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The Top Fiber Questions blog series is dedicated to answering the most frequently asked questions in the industry. We hope to provide great answers. Most importantly, we’d love for you to ask your fiber questions in the comments section below.
How long of a span can I suspend self-supporting fiber?
First, a definition: a self-supporting fiber cable is defined as a cable that was purposefully designed to support itself without attaching to a supporting strand. Typically, these include integrated messenger (also known as figure eight), flat drop and all dielectric self-support (ADSS). Each self-supporting fiber cable will have its own specification for maximum span length.
Most self-supporting fiber optic cables can mechanically withstand the loads of longer distances that are typically specified for each cable. However, the span lengths are often limited by the strain placed on the fiber optic glass inside the cable and/or by the minimum clearance requirements provided by the National Electric Safety Code (NESC).
A great example is the M-MN-109 self-support drop fiber which has a maximum span length under NESC Heavy Loading Conditions of 128 feet when attached at 18 feet high with a minimum clearance for areas accessible by vehicles 15 feet high. If there is enough clearance to allow for 18.6 feet of sag, and the span length could extend to 420 feet, this would require an attachment height of 33.6 feet in an area accessible by vehicles. This is not very common and why CommScope refers to this type of length as an “infinity” span.
Some manufacturers only provide their customers with infinity span lengths with no regard to NESC clearances. CommScope provides span lengths in three categories: NESC vehicular access, NESC pedestrian access, and infinity within each NESC loading category: heavy, medium and light.
When a self-supporting fiber cable is latched to a support strand such as a ¼” 6.6M EHS strand, the self-support span limitations no longer apply since the load is being placed on the strand and not the cable.
The only sure way to know the limitations is to review the specifications for the cable. What kind of limitations have you run into in deployment?