What Each Layer Does, from Conductor to Sheath

The low-voltage build

For a standard 0.6/1 kV XLPE power cable the manufacturer's construction statement is short:

Conductor — compact stranded copper conductor, Class 2 to IEC 60228 Insulation — XLPE, rated 90 °C Sheath — PVC or FR-PVC type ST to IEC 60502, black

Three layers, three jobs. The conductor carries current; compact stranding gives a smaller overall diameter than non-compacted stranding at the same cross-section. The insulation withstands the voltage. The sheath takes the mechanical and environmental abuse during installation and service, and in the FR-PVC variant contributes flame retardance.

What changes above 1 kV

At medium voltage the build gains three layers. The manufacturer's statement for a 3.6/6 kV and above cable reads:

Conductor — compact stranded copper, Class 2 to IEC 60228 Conductor screen — semiconducting layer Insulation — XLPE, rated 90 °C Insulation screen — semiconducting layer Metallic screen — copper tape Sheath — PVC or FR-PVC type ST to IEC 60502, black

The two semiconducting screens are the essential addition. At medium voltage the electric field at the conductor surface is high enough that any irregularity — the gaps between strands in a stranded conductor, for example — concentrates the field locally and can initiate partial discharge that erodes the insulation from the inside. A semiconducting layer over the conductor presents a smooth equipotential surface, so the field in the insulation is uniform. The screen over the insulation does the same at its outer boundary.

The metallic screen

The copper tape screen sits outside the insulation screen and performs two separate duties. It provides a defined path for earth-fault current, and it confines the electric field inside the cable so that the field does not extend into the surrounding materials.

This is also why single-core medium-voltage cables are not laid in magnetic ducts. An alternating current in the conductor induces current in any closed magnetic loop around it; a steel duct forms exactly such a loop, heats, and adds losses the installation was never rated for. The manufacturer's application note states this restriction explicitly for the XLPE power cable range.

Layers that appear on request

Beyond the standard build, layers are added to meet specific duties rather than as default. Armouring is applied where the cable must withstand external mechanical forces — the standard YJV family is described as able to bear traction during installation but not external mechanical force, so a directly buried or mechanically exposed run calls for an armoured construction instead.

Water-blocking layers serve the same targeted role for cables exposed to moisture along their length. In the LSOH fire-resistant build, a mica tape is applied over the conductor to hold circuit integrity under fire. Each of these is a response to a stated condition, which is why a specification should describe the duty rather than simply request a heavier cable.