WCA September 2021

-50°C ~ +70°C

-60°C ~ +70°C

Project

Unit

LLDPE

MDPE

HDPE

Standard requirement (dB/km)

No.

| Δα | 1,310 0.004 0.007 0.000 0.003 0.005 0.001 0.001 0.006 0.001 0.001 0.008 0.002

| Δα | 1,550 0.009 0.009 0.006 0.008 0.010 0.005 0.004 0.008 0.007 0.005 0.010 0.008

| Δα | 1,310 0.005 0.009 0.001 0.005 0.009 0.002 0.001 0.010 0.005 0.000 0.011 0.004

| Δα | 1,550 0.005 0.006 0.003 0.005 0.007 0.002 0.002 0.006 0.003 0.002 0.007

Density

g/cm 3

0.91~0.94

0.94~0.955 0.955~0.978

Tensile strength

Mpa

≥14

≥18

≥21.5

1 2 3 4 5 6 7 8 9

Percentage of breaking elongation

%

≥600

≥660

Tensile strength after ageing

%

≤20

≤15

| Δα | 1,310 ≤0.02 | Δα | 1,550 ≤0.02

Elongation at break after ageing %

≤20

≤10

Environmental stress cracking resistance

h

≥500

❍ Table 2 : Performance comparison of three sheath materials

10 11 12

3.2.2 PBT Low-temperature environments require PBT to have good low- temperature performance, which requires high creep embrittle- ment strength and elongation at break. Low-temperature special PBT usually achieves good low temperature in the material ratio by reducing toughening agent or adding other additives, but this 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 No. 1 2 3 4 5

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. 0.004 ❍ Table 3 : Temperature performance test results of cold-resistant cable

Standard requirement The fibre should not break and the sheath should be visible without cracking

Test results

Test conclusion

Test items

Unit

Low- temperature bending performance

The fibre is not broken, the sheath is not cracked

M pa

Qualified

Changing rate of tensile strength of sheath before and after ageing Changing rate of elongation at break of sheath before and after ageing

(100 ±2) °C, after 240 h ≤20%

%

15.9% Qualified

(100 ±2) °C, after 240 h ≥300%

%

688% Qualified

Thermal shrinkage

% 100°C, 4 h ≤5%

2%

Qualified

50°C , 96 h Failures/ samples: 0/10

Environmental stress cracking number

0/10

Qualified

❍ Table 4 : Environmental performance test results of cold-resistant cable

54

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Wire & Cable ASIA – September 2021