Assignment 11 The primary benefit of conduit systems is the ability to ground and bond. Grounding and bonding play a significant role in minimizing electromagnetic interference (EMI). Steel conduit reduces electromagnetic fields by up to 95%, effectively shielding computers and sensitive electronic equipment from the electromagnetic interference (EMI) caused by power distribution systems. Cable tray advantages include wiring system design flexibility, simplicity, and lower installation cost. In plants where equipment is added, taken away, or is moved, cable trays provide a flexible advantage. Cable trays can typically adapt to complex configurations with a simple set of tools. The cost of material procurement for cable tray systems is not necessarily lower than that of conduit systems in all cases. However, compared to labor cost of conduit installation, cable trays present significant savings. Direct buried cable is placed underground without conduit. Here the cable must be designed to withstand the rigors of being buried in dirt, so it is generally a more rugged cable, armored to prevent harm from rodent chewing or the pressures of dirt and rocks in which it is buried. Direct burial is generally limited to areas where the ground is mostly soil with few rocks down to the depth required so trenching or plowing in cable is easily accomplished. Aerial cables are for outside installation on poles where consideration must be given to continual tension from the cable weight as well as wind and ice loads. Regular outdoor loose tube cables can be helically lashed to a messenger or another cable (common in CATV.) Some cables have heavier jackets and stronger metal or aramid strength members to make them self-supporting (called all-dielectric self-supporting or ADSS cable if the strength members are nonconductive.) Another "cable" type is not really cable at all. By installing a "cable" which is just a bundle of empty plastic tubes, you can "blow" fibers into the tubes using compressed gas as needed. If you need to upgrade, blow out the old fibers and blow in new ones. Both indoor and outdoor versions of air-blown fiber cables are available and it’s even been used for FTTH.
Every cable installed indoors must meet fire codes. That means the jacket must be rated for fire resistance, with ratings for general use, riser (a vertical cable feeds flames more than horizontal) and plenum (for installation in air-handling areas. Most indoor cables use PVC (polyvinyl chloride) jacketing for fire retardance. In the United States, all premises cables must carry identification and flammability ratings per the NEC (National Electrical Code) paragraph 770. Any cable that includes any conductive metal must be properly grounded and bonded per the NEC for safety. Indoor cables rated OFC, OFCG, OFCR or OFCP and outdoor cables with metallic strength members or armor must be grounded and bonded. All composite cables must be properly grounded and bonded also.