1. Group Members Usman Rehman M. Saleem M. Zeeshan
2. Content Introduction to Circuit breaker Basic Defination Of
Circuit Breaker Working Principle of Circuit Breaker Types Of
Circuit Breakers 1. Oil Circuit Breakers 2. Vacuum Circuit Breakers
3. Air Blast Circuit Breakers 4. SF6 Circuit Breakers 5. Miniature
Circuit Breakers 6. Earth Leakage Circuit Breakers Precautions with
CB CONCLUSION
3. Introduction to Circuit Breaker The modern power system
deals with huge power network and huge numbers of associated
electrical equipment. During short circuit fault or any other types
of electrical fault these equipment as well as the power network
suffer a high stress of fault current in them which may damage the
equipment and networks permanently. For saving these equipments and
the power networks the fault current should be cleared from the
system as quickly as possible.
4. Basic Defination Of 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.
5. Working Principle of Circuit Breaker The circuit breaker
mainly consists of fixed contacts and moving contacts. In normal
"on" condition of circuit breaker, these two contacts are
physically connected to each other due to applied mechanical
pressure on the moving contacts. The potential energy can be stored
in the circuit breaker by different ways like by deforming metal
spring, by compressed air, or by hydrolic pressure. But whatever
the source of potential energy, it must be released during
operation. Release of potential energy makes sliding of the moving
contact at extremely fast manner. When fault occurs on any part of
the system, the trip coil of the circuit breaker get energized and
contacts are separated.
6. Breaker Components All breakers contain the same basic
components: Stationary Main Line Contacts Moveable Mainline
Contacts Arcing Contacts Auxiliary Contacts Arc Interrupting Device
Closing Mechanism Opening Mechanism Control Mechanism Interlocks
Frame Assembly
7. Arc Phenomenon An arc is struck when contacts are separated.
The current is thus able to continue. Thus the main duty of a
circuit breaker is to distinguish the arc within the shortest
possible time. The arc provides the low resistance path to the
current and the current in the circuit remains uninterrupted.
8. A breaker is designed to provide quick isolation of a
component from a voltage source. Circuit Breaker selection process
is based upon the use and type of Arc Control desired.
9. Frequency Frequency selection is solely based on the
application that the breaker will serve. Operating at the improper
frequency could change the characteristics of how the breaker would
operate. This is primary concern for breakers that rely upon
inductive trip mechanisms. Breakers are generally designed for 50
and 60 Hz. Other models are available based of specific
applications.
11. OIL CIRCUIT BREAKER It is designed for 11kv-765kv. These
are of two types BOCB (Bulk oil Circuit Breaker) MOCB (Minimum oil
Circuit Breaker) The contacts are immersed in oil bath. Oil
provides cooling by hydrogen created by arc. It acts as a good
dielectric medium and quenches the arc.
12. Advantages: Oil has good dielectric strength. Low cost. Oil
is easily available. It has wide range of breaking capability.
Disadvantages: Slower operation , takes about 20 cycles for arc
quenching. It is highly inflammable , so high risk of fire. High
maintenance cost.
13. VACCUM CIRCUIT BREAKER It is designed for medium voltage
range (3.3-33kv). This consists of vacuum of pressure (1*10-6)
inside arc extinction chamber. The arc burns in metal vapour when
the contacts are disconnected. At high voltage , its rate of
dielectric strength recovery is very high. Due to vacuum arc
extinction is very fast. The contacts loose metals gradually due to
formation of metal vapours.
14. Advantages: Free from arc and fire hazards. Low cost for
maintenance & simpler mechanism. Low arcing time & high
contact life. Silent and less vibrational operation. Due to vacuum
contacts remain free from corrosion. No byproducts formed.
Disadvantages: High initial cost due to creation of vacuum. Surface
of contacts are depleted due to metal vapours. High cost & size
required for high voltage breakers.
15. AIR BLAST CIRCUIT BREAKERS This operates using high
velocity blast of air which quenches the arc. It consists of blast
valve , blast tube & contacts. Blast valve contains air at high
pressure. Blast tube carries the air at high pressure & opens
the moving contact attached to spring. There is no carbonization of
surface as in VCB. Air should be kept clean & dry to operate it
properly.
16. Advantages: High speed operation as compared to OCB.
Ability to withstand frequent switching. Facility for high speed
enclosure. Less maintenance as compared to OCB. Disadvantages:
Little moisture content prolongs arcing time. Pressure should be
checked frequently for frequent operation. Risk of fire hazards due
to over voltages. It cant be used for high voltage operation due to
prolonged arc quenching.
17. SF6 CIRCUIT BREAKERS It contains an arc interruption
chamber containing SF6 gas. In closed position the contacts remain
surrounded by SF6 gas at a pressure of 2.8 kg/cm2 . During opening
high pressure SF6 gas at 14 kg/cm2 from its reservoir flows towards
the chamber by valve mechanism. SF6 rapidly absorbs the free
electrons in the arc path to form immobile negative ions to build
up high dielectric strength. It also cools the arc and extinguishes
it. After operation the valve is closed by the action of a set of
springs. Absorbent materials are used to absorb the byproducts and
moisture.
18. Advantages: Very short arcing period due to superior arc
quenching property of SF6 . Can interrupt much larger currents as
compared to other breakers. No risk of fire. Low maintenance, light
foundation. No over voltage problem. There are no carbon deposits.
Disadvantages: SF6 breakers are costly due to high cost of SF6. SF6
gas has to be reconditioned after every operation of the breaker,
additional equipment is required for this purpose.
