We know that stress and overstrain is a major enemy of the fiber’s lifespan, so cable installers must ensure that during the installation, fiber cables would not suffer from undesirable stretching or bending. Pulling, pushing and blowing are the three techniques used in wire management, which usually cause minor installation strains even for a seasoned installer. It is unavoidable, but fiber optic cables made of glasses have a limitation of bending ratio or tight diameter. If it does not exceed the certain diameter, the fiber will function well. Therefore it is essential for us to know how far cables can be bent. In fact, the maximum transmission distance of the fiber optic cable depends on the aspects like bend radius, tensile strength, and usable duct space, which will be clearer illustrated in the following article.
Bend radius is the curvature an optical fiber can bend without damage or shortening its lifespan via kinking. The smaller the rated bend radius, the more flexible the fiber. Just check the manufacturer’s spec for bend radius before purchasing the products. If no recommendations are available from the cable manufacturer, the cable should not be pulled into a bend radius smaller than twenty times the cable diameter. For example, a cable with an outer diameter of 5 mm, should not be bent smaller than 100mm radius during installation.
Maintaining proper bend radius is key in preventing service calls due to damage and signal loss in your optical fiber. Bend loss usually occurs when the fiber cable bends are tighter than the cable’s maximum bend tolerance, which might damage the fiber by causing microcracks.
There are a couple of factors that may mitigate the problems of bend radius and the angles within them. If a fiber cable is being pulled or pushed through an empty duct or mini-duct, obviously there is less resistance to the cable and you can pull/push greater distances. If the coating of the outside of the cable and the inside of the duct is designed to reduce friction, you will be able to achieve greater distances as well.
With well-designed bend insensitive fiber patch cables (seen in the above image), you can usually push a fiber cable as far as 90-degree angles in the run with minimized signal loss compared with the traditional fiber patch cables.
Just as the bend radius, fiber optic cables also have maximum tensile strength. If the cable is being pulled through, it is better not to exceed the maximum tensile load. However, if the cable is being installed within a microduct or conduit, pushing the cable will apply no tensile force.
Over-stressing the fibers will not be noticeable until after installation since the cable outer sheath will elongate, whereas glass optical fiber will not. Ideally, a breakaway swivel should be used, however, where this is not possible, the installation crews should use a tension gauge attached to the pull-cord. As a rough approximation, 100 Newtons is equivalent to a 22lbf (pounds-force) load applied directly to the cable.
Usable Duct Space
The full space in the duct is not usable for cables because of horizontal and vertical bends and joints. Usable duct space should be at least 60%. 900µm and 250µm fibers are basically two kinds of fiber patch cables. 900µm fibers can easily be damaged with respect to storage and wire management. 900 µm kinks easily, but is very flexible and installs easily. Sometimes, you have to balance ease of installation with toughness. The difference between 250um Loose-tube and 900um Tight Buffered Fiber have illustrated in the above article.
During the cable installation, you must get the fiber through walls—possibly in basements, attics, or crawl spaces, and then through floors, walls and, eventually, through a room to the CPE location. For rugged spaces where your fiber may suffer damage, such as basements or in conduits sharing space with electrical or other wirings, you may need a 3mm rugged sheathed cable that is flexible and crush resistant. The image below shows fiber optic cables being kinked. The flat cat5e cables (left) and common Ethernet cable (right) have different effect after bending.
Therefore, a good rule of thumb to use in planning your fiber drop is below: Bends (angles) = Friction—maximum distance of the pushable fiber
In addition to the above bend radius, here are some other areas to focus on to achieve maximum distance without damaging the cable: signal power and performance requirements for each device or revenue generating unit, locations of required splices, list of cable lengths required. When using duct or cable in conduit, keep in mind that freezing water in the conduit can crush the cable—and it may be wise to use microduct.
One of the challenges of installing fiber is to choose a cable with a very small bend radius, and it is tough enough to handle many different installation conditions. Once you have selected the appropriate fiber, plan your route to minimize bends and friction. By planning ahead and thinking carefully, you can save money by reducing the costs of those services.