An insulating resin for switch gears on the basis of glycidyl ester contains methyl nadic anhydride/hydrogenated methyl nadic anhydride as hardener and an N-substituted imidazole as accelerator. Furthermore, platelet-shaped alumina is added as a filler. The resin has a substantially elevated glass transition temperature, while at the same time having a high mechanical level and being very tracking resistant. It is suitable as a casting resin in gas-insulated transmission line systems.