Circuit Breakers

During the operation of power system, it is often desirable and necessary

to switch on or off the various circuits (e.g., transmission lines,
distributors, generating plants etc.) under both normal and abnormal
conditions. In earlier days, this function used to be performed by a switch
and a fuse placed in series with the circuit. However, such a means of
control presents two disadvantages.
Firstly, when a fuse blows out, it takes quite some time to replace it and
restore supply to the customers.
Secondly, a fuse cannot successfully interrupt heavy fault currents that
result from faults on modern high-voltage and large capacity circuits.
Due to these disadvantages, the use of switches and fuses is limited to
low-voltage and small capacity circuits where frequent operations are not
expected e.g., for switching and protection of distribution transformers,
lighting circuits, branch circuits of distribution lines etc.
With the advancement of power system, the lines and other equipment
operate at very high voltages and carry large currents. The arrangement
of switches along with fuses cannot serve the desired function of
switchgear in such high capacity circuits.
This necessitates to employ a more dependable means of control such as
is obtained by the use of circuit breakers.
A circuit breaker can make or break a circuit either manually or
automatically under all conditions viz., no-load, full-load and short circuit
conditions. This characteristic of the circuit breaker has made it a very
useful equipment for switching and protection of various parts of the
power system.

Circuit Breakers
A circuit breaker is a piece of equipment which can,
(i) make or break a circuit either manually or by remote control under
normal conditions.
(ii) break a circuit automatically under fault conditions
(iii) make a circuit either manually or by remote control under fault
conditions
Thus a circuit breaker incorporates manual (or remote control) as well as
automatic control for switching functions. The latter control employs
relays and operates only under fault conditions.
Operating principle: A circuit breaker essentially consists of fixed and
moving contacts, called electrodes. Under normal operating conditions,
these contacts remain closed and will not open automatically until and
unless the system becomes faulty. Of course, the contacts can be opened
manually or by remote control whenever desired. When a fault occurs on
any part of the system, the trip coils of the circuit breaker get energized
and the moving contacts are pulled apart by some mechanism, thus
opening the circuit.
When the contacts of a circuit breaker are separated under fault
conditions, an arc is struck between them. The current is thus able to
continue until the discharge ceases. The production of arc not only delays
the current interruption process but it also generates enormous heat which
may cause damage to the system or to the circuit breaker itself. Therefore,
the main problem in a circuit breaker is to extinguish the arc within the
shortest possible time so that heat generated by it may not reach a
dangerous value.

Arc Phenomenon
When a short-circuit occurs, a heavy current flows through the contacts
of the circuit breaker before they are opened by the protective system. At
the instant when the contacts begin to separate, the contact area decreases
rapidly and large fault current causes increased current density and hence
rise in temperature. The heat produced in the medium between contacts
(usually the medium is oil or air) is sufficient to ionize the air or vaporized
and ionize the oil. The ionized air or vapor acts as conductor and an arc is
struck between the contacts. The p.d. between the contacts is quite small
and is just sufficient to maintain the arc. The arc provides a low resistance
path and consequently the current in the circuit remains uninterrupted so
long as the arc persists.
During the arcing period, the current flowing between the contacts
depends upon the arc resistance. The greater the arc resistance, the smaller
the current that flows between the contacts. The arc resistance depends
upon the following factors:
(i) Degree of ionisation— the arc resistance increases with the
decrease in the number of ionized particles between the
contacts.
(ii) Length of the arc— the arc resistance increases with the length
of the arc i.e., separation of contacts.
 (iii) Cross-section of arc— the arc resistance increases with the
decrease in area of X-section of
the arc.

Principles of Arc Extinction

The factors responsible for the maintenance of arc between the contacts:
(i) p.d. between the contacts.
(ii) Ionized particles between contacts
Taking these in turn,
(i) When the contacts have a small separation, the p.d. between them
is sufficient to maintain the arc. One way to extinguish the arc is to
separate the contacts to such a distance that p.d. becomes inadequate
to maintain the arc. However, this method is impracticable in high
voltage system where a separation of many meters may be required.
(ii) The ionized particles between the contacts tend to maintain the
arc. If the arc path is deionized, the arc extinction will be facilitated.
This may be achieved by cooling the arc or by bodily removing the
ionized particles from the space between the contacts.

