Circuit Breaker an Kit Bansal 2

7/31/2019 Circuit Breaker an Kit Bansal 2

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By:- Ankit bansal

UE104006

EEE


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Inspired by the works of American scientist Joseph Henryand English scientist Michael

Faraday, the circuit breaker was invented in 1836 by an American, Charles Grafton.

The American National Standards Institute (ANSI) defines a circuit breaker as: A

mechanical switching device, capable of making, carrying and breaking currents

under normal circuit conditions. Also capable of making and carrying for a

specified time and breaking currents under specified abnormal circuit conditions,

such as those of a short circuit.

A circuit breaker is an equipment which is designed to protect an electric circuits from

damage caused by short circuit or overload.

The basic circuit breaker working principle is that it is an automatically operated electric

switch.It detects the fault condition and interrupt the continuity to stop the flow of

electricity immediately.

It is not like a fuse which operates once and need to be replaced ,a circuit breaker can

be reset(either manually or automatically) to resume its normal operation .

Introduction
http://en.wikipedia.org/wiki/Michael_Faradayhttp://en.wikipedia.org/wiki/Michael_Faraday


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In every country the there will be a standard value for the voltagedistributed by the power plants. In INDIA it is 240 Volts. Though this is aconstant value, the value of the resistance and current will always varyaccording to our connections.

When we buy electrical components like bulbs and tubes, their companys

provide a standard resistance value for their effective working. This value,often referred to as the load, is the real reason for the appliances to work.

The components are designed in such a manner that the total voltage,current and the resistance across it are always constant.

If the current through a component increases, the heat may increase

causing the melting down of the wires inside. This causes a contact betweenthe neutral and phase wires.

The fire thus produced may even cause the whole building to collapse. Toavoid this, circuit breakers are used.

Why do we need Circuit Breakers?


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Construction of Circuit Breaker


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Frame:-The frame has a molded case exterior. Its primary function is toprovide a rigid,mechanically strong, insulated housing in which the othercomponents are mounted.

Operating Mechanism:- The operating mechanism provides the means of

opening and closing the circuit. It has a three-position switch (on, off,and tripped). It can only be in the tripped position as a result of anovercurrent. When power is removed manually, it can only be switched tothe off position. This makes it is easy for a maintenance person todetermine why power has been cut.

Trip Unit:- The trip unit is the brain of the miniature circuit breaker. It

activates the operating mechanism in the event of a prolonged overload orshort circuit. This type of circuit breaker uses a thermal magneticmechanism. This is the predominant trip unit technology used in thedomestic market. A bimetal and an electromagnet work together to provideoverload and short-circuit protection.

Contacts:- When an overload or short circuit situation occurs, the contacts

open to break the current flow. When this happens, an electrical arc isformed. The arc continues until the first possible zero point in the AC cycle.The zero point is the weakest point in the AC cycle and will not support thecontinuance of an arc. By breaking the arc, flow is stopped. This is calledzero point construction.


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Miniature Circuit Breakers are based on thermal magnetic technology. The protection isProvided by combining a temperature sensitive device (bimetal) and a current sensitiveelectromagnetic device. Both components trigger the mechanism mechanically.

Tripping Mechanisms A tripping mechanism is an assembly within the circuit breaker molded case that

causes the circuit breaker to open automatically under sustained overload or shortcircuit conditions.

The tripping mechanisms in two- and three-pole circuit breakers operate such that anovercurrent on any pole of the circuit breaker will cause all poles of the circuit breakerto open simultaneously.

Thermal Trip:-

The thermal trip element of a circuit breaker is a root-mean-squared (rms) currentsensing device.

The thermal element, or bimetal, is constructed from metals with dissimilar rates ofexpansion bonded together.

Working principle of circuit breakers


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The thermal element responds to overloads by reacting to the heat generatedboth by the current flowing through the circuit breaker and by the heatcontribution from ambient conditions. The bending force of the bimetal causes

the circuit breaker to trip.

The deflection of the bimetal is predictable as a function of current and time.This is the inverse time tripping characteristic of the thermal element, i.e., thetripping time decreases as the magnitude of the current increases.


