Medium and high voltage XLPE cables carry two thin black semiconducting layers, one directly over the conductor and one directly over the insulation. Neither carries load current. Their job is to control the electric field. A stranded conductor has ridges and gaps between the wires, and if the insulation sat straight on it the field would concentrate at every ridge and slowly erode the XLPE through partial discharge. The conductor screen presents the insulation with a perfectly smooth round surface instead. The insulation screen does the same job on the outside, giving the metallic screen a smooth surface to sit on and keeping the field contained within the insulation rather than in the air gaps around it.
In APAR's MV and HV XLPE cables up to 66 kV the conductor screen, insulation and insulation screen are extruded together in one pass, so there is no gap or contamination between the layers. Cables at 1.1 kV do not need screens because the field stress is far lower.