Asbestos and glass-fibre insulation, impregnated with silicone resins, is now widely used in the construction of dry-type transformers rated as class C in BS 171: 1959, and operating at a maximum winding temperature of 200°C. Applications range from mining transformers to packaged substations, wherever oil is prohibited because of fire hazard. It is noted that the costs of class C air-cooled transformers are usually about 50% higher than for oil-cooled transformers of equivalent rating, so that these units are only selected where other factors justify this price differential. The insulation components are described, with some notes on their thermal-aging performance.Though present designs are mostly restricted to voltages of 11kV or below, it is considered that the use of fluorinated-hydrocarbon gases for cooling may lead to further developments at much higher voltages, e.g. for indoor-generator transformers.