Semiconducting layer in MV cables — conductor screen and insulation screen per IEC 60502-2

The guide on cable terminations — stress cone & field control covers how a stress cone manages the local field at the point where the outer screen of a medium-voltage cable is cut back. This article covers the layer itself that gets cut back there: the semiconducting screens every medium-voltage cable contains per IEC 60502-2, and why they are a functional, not a cosmetic, part of the cable construction.

Two semiconducting layers, two different interfaces

A medium-voltage cable to IEC 60502-2 by default contains two extruded semiconducting layers, with a resistivity typically in the range of roughly 10⁻¹ to 10⁸ Ω·cm — far more conductive than the insulation, but not as conductive as the copper conductor or the metallic screen itself:

Conductor screen (inner semiconducting layer): applied directly around the stranded conductor, before the insulation.
Insulation screen (outer semiconducting layer): applied directly around the insulation, between the insulation and the metallic screen (copper wires, tape, or foil) that subsequently provides the earthing path for the cable sheath.

In modern cables, both layers are chemically bonded to the insulation, so no microscopic air gap can form between screen and insulation where partial discharge could otherwise develop.

Why these layers exist: a uniform, radial field

A stranded copper conductor, even after compacting, has an irregular, slightly ribbed surface. Without a conductor screen, the electric field would locally concentrate around every small irregularity or strand transition, producing sharp field peaks — precisely the condition under which partial discharge and, over time, electrical treeing through the insulation can develop. The conductor screen electrically smooths out this surface, so the insulation "sees" a uniform, purely radial field instead of a series of local field concentrations. The insulation screen performs the same function on the outside of the insulation, between the insulation and the earthed metallic screen.

Note: the semiconducting layer is therefore not an alternative name for the cable's metallic (copper) screen. The metallic screen carries earth current and any fault current; the semiconducting layers only smooth out the electric field on either side of the insulation. A cable needs both, each with its own distinct function.

The interface with cable terminations

Exactly at the point where a cable is stripped for a termination or joint, the insulation screen must be cut back to expose bare insulation for the connection. That cut-back point is inevitably a location where the carefully smoothed field suddenly concentrates again — precisely the problem the stress cone (see the related guide) then has to manage by gradually "grading out" the screen over a defined length, rather than letting it end abruptly.

Damage risk: why this is a common failure factor

Removing the insulation screen while preparing a termination or joint is precision work where two things can go wrong:

Incompletely removed screen residue: a thin layer of semiconducting material left adhering to the insulation beyond the intended cut-back boundary creates an uncontrolled, undefined transition point where a local field concentration and partial discharge can still develop.
Damage to the underlying insulation: the wrong stripping tool, or too much pressure during manual screen removal, can leave grooves or scratches on the insulation surface — precisely where the field is already most critical.

This is one of the reasons corona discharge and partial discharge (see the related guides) are relatively often detected specifically at a cable termination rather than along the straight cable run: the termination is the point where the factory-controlled field grading provided by the semiconducting layers is first interrupted.

Practical relevance

When assessing a medium-voltage cable installation — particularly while preparing or inspecting a termination or joint — it is important to verify that the insulation screen has been removed exactly to the manufacturer's specification (cut-back dimension, bevel angle), with no visible screen residue beyond the intended boundary and no grooving on the exposed insulation.

Common mistakes

Not fully removing insulation-screen residue beyond the intended cut-back boundary, leaving an undefined, critical transition point in the field.
Using the wrong stripping tool or applying too much force during manual screen removal, causing scratches or grooves in the underlying insulation.
Confusing the semiconducting layer with the cable's metallic screen — the two serve different, complementary functions.
Not following the manufacturer's cut-back dimension and bevel angle when preparing a termination or joint, so the stress cone cannot properly manage the field.