The basic designs of insulators evolve around three essential components. These are: a core, a sheath or weathersheds and metal end fittings. The three parts formed all kinds of insulators.

The end fittings are attached to the core in various ways to develop the required mechanical strength for the intended application. The core consists of axially aligned glass fibers bonded together by means of an organic resin.
The unprotected core with end fittings by itself is not suitable for outdoor high voltage application, as moisture, ultraviolet rays, contamination, acid, rain, ozone and voltage are conducive to the degradation of the core material and leading to electrical and mechanical failure. Hence, a protective sheath or weathersheds made from various polymer materials that have been compounded for outdoor electrical applications are applied over the core in various ways to protect the core and to provide maximum electrical insulation between the attachment ends.
It is quite clear that with such a diversity of possible constructions, the performance of polymer insulators depends on the selection of materials and on the design and construction of the insulators.

The designs of insulators have many changes now. Although the basic structure is the same, but the performance is improved a lot. 

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