Methods of Arc Extinction

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

1. High resistance method. In this method, arc resistance is made to

increase with time so that current is reduced to a value insufficient
to maintain the arc. Consequently, the current is interrupted or the
arc is extinguished.

The principal disadvantage of this method is that enormous energy is

dissipated in the arc. Therefore, it is employed only in d.c circuit
breakers and low-capacity a.c circuit breakers.

The resistance of the arc may be increased by:

(i) Lengthening the arc. The resistance of the arc is directly
proportional to its length. The length of the arc can be
increased by increasing the gap between contacts.
(ii) Cooling the arc. Cooling helps in the deionization of the
medium between the contacts. This increases the arc
resistance. Efficient cooling may be obtained by a gas blast
directed along the arc.
(iii) Reducing X-section of the arc. If the area of X-section of the
arc is reduced, the voltage necessary to maintain the arc is
increased. In other words, the resistance of the arc path is
increased. The cross section of the arc can be reduced by
letting the arc pass through a narrow opening or by having
smaller area of contacts.

(iv) Splitting the arc. The resistance of the arc can be increased by
splitting the arc into a number of smaller arcs in series. Each
one of these arcs experiences the effect of lengthening and
cooling. The arc may be split by introducing some conducting
plates between the contacts.

2. Low resistance or Current zero method. This method is employed

for arc extinction in a.c. circuits only. In this method, arc resistance is
kept low until current is zero where the arc extinguishes naturally and
is prevented from restriking inspite of the rising voltage across the
contacts.
All modern high power a.c. circuit breakers employ this method for arc
extinction.

In an a.c. system, current drops to zero after every half-cycle. At every

current zero, the arc extinguishes for a brief moment. Now the medium
between the contacts contains ions and electrons so that it has small
dielectric strength and can be easily broken down by the rising contact
voltage known as restriking voltage. If such a breakdown does occur,
the arc will persist for another half cycle. If immediately after current
zero, the dielectric strength of the medium between contacts is built up
more rapidly than the voltage across the contacts, the arc fails to
restrike and the current will be interrupted. The rapid increase of
dielectric strength of the medium near current zero can be achieved by:
(a) Causing the ionized particles in the space between contacts to
recombine into neutral molecules.
(b) Sweeping the ionized particles away and replacing them by
unionized particles.

Therefore, the real problem in a.c. arc interruption is to rapidly deionize

the medium between contacts as soon as the current becomes zero so
that the rising contact voltage or restriking voltage cannot breakdown
the space between contacts. The deionization of the medium can be
achieved by:

(i) Lengthening of the gap. The dielectric strength of the medium is

proportional to the length of the gap between contacts. Therefore, by
opening the contacts rapidly, higher dielectric strength of the medium
can be achieved.
(ii) High pressure. If the pressure in the vicinity of the arc is increased,
the density of the particles constituting the discharge also increases.
The increased density of particles causes higher rate of de-ionization
and consequently the dielectric strength of the medium between
contacts is increased.
(iii) Cooling. Natural combination of ionized particles takes place more
rapidly if they are allowed to cool. Therefore, dielectric strength of the
medium between the contacts can be increased by cooling the arc.
(iv) Blast effect. If the ionized particles between the contacts are swept
away and replaced by unionized particles, the dielectric strength of the
medium can be increased considerably. This may be achieved by a gas
blast directed along the discharge or by forcing oil into the contact
space.

Important Terms
The following are the important terms much used in the circuit breaker
analysis:
(i) 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, an arc is formed. The
voltage that appears across the contacts during 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 peak value
and this peak voltage tends to maintain the current flow in the form of
arc.

(ii) Restriking voltage. It is the transient voltage that appears across the
contacts at or near current zero during arcing period. At current zero, a
high-frequency transient voltage appears across the contacts and is
caused by the rapid distribution of energy between the magnetic and
electric fields associated with the plant and transmission lines of the
system. This transient voltage is known as restriking voltage. The
current interruption in the circuit depends upon this voltage. If the
restriking voltage rises more rapidly than the dielectric strength of the
medium between the contacts, the arc will persist for another half-
cycle. On the other hand, if the dielectric strength of the medium builds
up more rapidly than the restriking voltage, the arc fails to restrike and
the current will be interrupted.

