Design and Construction of Optical Ground Wires (OPGW Cables)

Optical ground wires or OPGW cables comprise an optic fiber housing unit to protect optical fibers from external mechanical stresses like compressive loads, bending, torsion, tensile strains, the effect of moisture, etc. Manufacturers have to ensure that the materials used to produce the optical ground wires must be of high quality. The raw materials used for the production of OPGW shall be free from non-conformities and must be tested periodically at their laboratory for compliance.

One of the important materials used for the construction of OPGW is Aluminium clad steel that is abbreviated to ACS generally in the industry. ACS –Aluminium (Aluminum) clad steel. These are aluminum coated steel wires also known as Alumoclad or Alumoweld. Aldrey wires are a highly conductive alloy of aluminum, silicon, and magnesium. This has two times higher tensile strength compared to aluminum.

A sample schematic diagram of OPGW Cable is shown below;

Cross section of OPGW - Optical ground wire cable

Aluminum alloy or AA wires are used for the outer conducting armor layer and inner conducting layer if the inner layer is required, Buffer tubes used in OPGW are metallic tubes. Hydrogen absorbent gel and moisture proof materials are used in the construction of OPGW cables.

The outer armor layer is designed to function as the conventional shield wire and to protect the fiber optic unit additionally. The housed optical fiber units and the stranded metallic conductors made of Aluminum clad steel wires together serve as an integral section to ensure the protection of optical fibers from degradation due to vibration effects from galloping, ice loading, the wind blows, extreme temperature variations, lightning, and fault currents. Also, this unit will protect the fibers from environmental effects that produce hydrogen.

Short circuit current and the heat produced due to its effect are one of the major concerns while designing Optical Ground Wires (OPGW). The material used for the Buffer tube shall be able to withstand a temperature of 200 deg C. This temperature is expected to reach when short circuits happen. The buffer tubes shall not be on the outer layer in order to be protected from mechanical forces and electrical influences.

Optical fibers are safely protected inside the tubes, which are filled with waterproof jelly. Waterproof jelly will take care to safeguard the optical fibers from the intrusion of moisture into the tube. Jelly provides an excellent cushion for optical fibers that prevent friction from the inner wall of the tube and also hold the excess fiber length (EFL) inside the tube.

Metallic tubes used to house optical fibers in OPGW cables are clad with aluminum to prevent corrosion. The excess fiber length inside the metallic buffer tubes ensures the maximum pulling strength that the cable can withstand. Optical fibers inside the metallic tubes are color coded to make the identification easier. Alternatively, a plastic tube can be used instead of a metallic buffer tube.

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