The water ingress and sealing treatment of the fiber cable splice closure, which is called fiber optic enclosure, used in underground optical cables are the key points of optical cable line construction and maintenance. Because underground optical cables are laid directly in the ground, they are less damaged by external factors, and their transmission quality is less affected by ambient temperature changes.
However, in the direct burial laying method, the problems of moisture and water ingress in the splice closure are quite common, and once the splice closure is flooded, its maintenance monitoring and fault handling are more complicated than other laying methods. If handled improperly, it will seriously affect the operation safety of the entire optical cable network.
Causes Of Water Ingress Into Direct Buried Optical Cable Splice Closure
1. Analysis of Water Ingress into the Optical Cable Closure
When the operator overhauled a 210 km direct buried optical cable line maintained by a certain institute, the insulation resistance of 74 splice closures to the ground was tested. Among them, 52% had an insulation resistance value of zero, and 48% of the splice closures had obvious water accumulation.
After consulting relevant technical data of the same period, the results showed that 67% of the optical cable splice closures laid directly underground would have water ingress failures.
The water ingress of the splice closures found during the renovation showed that the waterproof effects of the three types of optical cable closures were as follows: the waterproof rate of the dome-type closure was 83%, the waterproof rate of the horizontal-type closure was 75%, and the waterproof rate of the box-type closure was 45%. It can be seen that the use of the dome-type closure is the best to prevent water ingress of the direct buried optical cable closure.
2. Unreasonable Design of the Optical Cable Closure
The dome type closure is mainly designed for overhead and tunnel laying of optical cables. It does not meet the waterproof requirements of the regulations when used in direct buried lines, but it has a good moisture-proof effect in the line. According to regulations, the optical cable closure of common structures such as the box type are used in direct buried laying, but the moisture-proof effect is poor.
Although the structures and application scopes of these three types of optical cable closures are different, from the perspective of sealing technology, there are only two parts that need to be waterproofed: the closing part of the upper and lower covers of the closure and the introduction part of the line optical cable into the closure.
The dome type closure uses a heat shrink sleeve to seal the introduction part of the optical cable into the closure, connecting the line optical cable and the closure into one, and the upper and lower covers are closed to squeeze the rubber ring for waterproofing.
The other two types of closures are sealed with non-vulcanized rubber tape at the junction of the upper and lower covers and the introduction of the optical cable. Water and moisture will enter the closure through these two channels.
Non-vulcanized rubber tape is a polymer material with a certain viscosity. When subjected to a large external force, it will deform and fill the gaps between objects to achieve the purpose of moisture-proof.
However, non-vulcanized rubber tape is a medium- and low-viscosity chemical substance. Its physical properties are easily affected by temperature changes, especially when it is adhered to other impurities, its sealing performance will drop sharply.
For the dome type closure, the line optical cable enters the optical cable closure introduction part, which is sealed with a heat shrinkable sleeve. Compared with non-vulcanized rubber tape, its physical properties are more stable and the moisture-proof treatment process is more sophisticated, so the occurrence of water ingress failure is avoided.
3. The Treatment of Optical Cable Introduction Part of Closure is not Standardized
85.7% of all water ingress failures are caused by improper handling of the cable entry point of the line optical cable into the closure, of which the water ingress ratio of the horizontal closure and the box direct buried optical cable closure is as high as 88.3%.
Therefore, this problem must be solved in the project, so as to greatly improve the moisture-proof performance of the direct buried optical cable closure.
A careful study of the water ingress of the cable entry points of these three commonly used direct buried optical cable closures found that there are three main reasons:
- The line optical cable is warped at the cable entry point of the closure
- The non-vulcanized rubber belt used for sealing the closure is contaminated with impurities
- The line optical cable is not thoroughly roughened at the cable entry point of the closure
Prevention and Solution of Water Ingress into Direct Buried Optical Fiber Cable Splice Closure
If water enters the direct buried optical cable closure and is not treated in a short period of time, the loss at the optical fiber joint will increase. The unclean groundwater will not only cause the optical fiber coating to fall off, reduce the mechanical strength, but also cause permanent damage to the optical fiber.