(iii) Recovery voltage. It is the normal frequency (50 Hz) r.m.s. voltage
that appears across the contacts of the circuit breaker after final arc
extinction. It is approximately equal to the system voltage. When
contacts of circuit breaker are opened, current drops to zero after every
half cycle. At some current zero, the contacts are separated sufficiently
apart and dielectric strength of the medium between the contacts attains
a high value due to the removal of ionised particles. At such an instant,
the medium between the contacts is strong enough to prevent the
breakdown by the restriking voltage. Consequently, the final arc
extinction takes place and circuit current is interrupted. Immediately
after final current interruption, the voltage that appears across the
contacts has a transient part. However, these transient oscillations
subside rapidly due to the damping effect of system resistance and
normal circuit voltage appears across the contacts. The voltage across
the contacts is of normal frequency and is known as recovery voltage.
Classification of Circuit Breakers
There are several ways of classifying the circuit breakers. However, the
most general way of classification is on the basis of medium used for arc
extinction. The medium used for arc extinction is usually oil, air, sulphur
hexafluoride (SF6) or vacuum. Accordingly, circuit breakers may be
classified into:
(i) Oil circuit breakers which employ some insulating oil (e.g.,
transformer oil) for arc extinction.
(ii) Air-blast circuit breakers in which high pressure air-blast is used for
extinguishing the arc.
(iii) Sulphur hexafluroide circuit breakers in which sulphur hexafluoride
(SF6) gas is used for arc extinction.
(iv) Vacuum circuit breakers in which vacuum is used for arc extinction.
Each type of circuit breaker has its own advantages and disadvantages.

Oil Circuit Breakers

In such circuit breakers, some insulating oil (e.g., transformer oil) is used
as an arc quenching medium. The contacts are opened under oil and an
arc is struck between them. The heat of the arc evaporates the surrounding
oil and dissociates it into a substantial volume of gaseous hydrogen at high
pressure. The hydrogen gas occupies a volume about one thousand times
that of the oil decomposed. The oil is, therefore, pushed away from the
arc and an expanding hydrogen gas bubble surrounds the arc region and
adjacent portions of the contacts shown in Fig.

* Mainly hydrogen gas is produced as a result of oil decomposition. However, a small percentage of methane, ethylene and
acetylene is also generated.
The arc extinction is facilitated mainly by two processes.
Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus
aiding the de-ionisation of the medium between the contacts.
Secondly, the gas sets up turbulence in the oil and forces it into the space
between contacts, thus eliminating the arcing products from the arc path.
The result is that arc is extinguished and circuit current †interrupted.
Advantages.
The advantages of oil as an arc quenching medium are:
(i) It absorbs the arc energy to decompose the oil into gases which
have excellent cooling properties.
(ii) It acts as an insulator and permits smaller clearance between
live conductors and earthed
components.
(iii) The surrounding oil presents cooling surface in close
proximity to the arc.

Disadvantages.
The disadvantages of oil as an arc quenching medium are:
(i) It is inflammable and there is a risk of a fire.
(ii) It may form an explosive mixture with air
(iii) The arcing products (e.g., carbon) remain in the oil and its
quality deteriorates with successive operations. This necessitates
periodic checking and replacement of oil.

