How do optical cable lines do lightning protection measures?

Because the cable has good insulation performance, the importance of lightning protection cable is often ignored. The lightning protection of optical cable line is a key technology related to line safety which should be paid attention to in the whole process of route survey and design to engineering construction and installation.

Optical cable good protection performance makes its lightning protection work is not as obvious as coaxial cable and open circuit, so in the process of rapid development of optical cable lines, safe grounding is often misunderstood, even forgotten. With the extensive use of optical fiber, in recent years, optical cable lines are often struck by lightning. Optical cable lines have a large communication capacity, and the most vulnerable to lightning is directly buried lines, repair is difficult, so once the obstacle will cause huge losses.

1. cable line landfall causes and impact

Although the main component of the optical fiber is SiO2, it has no electrical conductivity and is not affected by lightning electromagnetic pulse. However, in order to make the optical fiber can withstand mechanical tensile load, and avoid the impact of external environment (such as corrosion, rat bite, rock extrusion collision, etc.), the directly buried optical cable must have cable core steel wire reinforcement and metal outer protective armor layer, these protective components are metal conductors. When the lightning falls near the buried optical cable, the lightning flow from the falling point to the earth increases the ground potential of the buried point of the optical cable, and the optical cable extends to a far place, the potential of the metal component should be regarded as zero potential. Such a great potential difference between the mine point and the cable metal components, if the potential difference exceeds the voltage strength of the cable protection layer, will break down the outer protective layer, the formation of the arc channel from the mine point to the metal components, so that quite strong lightning pouring into the optical fiber, will generate induction current in its outer protective armor layer and cable core metal reinforcement, impulse voltage, Melt metal components, breakdown of outer protective layer, fiber structure deformation.

2. optical cable lines should take lightning protection measures

In areas where the thunderstorm days are greater than 20 days and the soil resistivity is greater than 100 Ω∙m at depths of 10 m, the following lightning protection measures should be taken when the cable line encounters the following premises:

1) Places with sudden changes in geological structure.

2) There are boundary effects at the junction of stone mountain, paddy field and river, at the boundary of mineral deposits, and at the boundary of entering and leaving forests.

3) Sunny or windward slopes of mountains facing wide water, with high topography or prominent isolated hilltops.

4) Sites of lightning strikes.

5) The distance between the optical fiber and an isolated tree of more than 10 m, a power pole (including a cable) that is more than 6.5m above the ground, or a high-rise building and its protective grounding device is less than the clear distance specified in the following table. Minimum lightning-proof distance between directly buried communication fibers and isolated trees.

3. optical cable line lightning protection measures

1) Overhead optical cable lines

In the construction of overhead optical cable lines, galvanized steel stranded wires are generally used as the suspended wires of optical fibers. In order to reduce the impact of lightning on overhead optical fiber lines, the suspended cables of optical fibers should be grounded by lightning rod or cable every 300 to 500m, and insulators should be installed every 1km or so for electrical disconnection. Disconnect the metal component in the optical fiber from front to back at the optical fiber connector, do not do electrical connection, and do not ground treatment. When routing a new optical fiber, it should avoid parallel approaches to high-voltage transmission lines and AC electrified railway as much as possible, and the crossover Angle should be at least 30˚ when intersecting them. For the overhead optical fiber lines in areas with particularly serious lightning damage, the structure form of non-metallic reinforced core or no metal components can be used. Overhead lightning protection ground wire can also be set up above the overhead optical fiber lines (overhead ground wire is made of 4.0mm galvanized iron wire, which is set up in a position 30~60 cm higher than the top of the pole).

2) Directly buried optical cable lines

Direct buried optical cable lines from the investigation and design to the whole process of construction laying should be selected reasonable optical fiber path, as far as possible to avoid lightning strikes in the area, such as optical fiber line laying location must pass through the mine section, in the design and construction should take effective lightning protection measures. For example, lightning protection flow line, arc suppression line, flash rod, etc.

4. lightning protection flow line

According to the laboratory experiment and practical application, among the many protective measures for directly buried optical cable lines, the lightning protection and drainage streamline is the most effective lightning protection measure. In areas where the average annual thunderstorm days are greater than 20 and the soil resistivity is greater than 100Ω∙m, if underground communication lines cannot circumvent the above sections, lightning protection and drainage lines (also known as underground lightning protection and lightning shielding lines) can be set up according to the following principles:

1) A drainage line shall be provided for the sections with soil resistivity ranging from 100 to 500 Ω∙m;

2) Two drainage lines (one if protected by plastic pipe) shall be provided for sections with soil resistivity greater than 500 Ω∙m.

The common practice in laying lightning protection and drainage lines is to use two 7/2.2 galvanized steel strand wire or two Φ6.0mm galvanized steel bar. In some areas, in order to ensure the lightning protection effect and the service life of the lightning protection ground wire, there are also two Φ4.0mm copper clad steel wire as drainage lines. 

3) Arc suppression

When there are independent lightening objects such as trees, poles and tall buildings near the optical fiber line, the possibility of direct lightning strikes on the optical fiber is small. However, if the high target is hit, the lightning will flow through the root or the ground body of the lightning rod and leak into the optical fiber, or the optical fiber will be damaged by electric arc through the soil. The most effective way of protection is to make the lightning protection flow line into the form of arc suppression. The arc-extinguishing line is a lightening and streamer line, but it is not straight, but faces the optical fiber to form a semi-circular arc around the tree. Grounding devices must be used at both ends of the arc suppression cable. The distance between the grounding device and the optical fiber must be at least 15 m, and the grounding resistance must be less than 10 ohms. However, it should be noted that when the distance between the optical fiber line and the mine target is less than 5m, arc suppression is not suitable (therefore, the optical fiber is likely to be in the arc area), and steel pipe can be used for protection. The laying method of arc suppression line is shown in the figure.

4) Connect the flash rod

Lightning rod is a widely used device to prevent direct lightning strike. It can direct lightning discharge to itself and prevent the protected object from direct lightning strike. The application range of lightning protection with flash rod is the same as that of lightning protection with arc suppression method. It can also be used in the tuyere area between two mountains and other favorable terrain.

It is not suitable to use cement poles as supports, because cement poles have steel bars and low insulation to the ground. A flash rod is installed on the top of the support, and the lead down line of the flash rod can be 40 mm × 4 mm hot-dip galvanized flat steel or Φ12 mm galvanized round steel. The lead down line must be at least 15 m away from the optical fiber. Therefore, the lead down line must be drawn vertically and horizontally in the direction opposite to the optical fiber. It cannot be drawn into the ground by connecting the rod body of the flash rod support. If you need to use a cable to fix the wooden pole that connects the flash pole, then the ground anchor that fixes the cable must also have a distance of more than 15 m from the optical fiber, otherwise you can not use a cable, only use a pole. The grounding resistance of the flash rod shall meet the following requirements: When the soil resistivity is less than 100 Ω∙m, the grounding resistance shall not exceed 5 Ω; When the soil resistivity is greater than 100 Ω∙m, the ground resistance is less than 10 Ω.