Technology changes can lead to unintended consequences. Recently, I’ve heard stories from the field about problems with fiber optic cables that can be traced to new cable technology. The biggest issue is breaking fibers in high-fiber-count outside plant cables.
Changing cable sizes
For practically the entire history of fiber optics, a big problem has been finding room for new fiber optic cables in crowded underground ducts. In the early days, room for fiber was often created by removing bulky copper cables, but most of those are now gone. Fabric ducts and microducts have been successful at adding more fiber optic cables. One solution was to make fiber optic cables smaller.
In the last five years, cable manufacturers have been in a race to make regular cables with more fibers in smaller cables that fit in crowded ducts. Much of the technology comes from the development of microcables. Reducing fiber coating diameters from 250 microns to 180–200 microns and changing from solid fiber ribbons to flexible ribbons means you can squeeze more fibers into smaller spaces. Reducing the thickness of plastic jackets and loose tubes, including fewer or no Kevlar strength members and making strength members smaller helped reduce cable size, too.
Water blocking necessary for outside plant cables has been changing as well. Everybody was happy to see messy water-blocking gel or grease replaced by dry powder or tape that would expand in water and do the same thing.
Cable size is an issue. Microcables are usually limited to 288 fibers, although a few are larger. Meanwhile, regular outside plant cables are being made with higher fiber counts. The biggest ones were once 864 fibers, but now we see cables with 1,728, 3,456 and even 6,912 fibers. Even with new cable construction designs, these cables are much bigger and heavier than microcables or conventional outside plant cables.
Blowing versus pulling fiber
All these design changes worked great with microcables, but microcables and regular cables are installed differently. Microcables are “blown” into microducts. Blowing cables really means you use high-pressure air flow to float the cable so it can be pushed down the microduct. Lightweight microcables can be blown for several kilometers.
Regular outside plant cables are usually pulled into ducts, not blown, so they are usually rated for high pulling tension (600 pounds or more). They are often pulled with a capstan that grabs a loop of the cable to provide pulling tension and pulls cable in ducts buried underground.
Techs pulling some of these new, smaller, higher-fiber-count cables are reporting that fibers are being broken during installation. While the problem is still under investigation, the Fiber Optic Association has been told that manufacturers are concerned and are working on guidelines for installers to prevent damage.
The problem seems to be related to the newer cable designs, with tighter construction, higher fiber density, fewer strength members, thinner buffer tubes and jackets and no gel to cushion fibers when stressed. In addition, there may be a problem with tools that were never intended for use with some of these larger cable designs.
Fibers are designed to be stressed when pulled, but they do have tension and bending limits. Traditional cable designs have lots of room for error, but some of these new, denser cables may not be as forgiving.
Bend diameter and radius
In addition, the high-fiber-count cables with a larger diameter have a larger acceptable minimum bend diameter. Often, that diameter is larger than the size of currently used tools such as capstans, pulleys and blocks.
The industry standard for bend diameter under tension is still 40 times the cable diameter. Many documents still say a bend radius is 20 times the cable diameter, but the industry is moving to specifying bend diameter because the definition is clearer when applied to tools such as capstans, pulleys and blocks.
Many installers today are still using the equipment they have been using for years for pulling 144- or 288-fiber cables, typically 24 or 30 inches in diameter. Manufacturers are now introducing capstans and sheaves (pulleys) up to 50 inches in diameter for the larger cables and pulling blocks, with large-radius plastic blocks that are specifically designed for these new large-diameter cables replacing small-diameter rollers.
If you have not yet upgraded your equipment for these new cable types, now is the time to do it, before you discover the older equipment can damage the new, larger cables. These high-fiber-count cables are expensive, and damaging one can become a very costly learning process.
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