Types of Oil Circuit Breakers

The oil circuit breakers find extensive use in the power system. These can
be classified into the following types:
(i) Bulk oil circuit breakers which use a large quantity of oil. The oil has
to serve two purposes.
Firstly, it extinguishes the arc during opening of contacts and secondly, it
insulates the current conducting parts from one another and from the
earthed tank.
Such circuit breakers may be classified into:
(a) Plain break oil circuit breakers (b) Arc control oil circuit breakers.
In the former type, no special means is available for controlling the arc
and the contacts are directly exposed to the whole of the oil in the tank.
However, in the latter type, special arc control devices are employed to
get the beneficial action of the arc as efficiently as possible.
(ii) Low oil circuit breakers which use minimum amount of oil. In such
circuit breakers, oil is used only for arc extinction; the current conducting
parts are insulated by air or porcelain or organic insulating mat
Plain Break Oil Circuit Breakers
A plain-break oil circuit breaker involves the simple process of separating
the contacts under the whole of the oil in the tank. There is no special
system for arc control other than the increase in length caused by the
separation of contacts. The arc extinction occurs when a certain critical
gap between the contacts is reached. The plain-break oil circuit breaker is
the earliest type from which all other circuit breakers have developed. It
has a very simple construction. It consists of fixed and moving contacts
enclosed in a strong weather-tight earthed tank containing oil upto a
certain level and an air cushion above the oil level. The air cushion
provides sufficient room to allow for the reception of the arc gases without
the generation of unsafe pressure in the dome of the circuit breaker. It also
absorbs the mechanical shock of the upward oil movement.

Fig. shows a double break plain oil circuit breaker. It is called a double
break because it provides two breaks in series. Under normal operating
conditions, the fixed and moving contacts remain closed and the breaker
carries the normal circuit current. When a fault occurs, the moving
contacts are pulled down by the protective system and an arc is struck
which vaporizes the oil mainly into hydrogen gas. The arc extinction is
facilitated by the following processes:
(i) The hydrogen gas bubble generated around the arc cools the arc column
and aids the deionization of the medium between the contacts.
(ii) The gas sets up turbulence in the oil and helps in eliminating the arcing
products from the arc path.
(iii) As the arc lengthens due to the separating contacts, the dielectric
strength of the medium is increased. The result of these actions is that at
some critical gap length, the arc is extinguished and the circuit current is
interrupted.
Disadvantages
(i) There is no special control over the arc other than the increase in length
by separating the moving contacts. Therefore, for successful interruption,
long arc length is necessary.
(ii) These breakers have long and inconsistent arcing times.
(iii) These breakers do not permit high speed interruption.
Due to these disadvantages, plain-break oil circuit breakers are used only
for low-voltage applications where high breaking-capacities are not
important. It is a usual practice to use such breakers for low capacity
installations for voltages not exceeding 11 kV.
Arc Control Oil Circuit Breakers
In case of plain-break oil circuit breaker, there is very little artificial
control over the arc. Therefore, comparatively long arc length is essential
in order that turbulence in the oil caused by the gas may assist in
quenching it. However, it is necessary and desirable that final arc
extinction should occur while the contact gap is still short. For this
purpose, some arc control is incorporated and the breakers are then called
arc control circuit breakers. There are two types of such breakers, namely
:
(i) Self-blast oil circuit breakers— in which arc control is provided by
internal means i.e. the arc itself is employed for its own extinction
efficiently.
(ii) Forced-blast oil circuit breakers— in which arc control is provided by
mechanical means external to the circuit breaker.
(i) Self-blast oil circuit breakers. In this type of circuit breaker, the gases
produced during arcing are confined to a small volume by the use of an
insulating rigid pressure chamber or pot surrounding the contacts. Since
the space available for the arc gases is restricted by the chamber, a very
high pressure is developed to force the oil and gas through or around the
arc to extinguish it. The magnitude of pressure developed depends upon
the value of fault current to be interrupted. As the pressure is generated
by the arc itself, therefore, such breakers are sometimes called self-
generated pressure oil circuit breakers.
The pressure chamber is relatively cheap to make and gives reduced final
arc extinction gap length and arcing time as against the plain-break oil
circuit breaker. Several designs of pressure chambers (sometimes called
explosion pots have been developed and a few of them are described
below :

(a) Plain explosion pot. It is a rigid cylinder of insulating material

and encloses the fixed and moving contacts (See Fig.).

