A high voltage surge arrester, including a varistor element arranged so as to be connected to a high voltage source and to carry a high voltage when being positioned in its operative position, and an electric insulator that encloses and is in contact with the varistor element and forms an outer surface of the apparatus, wherein the electric insulator includes a silicone-based rubber. The silicone based rubber includes particles chosen from the group consisting of Al2O3, BN and ZnO, to such an extent that the thermal conductivity of the silicone-based rubber is equal to or above 0.8 W/mK.