In the cable manufacturing process, two issues are frequently encountered: the insulating sleeve and the inner liner. The insulating sleeve refers to the inner sheath between two different metal coating layers, commonly known as the inner sheath. The inner liner is a non-metallic coating surrounding the cable core and filler (if any) of a multi-core cable, located below the protective layer.

Insulating Sleeve: The key word is "insulating." Insulation refers to a layer of PVC sandwiched between two different metals—for example, in a three-core YJV22 structure, between the copper conductor and the steel strip. In this case, the PVC acts as an insulating layer. From a chemical and physical perspective, copper and iron have different reactivity; when they come into contact, it accelerates the oxidation process of copper. Therefore, the insulating sleeve must be extruded, not wound, because the winding process creates gaps that could lead to direct metal-to-metal contact.

Inner Liner: Copper wire shielding, copper tape armor, single-core power cables—this structure also has a PVC layer between the shielding and armor layers. However, since these two metals are the same, they do not accelerate oxidation; therefore, the PVC does not need to act as an insulating layer. In this case, it is not called an isolation sleeve, but an inner liner. Furthermore, the inner liner can also be wound.

Buffer Layer: Usually, the word "ribbon" is added before "buffer layer," and its function is described in many standards. When using armored cables, it is used in conjunction with the inner liner to enhance its strength.

The inner liner can be extruded or wrapped, while the isolation sleeve can only be extruded. For cables with a cross-sectional area exceeding 10 square millimeters, wrapping saves material costs. When the metal layer beneath the armor differs from the armor material, an isolation sleeve must be extruded to separate it. In this case, the isolation sleeve can replace the function of the inner liner. Because the cores of medium-voltage power cables are shielded with copper tape, while the cores of low-voltage cables are unshielded, we typically see that medium-voltage armored cables require an extruded inner liner (isolation sleeve), while low-voltage armored cables require a wrapped inner liner.

The thickness of the inner liner is approximate and does not require guarantee or testing. The minimum thickness can be used as long as it effectively protects the cable core from damage by the armor metal. Depending on the manufacturing process, the inner lining can be either wound or extruded. For the wound process in power cable production, when the cable core diameter is assumed to be 40 mm or less, the approximate thickness is 0.4 mm; when the diameter is greater than 40 mm, it is 0.6 mm.

For control cables, the nominal thickness of the inner lining is 1.2 mm, but a negative deviation of 20% is allowed, and the minimum thickness must not be less than 1.0 mm. The thickness of the insulating sleeve should be the nominal thickness and is a mandatory testing item. Its thickness calculation formula is: Ts = 0.02Du + 0.6, where Du is the assumed diameter before extruding the insulating sleeve; its measured minimum thickness must not be less than 80% of the nominal thickness - 0.2 mm.

The material for extruded inner linings or insulating sleeves can be polyvinyl chloride or polyethylene, and its operating temperature should match that of the cable's insulation and sheath. When the cable is a halogen-free cable, halogen-free materials should also be used, usually recycled PVC tape as the covering material.

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