Why High-Voltage Accessories Need Special Insulation Design

High-voltage cables are impressive pieces of engineering. Their insulation is carefully designed to withstand the electric field, handle thermal stress, and resist aging for decades. Yet, when these cables end at a termination or are joined at a splice, the insulation challenge changes dramatically. The accessories—joints and terminations—must operate under much more severe conditions than the cable itself. They require special insulation design that goes far beyond simply extending the cable’s insulation. This article explains why high-voltage accessories need unique insulation systems and what makes them different from the cable.

1. The Cable Insulation: A Controlled Environment

To understand why accessories need special design, it helps to first understand the cable insulation. A high-voltage cable is manufactured in a factory under tightly controlled conditions:

• Clean environment – Dust and contaminants are minimised.

• Precise extrusion – Insulation thickness is uniform, with no voids.

• Homogeneous material – The same material throughout the insulation layer.

• No interfaces – The insulation is a continuous layer from the conductor to the shield.

The electric field in the cable is radial and uniform. The stress is highest at the conductor surface and decreases smoothly towards the shield. The insulation is designed to handle this predictable stress profile over its lifetime.

In a cable, the insulation is a single, uninterrupted layer. There are no interfaces, no abrupt changes in geometry, and no field distortions. The challenge is managing the bulk stress—the average stress through the insulation thickness.

2. The Accessory Insulation: A Complex Challenge

In a termination or joint, the situation is very different. The insulation is no longer a continuous, homogeneous layer. Instead, it is an assembly of multiple components:

• The original cable insulation.

• The stress control elements (cones, Hi-K layers, NLR materials).

• The insulation body of the accessory (silicone or EPDM).

• The interfaces between these different materials.

Each of these components has different electrical properties (permittivity, dielectric strength, conductivity). The interfaces between them are potential sites for field distortion, partial discharge, and moisture ingress.

Furthermore, the geometry is no longer simple. There are sharp edges (at the shield cut), changes in diameter (at the connector), and surfaces exposed to air or other insulating media. The field is no longer radial; it has significant longitudinal (axial) and tangential components.

The insulation in an accessory must manage all of these complexities. That is why it requires special design.

3. The Field Distortion Problem

In a cable, the electric field is radial and uniform. In an accessory, the field is distorted at several points:

The peak stress at a shield cut can be 5 to 10 times higher than the average stress in the cable. This is far beyond what the cable insulation is designed to withstand. The accessory must reduce this peak stress to a safe level—typically by spreading the voltage drop over a longer distance and by using materials that can withstand higher surface stresses.

4. Interfaces: The Achilles' Heel

The most critical difference between cable insulation and accessory insulation is the presence of interfaces. A cable has no interfaces within its insulation; an accessory has several.

At an interface between two materials with different permittivities, the electric field lines are refracted (bent). This can lead to:

• Increased tangential stress along the interface.

• Charge accumulation at the interface.

• Initiation of partial discharge if the interface is not perfect.

To manage interfaces, accessory designers use materials with matched permittivities (to reduce refraction) or introduce stress grading layers that smooth the transition.

The interface between the accessory body and the cable insulation is particularly critical. If there is a gap (air void) at this interface, the field will concentrate in the void, causing partial discharge. That is why cold-shrink accessories are popular—they provide a tight, void-free interface through radial pressure.

5. Insulation Thickness: Not a Simple Extension

One might think that the insulation in an accessory should simply be as thick as the cable insulation. But this is not the case. The optimal thickness for an accessory is different because:

• The field distribution is different (non-radial).

• The materials have different dielectric strengths.

• The accessory must also provide mechanical support and sealing.

In fact, making the insulation too thick can be detrimental. A thick layer of insulation at a termination increases the stress at the shield cut (because the surface is farther from the ground plane). The geometry must be carefully optimised to balance stress at all points.

Engineers use finite element analysis (FEA) to calculate the optimal insulation shape and thickness for each accessory design.

6. Materials: A Different Set of Requirements

The materials used in accessory insulation are different from those used in the cable. While the cable insulation is typically XLPE (cross-linked polyethylene) for high-voltage cables, the accessory insulation is often silicone rubber or EPDM.

Why different materials?

Silicone rubber, for example, has excellent hydrophobicity (water repellency) and tracking resistance, making it ideal for outdoor terminations. EPDM has good mechanical strength and is often used in joints.

7. Thermal Management: A Shared Challenge

Both the cable and the accessory must manage heat generated by the conductor (I²R losses). However, the accessory often has poorer heat dissipation because it is bulkier and may be enclosed in a housing.

The insulation materials in the accessory must withstand the same operating temperatures as the cable (typically 90°C for XLPE, up to 105°C for some accessories). But they must also withstand localised heating from the connector, which may run hotter than the conductor itself.

The insulation design must ensure that the temperature at all points remains within the material's rating. This often requires adding heat-sink features or using materials with high thermal conductivity.

8. Electrical Stress vs. Mechanical Stress

Accessory insulation is subjected to both electrical and mechanical stresses. The cable insulation is primarily an electrical component; the accessory insulation must also provide:

• Sealing – against moisture ingress.

• Mechanical support – to hold the connector and cable in place.

• Stress relief – to protect the connector from bending forces.

• Creepage distance – for outdoor terminations, to prevent surface flashover.

This means the insulation design must balance electrical, mechanical, thermal, and environmental requirements—a much more complex task than cable insulation design.

9. Testing and Qualification

The special nature of accessory insulation is reflected in the testing and qualification standards. High-voltage accessories are tested to a different set of standards than cables:

• IEC 60840 / IEC 62067 – for cable accessories, with tests for partial discharge, dielectric withstand, and thermal cycling.

• IEC 60502-4 – for medium-voltage accessories.

• IEEE 48 – for terminations.

• IEEE 404 – for joints.

These tests include electrical, mechanical, and environmental conditions that mimic the real-world stresses the accessory will face. A cable may pass its tests, but an accessory must pass a more extensive set—because it is asked to do more.

The cable insulation is designed to manage a uniform, radial electric field in a controlled, homogeneous material. The accessory insulation must manage a distorted, multi-directional field in an assembly of different materials with interfaces, exposed surfaces, and mechanical loads.

That is why high-voltage accessories need special insulation design. It is not a matter of simply extending the cable insulation; it is a fundamentally different challenge that requires different materials, different geometries, and different testing. The next time you see a termination on a high-voltage cable, remember: the insulation inside is not just a copy of the cable—it is a carefully engineered solution to a much more complex problem.