The composite insulators for overhead ?transmission lines have been used in ?worldwide and have been replaced the ?traditional strings of glass and ceramic ?insulators due to their several advantages ?such as more resistance to pollution and ?vandalism, light weight. However, some of ?these new insulators have been failed by ?the catastrophic and not well understood ?mechanical brittle fracture. Many studies ?have attributed the cause of brittle fracture ?as the sum of several factors such as ?pollution, humidity, UV radiation, intense ?electric field, corona discharges, which ?could cause the formation of acids (nitric, ?sulphuric acids) that would attack the ?composite rod of insulators due to the ?stress corrosion crack. Nowadays, there are ?several types of composite insulators but, ?basically, they are made of three parts: (i) ?composite rod of glass fiber reinforced ?polymer, (ii) housing and sheds of silicone ?rubber to protect the composite rod and ?increase the electrical breakdown voltage; ?respectively and (iii) metallic end-fittings.?

Composite insulator components:?
A composite insulator is made of at least ?two insulating parts – a core and housing. ?It is equipped with end fittings.?
?? Core: The core is the internal insulating ?part of a composite insulator. It is intended ?to carry the mechanical load. It consists ?mainly of glass fibers positioned in a resin ?matrix so as to achieve maximum tensile ?strength.?

?? Housing: The housing is external to the ?core and protects it from the weather. It ?may be equipped with watersheds. Some ?designs of composite insulators employ a ?sheath made of insulating material between ?the weathersheds and the core. This sheath ?is part of housing.?
? ? End fittings: End fittings transmit the ?mechanical load to the core. They are ?usually made of metal.?
Advantage of composite insulator:?
Polymeric composite insulators offer ?significant advantages over porcelain and ?glass insulators, especially for ultra HV ?transmission lines.?
• Light weight, and lower construction ?and transportation costs.?
• Vandalism resistance, less gunshot damage.?
• Better contamination performance.?
• Improved transmission line aesthetics.?
• Hydrophobicity of insulator surfaces.?

Disadvantage of composite insulator:?
Composite insulators also have weaknesses
• They are subjected to chemical changes ?on the surface due to weathering and ?from dry band arcing or flashover.?
• Long reliability is unknown.?
• Faulty insulators are hard to find, for it ?is difficult to check for inconsistencies ?or damages.?
• Less experience in the field, still ?changing as more polymer products ?come out.?

China has hundreds of composite insulator ?manufacturers, and these manufacturers ?supply all Chinese power system and also ?export most of the countries in the world. ?
Insulator manufacturer types according to ?voltage: ?
• Manufacturers only make 11kv – 33kv. ?
• Manufacturers make 10kv – 110kv.?
• Manufacturers make 10kv – 500kv.?
• ? Manufacturers make 10kv – 1000kv.?
• ? Manufacturers make AC and DC voltage.?