October 2009 Electrical Apparatus

October 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus October 2009 featured technical article,  by Richard L. Nailen, P.E.  

Insulation materials or “dielectrics” are essential to all electrical technology from microcircuits to the utility grid. A common assumption is that insulators block electric current, whereas conductors allow its free passage (although resistance does vary with the material).

In all materials, however, dielectric stress caused by application of voltage causes electrons to be stripped from atoms and circulate in a closed path of one is present. Little of that can take place in an insulator; much in a conductor. Electron movement requires energy and causes heat in the material, and insulation will break down–become highly conductive and thermally damaged–under excessive dielectric stress.

The first electrical insulation was air, still a basic dielectric. Its characteristics are the norm to which any other dielectric is compared by a dielectric constant ratio. Dielectric materials can be solids (sheet and tape, plates, and molded shapes); liquids (such as transformer oil); or gases (air or sulfur hexafluoride).

As devices and circuits were developed, readily available materials such as wood, cloth, and glass were used to confine current flow to desired paths. However, the requirements of long-distance telegraph circuits in the mid-19th century required practical insulations for lengths of round wire. Natural rubbers came first, followed by shellac and oil-based enamels, then cotton fibers. In the early 1900’s, organic polymer chemistry began developing the synthetic enamels and plastics that led to the amide, imide, and polyvinyl formulations now common.

Modern dielectrics involve complex manufacturing processes as well as sophisticated materials, to combine resistance to heat, chemicals, solar radiation, and moisture with physical strength, conformability, voltage endurance, and compatibility with resins used for impregnation and encapsulation.

One natural material that has survived is mica. Offering up to eight times the dielectric stress withstand capability of air, and four times that of most plastics, mica remains the most popular insulation material for medium-voltage rotating machine windings. Another dielectric material found in nature is asbestos, once favored for high temperature service. The health hazard inherent in mining and processing asbestos has resulted in its disappearance from many applications.

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