Download Definition of Circuit Breaker

Circuit Breaker ;
A circuit breaker is an automatically-operated electrical switch designed
to protect an electrical circuit from damage caused by overload or short
circuit. Its basic function is to detect a fault condition and, by
interrupting continuity, to immediately discontinue electrical flow. Unlike
a fuse, which operates once and then has to be replaced, a circuit
breaker can be reset (either manually or automatically) to resume
normal operation. Circuit breakers are made in varying sizes, from small
devices that protect an individual household appliance up to large
switchgear designed to protect high voltage circuits feeding an entire
city.
Origin :
An early form of circuit breaker was described by Edison in an 1879
patent application, although his commercial power distribution system
used fuses. Its purpose was to protect lighting circuit wiring from
accidental short-circuits and overloads.
Operating Principle
The primary function of the circuit breakers mechanism is to provide the
means for opening and closing the contacts. Initially, this seems to be a
rather simple and straightforward requirement. However, when one
considers the fact that most circuit breakers, once placed into service,
will remain in the closed position for long periods of time, and yet on the
few occasions when they are called upon to open or close, they must do
so reliably, without any delay or sluggishness, then one realizes that the
demands on the mechanisms are not as simple as was first thought.
A circuit breaker essentially consists of fixed and moving contacts.
These contacts are called electrodes. The need for carrying the
continuous current and for withstanding a period of arcing makes it
necessary to use two sets of contacts in parallel, one is the primary
contact and the second is the arcing contact. The primary contact is
always made of a high conductive material such as copper and the
arcing contact is made of arc resistance material such as tungsten or
molybdenum, which has a much lower conductivity than those used for
primary contacts. When the circuit breaker opens to interrupt the
current, the primary contacts open before the arcing contacts.
Under the normal operating conditions, these contacts remain closed
and are not open automatically until and unless the system becomes
faulty. Of course, the contacts can be opened manually or by remote
control when ever 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 break 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 the 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 the circuit breaker is to extinguish
the arc within the shortest possible time so that heat generated by it
may not reach a dangerous value.
In single phase (1-phase) circuits (i.e., lighting circuits etc.), a switch is
located in only one of the two conductors to the load. However in the
power circuits, a circuit interrupting device (i.e., circuit breaker) is put in
each phase or conductor. These are, sometimes, called 3-pole circuit
breakers.
Arc Phenomenon
Arc in an ac circuit breaker occurs in two ways:
1. When the current-carrying contacts are being separated, arcing is
possible even when the circuit e.m.f. is considerably below the minimum
cold electrode breakdown voltage, because of the ions neutralizing the
electronic space charge and thus allowing large currents to flow at
relatively low voltage gradients. This way of occurrence of an arc is
common to both dc and ac circuit breakers.
2. The other way of occurrence of an arc happens only in ac circuit
breakers. In such case, the arc is extinguished every time the current
passes through zero and can restrike only if the transient recovery
voltage across the electrodes already separated and continuing to
separate, reaches a sufficiently high value causing breakdown. The
function of an ac circuit breaker is to prevent restriking of the arc, which
depends upon the following important factors:
The nature and pressure of the medium of arc.
The external ionizing and de-ionizing agents present.
The voltage across the electrodes and its variation with time.
The material and configuration of the electrodes.
The nature and configuration of the arcing chamber.
Classification of Circuit Breakers:
Based on voltage :
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Low voltage
Medium voltage
High / Extra voltage
Ultra voltage
Based on location :
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Indoor
Outdoor
Based on external design :
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Dead tank
Live tank
Based on interrupting media :
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Air Break
Air Blast
Oil
SF6
Vacuum
However, the most general way of classification is on the basis of
medium used for the arc extinction , so we will talk about this type .
Oil circuit breaker :
In such circuit breakers, some insulating oil (i.e., 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
hydrogen gas at a high pressure. This large volume of the hydrogen
gas pushes the oil away from the arc. In an oil circuit breaker, the arc
quenching process is entirely dependent on arc energy generated. The
arc drawn across the contacts is contained inside the interrupting pot,
and thus the hydrogen gas formed by the vaporized oil (gas) is also
contained inside the chamber. As the contacts continue to move, and
the moving contact rod separates itself from the orifice of the chamber,
an exit similar to a nozzle allows escape of the hydrogen gas trapped
inside the interrupting chamber. The escaping high pressure hydrogen
gas, having a high thermal conductivity, takes away the heat, thus
making the contact gap cool and free from ionization, immediately after
current zero.
Advantages :
1. Oil absorbs the arc energy to produces hydrogen gas during arcing.
The hydrogen has excellent cooling properties and helps extinguish
the arc. (In addition to hydrogen gas, a small proportion of methane,
ethylene, and acetylene are also generated in oil decomposition.)
2. The oil provides insulation for the live exposed contacts from the
earthed portions of the container.
3. Oil provides insulation between the contacts after the arc has been
extinguished.
