Over the years, three different installation technologies and three different insulation materials have become widely accepted in the cable accessory business. In regard to functionality and service life, the requirements for such accessories are similar to those placed on the cable itself. At the same time, it should also be noted that installation technology is becoming increasingly important.
Application of these different technologies and choice of optimal insulation material both depend mainly on voltage level as well as on the specific type of accessory involved. For example, heat shrink is the technology most often used for medium voltage cable accessories. The basic production principle here is extruding polyethylene compounds as a tube followed by cross-linking of the material. After heating, expansion and subsequent cooling, the heat shrink tube is ready for use.
A key advantage of such tubes is the wide range of possible application diameters. Moreover, the trend in this technology is combining different materials, such as the heat shrink material itself, with various rubber compounds to produce a layered rubber and polyethylene tube. Indeed, these are now often being used for cable joints. A related development of interest is combination of heat shrink tubes with silicone sheds to produce terminations.
Slip-on technology is suitable for all voltage levels and relies on pre-fabricated cable accessory components made mostly from silicone or EPDM material. A joint or termination can then be a one-piece solution or a combination of several parts (as in the photo below which shows a modular such termination composed of EPDM sheds). A stress control cone is also visible, but conducting carbon paths are evident on the upper shed. This is, in fact, one of the main disadvantages of using EPDM for termination applications.
Cold shrink is the newest technology and, like slip-on technology, applicable from medium to extra high voltages. The insulation material used most often is also silicone or EPDM.
Requirements for cable terminations especially for outdoor use are:
• Only one size of termination for a wide range of cross-sections,
(especially important for medium voltages);
• High flashover voltage;
• Long creepage distance;
• Resistance to tracking and UV radiation;
• Excellent behaviour under pollution, rain and fog;
• Easy installation.
In general, all these requirements can be satisfied by single piece terminations or by ones with modular construction. Furthermore, the clear trend when it comes to insulation materials for cable termination applications is toward use of silicone rubber.
At medium voltages, the main application technology is heat shrink but also in combination with silicone sheds. Slip-on as well as cold shrink technology both use silicone about 90% of the time. At the high voltage level, virtually only silicone is now being used.
The key advantages of silicone rubber material for terminations include:
1. Hydrophobicity leads to low leakage current and high flashover voltage;
2. Recovery of hydrophobicity is always possible, even after extreme surface stress by discharges;
3. Transfer of hydrophobicity into any kind of contamination layer;
4. Resistance to UV radiation leads to very long life cycle, even for outdoor applications;
5. Resistance to tracking means no reduction in ‘effective’ creepage distance from conductive leakage paths;
6. Permanent elasticity and high interface pressure;
7. Flame resistance and no toxic gases;
8. Good environmental performance.