Currently, the focus of home broadband construction is gradually shifting from cities to rural areas. Since China Mobile’s own pole roads are relatively scarce in rural areas, and the cost of building self-built pole roads and attached fiber optic cable is as high as 20,000 yuan/km, many rural areas do not have the resources to build home broadband due to the high construction costs per household.
If lightweight self-supporting optical cable are attached to poles, the cable construction cost can be reduced to less than 5,000 yuan/km, thus greatly reducing the construction costs of rural broadband.
Structural characteristics and main mechanical properties
Structure
The lightweight self-supporting fiber cable is a non-metallic reinforced hanging wire, central tube filled type, non-metallic fiber reinforced polyethylene sheathed figure-8 self-supporting structure, the model is GYFXTC8F, as shown in Figure 1. The cable itself has its own hanging wire, so there is no need to rely on steel strands for hanging during laying.
Figure 1. GYFXTC8F Figure 8 Cable Structure
| Fiber count | Cable diameter(short axis) | Cable diameter (long axis) | Cable weight(kg/km) |
| 2~12core | 6.5 | 14 | 56 |
| 14-24core | 7.2 | 14.7 | 106 |
Table 1. Fiber cables Figure 8 dimensions and weight
| Bending radius (static) | Bending radius (dynamic) | Allowable tension (long term)(N) | Allowable tension (short term)MAT(N) | Allowable lateral pressure(long term)(N) | Allowable lateral pressure(short term)(MAT)(N) |
| 25D | 30D | 1000 | 2000 | 1000 | 2000 |
Table 2. Figure 8 cable main mechanical properties
Note: D is diameter of the side with fibers, the cable size, the minimum curvature radius of the cable after subtracting the reinforcement should be 10D (static) and 20D (dynamic).
Due to the poor flexibility of the reinforcements in the lightweight self supporting fiber optics cable material, the curvature radius of the cable is larger; although the allowable tension of the cable is larger than that of ordinary cables, it is only 1/15 of that of ordinary aerial with cable steel strands (ordinary cable long-term allowable tension is 600N, and the breaking force of 7/2.2 steel strand is about 30,000N).
Application
The performance characteristics of lightweight self-supporting optical cables and the laying conditions for hanging under power lines make the scope of application significantly different from ordinary overhead optical cable.
Light weight self-supporting cables are suitable for access projects such as home broadband, customer service or wireless small and micro station access. They are not suitable for use in backbone cable lines and backbone access cable line projects; they are suitable for use in terminal sections of access lines, especially It is at the end of rural home broadband access projects and in urban villages and other scenarios where power distribution lines are relatively messy;
This figure 8 cables should be laid overhead on power poles, walls or other buildings. Other laying methods are not suitable. They can be attached to power poles of 10kV and below, and must not be hung on power poles above 10kV.
The following fiberoptic cable sections should not use lightweight self-supporting cables.
(1) Macro base stations and dedicated line access lines with high service levels;
(2) Sections where more than 2 optical cables need to be laid on the same route or optical cables need to be expanded within 2 years;
(3) Sections in the same cable with a large number of connectors and a small average spacing between connectors;
(4) Sections that cross traffic arteries or cross roads multiple times.
In order to reduce the loss when the fiber optic cable fails, the core number of the light self-supporting optical cable should be mainly 6 cores and 12 cores, and the maximum should not exceed 24 cores.
Laying and installation design
Laying sag and span
Since the allowable tension of lightweight self-supporting optical cables is small, sag should be left during installation; sag is the ratio of sag to span, as shown in Figure 2. The greater the sag, the smaller the tension on the optical cable. However, since the light self-supporting cable is laid under the power line, when the sag is large, the distance between the cable and the ground may be insufficient. Generally, the sag of the cable in winter is required to be 1.0% and 1.5% in summer.
The attachment distance of lightweight self-supporting cables generally does not exceed 50 meters, and the maximum distance does not exceed 100 meters; the maximum distance across highways does not exceed 70 meters. For sections where the span exceeds the requirements, appropriate reinforcement measures should be taken.
Main supporting materials
When the cable is attached to the pole, steel belt hoops should be used as the fixing point. Steel belt hoops mainly include stainless steel locks, hook seats and cable hooks; generally, steel belt hoops with an adaptive range of φ100-200mm are used for fixing on communication poles, and steel belts with an adaptive range of φ200-300mm are used for fixing on power poles.
When laying the cable along the wall, C-ring fasteners can be used to fix them when the span is small. When the span is large, traditional U-shaped pull tabs and straight pull tabs can be used as fixing points.
At the fixed point, we should use wedge-shaped clamps to fix. See figure 5 below.
Before splicing, we should cut the messenger wire of the self-supporting cable, so the optical cable joint closure can only be installed in places where the cable suspension wire is easy to terminate, such as on walls and poles, but cannot be installed in suspended sections of the cable. In order to facilitate the fixation of the joint box, a dome closure should be used.
Laying of optical cables on power lines
When install cable attached to a power pole, it should be erected below the power line. In order to ensure the safety of construction workers during construction operations, a certain distance should be maintained between the optical cable and the power line. Optical cables must not directly pass through transformers, and it is strictly prohibited to work on poles directly under power lines.
Table 3: The minimum clear distance between the optical cable and the power line
| Item | Below 1KV | 1KV~10KV | Power supply line to household line | Remark |
| Min. vertical clear distance | 2.0 (note 1) | 2 | 0.6 (note 2) | Top cable to power line |
| Min. horizontal distance | 2.0 (note 1) | 2 | – | Cable and wire edges |
Note 1: During construction when the power supply line is out of power, the minimum horizontal distance and vertical clear distance can be reduced to 1 meter.
Note 2: When the power supply line is a covered wire, the optical cable can also cross over the power supply line.
Due to its small tensile strength, it is difficult to reserve the cable in the middle of the power pole; if reserved on the power pole, it will affect the operation of the power maintenance personnel on the pole; therefore, there is no reservation for light weight self-supporting cable when hooking up power lines.
Conclusion
Due to lower construction costs, light weight self supporting cable have begun to be used on a large scale in rural broadband projects in many provinces and cities. However, this cable are mainly laid through power poles, please do pay attention to the safety during construction and future maintenance. Because the allowable tension is small, if the cable line encounters natural disasters such as strong winds and ice, it may cause damage to the optical cable line. These are unavoidable problems in engineering construction.