4. The oil close to the arc region provides cooling surface.
Disadvantages :
1. Oil is inflammable and may cause fire hazards. When a defective
circuit breaker fails under pressure, it may cause an explosion.
2. The hydrogen generated during arcing, when combined with air,
may form an explosive mixture.
3. During arcing, oil decomposes and becomes polluted by carbon
particles, which reduces its dielectric strength. Hence, it requires
periodic maintenance and replacement.
Sulphur Hexafluoride (SF6) Circuit Breakers :
In these circuit breakers, Sulpher hexafluoride gas (SF6) is used as the
arc quenching medium. The SF6is an electronegative gas and has a
strong tendency to absorb free electrons. The contacts of the breaker
are opened in a high pressure flow of SF6gas and an arc is struck
between them. The conducting free electrons in the arc are rapidly
captured by the gas to form relatively immobile negative ions. This loss
of conducting electrons in the arc quickly builds up enough insulation
strength to extinguish the arc. The SF6 circuit breakers have been found
to be very effective for high power and high voltage services.
The excellent insulating properties of SF6 gas make it possible to
design circuit breakers with smaller overall dimensions, shorter contact
gaps, which help in the construction of outdoor breakers with fewer
interrupters.
Advantages:
1. Because of the high conductivity of the arc in the SF6 gas, the arc
energy is low. (Arc voltage is between 150 and 200V.)
2. Due to the low energy the contact erosion is small.
3. The gaseous medium SF6 possesses excellent dielectric and arc
quenching properties. After arc extinction, the dissociated gas
molecules recombine almost completely to reform SF6. This
means that practically no loss/consumption of the quenching
medium occurs.
4.
Due to the superior arc quenching property of the SF6 gas, such
circuit breakers have very short arcing time. Furthermore, they
can interrupt much larger current.
5. These breakers give noiseless operation due to its closed gas
circuit and no exhaust to atmosphere unlike the air-blast circuit
breaker.
6. The SF6 gas is not inflammable, so there is no risk of fire in SF6
breakers.
7. Since SF6 breakers are totally enclosed and sealed from the
atmosphere, they are particularly suitable where explosion
hazards exist, i.e. in coal mines.
Disadvantages:
1. These circuit breakers are expensive due to the high cost of SF6
gas.
2. Since SF6 gas has to be reconditioned after every operation of the
breaker, additional equipment is required for this purpose.
3. The SF6gas has been identified as a greenhouse gas, and safety
regulations are being introduced in many countries in order to
prevent its release into the atmosphere.
Vacuum circuit breaker :
In such circuit breakers, the vacuum is used as the arc quenching
medium. The vacuum circuit breaker takes the advantage of nonsustainability of electric arc in vacuum, and employs the principle of
contact separation under vacuum where there is no ionization due to
medium. The initial arc caused by field and thermionic emissions during
the contacts separation, will die away soon, as there is no further
ionization because of vacuum.
The degree of vacuum in these circuit breakers is in the range from 10-7
to 10-5 torr . Since vacuum offers the highest insulating strength, it has
far superior arc quenching properties than any other medium. When the
contacts in the vacuum circuit breakers are opened in vacuum, an arc is
produced between the contacts by the ionization of metal vapors of
contacts. However, the arc is quickly extinguished because the metallic
vapors, electrons and ions produced during arc rapidly condense on the
surface of the circuit breaker contacts.
Advantages:
1.The vacuum circuit breakers are compact in size and have longer
lives.
2. Operating energy requirements are low, because the mechanism must
move only
relatively small masses at moderate speed, over very short distances.
3. Because of the very low voltage across the metal vapor arc, energy is
very low.
(Arc voltage is between 50 and 100V.)
4.Due to the very low arc energy, the rapid movement of the arc root
over the contact and to the fact that most of the metal vapor recondenses on the contact, contact erosion is extremely small.
5.There is no generation of gases during and after the circuit breaker
operation.
6. The outstanding feature of these breakers is that it can break any
heavy fault current perfectly just before the contacts reach a definite
open position .
7.They can successfully withstand lightning surges
Air Circuit Breakers :
Air circuit breaker is defined as a circuit breaker, in which the contacts
open and close in air at atmospheric pressure. In general, the use of
this type of circuit breakers is restricted to low voltage applications or
high security installations where the risk of an oil fire or oil
contamination of the environment is too high to be tolerated. Countries
following the American practice used air circuit breakers almost
exclusively for systems up to 15 kV until the advent of the new vacuum
and SF6technologies.
The principles of arc interruption used in an air circuit breaker are rather
different from those in any other type of circuit breaker. However, the
objective is the same for both categories of the circuit breakers, i.e. to
prevent the resumption of arcing after current zero by creating a
situation where in the contact gap will withstand the system recovery
voltage; the air circuit breaker does this by creating an arc voltage in
excess of the supply voltage. This can be achieved in three ways:
1.
2.
3.
Intense cooling of the arc plasma, so that the voltage is very
high .
Lengthening the arc path to increase the arc voltage
Splitting up the arc into a number of series arcs
It is inflammable and there is a ris
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