Prevent direct buried optical cable splice box from water ingress

The direct-buried optical cable splice box got water without knowing what happened, and I don’t know how to prevent the direct-buried optical cable splice box from getting water. Let's talk about why the direct buried optical cable splice box gets water and how to prevent the direct buried optical cable splice box from getting water.

Why does the direct buried optical cable splice box get water

1.1 The structural design of the splice box is not suitable for direct burial of optical cables.

The cap-type splice box is mainly designed for laying optical cables in overhead and tunnels. It does not meet the waterproof requirements of the regulations when used in direct-buried lines, but the moisture-proof effect in lines is better. According to regulations, the open type and other three commonly used joint boxes are all used in direct burial laying, but the moisture-proof effect is poor.

Although the structures and scopes of application of these four types of optical cable splice boxes are different, from the perspective of sealing technology, there are only two parts that need to be waterproofed: the joint of the upper and lower covers of the splice box; the place where the line optical cable enters the splice box Introduction site.

The cap-type splice box uses a heat-shrinkable sleeve to seal the lead-in part of the optical cable into the splice box, and connects the line optical cable and the splice box as a whole. The upper and lower covers squeeze the rubber ring to make it waterproof. The other three joint boxes are sealed with non-vulcanized rubber belts at these two parts, and water and moisture can only enter the joint box through these two channels. The optical cable lead-in parts of these three types of optical cable joint boxes are sealed with non-vulcanized rubber bands to prevent moisture.

The non-vulcanized rubber belt is a polymer material with a certain viscosity. When it is subjected to a large external force, it will deform and fill the gaps between objects to achieve the purpose of moisture resistance. However, it is a medium and low viscosity chemical substance, and its physical properties are easily affected by temperature changes, especially when other impurities are attached, its sealing performance will drop sharply. In the cap-type splice box, the line optical cable enters the cable introduction part of the splice box and is sealed with a heat-shrinkable sleeve. Compared with non-vulcanized rubber belts, its physical properties are more stable, and the moisture-proof treatment process is also more refined, so the occurrence of water ingress failures is avoided.

1.2 The treatment of the lead-in parts of the optical cable in the joint box is not standardized.

The water ingress faults caused by the non-standard treatment of the line optical cable entering the joint box optical cable lead-in part accounted for 85.7% of the total water ingress faults, and the proportion of water ingress in the three commonly used direct-buried optical cable joint boxes was as high as 88.3%. After careful study of the water ingress in the cable lead-in parts of these three commonly used direct-buried optical cable splice boxes, it was found that there are three main reasons:

(1) The line optical cable is warped at the cable introduction part of the joint box;

(2) The non-vulcanized rubber tape used for the sealing of the joint box is stuck with impurities;

(3) The line optical cable is not completely roughened at the cable introduction part of the joint box.

How to prevent direct buried optical cable splice box from entering water

2.1 Treatment of the lead-in part of the optical cable in the joint box

Commonly used optical cable splice boxes for direct burial laying, no matter what kind of structure, as long as they have a network access permit, they are equipped with instructions for their application characteristics. The operator should carefully read the manual before use, and pay close attention to the parts with a high probability of problems, especially the lead-in parts where the line optical cable enters the splice box. The following work should be done in the treatment of this part:

a. It is necessary to ensure that the line optical cable is thoroughly roughened at this part to enhance the adhesion between the non-vulcanized rubber tape and the optical cable. In the project, the optical cable at this place should be sanded with No. 8 sandpaper, and the fiber optic cable should be used as the axis to be evenly roughened in a circular shape until the luster of the outer sheath of the optical cable completely disappears, and then use cotton yarn to remove the remaining sandpaper particles on the outer sheath. Finally, the non-vulcanized rubber tape is stretched in the horizontal direction and evenly wound on the optical cable. The thickness of the winding should be higher than the extrusion recess between the upper and lower covers of the splice box. After the joint box is closed, traces of the non-vulcanized rubber band being extruded out of the box can be clearly seen from the outside of the joint box.

b. It is necessary to ensure that the non-vulcanized rubber belt is clean. Due to the limitation of the field geographical environment of some optical cable splicing points and the negligence of the operator, if the non-vulcanized rubber tape is stuck with impurities before use, its viscosity will decrease, resulting in a decrease in the moisture-proof ability of the splice box after packaging.

c. It is necessary to ensure that the non-vulcanized rubber belt is stretched and wound in accordance with the operating specifications. In order to ensure that the non-vulcanized rubber tape can connect the splice box and the line optical cable as a whole during packaging, and there is no gap between layers, the operator should stretch it evenly when winding it. If the stretching force is not enough, it is impossible to make it evenly wound, and gaps will inevitably appear after the joint box is packaged; if the stretching force is too large, it will break.

2.2 Prevent warping of the optical cable at the cable introduction part of the splice box

After the optical cable splice box is packaged, the link of backfilling and soil covering is generally not paid attention to, but unqualified backfilling method is the root cause of the warping of the line optical cable in the cable introduction part of the splice box. According to statistics, 59% of direct-buried optical cable splice boxes are caused by water ingress. This is mainly because the openings of all direct-buried optical cable splice boxes leading into optical cables are located in the middle of the longitudinal thickness of the splice box. Before backfilling the joint box with soil, if there is no bottom of the optical cable here, use a hard object to pad the optical cable to the same height as the opening of the joint box, and the optical cable at this place will be in a suspended state. Especially when the loose backfill soil sinks and becomes more and more compact, it will cause the optical cable at this place to be unable to withstand the huge sinking force of the backfill soil and warp, which will drive the original seal winding deformation of the non-vulcanized rubber belt, and eventually Causing water to enter the connector box. Therefore, before the cable splice box is backfilled with soil, hard stones should be used to pad the bottom of the cable at the lead-in part of the splice box so that the cable can be at the same height as the opening of the splice box when backfilling the soil, and when backfilling the soil, avoid Do not drop the optical cable at this part.

2.3 Enhancing the moisture-proof performance of the directly buried optical cable splice box

On the basis of maintaining the structure of the existing optical cable splice box, the non-vulcanized rubber tape is replaced by the expansion water-blocking yarn tape as the sealing material to improve the moisture-proof performance of the splice box. The water-blocking function of the expansion water-blocking yarn tape is to use the main body of the water-blocking yarn tape to swell rapidly to form a large jelly (the water absorption can reach dozens of times its own volume, and the first minute it meets water) The inner diameter can rapidly expand from about 0.5 mm to about 5.0 mm), and the water retention capacity of the jelly is quite strong, blocking the water channel space between the optical cable splice box and the outside world, preventing moisture from continuing to infiltrate and spread. Compared with non-vulcanized rubber tape, water-blocking yarn tape has the following characteristics: a. Reusable, reducing the operation and maintenance costs of optical cables. b. Good chemical stability, no corrosive ingredients, resistant to bacteria and mold. c. It has a certain mechanical strength, which can prevent the damage to the sealing performance of the joint box caused by vibration or terrain changes. d. The sealing requirements and procedures and operation methods are consistent with those of non-vulcanized rubber belts, and no special skills are required.