19. MINIATURE CIRCUIT BREAKER An MCB is an Electromagnetic
Device which Operates and Disconnects the Circuit when the Current
reaches a pre determined value. It is most modern substitute for
conventional rewireable fuse with most accurate & efficient
system in over-load & short circuit protection. In fact, it
provides over-load protection & short circuits protection
whereas FUSE offers short circuit protection only. An MCB will
normally operate at 1.25 times it rated current i.e. 20 A. MCB
operates at 25 A compared with 30 A for cartridge fuse or 40 A for
rewireable type of fuse of the same rating.
20. Also the MCB which has tripped due to over-load or short
circuit can be readily identified as the dolly or toggle or
operating knob automatically moves to the OFF position. PRINCIPLE
OF OPERATION : MCB is a device which operates on the two metallic
plates for the protection against over-load & short circuit. 1.
Thermal for normal overload protection. 2. Magnetic for the short
Circuit protection .
21. The thermal operation is achieved by metallic strip, which
defects when heated by any over current flowing through it . In
doing so release the latch mechanism and causes the contacts to
open. When short Circuit occurs, the rising energizes the solenoid
operating plunger to strike the trip lever causing immediate
release of the latch mechanism
22. CLASSIFICATION OF MCBs: 1.L-Series MCB for Lighting
Circuits:- Suitable for resistive Load installation with Low &
Steady Currents like heaters , ovens, geysers, electric irons ,
etc. The L-Series MCBs are also used for protection of distribution
equipment like wires, Cables, metering equipment etc. 2.G-Series
MCB for Motor Circuits:- It is suitable for use in installation
with high inrush current peaks which require closer over-load
protection. These include inductive Loads such as motors, A.C.
transformers , halogen fluorescent sodium vapor lamps , machine
tools etc.
23. APPLICATION OF MCB: They are required in homes, offices,
shops distribution boards for protection of individual circuit
& complete wiring system in that buildings through main
circuits. Whatever be the application, these MCBs offer perfect
over-load & circuit protection. ON OFF
24. EARTH LEAKAGE CIRCUIT BREAKER : An Earth Leakage Circuit
Breaker is a safety device used in electrical installations with
high earth impedance to prevent Shock. Many electrical
installations have a relatively high earth impedance. These may be
due to use of local earth rod to dry ground conditions. These
installations have a major problem if no ELCB is used. During live
to earth fault current,two conditions occur. Because earth
impedance is high,
25. 1. Not enough current exist to trip a fuse or circuit
breaker, so condition persists uncleared indefinitely . 2. The high
impedance earth can not keep the voltage of all exposed CPC
connected metalwork to safe voltage all such metalwork may rise to
close to live conductor voltage. TYPES OF ELCB: I. Voltage
Operated:- They Provide a major advance in safety for mains
electrical supplies with inadequate earth impedance. II. Current
Operated:- They Provide protection against earth leakage, through
the details & method of operation are different.
26. PRINCIPLE OF OPERATION: It is a current operated device
designed to operate when a leakage current exceeds a pre-determined
value. It essentially consist of an operating coil & a trip
mechanism which operates contacts controlling the supply to the
circuit concerned. Consider a single-phase circuit. The two supply
wires i .e. phase & neutral are fed through separate winding of
current transformer as shown in the fig: another winding controls
the tripping mechanism.
27. Ip = In Therefore, the fluxes linking the phase &
neutral conductors are equal in magnitude & opposite in
direction. Thus, no fluxes links with tripping. Winding. ADVANTAGES
: They are less sensitive to fault conditions , therefore have less
nuisance strips. They can be arranged to protect against cable
damage only. And not strip on faults in down line
installations.
28. DISADVANTAGES: They do not detect faults that dont pass
current through the CPC to the earth rod. They do not allow a
single building system to be easily spilt into multiple sections
with independent fault protection, because earthing system are
usually bounded to pipe work. ELCBs introduce additional resistance
& an additional point of failure into earthing systems.
29. Precautions with Breakers Always completely de- energize
any breaker before servicing it. Always check voltages between
phases and between each phase and ground. Always ensure that the
Control Power source has been removed prior to Racking In or Out
any breaker. Always use a procedure when performing any type of
inspection on a breaker.
30. Breaker Precautions Precautions with Breakers Avoid
touching contact surfaces with bare hands. Oil from your finger
tips will damage the contact face. Avoid setting a breaker on its
side or back. This could damage the Secondary Disconnects,
Auxiliary Contacts, or any of the control modules. Some breakers
are equipped with oil filled over current devices. This oil may
leak out and damage the breaker.
31. Precautions with Breakers Breaker Precautions Arc Chutes
should be kept clean and generally free of defects.
32. Ensure that the breaker charging mechanisms is completely
discharged prior to breaker disassembly. Precautions with
Breakers
33. Precautions with Breakers Never override an interlock. A
breaker in the TEST position is still as dangerous as a breaker
that is racked in. Ensure the proper lubricant is used for the
right breaker. There is a difference!
34. For SF6 Breakers, ensure that O2 concentrations are kept
less than 18%. This will ensure that there is not a sustained arc.
Precautions with Breakers Breaker Precautions
35. Precautions with Breakers Breaker Precautions For Vacuum
Breakers, ensure that each interrupter has the proper vacuum IAW
its associated tech manual.
36. CONCLUSION: Therefore, we conclude that circuit breaker is
the most essential part of the electrical networks as it protects
every device from damage. It helps us to detect the fault and area
affected by it. Nowadays vacuum and SF6 circuit breakers are widely
used due to their reliable and fast operations.