a solid insulator for an electric equipment coil is proposed which comprises a base film of a polyester , and a protective film of a resin selected from the group consisting of polypropylene , polyester , fluoropolymer , and aramid resin , which is formed on at least one major surface of said base film . there is also proposed an electric equipment coil comprising a winding which is resistive to hydrolysis and chemicals , an insulating film surrounding said winding , and a water - soluble insulating varnish filled and cured under heating between turns of said winding , wherein said insulating film comprises above said solid insulator .