A method in the manufacturing of an insulated electric high voltage DC termination or joint includes providing an insulated electric high voltage DC cable including a high voltage DC conductor, a polymer based insulation system surrounding the high voltage DC conductor, the polymer based insulation system including an insulation layer and a semiconducting layer surrounding the insulation layer, and a grounding layer surrounding the semiconducting layer; removing the grounding layer and the semiconducting layer in at least one end portion of the high voltage DC cable, mounting a field grading adapter or joint body in the at least one end portion of the high voltage DC cable; and subjecting the insulation layer of the polymer based insulation system in the at least one end portion of the high voltage DC cable for a heat treatment procedure, while being covered by the mounted field grading adapter or joint body.