Different types of insulators with hydrophobic properties have now become commercially available, including composite insulators, hybrid insulators and silicone greased ceramic insulators.

Composite insulators have become the dominant type selected for use on overhead lines and have also been growing rapidly in application at substations.

In the past, the main options for support insulators at air-insulated HVAC substations were either solid core porcelain or composite insulators up to 245 kV. In addition, RTV coatings have at times been applied to solid core porcelain post insulators to improve their pollution performance.

Test stations have been acknowledged as an important tool for evaluating insulator performance under different service environments. Typically, these stations are sited in areas having severe pollution problems and therefore allow accelerated accumulation of contamination.

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Transmission lines often must traverse mountain ranges or plateaus situated at least 1000 m above sea level. In such cases, design of transmission and distribution systems has to take into account fundamental changes in insulation performance associated with high altitude.