanned substation operations'

• )ressure relief de&ice'

• High seismic withstand capability - earthquake safety'

Const%uction ope%ation:

ll our #>? Circuit breakers ha&e a similar interrupter design' The range of breakers from .<':

k2 to <: k2 is manufactured with single break interrupter design while <= k2 breakers are

manufactured with double break interrupters' These breakers are of li&e tank design and employ

puffer action for interruption ensuring higher operational reliability and safety of power

transmission and distribution systems' The interrupting unit filled with #>? gas is placed at the

top of the pole and contains #tationary

Contact, 7o55le, $o&ing Contact, )uffer Cylinder and >i*ed )iston' During opening operation

the $o&ing Contact along with the )uffer Cylinder is pulled down' The )uffer Cylinder, which

mo&es along with the $o&ing Contact, compresses the #>? gas against the >i*ed )iston thus

generating a powerful #>? gas blast through the no55le and o&er the arc' fter tra&elling through

some distance, the dielectric strength of the gap is sufficient to withstand the &oltage and thus the

arc e*tinguishes' The reliability of the system is further increased by the single pressure dual

flow #>? gas puffer interrupter which reduces the number of mo&ing parts and au*iliary systems

in the circuit breaker'





4i ci%cuit %ea.e%s

In modern installations, oil circuit breakers, which are becoming obsolete, are being replaced by

&acuum and #>? breakers' Howe&er there are many installations, which still employ these

breakers where replacements are found to be a costly proposition' In this design, the main

contacts are immersed in oil and the oil acts as the ioni5ing medium between the contacts' The oil

is mineral type, with high dielectric strength to withstand the &oltage across the contacts under

normal conditions'

"a% Double break "used since 8=%,'

"b% #ingle break "more popular in earlier days as more economical to produce Oless copper,

arc control de&ices, etc', rc energy decomposes oil into .=B hydrogen, <<B acetylene,

:B methane and /B ethylene' rc is in a bubble of gas surrounded by oil'

Doue %ea. oi ci%cuit %ea.e%



Sin#e %ea. oi ci%cuit %ea.e%



4i has the foowin# adanta#es:

bility of cool oil to flow into the space after current 5ero and arc goes out

Cooling surface presented by oil

bsorption of energy by decomposition of oil

ction of oil as an insulator lending to more compact design of switchgear'

Disadanta#es:

Inflammability "especially if there is any air near hydrogen%

$aintenance "changing and purifying%'

In the initial stages, the use of high-&olume "bulk% oil circuit breakers was more common' In this

type, the whole breaker unit is immersed in the oil' This type had the disad&antage of production

of higher hydrogen quantities during arcing and higher maintenance requirements' #ubsequently

these were replaced with low oil "minimum oil% types, where the arc and the bubble are confined

into a smaller chamber, minimi5ing the si5e of the unit'

acuu+ ci%cuit %ea.e%s and contacto%s: 2acuum circuit breakers and contactors were

introduced in the late ?=s' circuit breaker is designed for high through-fault and interrupting

capacity and as a result has a low mechanical life' 3n the other hand, a contactor is designed to

pro&ide large number of operations at typical rated loads of <==+==+?== at &oltages of

:==+//==+??==+ === 2'

The foowin# tae iust%ates the +ain diffe%ences etween a contacto% and a ci%cuit

%ea.e%





Hence, it is necessary to use back-up fuses when contactors are employed to take care of the high

fault conditions' 2acuum breakers are also similar in construction like the other types of breakers,

e*cept that the breaking medium is &acuum and the medium sealed to ensure &acuum' >igures

below gi&e the components of a &acuum circuit breaker'



ene%a const%uction of a acuu+ ci%cuit %ea.e%

Dept of EEE, S!IT "a#e 5$


The modern &acuum bottle, which is used in both breakers and contactors, is normally made

from ceramic material' It has pure o*ygen-free copper main connections@ stainless steel bellowsand has composite weld-resistant main contact materials' typical contact material comprises a

tungsten matri* impregnated with a copper and antimony alloy to pro&ide a low melting point

material to ensure continuation of the arc until nearly current 5ero'

