A power transformer having an operating temperature in excess of 200.degree. C. and including a casing or tank, an electrical core and coil assembly having an asbestos based barrier material and being supported in said casing, and a low cost saturated hydrocarbon insulating fluid substantially filling the casing and having a vapor pressure at 200.degree. C. less than 10 millimeters of mercury and a flash point in excess of 230.degree. C.