The high and low voltage bushing insulators can be in glazed porcelain, in resin or can be of the silicone type. In oil transformers and reactors, the through insulators can be positioned on the lid (typical European design) or on the vertical sides of the tank, provided or not with cable box (British Design). They can be replaced without removing the active part.
On dry transformers the through insulators are positioned on the supports fixed to the carpentry of the transformer itself or on the protection box where the desired IP is guaranteed.
The high and medium voltage insulators can be equipped with spark gap rods or dischargers to protect against any overvoltages. The insulators can be of the elastimold type both for oil and air machines.