The moving contact is a cylindrical rod passing through a restricted

opening (called throat) at the bottom. When a fault occurs, the contacts
get separated and an arc is struck between them. The heat of the arc
decomposes oil into a gas at very high pressure in the pot. This high
pressure forces the oil and gas through and round the arc to extinguish it.
If the final arc extinction does not take place while the moving contact is
still within the pot, it occurs immediately after the moving contact leaves
the pot. It is because emergence of the moving contact from the pot is
followed by a violent rush of gas and oil through the throat producing
rapid extinction.
The principal limitation of this type of pot is that it cannot be used for
very low or for very high fault currents. With low fault currents, the
pressure developed is small, thereby increasing the arcing time. On the
other hand, with high fault currents, the gas is produced so rapidly that
explosion pot is liable to burst due to high pressure. For this reason, plain
explosion pot operates well on moderate short-circuit currents only where
the rate of gas evolution is moderate.
(b) Cross jet explosion pot. This type of pot is just a modification of plain
explosion pot and is illustrated in Fig 19.5. It is made of insulating
material and has channels on one side which act as arc splitters. The arc
splitters help in increasing the arc length, thus facilitating arc extinction.
When a fault occurs, the moving contact of the circuit breaker begins to
separate. As the moving contact is withdrawn, the arc is initially struck in
the top of the pot. The gas generated by the arc exerts pressure on the oil
in the back passage. When the moving contact uncovers the arc splitter
ducts, fresh oil is forced across the arc path. The arc is, therefore, driven
sideways into the “arc splitters” which increase the arc length, causing arc
extinction.

The cross-jet explosion pot is quite efficient for interrupting heavy fault
currents. However, for low fault currents, the gas pressure is small and
consequently the pot does not give a satisfactory operation.
Self-compensated explosion pot. This type of pot is essentially a
combination of plain explosion pot and cross jet explosion pot. Therefore,
it can interrupt low as well as heavy short circuit currents with reasonable
accuracy. Fig. 19.6 shows the schematic diagram of self-compensated
explosion pot. It consists of two chambers, the upper chamber is the cross-
jet explosion pot with two arc splitter ducts while the lower one is the
plain explosion pot. When the short-circuit current is heavy, the rate of
generation of gas is very high and the device behaves as a cross-jet
explosion pot. The arc extinction takes place when the moving contact
uncovers the first or second arc splitter duct. However, on low short-
circuit currents, the rate of gas generation is small and the tip of the
moving contact has the time to reach the lower chamber. During this time,
the gas builds up sufficient pressure as there is very little leakage through.
arc splitter ducts due to the obstruction offered by the arc path and right
angle bends. When the moving contact comes out of the throat, the arc is
extinguished by plain pot action. It may be noted that as the severity of
the short-circuit current increases, the device operates less and less as a
plain explosion pot and more and more as a cross-jet explosion pot. Thus
the tendency is to make the control self compensating over the full range
of fault currents to be interrupted.
(ii) Forced-blast oil circuit breakers. In the self-blast oil circuit breakers
discussed above, the arc itself generates the necessary pressure to force
the oil across the arc path. The major limitation of such breakers is that
arcing times tend to be long and inconsistent when operating against
currents considerably less than the rated currents. It is becasue the gas
generated is much reduced at low values of fault currents. This difficulty
is overcome in forced-blast oil circuit breakers in which the necessary
pressure is generated by external mechanical means independent of the
fault currents to be broken. In a forced -blast oil circuit breaker, oil
pressure is created by the piston-cylinder arrangement. The movement of
the piston is mechanically coupled to the moving contact. When a fault
occurs, the contacts get separated by the protective system and an arc is
struck between the contacts. The piston forces a jet of oil towards the
contact gap to extinguish the arc. It may be noted that necessary oil
pressure produced does not in any way depend upon the fault current to
be broken.
Advantages
(a) Since oil pressure developed is independent of the fault current to be
interrupted, the performance at low currents is more consistent than with
self-blast oil circuit breakers.
(b) The quantity of oil required is reduced considerably.
Low Oil Circuit Breakers
In the bulk oil circuit breakers discussed so far, the oil has to perform two
functions. Firstly, it acts as an arc quenching medium and secondly, it
insulates the live parts from earth. It has been found that only a small
percentage of oil is actually used for arc extinction while the major part is
utilized for insulation purposes. For this reason, the quantity of oil in bulk
oil circuit breakers reaches a very high figure as the system voltage
increases. This not only increases the expenses, tank size and weight of
the breaker but it also increases the fire risk and maintenance problems.
The fact that only a small percentage of oil (about 10% of total) in the
bulk oil circuit breaker is actually used for arc extinction leads to the
question as to why the remainder of the oil, that is not immediately
surrounding the device, should not be omitted with consequent saving in
bulk, weight and fire risk. This led to the development of low-oil circuit
breaker. A low oil circuit breaker employs solid materials for insulation
purposes and uses a small quantity of oil which is just sufficient for arc
extinction. As regards quenching the arc, the oil behaves identically in
bulk as well as low oil circuit breaker. By using suitable arc control
devices, the arc extinction can be further facilitated in a low oil circuit
breaker.
Construction. Fig 19.7 shows the cross section of a single phase low oil
circuit breaker.

