CIRCUIT BREAKERS...Factors Affecting the Arc Resistance Degree of ionization •Less number of...
Transcript of CIRCUIT BREAKERS...Factors Affecting the Arc Resistance Degree of ionization •Less number of...
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CIRCUIT BREAKERS
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MCB
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What is Arc ?Ionization of air molecules cause interrupting current flow from one contact to other contact becomes so heated that it glows is called arc.
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• Arc phenomenon
• when a short circuit occurs, the
contacts of circuit breaker begin to
separate
• The p.d between the contacts is
sufficient to ionize the air or oil or gas
• This ionized air acts as conductor and
ARC is struck between them even
though contacts of breaker are
opened
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Factors Affecting the Arc Resistance
Degree of ionization
• Less number of ionized particles between the contactsincreases the arc resistance.
Length of arc
• Arc resistance is a function of arc length. More the length ofarc, more will be the arc resistance.
Cross section of arc
• Arc resistance is large when the cross section of arc is less.
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High Resistance Method
Low Resistance Method
•ARC INTERRUPTION METHODS:
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High Resistance Interruption
The resistance of arc is increased by
1.Cooling of arc
2.Increasing the length of arc
3.Reducing the cross sectional area of arc
4.Spliting of arc
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•Recovery rate theory (Slepian’s theory)
•Energy balance theory (Cassie’s theory)
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Low Resistance Interruption
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Recovery Rate Theory (Slepian’s Theory)• The arc is a column of ionized gas.
• To extinguish the arc, the electrons and ions are tobe removed from the gap at current zero.
• Ions and electrons can be removed by sweepingthem away by using gas or liquid in the gap.
• The arc is interrupted if the ions are removed fromthe gap at a faster rate than the rate of ionization.
• Slepian described the process as a race between thedielectric strength and restriking voltage.
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Recovery Rate Theory (Slepian’s Theory)• In this theory, the rate at which the gap recovers its dielectric strength iscompared with the rate at which restriking voltage across the contacts rises.
• If the dielectric strength increases more rapidly than the restriking voltage, thearc will be extinguished.
• If the restriking voltage increases more rapidly than the dielectric strength, theionization continues and arc will exist for another half cycle.
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Energy Balance Theory (Cassie’s Theory)
• Immediately after current zero, the contact space contains ionized gasand the space will provide some resistance (post zero resistance).
• A current flows through this resistance and causes a power dissipation.
• This power is zero when the initial restriking voltage is zero.
• This power again becomes zero after the complete de-ionization of thecontact space.
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Energy Balance Theory (Cassie’s Theory)• Between these two limits, the power dissipated will rise to maximum andcomes back to zero.
• Power dissipation results in production of heat and dissipation of heat.
• If the heat generated exceeds the rate at which heat can be dissipated, theionization will persist and an arc is formed again.
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Terms Associated with Circuit Breaking
Arc voltage
• The voltage across the contacts of C.B. when the arc persists is calledarc voltage. This voltage becomes system voltage when the arc isextinguished.
Restriking voltage
• After the arc is extinguished, the voltage across the breaker contactsdoes not become normal instantaneously. But a transient oscillation isproduced.
• The transient voltage that appears across the breaker contacts at theinstant of arc extinction is called restriking voltage.
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Terms Associated with Circuit Breaking
Recovery voltage
• The normal power frequency voltage that appears across the breakercontacts after the final arc extinction is called the recovery voltage.
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TYPES OF CIRCUIT BREAKERS
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TYPES OF CIRCUIT BREAKERS
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Classification of Circuit Breakers• Based on Voltage
• Low voltage C.B (less than 1 kV)
• Medium voltage C.B (1 kV to 52 kV)
• High voltage C.B (66 kV – 220 kV)
• Extra high voltage C.B (300 kV – 765 kV)
• Ultra high voltage C.B (above 765 kV)
• Based on Medium used for Arc Quenching• Air-Break /Blast C.B
• Oil C.B
• Vacuum C.B
• Sulphur Hexafluoride C.B
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AIR BLAST/BREAK CIRCUIT BREAKERS
• suitable for high current interruption at low voltage.
• Compressed air 20-30 kg/cm^2 for Air blast type
• air gap between contacts=(-1.75cm)
• used up to 11 kV with air break
• Used up to 1100kV with Air blast
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• Types
• Cross blast C.B
• Axial blast C.B
• Radial blast C.B
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AIR BLAST/BREAK CIRCUIT BREAKERS
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fig: cross flow ABCB
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Axial-blast typeCross-blast type
Radial-blast type
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Disadvantages
1.The risk of fire is eliminated.2.Cheap compared to oil circuit breaker3.Arcing time is very less thereby less burning of contacts
Advantages
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1.Very sensitive to the variations in the rate of rise of restriking voltage.
2.Maintenance is required for compressor plant
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Oil Circuit Breakers
• Mineral oil has better insulating properties than air.
• It has good cooling property.
• When arc is formed, it decomposes the oil into gases.
• Hence the arc energy is utilized in decomposing the oil.
• Less suitable for frequent operation.
• Interruption of capacitive and inductive currentscreates a problem.
• Hence resistance switching is necessary.
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Oil Circuit Breakers
• Types
• Bulk Oil C.B
• Minimum Oil C.B
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Bulk oil circuit breakers
• which use a large quantity of oil.
• The oil has to serve 2 purposes.
1. it extinguishes the arc
2. it insulated the current conducting parts
Cross sectional view of
bulk oil CB
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Oil Circuit Breakers
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Minimum oil circuit breakers
• use minimum amount of oil, only for arc extinction
• conducting parts are insulated by air or porcelain or organic insulating material.
