While fiber optic cabling offers superior transmission speed and distance over copper cabling, it’s also surprisingly durable. Despite being made of glass, fiber cables can last much longer than copper cables when installed correctly. However, they require careful handling and specific installation practices to maintain their durability. Selecting the correct type of fiber for the application and environment is also essential.
What is the Bend Radius of Fiber?
The minimum bend radius is the smallest allowed curve that you can bend a cable without causing damage. Fiber optic cables typically have a minimum bend radius of 20 times the cable’s diameter during installation, sometimes called bend radius under tension, dynamic bend radius, or short-term bend radius. The minimum bend radius for fiber cables after installation (long-term or static bend radius) is typically 10 times the diameter. The minimum bend radius during installation is larger due to additional stresses applied to the cables under tension.
Because fiber is comprised of glass, it requires a larger bend radius than copper cables. Most copper cables have a minimum bend radius of 4 times the diameter for unshielded construction and 8 times the diameter for shielded construction. While this may seem like a substantial difference, the overall diameter of a fiber cable is much smaller. For example, a duplex fiber cable with a diameter of 0.19 inches has a minimum bend radius of 1.9 inches after installation. In comparison, a Category 6A UTP cable with a diameter of 0.57 inches has a minimum bend radius of 2.28 inches.
It’s important to note that the minimum bend radius of a fiber cable can vary based on the construction of the cable and environmental factors. For example, some bend-insensitive fiber cables can handle a minimum bend radius of 15 times the diameter under tension. Increasing the bend radius during installation is often recommended in colder environments where cables can become more rigid. Fiber cable specifications state the minimum bend radius during and after installation and the temperature range for storage, operation, and installation. For fiber cables available in various fiber counts and diameters, it’s helpful if the specifications include bend radius information for all options to avoid confusion.
When fiber optic cable bends exceed the minimum bend radius, it can cause light signals to leak out of the fiber, significantly increasing insertion loss (i.e., attenuation) and degrading transmission performance. Exceeding the minimum bend can even cause the glass of the fiber to break. Sometimes, the impact of exceeding the minimum bend radius is not immediately apparent, but it can degrade the performance of the fiber over time.
How Strong is Fiber Optic Cable?
One key factor that indicates the durability of fiber cable is tensile strength, which can vary based on cable type and construction. Tensile strength represents the highest load that can be placed upon the cable without causing damage or breakage, calculated as pounds (lbs.) or Newtons (N), where 1 lb. of force equals 4.5 N. It can also be specified per unit area, such as pounds per square inch (PSI) or Newtons per square meter (N/m²). Like bend radius, tensile strength varies for installation (short term) and after installation (long term).
Maximum tensile strength is specified by cable manufacturers and included in product documentation. Most fiber cables have a tensile strength between 200 and 600 lbs. (890 to 2700 N), much stronger than copper cables that are typically around 25 lbs. (100 N). Once installed, fiber cables have a lesser tensile load.
For example, a
12-strand tight buffered OM4 multimode cable has an installation tensile load of 1800 N and an operational (static) tensile load of just 600 N (135 lbs.).
The length of a pull, the weight of the cable, and the amount of friction and twisting when pulling fiber cable can affect tension. In general, fiber optic cables should be pulled as straight as possible with the least amount of tension in a uniform, continuous motion to avoid exceeding the load. Using fiber innerduct can help reduce tension since it reduces surface contact. Vertical cables should also be installed from the top down rather than pulling them up to reduce tension. While fiber cable is stronger than copper cable and can handle a higher tension load, it’s imperative to never pull fiber cable by the glass or the cable jacket—only pull by the internal strength members (Kevlar material or aramid yarns).
Pre-terminated fiber assemblies commonly deployed in the data center environment often come with an integrated pulling eye that manufacturers should properly integrate to prevent too much force on the jacket and eliminate any force from being placed on the fibers. For example, inside our HD8² HDReadyLink Cassettes, the internal strength members are separated from the fiber and connected to an integrated strain relief to prevent any tension on the fibers themselves.
What Factors Affect Fiber Optic Cable Durability?
Another factor that affects fiber optic cable durability is vertical rise, which is the perpendicular distance that should not be exceeded for an unsupported fiber cable. Vertical rise is a factor of the weight of the cable and the tensile strength. If the length is too long, the weight of the cable creates too much tensile load. Most cable manufacturers specify vertical rise for cables intended for vertical applications (riser rated), but it can also be calculated based on the tensile load and weight per distance. For example, a cable with a tensile load of 285 lbs. weighing 30 lbs. per 1000 ft has a maximum vertical rise of 9500 feet (285/30 X 1000). Longer vertical pulls can be secured at intermediate points to avoid exceeding vertical rise specifications.
Other mechanical factors to consider are crush, impact, and flex resistance. These critical factors are determined by manufacturers using industry-standard fiber optic test procedures (FOTPs). Crush resistance specifies how much pressure can be placed on the cable without causing damage, which is typically defined as Newton per inch (N/in) or per centimeter (N/cm). Cable construction and application play a part in crush resistance. For example, direct burial cables must have a high crush resistance, so they are typically armored fiber cables. A 72-strand outdoor burial armored singlemode fiber offers a crush resistance of 1800 N/cm or higher, while an indoor 72-strand multimode distribution cable has a crush resistance of no more than 1500N/cm.
Impact resistance is a fiber cable’s ability to withstand repeated impact loads as indicated by the maximum number of impacts. In contrast, flex resistance is a fiber cable’s ability to withstand repeated bends as indicated by the maximum number of bends. While industry standards determine crush, impact, and flex resistance, these factors can vary based on cable type. For example, fiber cables for dynamic broadcast applications that are repeatedly handled have a higher flex resistance than cables deployed in static environments.
Choose the Right Fiber Cable for the Application
Different fiber cable types have specific characteristics that impact durability, including flame-retardant rating and the ability to withstand environmental factors such as extreme temperatures, UV, moisture, chemicals, and abrasion.
Certain applications have specific requirements regarding these factors, and selecting the right fiber cable type for your application is essential. For example, outside plant (OSP) fiber cables have a broader operating temperature range, UV-resistant jacketing, and water-blocking materials compared to indoor distribution fiber cables. Specialty cables used in mining or military applications have higher flame retardant ratings and jacketing that offers superior abrasion, cut, and chemical resistance to withstand harsh environments.
Fiber Cable Installation Check List
The following installation check list will help maintain the durability of your fiber cables:
- Never exceed the minimum bend radius of 20 times the cable diameter when pulling fiber cable
- Never pull a fiber cable by the glass or outer jacket--always pull by the internal strength members
- Pull fiber cables as straight and uniformly as possible
- Avoid twisting the fiber cable
- When installing fiber cable vertically, always pull down instead of up
- Secure fiber cable at regular intervals in a riser application to maintain vertical rise specifications and reduce tension
- Do not crush fiber optic cable and avoid any potential crushing by vehicles or other means on job sites
- Limit repeated flexing of the cable and avoid impacts
- Always check your cable specifications for proper installation, environment, and applications
- Always follow industry cabling standards and installation best practices
The good news is that CablesPlus offers a wide range of highly durable fiber optic cables that comply with industry standards and the specifications you need for your next project—whether it’s indoor, outdoor, military, mining, broadband, or other. Contact us today for all your fiber optic cable needs.
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