I've got to replace a section of 2 fiber cable that was apparently damaged by water freezing in the raceway near where the conduit exits the ground. The fiber is of loose tube construction and is designed for direct burial or underground duct installation. The tube that contained the fibers was crushed and the fibers broken. There is no evidence of damage to the above grade part of the conduit run. The conduit was full of water up to about 2' above grade before I removed the cable, so I don't think there is ice damage to the conduit below grade. I have to install a splice box about 200' feet away and install new fiber in the existing underground conduit. I'm looking for suggestions on how to avoid this same type of damage to the new fiber. I've been thinking about digging down to the 90s at each end of conduit and putting in some washed gravel for a drain bed and drilling some drain holes in the conduit. Don(resqcapt19)
That is a tough one because what if there is a lot of rain water soaks into the conduit, and becomes trapped because of dirt, clay washing into the weep hole. Is there a way to determine where the water entered the conduit? By the way, what type of conduit? Your first suggestion may work but I'd be worried about creepycrawlies, weeds, and such entering the conduit.
Re: fiber optic cable damage#889 03/31/0101:54 AM03/31/0101:54 AM
Steve, The conduit is rigid. As far as I know there is no way to keep water out of an underground conduit, and this location is even worse because it is in a flood plain. I'm also thinking about pulling some small plastic tubing, like the pipe fitters use to connect instruments and valves, in with the fiber when I replace the fiber. This would help in 2 ways, I think. It would reduce the space that the water could take up and when it freezes the tubing would absorb some of the crushing force. Don(resqcapt19)
How about a combination of the two ideas. Drill your holes and can you put the fiber inside a larger plastic tubing? This would give you some airspace around the fiber. If the conduit did fill with water and freeze again there would be some give in the tubing.
I don't want to sound too nutty but, maybe you could try a test run using a short section of conduit with the fiber and tubing and putting it in a freezer to see if it would work. Hate to do all that and find out the ice still expanded too much and caused the damage any way.
Can fiber optics and 120V AC circuits be placed in the same conduit? Don't have the "Book" in front of me.
Re: fiber optic cable damage#893 04/04/0110:58 AM04/04/0110:58 AM
Steve, The test idea is good, but i've got to replace it no matter what. If it fails again the repair will be easier as I am installing a j-box to make this repair. Non-conductive fiber is permitted in with ANY voltage. I've though about some heat trace, but this is in the middle of nowhere and I don't think the existing power source will support any additional load. The run is 150' and so 3 watt per foot heat trace would add 450 watts and this would push the voltage drop up too much. Don(resqcapt19)
Re: fiber optic cable damage#894 06/30/0102:29 AM06/30/0102:29 AM
I just got back from a Fiber Training course and I had the opportunity to ask the instructor about this situation. If the cable was designed for underground it should have been OK. Specs would be generally loose tube design, like you said, and gel filled. The gel serves to keep out water. Could it have been old cable? (maybe gel dried out?) Or perhaps other cables would have greater crush resistance. Some cables could also have some empty tubes in them that might help absorb some crushing force.
Bill, The cable is 5 or 6 years old. The tubes were still filled with the water blocking gel. The cable was just crushed by the water in the rigid conduit when the water froze. This cable had 5 solid fillers. I've never seen cable with empty tubes used as fillers. Thanks for taking this up with the instructor. I guess I'm not the only one to have this problem. I just found a product by Polywater for this. http://www.polywater.com/icefree.html Don(resqcapt19)