1. Treatment of Optical Cable Entry Part of the Closure
Regardless of the structure of the direct buried optical cable closure, it is equipped with a note of caution for its application characteristics. The operator should read the instructions carefully before use and pay close attention to the parts with a high probability of problems, especially the introduction part of the line optical cable into the closure. The following work should be done for the treatment of this part:
- It is necessary to ensure that the line optical cable is thoroughly roughened at this location to enhance the adhesion between the non-vulcanized rubber belt and the optical cable. In the project, the optical cable at this location should be roughened evenly in a circular shape with the optical cable as the axis using No. 8 sandpaper until the gloss of the outer sheath of the optical cable completely disappears, and then the sandpaper particles remaining on the outer sheath are removed with cotton yarn. Finally, the non-vulcanized rubber belt is stretched horizontally and evenly wrapped around the optical cable. The thickness of the wrapping should be higher than the extrusion concave between the upper and lower covers of the closure. After the closure is closed, the traces of the non-vulcanized rubber belt being squeezed out of the box can be clearly seen from the outside of the closure.
- Ensure that the non-vulcanized rubber tape is clean. Due to the geographical environment of some optical cable splicing points and the operator’s negligence, if the non-vulcanized rubber tape is contaminated with impurities before use, its viscosity will decrease, resulting in a decrease in the moisture-proof ability of the splice closure after packaging.
- It is necessary to ensure that the non-vulcanized rubber belt is stretched and wound in accordance with the standard. In order to ensure that the non-vulcanized rubber belt can connect the closure and the line optical cable as a whole when it is packaged, and there is no gap between the layers, the operator should stretch it evenly when winding it. If the stretching force is not enough, it is impossible to make it wound evenly, and gaps will inevitably appear after the closure is packaged; if the stretching force is too large, it will break.
2. Prevent Optical Cable from Warping at the Cable Entry Point of the Closure
After the optical cable closure is sealed, the backfilling and covering process is generally not taken seriously. However, unqualified backfilling methods are the root cause of the warping of the line optical cable at the cable introduction site of the closure.
According to statistics, 59% of the water ingress into the direct buried optical cable closure is caused by this. This is mainly because the openings for the introduction of optical cables into all direct buried optical cable closures are located in the middle of the longitudinal thickness of the closure.
Before backfilling the closure, if the optical cable is not padded to the same height as the opening of the closure with a harder object at the bottom of the optical cable, the optical cable at this location will inevitably be suspended. In particular, when the loose backfill soil sinks and becomes compact, the optical cable at this location will not be able to withstand the huge sinking force of the backfill soil and will warp, thereby causing the original sealing winding of the non-vulcanized rubber belt to deform, and eventually causing water ingress into the closure.
Therefore, before backfilling the soil of the optical cable closure, use harder stones to pad the bottom of the optical cable at the cable entry point of the closure so that the optical cable can be at the same height as the opening of the closure when backfilling the soil. And be careful not to hit the optical cable in this position when backfilling the soil.
3. Enhance the Moisture Resistance of Direct Buried Optical Cable Closures
On the basis of maintaining the existing structure of the optical cable closure, the expansion water-blocking yarn tape is used to replace the non-vulcanized rubber tape as the sealing material to improve the moisture-proof performance of the closure.
The water-blocking function of the swelling water-blocking tape is that after the main body of the water-blocking tape meets water, it can quickly expand to form a large volume of colloid (the water absorption can reach dozens of times of its own volume, and the diameter can quickly expand from about 0.5 mm to about 5.0 mm within the first minute of meeting water).
Moreover, the water-blocking ability of the colloid is quite strong, and it can effectively prevent the growth of water velocity for a long time, block the water channel space between the optical cable closure and the outside world, and prevent water from continuing to penetrate and spread. Compared with non-vulcanized rubber tape, the water-blocking tape has the following characteristics:
- Reusable, reducing the cost of optical cable operation and maintenance
- Good chemical stability, no corrosive components, resistant to bacteria and mold
- Has a certain mechanical strength, can prevent damage to the sealing performance of the closure caused by vibration or terrain changes
- The sealing requirements, procedures and operation methods are consistent with non-vulcanized rubber belts, and no special skills are required
4. Improve the Structure of Direct Buried Optical Cable Closure
Judging from the sealing performance of the above three different structures of the closure, different moisture-proof effects are brought about because different sealing processes are used for the introduction part of the optical cable into the closure.
The cable entry part of the dome type closure is waterproofed by heat shrink tubing, while the other two commonly used direct-buried optical cable closures such as the box type use non-vulcanized rubber tape to prevent moisture in this part. From the overall effect, it is obvious that the moisture-proof effect of using heat shrink tubing is better than that of using non-vulcanized rubber tape.
