Common types of cable accessories

Heat shrink type
The most commonly used products for medium and low voltage cable accessories are heat shrink accessories, prefabricated accessories, and cold shrink accessories. They have the following characteristics:
The materials used are generally a blend of multiple material components such as polyethylene, ethylene-vinyl acetate (EVA) and ethylene propylene rubber. This type of product mainly uses stress tubes to deal with electrical stress concentration problems. That is, the parameter control method is used to alleviate the electric field stress concentration. The main advantages are light weight, easy installation, good performance, and low price. The stress tube is a heat shrink tube with special electrical parameters with moderate volume resistivity (1010-1012Ωcm) and large dielectric constant (20--25). The electrical parameters are used to force the stress at the cable insulation shielding break to be dispersed into a more uniform distribution along the stress tube. This technology is generally used in cable accessories of 35kV and below. Because the stress tube will heat up and cannot work reliably when the voltage level is high.
The key technical issues in its use are:
To ensure that the electrical parameters of the stress tube must reach the values ​​specified in the above standards to work reliably. In addition, attention should be paid to filling the air gap of the cable insulation semi-conductive layer fracture with silicone grease to remove gas and achieve the purpose of reducing partial discharge. Cross-linked cables will shrink greatly during operation due to poor internal stress treatment. Therefore, when installing accessories, attention should be paid to the overlap of the stress tube and the insulation shield by at least 20mm to prevent the stress tube from detaching from the insulation shield during shrinkage. Heat shrink accessories have low elasticity and may cause air gaps at the interface during thermal expansion and contraction during operation. Therefore, sealing technology is very important to prevent moisture from entering.
Prefabricated
The materials used are generally silicone rubber or ethylene propylene rubber. The main method of geometric structure, namely stress cone, is used to deal with stress concentration problems. Its main advantages are excellent material performance, simpler and faster installation, installation without heating, good elasticity, and greatly improved interface performance. The main form used for medium, low and high voltage cables. The disadvantage is that the outer diameter of the cable insulation layer is required to be high. The interference is usually 2 to 5 mm (that is, the outer diameter of the cable insulation should be larger than the inner diameter of the cable accessories by 2 to 5 mm). If the interference is too small, the cable accessories will fail; if the interference is too large, the cable accessories are very difficult to install (high process requirements). The problem is particularly prominent at the intermediate joint, which is not only inconvenient to install, but also often becomes a failure point. In addition, the price is relatively expensive. The key technical issues in its use are:
The size of the accessories must match the size of the cable to be installed in accordance with the specified requirements. In addition, silicone grease is also required to lubricate the interface for easy installation.
Cold shrink type
Cold shrink type cable accessories are made of elastomeric materials (commonly used are silicone rubber and ethylene propylene rubber) that are injection vulcanized in the factory, and then expanded and lined with plastic spiral supports to form various cable accessories. During on-site installation, these pre-expanded parts are put on the treated cable ends or joints, and the internally supported plastic spiral strips (supports) are pulled out and pressed against the cable insulation to form cable accessories. Because it relies on elastic retraction force at room temperature, rather than heat shrink cable accessories that need to be heated and shrunk by fire, it is commonly known as cold shrink cable accessories. Early cold shrink cable terminal heads only used silicone rubber cold shrink parts for additional insulation, and the electric field treatment still used stress cone type or stress belt wrapping type. Cold shrink stress control tubes are generally used, and the voltage level ranges from 10kv to 35kv. Cold shrink cable joints, 1kv level uses cold shrink insulation tubes for enhanced insulation, and 10kv level uses joint cold shrink insulation parts with inner and outer semi-conductive shielding layers. Cold shrink branch sleeves are used at the fork of three-core cable terminals.
Cold shrink cable accessories have the advantages of small size, easy and fast operation, no need for special tools, wide application range and few product specifications. Compared with heat shrink cable accessories, they do not need to be heated by fire, and moving or bending after installation will not cause the risk of internal interlayer detachment of accessories like heat shrink cable accessories (because cold shrink cable accessories rely on elastic compression force). Compared with prefabricated cable accessories, although both rely on elastic compression force to ensure internal interface characteristics, they do not correspond to the cable cross-section one by one like prefabricated cable accessories, and there are many specifications.
It must be pointed out that before being installed on the cable, the components of the prefabricated cable accessories are without tension, while the cold shrink cable accessories are under high tension. Therefore, it must be ensured that during the storage period, the cold shrink components should not have obvious permanent deformation or elastic stress relaxation, otherwise, after being installed on the cable, it cannot be guaranteed that there is sufficient elastic compression force, and thus good interface characteristics cannot be guaranteed.