Bends and Breaks

Published On
May 15, 2020
Mistreating fiver optic cable

I’ve said it before: whenever I see a fiber optic installation in progress, I stop to watch what the installers are doing. Recently, I saw two techs pulling a cable into conduit just a couple of blocks away from our home in Santa Monica. The total pull was one block in a straight line; you could see the manholes at both ends of the conduit, 500 feet apart.

They already attached a pull rope to the cable. The pull rope came up out of a manhole, over a pulley attached to their truck and then to a capstan on the other end of the truck bed. They parked their truck near the manhole at right angles to the cable route so the pull rope took a circuitous route.

While I watched, they started pulling the cable. As it exited the manhole, I could see it was a very large fiber count cable. As it reached the pulley on the truck, the direction changed more than 90 degrees, and I cringed. I could almost hear the fibers in the cable crunching as it was pulled over a 5-inch-diameter pulley instead of the approximately 24-inch-diameter pulley they should have been using. And from the angle of the cable in the manhole, it was probably being stressed as it exited the conduit. Didn’t these installers understand the cable has a minimum bend radius?

Every fiber optic cable has its limits; exceed them and you may permanently damage the cable and the fibers inside. When the manufacturer made the cable, it considered the methods of installation and the final placement’s environment and designed the cable accordingly. The company produced a specification sheet for the cable that covered the fiber performance and the cable’s design limits. Pulling tension, bend radius and crush loads are primary specifications for fiber optic cable installation.

Pulling tension is determined by the cable’s strength members. That usually includes a solid strength member or two, a fiberglass rod and aramid fibers—which are often referred to by the trade name Kevlar—outside the fibers, which are used to attach pulling eyes and absorb the cable’s tension during pulling. The fiberglass rod also helps to limit the cable’s bending.

The cable design includes room for the fibers to move around when pulled, bent or crushed; strength members to handle the pulling tension; stiffness to handle crush loads if laid in cable trays with other cables; and, of course, environmental extremes of temperature, moisture or exposure to other elements such as oils or solvents.

Cable can be damaged during installation when the cable is pulled at high tension or bent around tight corners, or, even worse, when both happen at the same time. Fiber optic cable is designed to withstand certain stress, including during installation or over its lifetime. But if the design limits are exceeded, the cable may be damaged permanently, reducing its ability to withstand long-term effects of the environment or losing its ability to protect the fibers. In extreme situations, fibers may be left in permanent stress or broken.

Last year, I had an experience that would make anyone sensitive to fiber-bending issues. I was invited to a cable manufacturer’s outside plant installation test site and training facility. While there, I learned how to install the new high-fiber-count cables without damaging them. It’s tricky because these cables are very resistant to bending and twisting, so simple processes, such as fitting them into a manhole, requires some maneuvering.

While holding the end of a 1,728-fiber cable, I asked if I could test the bend radius. I grabbed the cable at two points about 4 feet apart and started bending it. It was hard. I had never seen a fiber optic cable this stiff, but it was more than an inch in diameter and about 50% glass fibers, so it was understandable.

I kept bending it, until the bend radius was under a foot. When the bend radius was about 6 inches, it cracked loudly and sounded like a small tree limb breaking. The sound startled everyone, and I was left holding a cable that was broken, limp and worthless.

If you don’t believe that bend radius is important, grab a large cable and break it. I bet the feeling as it breaks will convince you to be more careful.

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor

Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at

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