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For a given electrode gap distance in uniform field a dielectric has the highestbreakdown strength. However, it is very difficult to realize a uniform field in practice.

It is accomplished only for experimental purposes in research laboratories with

tremendous effort and utmost care. The size of the electrodes may have to be

increased extraordinarily large, whereas the slightest irregularity on the electrode

surface may change the field characteristics in case of small gap distance.

Between the two extreme field configurations explained above, another importanttype of field is classified as 'weakly nonuniform field'. Like in uniform field, in

weakly nonuniform fields also no stable partial breakdown occur before the

breakdown (Ui=Ub). Electrodes like concentric spheres and coaxial cylinders having a

'radial field' are typical examples of weakly nonuniform fields, if the concentric

electrode dimensions are suitably designed. The exact value of , defining weakly

nonuniform field, depends upon the particular dielectric and its physical conditions.

Nevertheless, the main criterion must be fulfilled that no stable partial breakdown

occur before the breakdown.

Degree of Uniformity of Electric Fields

The degree of uniformity introduced by Schwaiger in 1922 as a measure of theuniformity of a field, is defined as following

or

and are the peak values of the Mean and the Maximum field Intensities in a

dielectric respectively. is the peak value of potential difference applied between the two

electrodes at a distance 'd' apart.

The value of also represents the degree of utilization of the dielectric in betweentwo electrodes. A higher value of represents better utilization of the insulating

properties of a dielectric. Thus , a dimensionless quantity enables a comparison of

the uniformity of field configuration formed between different electrodes. Table 3.1gives the values of for typical fields. The value of lies between, 0 1

Table 3.1

Field Classification UniformWeakly non-

uniform

Extremely non-

uniform

Electrode

Configuration

Parallel plates

Concentric cylinders

Needle-plane

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1 0.25

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Fig 2.2 Schwaiger curves for spherical, cylindrical and curved electrode field configurations.For a fixed value of 'p', the following important basic relations of dependency of are

observed from these curves,

fields between cylindrical electrode systems; cylinder-cylinder (3) , cylinder - plane(4) , concentric cylinders (5) etc., have a higher value of , that is , they are more

uniform than the fields in spherical electrode systems; sphere - sphere (6) , sphere -

plane (7) , concentric spheres (8) , etc.,

a symmetrical electrode system, for example, sphere - sphere or cylinder - cylinder,has a higher value of than the corresponding unsymmetrical system, that is sphere -

plane or cylinder - plane systems.

the field between two similar electrodes, cylinders or spheres, placed adjacent to eachother is more uniform or has a higher value of than when the electrodes are placed

coaxial or concentric.

Recap

In this lecture you have learnt the following

Types of Electric Fields Schwaiger Factor, a measure for the degree of utilization of dielectrics

Congratulations, you have finished Lecture 3. To view the next lecture select it from the left hand side

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