Fiber Optic Cable and Lightning

Quote of the Day

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.

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)

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)

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)

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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9 Responses to Fiber Optic Cable and Lightning

  1. Since optical fibres carry light, rather than electrical signals, will they are affected by any changes in weather?

    • mathscinotes says:

      Fiber optic cable is definitely affected by environmental conditions. In fact, I wrote a post on a bit error problem we had that was related to the cable stretching as the temperatures rose (link). There are two main issues that weather poses for a fiber optic cable.

      • The delay on the cable changes by 75 ps/(°C km). Many fiber networks are synchronous, and this delay change is large enough that we must compensate for it.
      • In the US, over 90% of the fiber is hung between poles. Wind can cause the fiber to bend, and fiber attenuation increases dramatically at the point of the bend.


  2. Seema says:

    it's such a great post.I am hardware & networking very helpful post.Thanks for sharing

  3. Star says:

    I went through this topic, fiber optic cable and lightning. Choosing fiber over copper has its advantage but requires high precision for operational and maintenance of the network implemented. Star FFS-9000 Optical Fiber Fusion Splicer Model offers the least reflectance and minimal loss during the process of Fusion Splicing.

  4. Bryan says:

    Hello, I'm a locating systems manufacturer and would like to consult with you on some R&D we're working on specifically relating to lighting strikes and tracer wire applications. We manufacture for conduit manufacturers that embed a wire for locating and stand off systems. Safety for line locators is an important topic to our company.

  5. Randy Desgranges says:

    I'm a repair technician for a telecommunications repair facility. It has always astounded me that we get those ONT's in that have blown phone and ethernet ports. I just worked on 7 blown units the customer wanted to know the lightning route. I had to tell them the best I can figure is that the surge was coupled to the phone lines somehow. It was obvious the surge entered POTS line 1 and exited signal ground.

    • mathscinotes says:

      I have personally looked at hundreds of ONTs with POTS lightning damage. There is much research to be done in this area. I personally am convinced that the ONT ground level is shifting relative to the house ground (most ONTs have a separate ground), which means large currents flowing into or out of the ground (the voltage differential can be positive or negative). Tying the ONT ground back to the house ground does not really help if the wire is longer than 20 feet or so (the inductance in the wire prevents the levels from equalizing fast enough).

      Personally, I would always put the ONT next to the service ground and tie to that. However, this often is not possible because of where the existing telcom service is available at the house. I recently suffered lightning damage at my cabin in northern MN. I have a model installation and I still suffered damage. It is a very tough problem.

      Thanks for replying. I have lived your situation and it is not pretty.

  6. frank wright says:


    I really like the post, where get the knowledge Fiber Optic cable and lighting.

    Thanks for sharing such information about the fiber optics

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