Installing Fiber in Conduit

innerduraFuturePathGroupI thought I would take a break today from complaining about the FCC and instead talk today about how fiber is put into conduit. I know a lot of the people who read this blog are not technical and I figured some of you would want to know a little more about how fiber actually gets to where it’s going.

I’m looking specifically today about fiber placed in conduit. Conduit is used when fiber is installed underground in an environment where you want to either protect the fiber from damage or else be able to easily get to the fiber in the future. It’s possible to bury fiber directly, but most carrier class fiber routes use conduit.  There are three basic options for getting fiber through a conduit – pulling, pushing, and blowing.

In the first step of the installation process a conduit will be buried in the ground. Some conduit consists of a large empty tube that can hold multiple fibers. But today it’s becoming more common to use what is called innerduct conduit, which contains multiple smaller tubes inside of a larger conduit.

For long outdoor fiber runs the primary method used to install fiber in conduit is pulling. In fact, if you are installing large count fiber or heavier fibers, this is the only real option. Conduits made for this purpose come with factory-installed cords inside. The pulling process then consists of tying the fiber to the cord at one end of a run of conduit and then pulling out the cord from the opposite end. For long fiber runs the pulling is done with specialized equipment that can pull steadily and evenly to minimize any damage to the fiber. Fiber is strong, but it can be damaged during the installation process, which is why it’s essential before accepting a new run of fiber to first test it by shining a laser through to make sure the fiber survived the installation process. Damage from pulling is probably the number one cause of late fiber problems on long fiber routes.

In short fiber runs, such as inside of a central office or a home, the fiber can be pulled manually by hand. While fiber has a lot of flexibility, the fiber can be damaged by pulling it around tight bends or other impediments.

Pushing fiber is a technique that is only used for short runs of fiber. It’s exactly what it sounds like and you literally feed the fiber into one end of a conduit and shove the fiber through the empty conduit and hope there are no snags or bends that will limit your ability to make it the whole way through. Pushing fiber is the safest method to use since it puts the least amount of stress on the fiber. If the run is a bit longer, but still pushable, there are pushing tools that can apply steady constant pressure to force the fiber through the conduit.

Blowing fiber is perhaps the most interesting method used. Blowing fiber involves using equipment at both ends of the conduit to be filled. The machines force air into one end of the fiber, increasing air pressure, while at the opposite end of the fiber another machine draws air out of the conduit to produce lower air pressure. The difference in the air pressure draws the fiber through the conduit.

Blowing fiber can be used on longer routes as long as the fiber to be fed is not too heavy, perhaps 8 or 12 pairs of fiber. It’s vital when blowing fiber for longer distances to have conduit with very low-friction lining and no physical impediments.

Both pulling fiber and blowing fiber take specialized equipment and require following specific techniques to do it right to get the fiber through the conduit both quickly and safely. If you watch a fiber installation team and they are just sitting somewhere along the road, chances are that they are not being idle but are instead pulling or blowing the fiber through the conduit. All of these methods require knowledge and skill to do right without harming the conduit.

3 thoughts on “Installing Fiber in Conduit

  1. We routinely used to blow 144 and higher fibre count cables when I was at BT a few years ago. I don’t think your explanation of how it works is correct – the air acts as a cushion around the fibre while it’s mechanically fed into the sub-duct reducing friction. It’s not drawn by air pressure.

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    • There are several manufacturers that make the equipment to move the fiber by the combination of simultaneous vacuum and air pressure. They claim it significantly reduces the time it takes to get the fiber through ducts. Of course, this also increases the cost by needing equipment at two ends of a conduit and it’s always a trade-off between equipment costs, crew costs and time required for any method.

      What you are talking about is yet a different option than what I talked about, which is to feed fiber into a conduit and basically use the air as a lubricant to ease the fiber through the empty conduit. There are almost as many different variances on how this works as there are vendors that make fiber-filling equipment.

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  2. Actually being the first engineer in the US to have actually blown more fiber than I care to admit, you are both correct. The flow of air, higher pressure at the blowing head hence requiring drive wheels to feed the fiber and the actual flow (high pressure to low pressure) allow the fiber to float on a cushion of air and onto the end point which is open. For the record I was the first engineer hired by Sumitomo Electric (Fiber Optics Corp), now Sumitomo Electric Lightwave to help engineer and market this ABF system that was licensed from BT. I engineered many facilities for ABF (FutureFlex- I hated this brand name) most notably Universities (U of U, UCF, UC Davis) and set this on a course to be used in Nuclear Facilities and on Naval Shipboard Systems. OK, I tooted my own horn!

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