Stress cone at a medium-voltage cable termination — why the cut-back screen itself creates an electric field problem

The guide on cable joints covers the mechanical construction and sealing requirements (per IEC 61442) of a complete connection between two cable ends. This article covers an underlying problem that must be solved at every medium-voltage cable termination — and at every joint — regardless of which type of joint or termination is used: the strong concentration of the electric field that arises at the point where a cable's conductive screen is cut back, and the techniques (grouped under the term stress control) used to remove it. IEC 60502-4 sets the test requirements for cable accessories — terminations, joints, and separable connectors — for cables rated from 6 kV up to and including 30 kV.

Why a cable's screen keeps the field uniform in the first place

A medium-voltage cable with extruded insulation (for example XLPE) has a conductive screen surrounding the insulation, held at earth potential. As long as this screen fully and continuously surrounds the insulation, the electric field between the core (at high voltage) and the screen (at earth potential) stays radial and uniformly distributed — exactly the field distribution the cable's insulation thickness was designed for.

The problem: a concentrated field at the point where the screen ends

At a termination or joint, the screen must be cut back at a certain point to expose the core for the connection. At exactly that point, the screen's earth potential ends abruptly while the core continues at full voltage — causing the equipotential lines of the electric field to bunch up sharply there. Without further measures, the local field strength at the edge of the cut-back screen can be many times higher than the field strength in the rest of the cable insulation — well beyond what the insulation can durably withstand at that point, with an increased risk of partial discharge (see the guide on PD and tan-delta testing) and, eventually, breakdown.

Four common methods for removing this field concentration

Geometric field grading (stress cone): a conductive, cone-shaped component is applied directly at the edge of the cut-back screen and tapers out over part of the exposed insulation. This shape forces the equipotential lines to detach from the insulation more gradually, over a larger area, instead of bunching up at a single sharp edge.
Resistive field grading: a layer with a non-linear electrical resistance around the transition distributes the voltage drop across the transition more gradually, instead of concentrating it at the edge of the screen.
Refractive field grading: a material with a differing dielectric constant around the transition "bends" the field lines in a way similar to optical refraction, so they converge less sharply at the edge of the screen.
Capacitive field grading (high-K stress control): a layer with a high dielectric constant around the screen transition spreads the electrical stress via a capacitive coupling along the surface of the insulation, instead of letting the voltage concentrate at the edge of the screen.

These four methods are not interchangeable at will: the type of cable accessory (pre-molded cold-shrink termination, resin joint, heat-shrink termination) typically determines which method — or which combination — the manufacturer applies, and the screen should therefore never be cut back without the specific stress-control component designed for that particular product.

Why this is specifically critical at medium voltage, and usually not at low voltage

At a low-voltage cable, the voltage across the insulation is low enough that the field concentration at a cut-back screen typically stays well within the insulation's breakdown strength even without special stress-control measures — which is why low-voltage connections rarely need a separate stress cone. At medium voltage (and above), the same field concentration, at the same geometric sharpness of the transition, does become critical, simply because the absolute voltage — and hence the absolute field strength at every point — is proportionally higher.

Note: the exact shape and dimensions of a stress cone, and the choice between geometric, resistive, refractive, or capacitive field grading (or a combination), follow from the design of the specific cable accessory and are tested by the manufacturer per IEC 60502-4; this article covers the underlying principle, not a ready-made installation instruction for every product.

Practical relevance

When installing a medium-voltage termination or joint, the stress-control component must always be positioned exactly as specified in the manufacturer's installation instructions relative to the edge of the cut-back screen — a stress cone shifted too far forward or backward relative to the screen edge does not correctly restore the uniform field distribution and can still lead to premature partial discharge and breakdown, even if every other part of the termination is correctly installed.

Common mistakes

Cutting back the screen of a medium-voltage cable without installing the specific stress-control component designed for that product — this leaves the sharp field concentration at the screen edge unresolved.
Using a stress cone from one product or brand on a cable accessory from a different brand — field grading is specifically matched to the geometry and materials of the corresponding product and is not simply interchangeable.
Mispositioning the stress-control component relative to the edge of the cut-back screen — a positioning error of even a few centimeters can largely undo the intended field grading.
Assuming a stress cone is as necessary at a low-voltage connection as at medium voltage — at low voltage, the field concentration at a cut-back screen typically stays well within the insulation's breakdown strength, and that does not mean the same assumption applies at medium voltage.