Structure
The cross-linked cable is composed of the outer sheath, metal armor, inner sheath, filler, copper shielding layer, outer semiconducting layer, insulating layer, inner semiconducting layer, conductor, money core of the cable. Therefore, what kind of cable structure needs to have what kind of material and technology of cable accessories and its one-to-one correspondence and matching.
Principle
The design principle of the cable joint should meet and reach the requirements: make the cable run safely in any natural environment. In order to achieve this, it is necessary to pay attention to four key factors, namely: (1) sealing, (2) insulation, (3) electric field, (4) process and other elements. This is also to solve the four important problems of the cable head.
Sealed
1) Because most of the cable joints are installed in outdoor overhead lines, underground and other environments. Therefore, waterproofing and moisture-proofing have become one of the keys to ensure the safe operation of cable joints. And its sealing performance and methods must also be considered.
At present, there are usually two sealing methods:
1. one is the method of potting with asphalt or epoxy resin. This method is complicated in process, difficult to control, and not conducive to maintenance.
2. Another new method, which is currently the preferred method of domestic and foreign professional manufacturers, is to use highly elastic sealants. The process is simple, the performance is reliable, and the maintenance and installation are convenient. These unique advantages also make it the mainstream of use.
To use this new method, the first thing to consider is the performance of the sealant. Because the quality and performance of the sealant directly affect the sealing performance of the cable joint. Choose a glue that can bond with the surface of the cable body and the surface of the accessory material very firmly. At the same time, it is very important to be able to meet the glue that can be used in different temperature change environments.
2) Because the fully cold-shrinkable power cable accessories are actually elastic cable accessories. That is to say, the elasticity of liquid silicone rubber is used to expand the plastic and support strips in the factory in advance. Set it to the designated position on the spot, and pull out the support bar to shrink it naturally. This kind of technology is cold shrinkage technology, and this kind of accessory is cold shrinkable cable accessory. Therefore, this cold shrinkable accessory has good “elasticity”. It can avoid the thermal expansion and contraction of the cable due to the atmospheric environment and the load level during cable operation. It is the breakdown accident caused by the gap between the insulation produced by the “cable breathing”. The biggest disadvantage of heat-shrinkable accessories is that they are not flexible. Therefore, the fully cold-shrinkable accessories are the best choice for use in areas with large temperature differences and large impacts by the climate environment
Insulation
The insulation requirement of the cable head is to meet the two major insulations of phase-to-phase insulation and opposite ground.
1. The insulation of phase-to-phase is two kinds of insulation materials: silicone rubber type and heat-shrinkable material. Generally, the insulation performance needs to meet the requirements based on the unit insulation index of the material combined with the thickness of the material. The performance index of the fully cold-shrinkable accessories is 24kV/mm, and the designed operating thickness should meet 12mm thickness. It can withstand the test of lightning shock and overvoltage. The heat-shrinkable accessory is a mixture of a variety of composite materials, and has a heat-shrinking function after irradiation and processing. The unit insulation index is 1.8-2.0kV/mm, so the design thickness is relatively thicker than the silicone rubber material by 3-4mm units. That is: 15-16mm thick can meet the operating requirements..
2. The insulation between the phase to the ground is to prevent the charge from climbing a safe distance from a high potential to a low potential. Cold-shrinkable silicone rubber material has good elasticity. As long as the design is reasonable, its strong resilience has enough holding force. Regardless of the thermal expansion or contraction of the environment, the cold-shrinkable accessories ensure that the cable is hugged tightly. In this way, the inhalation of water and moisture is prevented, and the safe inner creepage distance is well guaranteed. At the same time, the internal crawling distance of the fully cold-shrinkable cable head is theoretically sufficient as long as 70mm. For the sake of safety, the design running distance is still 90mm. The shrinkage temperature of the heat shrinkable cable head is 100℃-140℃, and the temperature can meet its shrinkage conditions only when it is installed. When the temperature is low, because the thermal expansion coefficient of the cable is different from that of the heat-shrinkable material, it is entirely possible that delamination will occur in the environment below 80 ℃, so cracks will appear. In this way, water and moisture will enter under the action of breathing, thereby destroying the insulation of the system. However, when environmental conditions change, there is no elasticity like silicone rubber, so it will also affect safety. This is the disadvantage of heat shrinkable materials.
Electric Field
The electric field of cold-shrinkable cable joints is treated by geometric method, which changes the electric field distribution through the stress cone. It is solved with a certain geometric shape and a precise R angle. This method is easier to control and test. It can be ensured and realized in the factory. The electric field treatment method of the heat shrinkable cable head is to change the electric field distribution by the linear parameter method. It must rely on two important parameters: a volume resistance, 108-11Ω, and b dielectric constant of 25. Due to its complex production process and large changes due to environmental factors, it is difficult to control the stability of parameters. Therefore, it will have an impact on the quality of the product.
Choosing the best cable termination or joints for the project should never be guessed. Your cable termination will soon become a key part of your operation. You cannot afford part of the business that is closed due to frequent repairs or breakdowns. A right cold shrink / heat shrink cable termination products will increase your productivity and make work much easier for employees. Just make sure you do the proper research to choose a cable termination that handles all your jobs, provides reliable service, and your employees are properly trained.