For this reason, we use the heat shrink tubing originally used for sealing dome type closures for the other two types of direct buried optical cable closures. The improved closure is consistent with the original closure in structure, and the biggest difference from the original closure is that the length of the bayonet for introducing the optical cable in the upper and lower covers of the original closure is extended and connected to the upper and lower covers as one.
In order to facilitate the sealing of the heat shrinkable sleeve, its extended length should generally be no less than 4 to 6 cm. In this way, when encapsulating the closure, the heat shrink sleeve can be used to seal the parts that originally required the non-vulcanized rubber tape, thereby further improving the moisture-proof performance of the direct buried optical cable closure.
5. Sealing Methods for Closures in Low Temperature Areas
If the closure is not sealed tightly, the closure may be soaked in water during the change of seasons. When it freezes in winter and melts in spring, it is very easy for the optical fiber in the closure to be squeezed and deformed. In mild cases, large attenuation will occur, and in severe cases, the optical fiber will break, resulting in communication interruption.
(1) Main Reasons for Poor Sealing
The sealing process of the direct buried optical cable closure is a very meticulous and rigorous job. Especially in frozen areas, for various types of closurees, even if the prescribed sealing method is strictly followed, it is difficult to achieve absolute sealing of the closure.
According to research, the reasons for the closure being soaked in water are:
1. The optical cable inlet and outlet holes and the joints of the upper and lower box bodies are on the same sealing plane, which increases the difficulty of sealing
2. The circumference of the joint point between the upper and lower parts of the entire box body is longer, that is, the required sealing length is longer
3. Due to the thermal expansion and contraction of the sealing strips and tapes, gaps are generated in the seal
(2) Injecting Glass Glue
Practice has shown that the method of injecting glass glue can solve the problem of poor sealing caused by thermal expansion and contraction of the sealing strip.
The specific method is as follows: evenly squeeze an appropriate amount of glass glue into the grooves of the upper and lower boxes, embed the self-adhesive strip into the groove of the lower box, align the upper and lower boxes with the fastening screws (the tightening degree of each screw should be consistent), and when the glass glue is evenly squeezed out from the plane position of the joint of the upper and lower boxes, the distance between the upper and lower box planes is about 1mm.
At this time, the sealing area of ​​the sealing strip is increased, which enhances the sealing effect. If the upper and lower boxes are tightened too much, that is, the upper and lower boxes are pressed together, the strips and glass glue at the plane position of the joint of the upper and lower boxes will be completely squeezed out. When the temperature drops and the strip shrinks, a gap will still appear between the upper and lower boxes, affecting the sealing effect.
(3) Pay Attention to Insulation of the Monitoring Tail Cable
For some reason, when the outer sheath of the closure is directly measured, the insulation is good. However, after the optical cable is connected, it is found that the insulation is not good.
The places related to the outer sheath of the optical cable are: the monitoring tail cable used for connection and the glass glue used to seal the closure. Sometimes, if there is a slight negligence, the monitoring tail cable will press the monitoring line between the upper and lower box bodies when sealing the closure, causing the closure to leak. After a period of time, the insulation layer of the monitoring line is squeezed and the insulation resistance drops to below the normal range of 1OMΩ·km.
Sometimes, in the process of finding the problem, the cause cannot be found after repeated searches. After opening the closure and removing the monitoring tail cable, it is found that it is unqualified.
After the monitoring tail cable is dried, the test is normal. The reason is that under the action of humid air, the monitoring cable is not sealed well, and moisture enters the tail cable, which affects the insulation. After replacing the tail cable, the insulation test is normal.
Conclusion
Although water ingress into the direct buried optical cable closure is a difficult problem in the operation and maintenance of the current optical fiber communication system, through a detailed analysis of the causes of water ingress, it is found that the fault is caused by the structural design of the optical cable closure, the lack of detailed operating procedures, and the inherent defects of the original sealing materials.
Therefore, to ensure that the direct buried optical cable closure does not get water ingress, comprehensive prevention and treatment are still required. Especially in the currently designed optical cable communication system, Bonelinks expected safe service life of the optical cable is more than 20 years, and the fault interval is more than 10 years.
In order to achieve or extend the expected design service life of the optical cable, in addition to selecting high-quality optical cables and strictly following the established standards for construction, it is also crucial to improve the maintenance level of the optical cable and solve the key and difficult problems in maintenance. Please drop us a message or visit us at bonelinks.com for more details.