Reliability is the most important property of an insulator whether it is a polymeric (composite) insulator or ceramic one. The reliability of an insulator depends upon its electrical and Polymer Line Post Insulators mechanical strengths. With the advent of modern manufacturing, mechanical molding and fixture technique, the mechanical strength is quite reliable. However the electrical strength over decades is not fully guaranteed. The modern style polymeric insulators were introduced about 25 years ago with most recent version about 13 years ago. The reason for this was not failure of ceramic insulators, but the other benefits such as 90% weight reduction, better pollution performance and low associated costs of polymeric insulators over ceramic ones. Experience of outdoor insulation started from the introduction of telegraphic lines. The pin and cap type insulators have been used since the last quarter of the 18th century. These insulators are very reliable. Glass and porcelain insulators were the only type available before the introduction of newer polymeric insulators.
Ruled over the market till late second half of the 20th century. The polymeric insulator has a fiber rod structure covered with weather resistant rubbers and fillers and fitted with end fittings. Such a type of insulator is also called composite insulator. The most critical thing to be considered in outdoor insulators is the interface between the solid insulting body and the surrounding air. The problem appears at the interface because it is the interfering point of air and the solid insulator. This problem arises due to the effects of pollution, rain, dust, salt, corona, arcing over surfaces, nitric acid in air, etc. These things increase the leakage current and deteriorate its performance. Surfaces of insulating bodies were therefore coated with glazed material for glass and porcelain insulators, and organic or semi-organic polymer rubbers for composite insulators. A typical composite insulator is composed of a glass fiber reinforced (GFR) epoxy or polyester core (rod), attached with metal end-fittings. This is the load bearing structure. GFR plastics are mechanically very strong but are not able to bear the outdoor environmental effects. The presence of dirt and moisture in combination with electrical stress causes the material to degrade by tracking and erosion. So the rod is covered by a coating that protects it from outside stresses such as rain, salt, fog, pollution, etc. This coating is referred to as housing. A housing material should be able to protect the load-bearing core and provide sufficient pollution with stand. The reason of use of rubbers instead of ordinary plastics is simply the fact that the housing must be flexible enough to follow the changes in dimension caused by temperature or mechanical load. The early developments of modern polymeric insulators started in 1964, and prototypes for field installations started in 1967, and a report from 1996 stated that insulators installed in 1969 were performing well.
One way is to first manufacture the sheds separately and push them onto the core.