Circuit Breakers : Concepts - 1

Introduction of circuit breakers

All electric circuits needs a switching device and also a protective device. Switchgear is the general term covering a wide range of equipment connected with switching and protection. A circuit breaker is a switching and circuit interrupting device. A circuit breaker serves two purposes:

(i) Switching on and off during normal operation for maintenance etc.

(ii) Switching during abnormal conditions- short circuits, earthing etc. to protect the associated equipment.

In short, a circuit breaker is a sort of automatic switch which can interrupt the fault currents. Two important parts of a circuit breaker that need consideration are:

(i) Arc extinction system

(ii) Relay for operation

Arc Extinction in circuit breakers:

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

Since arc is produced in every circuit breakers, therefore suitable energy dissipating device must be incorporated in the design of circuit breaker. Unless carefully controlled, arc can lead to danger of fire or explosion.

The arc consists of a column of ionized gas i.e. gas in which the molecules have lost one or more of their negative electrons, leaving positive ions. The negative electrons are attracted towards the positive contact and being light, more towards it very rapidly. The positive ions attracted towards the negative contact. Due to electron movement the current flows.

The ionization process is accompanied by the emission of light and heat. Also some portion of power is dissipated as heat. The temperature of arc may be as high as 60000 C.

Methods of arc extinction in circuit breakers :

Two methods commonly used are:

(i) High resistance interruption :

In this the arc is controlled in such a way that its resistance is caused to increase rapidly, thereby reducing the current until it falls to a value that is insufficient to maintain the ionization process. The arc resistance may be increased by

(a) Arc lengthening

(b) Arc cooling

(c) Arc splitting

(ii) Low resistance interruption :

In this the arc resistance is kept low, in order to keep the arc energy to a minimum and use is made of a natural or artificial current zero when the arc extinguishes itself and is then prevented from re striking.

Protection of contacts of circuit breakers :

During arcing mechanical as well as electrical erosion of contacts occurs. Therefore the resistance to erosion by arching is the important property of contact materials. In case of dc circuits the process of erosion is represented by loss of material from one contact and the deposition of part of this material on to the other contact. However, in case of ac circuits there is no marked direction of transfer, as either contact becomes successively positive and negative.

There are two distinct forms of protections which may be employed with the object of reducing the rate of erosion of contacts by arcing thereby prolonging their useful life.

(a) Arc dispersion in circuit breakers :

In this the destructive effects of the arc are minimized, using one of the following methods:

1. Oil immersion of contacts

2. Multiple break contacts

3. De ionization of arc path

4. Magnetic blow out of arc

5. Blast principle using air, oil, gas or water.

(b) Arc prevention in circuit breakers:

In this the occurrence or arc is prevented by reducing the current and voltage below the minimum arcing values or reducing its destructive effects as far as possible. The principle devices used to quench circuits of this kind are :

(i) Discharge resistance

(ii) Rectifiers

(iii) Condensers

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