Tracking is slow damage caused by small leakage currents running across a wet, contaminated insulating surface. On an overhead line, rain mixed with salt spray, industrial dust or bird droppings forms a conductive film. Current creeping through that film dries it in patches, small arcs form across the dry bands, and over time they carbonise the surface into a conductive track that can end in failure.
A covered conductor sits outdoors for decades in exactly those conditions, so its outermost layer has to resist tracking as well as sunlight. APAR covered conductors use an outer XLPE layer that is UV-protected and anti-tracking, over an inner carbon-free XLPE layer and a semiconducting screen, all co-extruded and bonded. The outer layer also resists salt, humidity, fungal growth and pollution, which is why covered conductor suits coastal and industrial routes. See the covered conductor product page.