The casing is held by friction on the core (2), and comprises a seamless inner layer of fibre-reinforced plastic, and a seamless outer one of electrically-conductive and elastic material with a precisely-cylindrical external surface. The woven conductive metal is in contact with the core and the elastic material of the casing. The strands of woven metal are electrically connected to each other where they cross. Rubber can be used as the material (3) for electrically-conductive and elastic material of the casing, or alternatively rubber-like plastic.
