Underground cable is placed into ducts which are being built below the ground surface. In urban areas where space for telecommunications cable is limited, it needs to be used more efficiently. In underground installation, the conduit provides protection from both physical and environmental abuse. The conduit protects cable from shifting rocks, aggressive rodents, and/ or damage from hand shovels.
Underground cable that is in conduit is easy to replace or upgrade. The old cable can be pulled out of the conduit and the new pulled in without extensive and expensive digging. Underground cable and ducts are part of the underground conduit system. Telecommunication conduits are made from various materials and buried directly into the soil or encased in concrete. Fiber optic cables have provided a more optimal use of available underground conduit space because of its small cable diameter and the much higher communications traffic capacity of each cable. Optical cable is usually placed in a 25 to 40 mm inside diameter (ID) sub-duct which is placed into an existing larger diameter communications conduit. Most communications conduits can be fitted with three or four sub-ducts. Sub-ducts are often referred to as innerducts. An innerduct provides a more efficient use of the conduit system space, with a clean low coefficient-of-friction pathway and an extra measure of mechanical protection for an optical cable. If micro-duct cables are used, they are usually placed into small diameter ducts that are placed inside of the innerducts, nested two levels within a communications conduit system. These small diameter ducts are usually from approx 5 mm to 14 mm inside diameter and called microducts. Micro-duct cable is blown (jetted) into the micro-duct. Normally, standard size fiber optic cables are pulled into innerduct (sub-ducts).
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