circuit breaker

1. Circuit
Breaker
as
a
protection
device

2. Main point will covered in this presentation
1- General
2- circuit breaker
2.1- General
2.2- Rate terms of circuit breaker
2.3- operating principle
2.4- the insulation fluids commonly used as insulation
medium during the separation of contact
3- Arc phenomena
3.1- General
3.2- initiation of an arc
3.3- Maintenance of arc
3.4- Arc voltage
4- Arc Extinction
4.1- General
4.2- Method of Arc phenomena
4.2.1- High Resistance Method
4.2.2- LOW Resistance Method

3. 5- Classification of circuit breaker
6- Main type of circuit breaker
7- scoop on some types with some details
8-Tesying of circuit breaker

4. 1. General
A switch is used for opening and
closing of an electric circuit. Every
electric circuit needs a switching
device and protective device.
SSwwiittcchhggeeaarr is a general term covering
a wide rang of equipment concerned
with switching and protection.
In a power system switchgear serves two basic purposes:
1. Switching during normal operating condition for purpose
of operation and maintenance.
2. Switching during abnormal condition such as short-circuits
and interrupting the fault current.

5. The first of above could be served by relatively simple
switches because it is relatively simple as it involves
normal current which are easy to interrupt. The second
function is however complex.
With the advancement of electrical power system the lines
and other equipment operate at very high voltage and
carry large current. Whenever a short-circuit occurs, a
heavy current flows through the equipment causing
considerable damage to the equipment and interruption
the service.
In order to avoid such damage every part of the power
system is provided with a protective relaying system and
an associated switching device. The relaying equipment is
aided in this task by circuit breaker.

6. 2. Circuit breaker
2.1 General
Circuit breaker are a mechanical
device designed to close or open the
contact members, thus closing or
opining of an electrical circuit under
normal or abnormal condition.
The automatic circuit breakers perform the following duties:
1. It carries the full-load current continuously without
overheating or damage.
2. It open and close the circuit on no load.
3. It makes and breaks the normal operating current.
4. It makes and breaks the short-circuit current of magnitude
up to which it designed for.

7. 2.2 Rate terms of circuit breaker
• Max voltage
• NO, of poles
• Frequency
• Maximum continuous current
carrying capacity
• Max interrupting capacity
• Max momentary

8. 2.3 Operating principle
A circuit breaker as a switching and
current interrupting consist of fixed and
moving contacts, which are touching
each other and carry the current under
normal condition. When the circuit
breaker is closed, the current carrying
contacts, called the electrodes, engage
each other under the pressure of a
spring.
Whenever a fault occurs on any part
of the power system, the trip coils of the
breaker get energized and the moving
contacts are pulled apart by some
mechanism, thus opening the circuit. The
separation of current carrying production
an arc. The current is thus able to
continue until the discharge ceases.

9. Therefore the main problem in circuit breaker is to
extinguish the arc within the shortest possible time so
that heats generated by it not reach a dangerous
value.
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 breaker itself.

10. The basic construction of circuit breaker requires the
separation of the contacts in an insulating fluid which
serve two functions:
1. Extinguishes the arc drawn between the contacts when
the circuit breaker opens.
2. Provides insulation between the contact and from each
contact to earth.
The insulating fluids commonly used for this purpose
are as follows:
1. Air and atmospheric pressure.
2. Compressed air.
3. Oil producing hydrogen for arc extinction.
4. Ultra high vacuum
5. Sulphur hexa-fluoride (SF6)

11. 3. Arc phenomena
The heats generated from the
arc ionize the air molecules. So
the +ve ions attracted to the –ve
contact and the –ve ions
attracted to the +ve contact.
Thus current flow is caused due
to movement of electrons.
3.1 Initiation of an arc
Initiating electrons are thought of
produced by the following two
processes:
1. By high voltage gradient at the
cathode resulting into field emission
2. By increase of temperature resulting
into thermal emission

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13. 3.2 maintenance of arc
The ionization is further by:
1. High temperature of the medium around the contacts
caused by high current densities.
2. The field strength or voltage gradient which increases
the kinetic energy of moving electrons and increases
the chances of detaching from neutral molecules.
3. An increase of mean free path-the distance through
which the electron moves freely.
All above three processes (thermal emission – ionization –
and field emission) may start either one after the other or
almost simultaneously and enable the arc to be initiated
and maintained.