Cross sectional view of
bulck oil CBPRKREDDY.GNITS
Oil Circuit Breakers
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MOCB – Operation
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MOCB – Operation
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MOCB – Operation
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MOCB – Operation
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Advantages:
1. It requires smaller space for installation.
2. Maintenance problems are reduced.
3. Requires less quantity of oil.
4. Risk of fire is reduced.
Disadvantages:
1. The degree of carbonization is increased due to less quantity of oil.
2. Difficulty of removing the gases from the contact space in time.
3. Possibility of explosion.
4. The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization.
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Vacuum Circuit Breaker
• Vacuum of the order of 10-5 to 10-7 torr is used
• At this low pressure (high vacuum), the mean free path of electron is
large of the order of few metres.
• Hence the electrons cross the gap without any collision.
• It provides highest dielectric strength than any medium.
• space between the contacts recovers its dielectric strength 1000 times
faster than any other medium.
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Advantages
1. VCB are compact, reliable and have longer life .
2. They have low arc energy.
3. Risk of fire is eliminated.
4. Maintenance is less.
5. There is no generation of gas during and after operation.
6. They can withstand lightning surges.
Disadvantages
1. very expensive .
2. For above 36KV VCB `s are not used.
3. Require high technology for production.
4. In case of loss of vacuum, the entire breaker is useless .PRKREDDY.GNITS
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Sulphur Hexafluoride (SF6) C.B
• SF6 an inert gas does not undergo any chemical
reactions and thermally stable
• good arc quenching and insulating properties, 100 times
more than air.
• Its density is 5 times that of air and heat convection is
1.6 times that of air.
• It is non–inflammable, no –poisonous and odourless.
• But by-products produced in SF6 during arcing is
dangerous for the operating personnel.
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Sulphur Hexafluoride (SF6) C.B – Operation
• In closed position, the contacts are surrounded by SF6 gas at a
pressure about 2.8 kg/cm2.
• When C.B operates, the contacts are open and arc is struck between
them.
• Valve between reservoir and arcing chamber opens up to allow SF6
gas at 14 kg/cm2 to the arc interruption chamber.
• High pressure SF6 gas absorbs the free electrons in the arc to
formimmobile negative ions which are ineffective.
• Medium between the contacts recovers dielectric strength and arc is
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1) The interrupter unit
This consists of fixed contacts which comprise a set of current carrying fingers and an arcing probe. As the contacts move further apart, the arc is extended and attenuated. It is finally extinguished by the gas flow the high pressure to the low pressure systems.
2) The gas system.
The low and high pressure system are fitted with low pressure alarms and a set of lock-out switches which give a warning the moment the gas pressure drops below a certain value
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Sulphur Hexafluoride (SF6) C.B – Construction
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Advantages
1. SF6 circuit breakers have very short arcing time.
2. Dielectric strength of SF6 is 2 to 3 times than that of air , these breakers can interrupt larger currents
3. Gives noiseless operation due to its closed gas circuit .
4. Risk of fire is eliminated because SF6 gas is non-flammable.
Disadvantages
1. SF6 breakers are costly due to high cost of SF6 .
2. Sealing problems arise , Imperfect joints give rise to leakage of gases.
3. After every operation SF6 gas has to be reconditioned, so additional equipment is required.
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Fuse and Types of Fuses
A fuse is an electric / electronic or mechanical device, which is used to protect circuits from over
current, overload and make sure the protection of the circuit.
Fuse Rating = ( Power / Voltage ) x 1.25
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Characteristics of a Fuse
Current Rating & Current Carrying Capacity of Fuse
Voltage Rating of Fuse
Breaking Capacity of a Fuse
I2t Value of Fuse
Response Characteristic
Rated voltage of Fuse
Packaging Size
METAL MELTING
POINT IN
°C
SPECIFIC
RESISTANE
IN µΩ-mm
VALUE OF FUSE
CONSTANT K
FOR d in m
SILVER 980 16 -
TIN 240 112 12.8
ZINC 419 60 -
LEAD 328 210 10.8
COPPER 1090 17 80
ALUMINIUM 665 28 59
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Classification of FUSES
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The size of DC Fuse is large than AC fuse
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Low voltage fuses:
Cartridge Fuses are available up to 600A and 600V ACwidely used in industries, commercial as well as home distribution panels.
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Low voltage fuses:
Link Type Fuse
The link type fuse is also known as high rupturing capacity (HRC) or BS type fuse
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Low voltage fuses:
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Rewirable Fuses
Low voltage fuses:
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Striker type Fuse
Low voltage fuses:
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Switch type Fuse
Low voltage fuses:
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Low voltage fuses:
Drop-out fuse
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These fuses are normally charged for voltages over 1500V to 138000V.
HV (High Voltage) Fuses
The fuse part in HV fuses are fabricated with either copper, silver or in some cases Tin is used, in order to offer consistent and steady performance.
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Cartridge Type HRC Fuse
HV (High Voltage) Fuses
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Liquid Type HRC Fuse
HV (High Voltage) Fuses
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Expulsion Type HRC Fuse
HV (High Voltage) Fuses
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Motors & Transformers
Air-conditions
Home distribution boards
General electrical appliances and devices
Laptops
Cell phones
Game systems
Printers
Digital cameras
DVD players
Portable Electronics
LCD monitors
Scanners
Battery packs
Hard disk drives
Power convertors
Uses and Applications of Fuses
Different types of Electrical and Electronic Fuses can be used in all types of electrical and electronic systems and applications including:
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