Fiber Optic Cable and Lightning

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Many spend their time berating practitioners for not applying their method. We all need to disseminate our ideas, but most of our time should be spent applying and improving our methods, not selling them. The best way to sell a mouse trap is to display some trapped mice.

- David Parnas. This has been my approach to selling people on Computer Algebra Systems (CASs). I believe the best way to sell CASs to engineers is to show them solved, real-world engineering problems – which are my trapped mice.

Figure 1: Example of Lightning Damage From a Surge Coming Through the Ethernet Ports.

Lightning is a tough problem. All of my personal electronic systems are well grounded and have the best surge protection I can buy. Yet I still suffer occasional losses due to lightning – for example, this weekend I replaced a surge-blown power adapter at my cabin in northern Minnesota. Intuitively, you would think that fiber optic systems should be better protected against lightning strikes than copper-based systems because glass fiber does not conduct electricity. This is not necessarily true.

Unfortunately, the story is a bit more complicated than just copper versus glass. There are literally dozens of fiber optic cable types, however, for the sake of this discussion, I will assume that there are two types of fiber optic cable: those that contain no metal and those that do.

Engineers generally refer to a fiber optic cable that contains no metal as a dielectric cable (Figure 2). My personal belief is that homes connected with dielectric cable experience less surge damage than homes connected with metal-bearing cable – I am in the process of testing this hypothesis. Notice that dielectric cable contains strength members made of Kevlar that allow it to be pulled into position.

Figure 2: Standard Dielectric Cable. (Source)

One issue with dielectric cable is that it does not contain a tracer wire, which allows people digging to determine the location of underground cables by using a wire tracer. These tracer wires are commonly used with standard utility services, such as gas, water, and electricity. Thus, a buried dielectric cable is more likely to experience an accidental cut than a cable with an embedded tracer wire. You can run a tracer wire outside of the dielectric cable, but then you need to make sure it is grounded properly.

While we are seeing service providers use more dielectric cable (example: all-dielectric, self-supporting cable [ADSS]), the vast majority of deployments use cable that contain metal and for good reasons. In Minnesota, over 90% of the deployments involve the use of aerial fiber cable – cable strung in the air along poles. Figure 3 shows the construction of a typical aerial fiber cable. This cable contains a heavy metal strength member that provides the cable sufficient tension resistance to survive hanging between poles. Pole deployments require very strong cables because the cables must not only bear their own weight, but the stresses added by accumulated ice and wind.

Figure 3: Typical Aerial Fiber Optic Cable. (Source)

Unfortunately, metal in the cable provides a path for lightning to travel. This metal is always grounded for safety, but even a grounded cable will develop some surge voltage on it when lightning strikes.

When a fiber optic cable is run to a home, it frequently has metal strength members along its sides (Figure 3). These strength members make it easy to pull the cable through conduit or trenches. The strength members can also be used by wire tracers to locate the cable.

Figure 4: Commonly Used Fiber Optic Cable with Two Strength Members. (Source)

Lightning can also travel along the metallic path provided by these strength members. As with cable deployed on poles, the strength members are always grounded for safety. However, even a grounded cable will develop some surge voltage on it when lightning strikes. The key might be found in using hermetically sealed wire feedthrough cables that can be found on the likes of this page, as they might be able to withstand most kinds of damage, if not all.

Be that as it may, we continue to work to reduce the likelihood of lightning damaging fiber optic systems. ADSS cable is a big step forward and will help, but the need for a tracer wire near the home still complicates the issue. I think putting fiber optic hardware indoors and feeding it with dielectric cable within the home and with grounded, metal-bearing cable outside the home is probably the long-term answer.

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