Example of a Specification Common to Loosetube Fiber Optic Cable

The buffer tubes containing one or more optical fibers shall be stranded over a central strength member using reverse helical method. Reverse helical also known as SZ stranding is the most commonly employed method for stranding tubes around the central strength member. The central strength member shall be made of either metallic or non-metallic material.

The central anti-buckling member of the cable shall consist of a glass reinforced plastic rod. The purpose of the central member shall be to prevent buckling and to provide tensile strength that is required during pulling installation of the cable.

Suitable number of fillers may be included in the cable core to lend symmetry to the cable cross-section where needed. Binder yarns or tapes are used to bind the tubes and fillers tightly together with the central strength member to form stranded core.

Water blocking performance of the cable shall be ensured by the use of water blocking compound, which is usually known as filling compound/flooding compound. Purified petroleum gel is used as water blocking compound in fiber optic cables. Alternative method is to use water swellable yarns and water blocking tapes. If water blocking compound is used, polyester tape is used to wrap around the stranded core. Longitudinally applied water blocking tape or polyester tape needs to be bind with a binder yarn.

For single layer cables, a water blocking tape shall be applied longitudinally around the outside of the strand tubes/fillers. The water blocking tape shall be non-nutritive to fungus, electrically non-conductive homogenous. It shall also be free from dirt and foreign matter.  Gel filled water-blocking compound is not suitable for some type of cables, that are used in the backbone area.

Binders shall be applied with sufficient tension to secure the buffer tubes to the central member without crushing the buffer tubes. The binders shall be non-hygroscopic, non-wicking (or rendered so by the flooding compound), and dielectric with low shrinkage.

The cable shall contain at least one ripcord under the sheath for easy sheath removal. Supplier shall provide documents showing instruction to rip the cable sheath and preparation of the cable end for splicing of optical fibers in the field.

In addition to the central strength member, if more strength is required, tensile strength shall be provided by the use of high tensile strength dielectric yarns and shall be helically stranded evenly around the cable core. These yarns could be glass yarns or Aramid yarns.

Outer cable jacket shall have a consistent thickness throughout the entire cable length and shall be sheathed with plastic material. Typically used plastic mateiral is polyethylene. Varieties of polyethylene are available in the market ranging from LDPE, LLDPE, MDPE, HDPE, XLPE etc. Indoor cables use PVC and LSZH sheath material.

The minimum nominal jacket thickness shall be suitable to support pulling installation of the cable in the field. Usually a thickness of 1.5 mm is provided for duct cables if polyethylene is used.  For outdoor cables, the polyethylene shall contain carbon black to provide ultraviolet light protection and shall not promote the growth of fungus. Alternatively, color jackets may also be required, which shall be ultraviolet resistant.

The cable jacket shall be free of holes, splits and blisters. Cable jackets shall be marked with sequential meter markings. Sheath marking shall be done preferably using hot foil indentation. Manufacturer name, year of manufacture and meter makring shall be done as a minimum. Additional information for sheath marking shall be mutually agreed. The actual length of the cable shall be within 0 to 1% of the length markings. The marking shall be in contrasting color to the cable jacket. The height of the marking shall be easily readable.


Author: Fiber

Chief Editor of Fiber Optic Mania Magazine

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