Published In June 2001
We all know about maintenance. We know things wear out or get dirty and need replacing or cleaning. This is especially true about cars. We regularly change the oil and filter, rotate the tires, and get the engine tuned up to keep our vehicles running well. It’s also true for our tools, our homes, and more. Preventative maintenance is a good idea for most things. For some things however, the old adage “if it ain’t broke, don’t fix it” could apply. In fact, that statement applies to fiber optic networks. The properly installed fiber optic cable plant needs no periodic maintenance. In fact, attempting maintenance may harm more than help! Let’s consider a few factors that harm fiber and see how to prevent problems. Dirt is your enemy Perhaps the biggest problem with fiber optic cabling systems is dirt. Dirt can scratch the ends of the connectors during the termination process. These scratches produce high loss in mated pairs of connectors. Dirt can get on the ends of the connectors when they are exposed to the air and cause both high loss and high back reflection. Unless you store unconnected fiber optic cables and patch panels with dust caps, dust will get into the mating adapters and onto the ends of the connectors. While connectors can be cleaned with lint-free pads and isopropyl alcohol (no cotton swabs, please, as they leave threads that can be as harmful as dirt), the mating adapters are very difficult to clean properly. The best solution we have found is the “Curtis Blaster,” available in office supply stores, which is designed to clean computer parts. The same warning about dirt applies to networking equipment too. Keep dust caps on every fiber optic interface! Fiber optic cable is rugged, but... Fiber optic cable is really very rugged. It can be pulled to hundreds of pounds of tension during installation, as long as it is not bent too tightly, and crushed with large loads. Too much tension or too-tight bends might not break the glass fiber, but it may cause degradation in the fiber. Excess tension causes the cable to stretch and, when it relaxes, the fiber may be pinched as the cable contracts. This can cause stress losses in the fiber along the length of the cable. Tight bends cause loss in the fiber also. Bending the fiber tightly causes light loss, with greater loss at longer wavelengths (1,300nm and 1,550nm). Bends can occur as the fiber is pulled around corners or through bends in conduit. But even careful installation can be undone by later workers who lay other things, not just cables, on top of the installed fiber optic cables. Even worse, workers removing old cables may kink or cut perfectly good cables by mistake. It happens! In the outside plant cable world, the most common failure is referred to as “backhoe fade!” The connectors on the end of the cable are the most vulnerable part. They cannot withstand the pulling or twisting that a cable can. Bending the smaller cable used with connectors just behind the connector may break the fiber, and it is a very hard failure to find. Protecting your cable plant Your fiber optic cable plant is a big investment, so protecting it makes good sense. Fortunately, doing so may not add much cost to the installation. Here are a few ideas. If you are installing fiber optic cables in an area with lots of other cables already there (or anticipated, as for most office buildings), install the cables inside innerduct. Innerduct is large plastic tubing, usually in a bright orange color, designed to ease cable pulling and protect the fiber. The large size, hardness, and color of innerduct is good insurance against harm from other objects or someone accidentally cutting the cable. Innerduct is relatively inexpensive and available as rated for inside installations, even in plenum areas. Not only will it protect the cables, but it may actually speed installation. If you are going to use innerduct, get it with an installed pulling tape so it will be ready to pull the fiber cable. Pulling the innerduct is fast and easy, as it’s rugged, strong, and hard to kink. Pullers for innerduct do not need the skills and training of a fiber optic installer. After the innerduct is installed, pulling the fiber optic cable is easy, as the innerduct maintains a large bend radius for the cable and the inside is a special low-friction surface to minimize pulling tension. In many jobs, the shorter total time spent pulling innerduct than cable will pay back the extra cost of the innerduct. To protect the connectors, only use patch panels that are enclosed, and where the doors have locks. If you want to show off your fiber optics, you can get panels with see-through doors, but still with locks. This protects the connectors from accidental bumping that can break the fiber just behind the connectors where they are most vulnerable. It will also stop curious people who want to look at the end of the fiber to see the light, which is in the infrared and invisible anyway. So when you think about maintaining your fiber optic network, think about protecting it from dirt and harm. Otherwise, leave it alone and enjoy its high performance in your network. HAYES is the founder of Fotec, the fiber optic test equipment company and the Cable U training programs. He can be contacted at Jh@jimhayes.com.