EuroWire July 2021

Technical Article

• The cable sheath is retracted from the splice closure such that a length of cable core is exposed to unprotected conditions, affecting transmission 3 – Cable sheath ageing caused by strong illumination and high radiation The performance of various sheath materials changed under solar radiation, and the tensile strength and elongation at break decreased to about half of the original in ten years. At the same time, under the Sun’s exposure, the cable is prone to cracking under the action of bending and thermal stress. 2.3 Requirements for optical cable The special climate and temperature environment in the alpine region have certain influence on the safety of the optical cable line. In order to eliminate these effects, it is necessary to carefully analyse the problems existing in the actual operation of the optical cable, and conduct in-depth discussions on the structural selection, production, raw materials and laying of the optical cable. At present, overhead laying is still a common construction scheme in natural areas, namely metal reinforcing members, loose tube strand-filled, and steel-polyethylene bonded sheath communication outdoor optical cables. There is also the use of the GYTA type as an overhead construction optical cable in actual engineering. Straight-buried optical cables are usually selected from GYTA53 type optical cable (external addition of a layer of steel-polyethylene bonded sheath). There are currently two types of mainstreamcore structures: central tube type and stranded type cable. The loose tube in the stranded cable is spirally wound around the reinforcing member. When the loose tube is stretched due to temperature, this state can increase the friction between the members, which is beneficial to prevent the loose tube from expanding and contracting due to its core. The presence of a stretch window in the structure greatly improves the tensile properties of the cable. The central tube cable loose tube is in a straight line. This structure is not conducive to preventing the loose tube from expanding and contracting. In addition, due to the cable chromatography, the central tube structure cannot be used in high fibre-count cables. From a comprehensive comparison, the current accepted view in the industry is that the loose-sleeve structure is the most suitable in the alpine region and in the large-temperature-difference environment. It is not suitable to use the central tube structure. In terms of the sheath structure, the polyethylene sheath material has a large expansion coefficient and is alsoprone toheat shrinkage, so the expansion and contraction capability of the sheath is relatively large. In practice, the sheath is pasted with a lapped metal composite tape to limit the expansion and contraction of the sheath. The steel-plastic composite tape in the sheath is stronger than the aluminium-plastic composite tape, so the steel-plastic composite tape should be used as much as possible. Therefore, GYTS type optical cable should be selected for overhead laying, and GYTA53 type optical cable is chosen for direct buried optical cable. 3 Design of cold-resistant optical cable 3.1 Design of cable structure

PE sheath Loose tube Fibre gel Steel wire Optical fibre Water-blocking gel Steel-plastic tape

▲ Figure 1: Structural diagram of central tube optical cable

Optical fibre Fibre gel

Loose tube Cable gel Steel-plastic tape PE sheath Central strength member

▲ Figure 2: Structural diagram of stranded optical cable

protection for the fibre in the loose tube: preventing moisture in the air from eroding the fibre; and protecting the fibre. The buffer fibre is affected by mechanical forces such as vibration, impact and bending. If you want to enhance the low-temperature performance of the cable, you need to reduce its minimum operating temperature to below -60°C. The low-temperature resistance of the cable is mainly related to the temperature characteristics of the fibre gel and the cable gel. This requires the fibre gel to be hard under low- temperature conditions, that is, it has a large penetration at a low temperature, otherwise the fibre will be in a stiff grease. Severe micro bend loss, even mechanical stress of the fibre paste, and the viscosity of the fibre paste should not be too small, otherwise it will affect the dripping performance. The specific performance requirements are shown in Table 1 . 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

Low- temperature gel

Ordinary fibre gel

Project

Unit

Viscosity (25°C) 200 S -1

mPa.s

2,200 ±500

Viscosity (25°C) 50 S -1

mPa.s

4,050 ±950

Viscosity (25°C) 6 S -1

mPa.s

22,000 ±6,600

Shear thinning index

/

≥8

25°C penetration

0.1mm

430 ±20

3.2 Low-temperature material selection 3.2.1 Fibre gel

-40°C penetration

0.1mm

250 ±20

In optical cables, among the structural materials other than optical fibres, the most important influence on the performance of the cabled fibres is the fibre gel. The fibre gel has two aspects of

-60°C penetration

0.1mm

/

≥200

▲ Table 1: Comparison between ordinary fibre gel and low-temperature special fibre gel

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July 2021