EuroWire July 2021

Technical Article

method will cause negative effects such as lower modulus and lower bending strength. In addition, it is necessary to minimise the axial back contraction of the loose tube to avoid shrinkage of the sleeve caused by temperature difference changes. 3.2.3 PE sheath material The cable sheath is the first line of defence against external erosion and damage. Its performance is related to the long life of the cable and is one of the important factors for stable and reliable transmission within the service life. The ageing process of the cable sheathing material is mainly caused by thermal oxidation, ultraviolet irradiation and external force. At the same time, the cable sheath is exposed for a long time in the laying environment, and its mechanical properties will gradually decrease. Tensile strength and elongation at break are the characteristics of the mechanical properties of the sheath material. The lower the value, the easier it is to break under external force conditions. Therefore, the tensile strength and elongation at break are also indicators for characterising the stability of the cable sheath. The polyethylene (PE) used for cable sheath production mainly includes linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE) and medium-density polyethylene (MDPE). The properties of the three materials are shown in Table 2 . The higher the density, the higher the relative hardness. Compared

with the other materials, LLDPE has more branches, so the crack resistance is better than HDPE and MDPE, but the abrasion resistance, tensile strength and elongation at break of HDPE sheath gives it advantages over the other two materials. In addition, the wind and sand in the plateau area is great, and the wind/sand may blow the surface of the cable sheath, so the cable sheath is also required to have better wear resistance. For high UV protection, it depends mainly on the carbon black content in the sheath material. It is also possible to add UV protection additives to the material to cope with the ageing of the outer sheath caused by exposure. On the whole, it is more suitable to choose HDPE as the sheath material. 3.3 Measures to do with low-temperature environment The retraction of the loose tube and the outer sheath of the cable is a significant phenomenon of the overhead cable in the large- temperature-difference area. At present, the raw material of the main loose tube in the industry is PBT, and the sheath material is PE. There are mainly three kinds of stretching mechanisms in the cable – thermal retraction, thermal expansion and contraction, and stress expansion. Specific responses to these issues are as follows. Thermal retraction ismainlydue to thecrystallisationof thematerial, which is the process of rearranging the molecular structure. The crystallinity of PE and PBT during extrusion depends on the cooling rate. In the production and extrusion process, the cooling rate is related to the cooling water temperature, the extrusion speed of the extruder, and the length of the water tank. The ideal state is to perform segmental cooling, where thematerial is just extruded and cooled with warm water, and the water temperature is gradually lowered, and the traction speed is required. This is slow, and the sink is long. After the crystalline material is fully crystallised, heat shrinkage does not occur again. Thermal expansion and contraction are inherent properties of all materials. The cable sheath and casing are all plastic materials, and the coefficient of linear expansion is large – more than ten times that of metal materials. The loose tubematerial can bemade of low- shrinkage PBT to minimise the change in retraction rate and thus reduce the elongation and retraction of the member. A reserved bend can be made during construction to cope with the effect of thermal expansion and contraction on the joint. Material stress occurs during the construction process and can be eliminated by effectively controlling the traction tension and the tension of the thread during the production process. During the construction process, the construction workers drag the cable and easily generate tensile stress on the outer sheath of the cable.When the cable is laid, the tensile stress is gradually released, causing the sheath to retract. This kind of stress can be used in engineering to park the cable after laying, so that the internal components of the cable can be stretched as much as possible under natural conditions, thereby reducing the influence of residual stress.

Project

Unit

LLDPE

MDPE

HDPE

Density

g/cm 3

0.91~0.94 0.94~0.955 0.955~0.978

Tensile strength Mpa

≥14

≥18

≥21.5

Percentage of breaking elongation

%

≥600

≥660

Tensile strength after ageing

%

≤20

≤15

Elongation at break after ageing %

≤20

≤10

Environmental stress cracking resistance

h

≥500

▲ Table 2: Performance comparison of three sheath materials

-50°C ～ +70°C

-60°C ～ +70°C

Standard requirement (dB/km)

No.

| Δα | 1,310

| Δα | 1,550

| Δα | 1,310

| Δα | 1,550

1

0.004

0.009

0.005

0.005

2

0.007

0.009

0.009

0.006

3

0.000

0.006

0.001

0.003

4

0.003

0.008

0.005

0.005

4 Cold-resistant cable performance tests With the previous design and analysis, we produced cold-resistant cables according to the above-mentioned cable structure design and material selection to perform the following performance tests. 4.1 Temperature performance test We select 12 random fibres in the fibre optic cable to test the attenuation of fibre

5

0.005

0.010

0.009

0.007

| Δα | 1,310 ≤0.02 | Δα | 1,550 ≤0.02

6

0.001

0.005

0.002

0.002

7

0.001

0.004

0.001

0.002

8

0.006

0.008

0.010

0.006

9

0.001

0.007

0.005

0.003

10

0.001

0.005

0.000

0.002

11

0.008

0.010

0.011

0.007

12

0.002

0.008

0.004

0.004

Figure 3: Cold-resistance cable

▲

▲ Table 3: Temperature performance test results of cold-resistant cable

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July 2021