There are two compartments

separated from each other but both
filled with oil. The upper chamber is
the circuit breaking chamber while the
lower one is the supporting chamber.
The two chambers are separated by a
partition and oil from one chamber is
prevented from mixing with the other
chamber.
This arrangement permits two advantages.
Firstly, the circuit breaking chamber requires a small volume of oil which
is just enough for arc extinction.
Secondly, the amount of oil to be replaced is reduced as the oil in the
supporting chamber does not get contaminated by the arc.

(i) Supporting chamber. It is a porcelain chamber mounted on a metal

chamber. It is filled with oil which is physically separated from the oil in
the circuit breaking compartment. The oil inside the supporting chamber
and the annular space formed between the porcelain insulation and
bakelised paper is employed for insulation purposes only.
(ii) Circuit-breaking chamber. It is a porcelain enclosure mounted on the
top of the supporting compartment. It is filled with oil and has the
following parts:
(a) Upper and lower fixed contacts
(b) moving contact
(c) turbulator
The moving contact is hollow and includes a cylinder which moves down
over a fixed piston. The turbulator is an arc control device and has both
axial and radial vents. The axial venting ensures the interruption of low
currents whereas radial venting helps in the interruption of heavy currents.
(iii) Top chamber. It is a metal chamber and is mouted on the circuit
breaking chamber. It provides expansion space for the oil in the circuit
breaking compartment. The top chamber is also provided with a separator
which prevents any loss of oil by centrifugal action caused by circuit
breaker operation during fault conditions.
Operation. Under normal operating conditions, the moving contact
remains engaged with the upper fixed contact. When a fault occurs, the
moving contact is pulled down by the tripping springs and an arc is struck.
The arc energy vaporises the oil and produces gases under high pressure.
This action constrains the oil to pass through a central hole in the moving
contact and results in forcing series of oil through the respective passages
of the turbulator. The process of turbulation is orderly one, in which the
sections of the arc are successively quenched by the effect of separate
streams of oil moving across each section in turn and bearing away its
gases.
Advantages. A low oil circuit breaker has the following advantages over
a bulk oil circuit breaker:
(i) It requires lesser quantity of oil.
(ii) It requires smaller space.
(iii) There is reduced risk of fire.
(iv) Maintenance problems are reduced.
Disadvantages. A low oil circuit breaker has the following
disadvantages as compared to a bulk oil circuit breaker :
(i) Due to smaller quantity of oil, the degree of carbonisation is
increased.
(ii) There is a difficulty of removing the gases from the contact space in
time.
(iii) The dielectric strength of the oil deteriorates rapidly due to high
degree of carbonisation.

Maintenance of Oil Circuit Breakers

The maintenance of oil circuit breaker is generally concerned with the
checking of contacts and dielectric strength of oil. After a circuit breaker
has interrupted fault currents a few times or load currents several times,
its contacts may get burnt by arcing and the oil may lose some of its
dielectric strength due to carbonisation. This results in the reduced
rupturing capacity of the breaker. Therefore, it is a good practice to inspect
the circuit breaker at regular intervals of 3 or 6 months. During inspection
of the breaker, the following points should be kept in view:
(i) Check the current carrying parts and arcing contacts. If the burning is
severe, the contacts should be replaced.
(ii) Check the dielectric strength of the oil. If the oil is badly discoloured,
it should be changed or reconditioned. The oil in good condition should
withstand 30 kV for one minute in a standard oil testing cup with 4 mm
gap between electrodes.
(iii) Check the insulation for possible damage. Clean the surface and
remove carbon deposits with a strong and dry fabric.
(iv) Check the oil level.
(v) Check closing and tripping mechanism.