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. FO-VC2 JOINT USE - VERICAL MIDSPAN CLEARANCES 48. FO-RI JOINT USE RISER. Underground cables are pulled in conduit that is buried underground, usually 1-1. 2 meters (3-4 feet) deep to reduce the likelihood of accidentally being dug up. The charter of the FOA was to promote professionalism in fiber optics through education, certification, and. Where reels are supplied with protective material fitted over the cable, the protection should remain in place until the cable will be installed. Turn-backs and all sharp changes of direction. There are three common laying methods for outdoor optical cables, namely: underground pipeline laying (that is, laying optical cables in underground pipelines), direct underground laying and overhead laying (that is, laying from utility poles to utility poles in the air.