In a high voltage cable as now known, an inner semiconducting tape or layer is wound or extruded about the metal conductor of the cable and a layer of insulation is extruded about this inner layer. A ground screening shielding element is then applied concentrically about the insulation layer. This element usually consists of a semiconducting layer and a metallic ground return screen, whereby an even equipotential surface about the insulation layer is provided. A careful examination of the current proportions shows that the outer semiconducting layer conducts a capacitive current across the layer in radial direction from the metal conductor to the surrounding screen. Furthermore, a resistive current appears in the layer. This current equalizes the voltages which, due to possible non-uniform field distribution, appears in peripheral direction. On the metallic shield or screen nearest to the outer semiconducting layer the surrounding cable sheath can be applied.
With cables having some form of extruded plastic or rubber insulation, the inner semiconductor is usually applied by the same operation as the insulation layer. It has been found to be preferable to apply also the outer semiconducting layer in the same operation, that is, as a so-called triple extrusion. The semiconducting layer and the insulation material then adhere well together and thus result in a mechanically and electrically reliable product. Triple extrusion has primarily been used at the highest voltages but only if specialist installation personnel is available.
One problem with high voltage cables as previously known, is to have available the expertise and equipment to assemble the cable reliably and yet economically. Preparation of the cable assembly requires that parts of the cable sheath are removed together with the screening layers and the insulation layer to be able to connect the conductor. With now known cable constructions, it is the practice, in order to facilitate the preparation of the cable, to manufacture the outer semiconductor as tapes which are directly applied upon the insulation or as layers painted or sprayed outside the insulation layer and semiconducting tapes outside the painted layer.
It is also known to extrude upon the insulation a "tire" of, for example, semiconducting rubber which tightens around the insulation layer. The disadvantage of these measures is primarily that corona can occur in the air gaps which are left at the overlap of the semiconducting layers. Also, gaps can occur between loosely applied semiconducting layer and the insulation due to mechanical and thermal stresses. The semiconducting paint may be difficult to remove, specially if it has burnt onto the underneath laying layer due to overheating. At the ends of the cable, where the semiconductor according to known methods has to be removed for a certain length from the connection point, high longitudinal field forces may appear at the thus formed screen edge. It is previously known to decrease the field force at an abruptly ending shield or screen by arranging layers having a selected resistivity outside the insulation and a length extending from the screen edge to the conductor. As a result, part of the ground return current of the cable will flow through the resistive layer and thus causes a spread potential rise which decreases the field force and prevents corona in the air. Other field force equalizing modes are also known which require special material or special accessories, high skill of the asembler and time-consuming work. Particularly troublesome is the complete removal of the semiconducting layer, especially when the layer material adheres to the insulating surface. Accordingly, attempts have been made to manufacture semiconducting layers which can be easily and completely separated from the insulation surface. However, the easier it is to strip the semiconductor material, the greater is the risk for damages due to stresses.