14. 3.3 Arc voltage
The voltage that appears across the contact of circuit
breaker is called the arc voltage.
For moderated values of current and voltage the arc
characteristic can be expressed by Ayrton's equation
Ea = A + B/ia
:Where
The constant A and B vary
linearly With the arc length l
A=a+g l
B= b+δ l
Average values of a,g,b,δ
for arcs In air between copper electrodes are as following:
a=30v ,g=10v/cm ,b=10VA ,δ=30VA/cm

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16. 4. Arc extinction
The conductance of the arc is proportional to
1. The number of electrons per cubic centimeter
produced by ionization
2. The square of the diameter of the arc
3. Reciprocal of the length
The arc extinction can, therefore, be facilitated by
deionizing the arc path. This may be achieved by cooling
the arc or bodily removing the ionized particles from the
space between the circuit breaker contacts.

17. 4.1 Method of arc extinction
High Resistance Method 4.1.1
In this case the arc is controlled in such a way that its effective
resistance increases with the time so that the current is
reduced to such a value that heat produced by it is not
sufficient to maintain the arc and the current is interrupted or
the arc is extinguished.
The resistance of the arc can be increased by:
1. Cooling of arc
2. Increasing the length of arc
3. Reducing the cross-section of arc
4. Splitting of arc

19. 4.1.2 Low resistance OR Current zero interruption
This method is applicable only
in ac circuit interruption
because there is natural zero
current 100 times in a second
for 50Hz supply system.
In this method the arc resistance is kept low until the current is
zero where the arc extinguishes naturally and is prevented
from restriking after it has gone out at current zero.
This method of arc extinction is employed in all modern
high power ac circuit.

20. 5. Duties of circuit breakers
1. Interruption of small inductive current
2. Switching of unloaded transmission lines and unloaded
cables
3. Switching of capacitor banks
4. Interruption of terminal faults
5. Interruption of short-line faults (kilometric fault)
6. Asynchronous switching

21. 6. Resistance switching
It is a deliberate connection of a
resistance in parallel with the
contact space (or arc).
On occurrence of fault, the contacts
of the circuit breaker open and an
arc is struck . between the contacts
With the shunted by resistance R a part of arc current
is diverted through this resistance. This result in the
decrease of arc current and increase in the rate of
deionization of the arc path.

22. 7. Classification of circuit breaker
1. Oil circuit breakers
2. Water type circuit breaker
3. Air-Break circuit breaker
4. Air blast circuit breaker
5. Sulphur Hexafluoride (SF6 ) circuit breaker
6. Vacuum circuit breaker

23. Oil circuit breakers 7.1
· Bulk oil circuit breaker :
using a large quantity of oil, also
called the dead tank type
because the tank is held at
ground potential
· Low oil circuit breaker: which
operate with a min amount of oil,
also sometimes called the live tank circuit breaker because
the oil tank is insulated from the ground.

24. Advantage:
1. Arc energy is absorbed in decomposing of oil.
2. The gas formed which mainly hydrogen is provided good
cooling properties.
3. The oil has a high dielectric insulating between the
contacts after the arc extinguished.
4. The very good insulation medium allows smaller clearance
between line conductors and earth components

25. Disadvantage
1. Oil is inflammable and may cause fire hazards.
2. There is a risk of formation of explosive mixture with air.
3. Due to decomposition of oil in the arc, the oil becomes
pouted by carbon particles, which reduce its dielectric
strength.

26. Maintenance of oil circuit breaker
1. All current carrying parts be checked and arcing
contacts be attended if necessary.
2. Dielectric strength condition and level of oil should be
checked.
3. Inspect the insulation for possible damage. Clean the
surface and remove deposits of carbon
4. Check closing, tripping and interlock mechanism
5. Check indicating devices and lamps

27. Automatic resisting soled state type (PTC)
A polymer PTC (for positive temperature coefficient) is a special
type of circuit breaker called thermistor ( thermal resistor). PTC
resistor increase resistance as its temperature in increase, PTC’s
are made of conductive polymer , are solid state device , which
means they have no moving parts.

28. Polymer PTC construction and operation
In its normal state the material in the polymer PTC is in the form of
a dense crystal with many carbon particles packet together. The
carbon particles provide conductive pathways for current flow.
This resistance is low. When the material is heated from excessive
current the polymer expands, pulling the carbon chains apart, in
this expanded ” tripped ” state there are few pathways for current.
When current floe exceeds the trip threshold, the device remain in
the “open circuit” state as long as voltage is remain applied to the
circuit. It reset only when the voltage is removed and the polymer
is cools.