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The magnetic (instantaneous) trip element uses an electromagnetic assemblyto trip the circuit breaker instantaneously (with no intentional delay) at orabove a predetermined current value.

During a short circuit of sufficient magnitude, the high-level current passingthrough the conductor rapidly increases the magnetic field of the

electromagnet that attracts the armature. As the armature is drawn towardthe electromagnet, it initiates an unlatching action and opens the circuitbreaker contact.

Magnetic Trip:-


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based of insulating medium employed in the circuit breakers , CBs are

of 4 types:-

Oil Circuit Breakers

Air Blast Circuit Breakers

Vacuum Circuit Breakers

Sulphur Hexafluoride (SF6) Circuit Breakers

Types of circuit breaker


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A circuit breaker in which the currentcarrying contacts operate in SulphurHexafluoride or SF6 gas is known asan SF6 Circuit Breaker.

SF6 has excellent insulating property.SF6 has high electro-negativity. That

means it has high affinity of absorbingfree electron.

Whenever a free electron collides withthe SF6 gas molecule, it is absorbed bythat gas molecule and forms a negativeion.

The attachment of electron with SF6gas molecules may occur in towdifferent ways,

1) SF6 + e = SF6-

2) SF6 + e = SF5- + F


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These negative ions obviously much heavier than a free electron andtherefore over all mobility of the charged particle in the SF6 gas is much lessas compared other common gases. We know that mobility of charged

particle is majorly responsible for conducting current through a gas .

SF6 gas is approximately 100 times more effective arc quenching mediathan air. Due to these unique properties of this gas SF6 Circuit Breakerisused in complete range of medium voltage and high voltage electrical powersystem.

Working of SF6 Circuit Breaker

the arc energy is utilized to develop pressure in the arcing chamber for arcquenching. Here the breaker is filled with SF6 gas at rated pressure .

There are two fixed contact fitted with a specific contact gap. A slidingcylinder bridges these to fixed contacts


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The cylinder can axially slide

upward and downward along thecontacts. There is one stationarypiston inside the cylinder which isfixed with other stationary parts ofthe breaker, in such a way that it

can not change its position duringthe movement of the cylinder.

As the piston is fixed and cylinder ismovable or sliding, the internalvolume of the cylinder changes

when the cylinder slides.


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During opening of the breaker the cylinder

moves downwards against position of the

fixed piston hence the volume inside the

cylinder is reduced which produces

compressed SF6 gas inside the cylinder.The cylinder has numbers of side vents

which were blocked by upper fixed contact

body during closed position.

As the cylinder move further downwards,

these vent openings cross the upper fixed

contact, and become unblocked and thencompressed SF6 gas inside the cylinder

will come out through this vents in high

speed towards the arc and passes through

the axial hole of the both fixed contacts.

The arc is quenched during this flow of

SF6 gas.


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Advantages: Disadvantages:

SF6 breakers are costly due to high

cost of SF6 .

Sealing problems arise , Imperfect

joints give rise to leakage of gases.

After every operation SF6 gas has

to be reconditioned, so additionalequipment is required.

Due to superior arc quenchingproperty, SF6 circuit breakershave very short arcing time.

Since the dielectric strength ofSF6 is 2 to 3 times than that ofair , these breakers can interruptlarger currents

The SF6 circuit breaker givesnoiseless operation due to itsclosed gas circuit .

Risk of fire is eliminated becauseSF6 gas is non-flammable.


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Ground Fault Interrupter Circuit Breakers:-

This breaker has a solid state trip unit. It detects ground currents(which are small short circuits from one phase to ground), andtrips to protect both people and equipment.

Ground fault interrupter (GFI) circuit breakers go one step further todetect problems due to electricity being "grounded" by somethingthat's not part of the electrical system. They're used in places thatare exposed to water; for instance, they're what prevent you fromgetting electrocuted even if you get a hairdryer wet .

Special applications


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By:-

Ankit bansal

UE104006

EEE

Circuit Breaker an Kit Bansal 2

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