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Operating Principle of Circuit breakers

Operating Principle


A Circuit breaker consist fixed and moving contact which are touching each other under normal conditions

i.e., when Circuit breaker is closed.

Whenever a fault occurs, the trip coils get energized, the moving contacts are pulled by some mechanism and, therefore, the Circuit breaker is opened and the Circuit is broken.

The basic Construction of a Circuit breaker Requires the separation of constant in an insulting fluid which serves two functions,
Viz.

(1) extinguishing the arc drawn between the contacts on the opening of Circuit breaker and Providing insulation between the contacts and from each contact to earth.

The fluids commonly used for this purpose are

a) Air at Atmospheric pressure

b) Compressed air

c) oil producing hydrogen for arc extinction

d) Ultra high vacuum

e) Sulphur hexafluoride (SF6)

The fluids used in Circuit breaker should have properties of

1) High Dielectric strength

2) noninflammability

3) High thermal stability

4) Chemical stability

5) Arc extinguishing stability and

6) commercial availability at moderate cost.



  • Arc Extinction 

whenever a circuit carrying Current is interrupted by a Circuit breaker an arc is inevitably formed between the contacts which prolongs to the current interrupting process for a duration ranging from 10 to 100 or more ms.


The resistance of an arc, so the Current flowing the constants depends upon the number of particles ionized, the X-section of the arc and the length of the arc.

The interruption of this arc within shortest possible duration is an important factor in the Design of the breaker. There are two methods of extinguishing the arc as given below.


(A) High Resistance Method


By increasing the arc resistance with the time the Current is reduced to such a value that heat Produced by it is not sufficient to maintain the arc and thus the current is interrupted or the arc is extinguished.

The method is employed in DC Circuit breakers and low and medium power industrial type air - ckt breakers.

The resistance of the arc can be increased by
 
(1) Reducing the concentration of the ionized particles,

(2) Increasing the arc length

(3) Reducing the arc X-section and

(4) Splitting the arc.



(B) Low resistance or Zero point Method


In this method of are extinction, 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 a current zero.

This method is used in all modern high power ac Circuit breakers. This is achieved by

(1) High pressure

(2) cooling and

(3) blast effect




  • Arc, Restriking and Recovery Voltages



Arc Extinction
Arc Extinction


As soon as the contacts separate out an arc is formed. The voltage across the contacts during the arcing period is known as the arc voltage and its relatively low with heavy Current arcs of short length.

At Current zero it rises rapidly to the peak value since a short - Circuit Current is almost 90° lagging.

The transient Voltage apprering across tj constant at Current zero during arc period is called the Restriking voltage.

This voltage will probably Restriking the arc so that it persists for another half cycle.

This voltage is given by the Equation

V = V max ( 1 - cos 1/√LC )

The normal frequency RMS Voltage that appears the breaker constants after final arc extinction has occurred is called the Recovery Voltage.




  • Rate of Rise of Restriking Voltage (RRRV)



RRRV is obtained by drawing a straight line through zero and tangential to the point of curve and is expressed in volts per micro Second

                                   Peak value of
                                 Restriking voltage
Average RRRV  =  -----------------------
                              Time taken in attaining
                                     Peak value

                                2 V max
                            =  ------------
                                  π√LC

RRRV max  =  V max / √LC

Now we can say that if RRRV is smaller than the rate at which the Dielectric strength between the contacts is developed,
The arc will be quenched, otherwise it will further Restrike in next half cycle.

The theory is called the Recovery rate theory advocated DR. J. Slepian.



  • Current Chopping 



When a Circuit breaker interrupts a low Inductive Current, current falls to zero before  the natural current zero. It is termed as Current chopping.

It happens because the Circuit breakers exerts the same force of arc quenching as it were with heavy short circuit Current.



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