Non-metallic Loose tube Polyethylene sheathed Duct Optical fiber cables with Aramid yarns are installed where more pulling and installation tension is required.
Generally, Non-metallic duct cables with improved tensile performance are installed in the long haul and metro access fiber optic networks. Non-metallic cables are good choices to install along with electrical cables. Cable design does not use any metallic element and therefore, non-metallic type duct fiber cables are safe from the adverse effects of electrical influences. These cables can be pulled or blown into ducts made of HDPE. Corrugated ducts are preferred as it will give added protection.
Non-metallic duct cables with Aramid yarns can go aerially for a short span if required. The use of Aramid yarn gives the fiber cable required pulling tension to meet if go aerially. When the terrain condition has small brooks and water streams, the fiber cable laying in the underground duct may not be possible or it will cost more to civil works. Low strength conventional non-metallic duct cables do not support the relatively high strength required for installation on poles. Generally installed in ducts and if required tightly lashed on poles, these kinds of cables are installed where rodent attacks are not expected.
These cables are installed in conduits and also tightly lashed on aerial poles for short spans. The Aramid yarns give linear strength to the cable. The cables installed in the duct are safe from outside environmental effects. For the portion of cables open in the air, if installed aerially, the Polyethylene used for outer sheathing is UV stabilized. Non-metallic Duct fiber optic cables will comply with IEC-60794-3-10 March 2002
The schematic diagram of a typical Non-metallic duct fiber optic cable with Aramid yarns is shown below:
Process & Construction details
Colored optical fibers are put into loose tubes simultaneously filled by the thixotropic jelly compound. Thixotropic jelly prevents moisture entry and also provides a cushion for optical fibers. The color and number of fibers depend on the construction of the non-metallic optical fiber cable. The loose tube dimensions are decided based on the number of fibers to be put into the tube. For example, for a 24 fiber cable shown in the diagram, 4 fiber per tube is used. 1.8mm outer diameter for loose tubes with 1.2 mm inner diameter equals a thickness of 0.3mm for the loose tube. Loose tube material can be PBT, PP, Polycarbonates, or Nylon. PBT is widely used.
Loose tubes are stranded around a non-metallic strength member using the reverse lay technique. The helical stranding for loose tube type cables is a technology of the past. Reverse lay technique use SZ stranding. The FRP and core interstices are filled with jelly compound for water blocking. Dry designs use Water blocking yarns or powder in between the elements and over the FRP. The stranded core is wrapped with polymer tapes or non-conductive non-woven tapes as the case may be.
The stranded core is layered with Aramid yarns. The number and dtex of Aramid yarns are decided by the fiber optic cable design engineer according to the tensile strength requirement. Dupont and Teijin are the famous suppliers for Aramid yarns. But there is one more supplier from South Korea, Kolon for the supply of Aramid yarns. D.S Holdings of South Korea also supply Aramid yarns, but they use basic yarns from Dupont. Dupont’s Kevlar and Teijin’s Twaron brands are popular in the fiber optic industry.
The young’s modulus of Aramid yarn is important to ensure the tensile strength of fiber optic cable. Aramid yarns are applied helically over the core and then sheathed with polyethylene having properties of UV stability. Since the cable goes aerially UV stability is a must. Black polyethylene sheaths are preferable.
Non-metallic duct cables with Aramid yarns are designed to meet a tensile strength of 2700N or 2.5W (W is the weight of 1 km cable) whichever is higher. The high tensile strength compared to the conventional duct cables makes these types of cables suitable for harsh installation conditions. The outer sheath thickness ranges from a minimum of 1.5 mm to 2.0 mm. There are instances that some customers specify for 2.5 or even 3.0mm Polyethylene sheath thickness. But such cases are rare and applied for special applications.
Non-metallic duct cables with Aramid yarns are tested for water penetration tests. The cable sample of 3-meter length is put under a tight water column of 1 meter for 24 hours. At the end of the test no water shall be traceable at the end of 3 meters is considered a positive test result.
The temperature range of duct nonmetallic cables is -40 deg C to +70 deg C, generally required for storage, transportation, and operation. For installation, a temperature range of -10 deg C to +50 deg C is preferred.