;ecause it is &irtually impossible for electricity to flow in a &acuum, the early designs displayed

the ability of current chopping i'e' switching off the current at a point on the cycle other than

current 5ero' This sudden instantaneous collapse of the current generated e*tremely high-&oltage

spikes and surges into the system, causing failure of equipment'

nother phenomenon was pre-strike at switch on' Due to their superior rate of dielectric

reco&ery, a characteristic of all &acuum switches was the production of a train of pulses during

the closing operation' lthough of modest magnitude, the high rate of rise of &oltage in pre-strike

transients can, under certain conditions produce high-insulation stresses in motor line end coils'

#ubsequent de&elopments attempted to alle&iate these shortcomings by the use of JsofterF contact

materials, in order to maintain metal &apor in the arc plasma so that it did not go out during



switching' (nfortunately, this led to many instances of contacts welding on closing'

Dept of EEE, S!IT "a#e 5(


Restrike transients produced under conditions of stalled motor switch off was also a problem'

9hen switching off a stalled induction motor, or one rotating at only a fraction of synchronousspeed, there is little or no machine back emf, and a high &oltage appears across the gap of the

contactor immediately after e*tinction' If at this point of time the gap is &ery small, there is the

change that the gap will break down and initiate a restrike transient, puncturing the motorFs

insulation'

$odern designs ha&e all but o&ercome these problems' In &acuum contactors, higher operating

speeds coupled with switch contact material are chosen to ensure high gap breakdown strength,

produce significantly shorter trains of pulses' In &acuum circuit breakers, operating speeds are

also much higher which, together with contact materials that ensure high dielectric strength at a

small gap, ha&e ensured that prestrike transients ha&e ceased to become a significant

phenomenon' These ha&e led to the use of &acuum breakers more common in modern

installations'

"%actica "owe% S/ste+s "%otection



Switch off of staed $B$ . '&& .W +oto%-escaatin# %est%i.e on 0 phase

Dashpots

In oil circuit breakers, when the breaker is closed, if the operation is not damped then contact

bounce may occur and the breaker may kick open' Dashpots pre&ent this' They may also pre&ent

unnecessary physical damage to the contacts on impact' Their use of course depends on the

design'

Contacts

>i*ed contacts normally ha&e an e*tended finger for arc control purposes' $o&ing contacts

normally ha&e a special tip "!lkonite% to pre&ent burning from arcing'



Co+pa%ison of insuatin# +ethods fo% C!s

Co+pa%ison of %ea.e% t/pes

>ollowing cur&e gi&es the requirement of electrode gaps for circuit breakers with different

insulating mediums

"%actica "owe% S/ste+s "%otection

The following table highlights the features for different types of circuit breakers'



Ci%cuit !%ea.e% 4pe%ation

In addition to the e&ents that cause a trip, a circuit breaker for switchgear applications must also

be selected for the method by which it opens when tripped' This is important, because when

contacts are opened quickly at high &oltage le&els, a conducti&e metallic &apor can form that

allows current to continue to tra&el between the open contacts' This phenomenon, known asarcing, creates the greatest obstacle to circuit interruption'

s a result, medium- and high-&oltage circuit breakers employ one of four different arc

interrupting technologies' ll take ad&antage of the fact that e&en the most powerful C

o&ercurrent cycles pass the 5ero current le&el twice in one cycle' ;y reducing the amount of

conducti&e gas between the contacts, the arc cannot be sustained when it passes through a current

5ero' #ince the current in DC circuits does not follow a sine-wa&e pattern, circuit interruption is

&ery difficult' This makes the DC interrupting rating for most breakers much lower than theinterrupting rating for C circuits'

Ai% +a#netic %ea.e%s

ir magnetic breakers use the arc to generate a magnetic field that forces the arc into arc chutes,which lengthen and cool the arc, allowing it to be e*tinguished at a current 5ero'



4i %ea.e%s

3il breakers are of se&eral types, including bulk oil, but they all work in a relati&ely similar way'

Here, the contacts are immersed in a container of non-conducti&e oil' 9hen an o&ercurrent

occurs, the arc heats the surrounding oil forcing it to flow &iolently' The rapidly flowing oil

displaces the arcing gases and breaks the arc path'

3il breakers always carry the ha5ards of handling and disposing of spent oil, and the potential for

oil fire' Different oil breakers are designed for different power le&els, with the highest rated for

/: k2 to :== k2'

Fuse Circuit Breaker

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