Wednesday, May 18, 2011

ELECTRICAL INSULATION

ELECTRICAL : INSULATION 
                 The electrical conductivity of bone dry textile fibers is extremely small . The conductivity shown under conditions of normal regain must be considered due to the solution of water soluble electrolytes in the regain moisture to form am more or less continuous conducting film over the internal and external surfaces of the fibers .

SURFACE LEAKAGE :
                 The conductance is largely due to the facility with which an electric current can pass through the system by leakage from the surface of one fiber to another, This surface leakage depends on the following factors:
                The physical structure of the yarn or fabric, and especially the amount of twist in the yarn the closeness of the weave of a fabric the tightness with which the yarn or fabric is wound upon the metal to be insulated and the pressure under which it is placed.
                The springiness and surface structure of the fibers of smooth surface low resilience or high plasticity will tend to give the largest area of surface contact of fibers .
                The moisture content of the fiber at the temperature of operation of the electrical apparatus . The higher the moisture content the thicker the surface film of moisture and the more likely it is to be substantially continuous:
              The moisture content will normally be due in the main to the natural regain of the fiber but may be increased by the presence of hygroscopic salts . In apparatus designed to run at high temperatures such as one hundred to 115 deg .c . the eventual moisture content of fiber insulators will be small as most of the natural regain will be driven off under conditions of use .The eventual current leakage will therefore be only a fraction of the initial leakage the first effect of heating however will be to drive moisture to the surface of the fibers which will result temporarily in a great increase in leakage. The higher the initial regain of the fibers the more pronounced this effect will be and in the interior of electrical plant it may take some time for moisture to dry out.
             The content of water soluble electrolytes present as impurities in the fiber these may be naturally occurring or may be residues from some chemical processing.
             In recent years various processes have been devised for producing washed textile fibers. These consist in washing with weak acids to remove water and acid soluble electrolytes the washing being then followed with water washes until the PH of the fiber reaches the is electric point for the fiber at which adsorption of electrolytes ( including acids ) is at a minimum. These processes aim at reducing the ash content to a minimum and at leaving the fiber at its is o electric point . It has been found possible to wash cotton to give better insulation than natural silk . It is however not the total ash content but the water soluble electrolytes which are important the author produced a true silk fiber of very high insulation value yielding 11.4% of ash none of which however was water soluble .
             The hydrophilic and or salt forming groupings in the molecular structure of the fiber . for example the etherized and hydroxy or car boxy grouping in cotton are hydrophilic and the cooh groups are salt forming cotton is therefore intrinsically less suitable as an insulator than Cotopa ( Acetylated cotton ) or more especially than polyvinyl chloride rayon which contains no hydrophilic or salt forming groups and in consequence has an insignificant regain and water soluble electrolyte content.

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