Conductor shielding material keeps the electric field inside a cable smooth, and its quality decides how long medium and high voltage insulation survives. Around every stranded conductor, a semiconductive layer pressed tightly against the strands evens out the sharp points between wires, so field strength stops concentrating where ionisation and partial discharge would start. Shielding conductor material in power cables is normally cross-linked polyethylene loaded with carbon black; the compound is cured together with the insulation so the three layers bond as one. Two numbers define acceptability: volume resistivity, typically between 10 and 1000 ohm-metres at operating temperature, and surface roughness, kept under a few tens of micrometres because a single protruding grain can ignite partial discharge. The outer insulation screen shares the job, and both must stay conductive when cold after water-tree-free service. Specify compatible materials across the whole insulation system, not the shield alone.
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