The cable sheath is a protective layer covering the outside of the cable insulation. Together with the conductor and the insulation layer, it is collectively referred to as the three major components of a cable. A typical sheath structure includes a jacket (inner sheath) and an outer sheath. Do you know the different functions of the jacket and the outer sheath?

The inner sheath is a protective covering wrapped around the cable insulation. It serves to prevent the insulation layer from getting damp, mechanical damage, as well as the effects of light and chemically corrosive media. Additionally, it can conduct short-circuit current.

Inner sheaths come in metal and non-metal types. Metal sheaths include lead sheaths, flat aluminum sheaths, corrugated aluminum sheaths, copper sheaths, and composite sheaths. Metal sheaths are mostly used in oil-immersed paper insulated cables and cross-linked polyethylene insulated power cables of 110kV and above. Plastic sheaths can be used in various plastic insulated cables, while rubber sheaths are generally used in rubber insulated cables.

The outer sheath is a protective covering wrapped around the cable jacket (inner sheath), mainly playing roles in mechanical reinforcement and corrosion resistance. Common cables have outer sheaths with either a metal inner sheath or a plastic inner sheath.

It generally consists of three parts: a cushion layer, an armor layer, and an outer covering. The cushion layer is located between the metal sheath and the armor layer, functioning as an armor cushion and providing corrosion protection for the metal sheath. The armor layer is made of metal strips or wires, mainly playing a mechanical protection role; metal wires can bear tensile force. The outer covering is outside the armor layer, protecting the metal armor from corrosion. The cushion layer and outer covering are made of materials such as asphalt, polyvinyl chloride (PVC) strips, impregnated paper, PVC sheaths, or polyethylene sheaths. The composition and structure of the outer sheath vary according to the environment and conditions in which various cables are used.

There are two structures for this type of outer sheath:

Traditional PVC outer sheaths are less suitable for high-voltage cross-linked polyethylene (XLPE) cables that operate at high temperatures and have sheath insulation requirements due to the low operating temperature of PVC. Therefore, it is now common to use high-density polyethylene (HDPE) or linear low-density polyethylene (LLDPE) as outer sheaths, though they are non-flame-retardant. Fire prevention measures should be considered during installation, or flame-retardant cables should be used. Using HDPE as the outer sheath can improve the insulation level of the sheath, and an adhesive should be applied between the outer sheath and the corrugated metal sheath.