A process for impregnating an electrical coil which comprises a winding comprising two or more layers consisting of more than one turn of an electrically conducting wire-form material with a polymer material which electrically insulates the individual turns of the wire-form material from one another with the aid of a thermally curable epoxy resin composition, which composition is solid at room temperature and comprises the following constituents:(a) an epoxy resin which is solid at room temperature, selected from(a1) polyglycidyl ethers based on novolaks;(a2) diglycidyl ethers based on bisphenols, and(a3) mixtures of more than one of components (a1) and (a2);(b) a crosslinking agent for component (a);(c) a suitable accelerator for the reaction of component (a) and component (b);(d) from 15 to 70 percent by weight, based on the overall weight of the composition, of filler selected from(d1) calcium carbonate,(d2) quartz flour,(d3) wollastonite whose particles have an average ratio of length to cross-section which is less than 5:1;(d4) mixtures of components (d1), (d2) and (d3) and(d5) mixtures of components (d1), (d2), (d3) and (d4) with other inorganic fillers; and which also preferably comprises(e) from 10 to 60 percent by weight, based on the overall weight of the composition, of inorganic reinforcements in the form of acicular or fibrillar particles with a length of from 0.05 to 2.5 mm which have an average ratio of length to cross-section of 5:1 or more,the overall amount of component (d) and (e) being not more than 80 percent by weight, based on the overall weight of the composition, inter alia does away with the need to use sacrificial moulds and reduces by a multiple the curing time required for completing the coating of the coils, reducing it to the region of a few minutes.