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 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 and thus had fully ruled over the market till late second half of the 20^th 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 withstand. 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. The early types had an epoxy bonded E-glass fiber core covered with a thin room temperature vulcanized (RTV) silicon rubber housing.

A major change in production technology occurred in 1978, when the housing material was replaced with ATH-filled high temperature vulcanized (HTV) silicon rubber.

Composite insulators can be manufactured by different techniques. One way is to first manufacture the sheds separately and push them onto the core.

This technique was abandoned because these insulators experienced a lot of problems. The weak spots were the interfaces between the sheds where moisture could penetrate into the insulator causing internal tracking. A better way is to first cover the core with housing, add the sheds onto it and then vulcanize the parts together. This reduces the number of interfaces where moisture can penetrate to the GFR rod.

Today the most commonly used technique is one-shot molding. The whole insulator housing is then injection molded directly around the core in one piece. In this way, the housing can be chemically bonded to the core, and the number of interfaces where moisture can penetrate is minimized. This technique is the most attractive to manufacturers because of the lower number of steps involved and short time of processing. There are three main types of silicone rubbers used in high voltage insulation applications: high temperature vulcanizing (HTV) silicon rubber, room temperature vulcanizing (RTV) silicone rubber and liquid silicone rubber (LSR). HTV is cured at high temperature and pressure, catalyzed by peroxide induced free radicals or by hydrosilylation catalyzed by a noble metal, i.e. platinum. RTV is cured at lower temperature, i.e. around room temperature, by condensation reaction as one component system. The one component system is

cured by moisture diffusion from the surrounding air into the material and is rarely used for the production of insulators.

Fillers are added to the rubbers to control different properties of the product, such as mechanical stability and resistance to tracking, as well as to reduce the cost. Fumed silica is necessary for achieving good mechanical properties during processing, and alumina trihydrate (ATH) is added as a flame-retardant. Adding ATH also has the positive effect of improving the dielectric strength and tracking resistance.

End fittings are made of metal and the most common materials are cast, forged or machined aluminum and forged iron or steel. The end fittings can be attached to the core by different methods. Today, the most common technique is swaging (crimping) and gluing. The wedges are inserted in fiber rod by some manufacturers. However, the swaging is the strongest type of attachment and hence most popular. The two modern designs, straight shed and alternate shed are shown in Figure.