composition and design shape and will be key factors in its survival. Bushings can therefore vary considerably in both material and design style. Types Porcelain insulation The earliest bushing designs use porcelain for both indoor and outdoor applications. Porcelain was originally used due to its properties of being impervious to moisture once sealed by fired glaze, and low manufacturing cost. The main disadvantage with porcelain is that its small value of linear expansion has to be accommodated by using flexible seals and substantial metal fittings, both of which present manufacturing and operational problems. A basic porcelain bushing is a hollow porcelain shape that fits through a hole in a wall or metal case, allowing a conductor to pass through its center, and connect at both ends to other equipment. Bushings of this type are often made of wet-process fired porcelain, which is then glazed. A semi-conducting glaze may be used to assist in equalizing the electrical potential gradient along the length of the bushing. The inside of the porcelain bushing is often filled with oil to provide additional insulation and bushings of this construction are widely used up to 36 KV where higher partial discharges are permitted. Where partial discharge is required to conform to IEC60137, paper and resin insulated conductors are used in conjunction with porcelain, for unheated indoor and outdoor applications. The use of resin (polymer, polymeric, composite) insulated bushings for high voltage applications is common, although most high-voltage bushings are usually made of resin impregnated paper insulation around the conductor with porcelain or polymer weather sheds, for the outdoor end and occasionally for the indoor end. Paper insulation Another early form of insulation was paper, however, paper is hygroscopic and absorbs moisture which is detrimental and is disadvantaged by the inflexible linear designs. Cast resin technology has dominated insulated products since the 1960s, due to its flexibility of shape and its higher dielectrical strength. Typically, paper insulation is later impregnated either with oil (historically), or more commonly today with resin. In the case of resin, the paper is film coated with a phenolic resin to become synthetic resin bonded paper (SRBP), or impregnated after dry winding with epoxy resins, to become resin impregnated paper or epoxy resin impregnated paper (RIP, ERIP). SRBP insulated bushings are typically used up to voltages around 72.5 kV. However, above 12 kV, there is a need to control the external electrical field