The individual phase conductors are assembled, wrapped with binder tape, then covered with an extruded lead sheath that also acts as a neutral. PILC cables have delivered very long service lives, but the older ones are now starting to have problems. Fatigue cracks in the lead let in moisture. That leads to failures in the paper insulation. Operating temperatures of PILC cables have to be conservative.
High temperatures can drive the insulating oil out of the paper.
The aging cables are mostly 5kV, 12kV and 25kV installations. The ducts they occupy were built for small cables. Conventional extruded cables won t fit in those ducts. PILC cable is still being manufactured and installed, but the lead sheath raises safety and environmental issues that were not even considered in the 1930s. Splicing the lead sheathing in the field requires a high degree of skill.