In my May 2020 Fiber Optics column, I mentioned seeing a crew pulling cable in my neighborhood in a way that was likely to do significant damage to the cable. Now I’ll share with you some of the photos I took and analyze what was being done wrong. Then I’ll try to explain how that cable should have been pulled properly.
Let’s start with a photo of the actual installation. It involved pulling a fiber optic cable through underground ducts under one city block, which is a total distance of around 500 feet, or 150 meters. The crew had already blown through the duct with a pull tape, attached it to the fiber optic cable and were pulling the cable when I arrived.
The first thing I noticed was the size of the pulley they were using to pull the cable out of the manhole and bend it toward the capstan. They were using it to apply the pulling force. That pulley is about 5 inches in diameter, or 125 mm. That brings up the topic of fiber optic cable bend radius.
Fiber optic cables have three primary specifications that must be followed to prevent damage when pulling cables: pulling tension, bend radius and crush load. Pulling tension is obvious and most outside plant cables have a maximum pulling tension limit of 600 pounds. That limit makes pulling several thousand feet relatively easy and can be done longer with cable lubricant. Crush loads are obvious; don’t walk or drive over a fiber optic cable.
Bend radius is more complicated. How tightly you can bend a fiber optic cable depends on the diameter and construction of the cable and whether the cable is loose or under tension, e.g. when it is being pulled. Bend radius limits for cables are generally calculated from the diameter of the cable with a minimum bend radius of 20 times the cable diameter under pulling tension, and 10 times the cable diameter when the cable is relaxed after being installed. Those two guidelines, 20 times and 10 times, apply to most, but not all cables, so when in doubt, check the manufacturer’s specifications.
Here is an example of how to calculate bend radius:
(Inches / mm)
|Under Tension (20 times)
(Inches / mm)
|Installed (10 times)
(Inches / mm)
|Radius Diameter||Radius Diameter|
|0.25 / 6||5 / 120 10 / 240||2.5 / 60 5 / 120|
|0.5 / 13||10 / 260 20 / 480||5 / 130 10 / 260|
|0.75 / 20||15 / 400 30 / 800||7.5 / 200 5 / 400|
|1.0 / 25||20 / 500 40 / 1,000||10 / 250 20 / 500|
Metric conversions are rounded off.
Radius is easier to understand when bending around a corner. Diameter is easier to understand when choosing pulleys or capstans.
For example, a cable 0.5 in (13mm) in diameter would have a minimum bend radius under tension of 20 X 0.5 inches = 10 inches (260mm). That means if you are pulling this cable over a pulley or capstan, it should have a minimum radius of 10 inches (260mm) or a diameter of 20 inches (520mm). Be careful you don't get radius and diameter mixed up! After pulling and under no stress, the cable could have a bend radius of 5 inches (130mm) or a diameter of 10 inches (260mm).
I believe the cable in the photo above was actually more than 0.5 inches in diameter, so that pulley should probably have been much bigger. I added a dotted red line on the photo version below so you can see how small the pulley being used really was.
That’s not the only problem here. I also added a dotted line following the cable route out of the manhole. Note how it is being pulled almost straight up out of the manhole, indicating the cable was being pulled at an angle almost 90 degrees to the conduit. Pulling that cable against the end of the conduit at that angle creates a bend angle even worse than the small pulley on the truck. If it were necessary to pull the cable that way, hardware exists to limit the bend radius coming out of the conduit, including a length of flexible fiber duct to limit the angle.
It would have been much better to have moved the truck and turned it 90d degrees so the capstan could pull the cable directly without needing to change the angle with the pulley. Had this job been done with some bend-limiting hardware on the conduit exit in the manhole and the truck angled properly, this cable could have been pulled simply and safely.
I’ve discussed this incident with several contacts in the industry. There are some interesting theories about how this happened. One person says that the term “bend radius” is partly to blame because some installers don’t know the difference between radius and diameter. Another put blame on hardware manufacturers for not specifying hardware for fiber optics, e.g. a small pulley is for pulling rope not fiber.
My theory is simpler; these techs were never properly trained in fiber installation and did not know what they were doing was wrong. They should have known how to properly treat fiber optic cable during installation and respected the specs for pulling tension, bend radius and crush loads. The only way to prevent problems like this is training and certification.