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.

In actual operation, accidents caused by cable accessories are relatively frequent. Therefore, our staff should pay attention to the following aspects when installing cable accessories. For details, please see the following:

Moisture inspection:
Rubber and plastic cables should be dissected layer by layer to check for rust on the armor, metal shielding layer and wire core, whether the filler is damp, whether the buffer water-blocking tape has changed, and whether there are water droplets on the surface of the outer shield and in the wire core. If there is moisture, the cable should be cut off until there is no moisture in the cable.

Straightening:
When straightening the cable, fix the cable firmly by buckling two angle aluminums together. If the cable is vertical, use nylon ropes to tie several guy wires to keep the cable in a vertical state all the time. If the cable is lying flat, place it on a straight bracket to keep it in a horizontal state all the time.

In addition, during straightening, protective measures should be taken, and the cable should be allowed to cool naturally to room temperature. The cooling time is generally not less than 24 hours.

Insulation treatment:
The common method for treating the surface of cable insulation is to scrape it with special tools, glass sheets, etc., and then polish it with sandpaper. Usually, a grinder is used for grinding first, then manual fine grinding. The sandpaper used for grinding ranges from 240 mesh to 1200 mesh. It should be noted that during grinding, sandpaper should be used in order from low mesh to high mesh, and the sandpaper used for grinding the semi-conductive layer must not be used for grinding the insulation layer again.

Conductor crimping for connection:
Generally, before crimping, the insulation layer should be stripped according to the required length for connection, which is the depth of the connecting terminal hole + 5mm or 1/2 of the length of the connecting pipe + 5mm. Remove oil stains or oxide films on the surface of the conductor. For aluminum stranded conductors, use a steel wire brush to brush the conductor until the surface of the conductor is shiny. Also, after rounding the end of the cable conductor, insert it into the connecting pipe or terminal cylinder. For intermediate connection, each end of the conductor is inserted until it reaches the stop pit (or oil-blocking grid). For terminal connection, the conductor should be fully inserted into the terminal cylinder before crimping.

At the crimping part, the formed edges of the circumferential crimping or the center lines of the crimping pits of the pit crimping should be on the same plane or straight line respectively.

Usually, for each crimping of the crimping die, after the die is closed in place, it should stay for 10-15 seconds to allow the plastic deformation of the crimped part to be basically stable before releasing the pressure. In addition, attention should be paid to the crimping sequence.

Lead coating:
When coating lead, the lead sealing strip should use lead sealing solder made of 65% lead and 35% tin, which is in a paste state at 185-250°C. To ensure good sealing, lead sealing is carried out in two layers. To prevent damage to the cable insulation layer, the total time from the start of cable heating to the completion of sealing must not exceed 15 minutes.

In conclusion, power accessories play a very important role in cable construction. They not only effectively control the movement of high voltage but also ensure a certain stability of voltage during use, which is helpful for people's daily life and production. However, since there are strict requirements for the installation technology of cable accessories, during the installation project, they must be installed in strict accordance with the construction requirements to avoid a series of problems caused by improper installation methods, which may lead to safety accidents.

The integral prefabricated joint is a prefabricated component where the semi-conductive inner shield, main insulation, stress cone, and semi-conductive outer shield of the joint are pre-manufactured as a whole in the factory. During on-site installation, it is simply sleeved over the cable insulation, making the installation process simple and time-efficient. Additionally, since the joint insulation is an integral prefabricated part, factory tests can be conducted to verify its manufacturing quality.

Although integral prefabricated joints produced by different manufacturers have similar structures, their installation processes vary, mainly including the following types:

(1) Before connecting the conductors, first sleeve the prefabricated joint onto the outer shielding layer of one side of the cable to be connected. After the conductors are connected, pull the prefabricated component to its final position. This process has a drawback: when the prefabricated joint moves back and forth on the outer semi-conductive layer, it may carry particles of semi-conductive material from the cable (left by sanding the cable insulation shielding layer during installation) onto the insulation, affecting the insulation level of the interface. Although silicone grease is applied to the interface between the prefabricated component and the cable during installation, and the process specifies that the time from sleeving to final positioning of the prefabricated component should not exceed 2 hours, this risk still exists, and special attention must be paid during installation. To avoid such problems, the following installation methods have emerged:

(2) Before connecting the conductors, use mechanical means to push a lining tube into the prefabricated component to expand its inner diameter. Then sleeve the expanded prefabricated component onto the outer semi-conductive layer of the cable. After the conductors are connected, move the prefabricated component to its final position and pull out the expansion tube. This eliminates the possibility of semi-conductive material being carried onto the insulation. Another method is to expand the inner diameter of the prefabricated component even further, directly sleeving the joint onto the outer sheath of the cable. This process not only solves the aforementioned problem but also shortens the stripping length of the outer sheath, thereby reducing the length of the joint.

(3) Use compressed gas (nitrogen) to expand the joint. Specifically, nitrogen is filled between the joint and the cable to form an air film, after which the joint is pushed to the predetermined position. The air film on the interface reduces friction, thus preventing semi-conductive material from being carried onto the insulation.