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The ins and outs of ribbon fiber optic cable
You would think that after being in a business for more than 25 years, you would have seen it all. But recently, I was introduced to the actual installation and splicing processes for ribbon fiber optic cable. While I had seen plenty of the cable over the years, I had never had the opportunity to work with it myself. Corning Cable Systems was training one of the trainers we work with and invited me to join the session.
Ribbon cable is widely used in outside plant telephone networks and submarine cables but, until recently, had rarely been seen in campus or premises applications. Ribbon cable is highly regarded for long-distance applications because it has several advantages over normal loose tube outside plant cable due to its unique construction.
In ribbon cables, the fibers really look like multicolored ribbons (see photo). The actual ribbons usually contain 12 fibers, although designs with 24 fibers are available. In the beginning, ribbons were made by placing fibers on double-stick plastic tape, but now they are more closely packed and held with adhesives. Consider the small size of a 12-fiber ribbon: 12 coated fibers, each 250 microns or 10-thousandths of an inch in diameter, create a ribbon only 120-thousandths of an inch—approximately ¹/8 of an inch—wide. Ribbons can be stacked up in a cable, so a ¹/8-inch square matrix of 12 ribbons equals 144 fibers. Cable designs with slotted cores can handle multiple ribbon fiber matrices and cables with up to approximately 2,000 fibers have been used.
Ribbon cable allows the largest number of fibers to be incorporated in the smallest diameter cable. This allows the use of smaller conduit or innerduct for a single cable or allows more cables to be placed in the same size duct. The smaller size also means lighter weight, especially if the cable is armored to prevent rodent penetration. The smaller size and weight allows longer cable runs to be pulled, reducing the number of splices necessary. Finally, ribbon cable is spliced by the ribbon, not the individual fiber, so splicing goes much faster.
All this means that ribbon cable is more cost-effective in longer outside plant runs, but ribbon cable is now finding a market in premises cabling. The most common application is for preterminated fiber assemblies, where racks of terminations are created from modules of 12 jacks of the connector style required by the customer. Each module has a multifiber connector that connects to a preterminated ribbon cable that runs between racks. Since the cabling is plug-and-play, the installer carefully pulls the cables, installs the racks, plugs it together and verifies the connections.
Preterminated premises cabling systems have obvious benefits in rapid installation and in creating large fiber count backbones in small spaces. Ribbon connectors are much smaller than a dozen traditional connectors, making the ends of the cable less bulky and easier to pull or place. Not having to terminate each connector can speed installation and using ribbon terminations saves space in the rack.
However, ribbon cable does require special tools and splicing equipment. Obviously, you have to open the cable by cutting the outer jacket and armor, if included, to expose the ribbons. Next the cable must be cleaned of any water-blocking gel or powder. Then each ribbon must be separated and handled individually.
Ribbon cable is never directly terminated; it is always spliced, either to another cable or to a breakout cable that has been factory-made with a bare ribbon on one end to splice to the cable being installed and connectors on the other end, made to the customer’s specification.
Special tools have been developed for ribbon cable splicing. Fiber strippers are designed to strip all the fibers in the ribbon at once. To reduce the stress on the ribbon, some strippers have heaters to soften the coatings for easy removal. Ribbon fiber cleavers can cleave 12 fibers as quickly as an individual fiber, with equal precision. Manufacturers offer custom cleavers that have fiber holders that transfer the fibers directly to the splicer. Fusion splicing machines align and fuse 12 fibers in practically the same amount of time as a single fiber. Splice protectors cover the entire ribbon. Since stripping, cleaving and splicing take about the same time as a single fiber, you can see how efficient and cost-effective ribbon cable installation can be.
The downside of installing ribbon cable, of course, is all these tools are expensive, especially the fusion splicer. The good news is that manufacturers such as Corning Cable Systems rent them, a much more viable alternative to owning for most contractors. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.jimhayes.com.