Hi everyone, We are going to install a fiber optic cable inside a tunnel 10km long. The tunnel is 2 meters in diameter which connects an intake to a hydro electric plant. The headrace tunnel is used to divert the water from a river and have it flow freely via this tunnel to the plant turbine. Now, does anyone know any tips/suggestion on how we may be able to implement this engineering feat with minimum risk? Should we hang this on the highest portion of the tunnel? How far apart will the clamps be on the f.o. cable? What type of f.o. cable should we use? Is the buried type, jelly filled, armored, good enough? Or should we use a duct? Regards, Andre Pablo "Apaul" G. Fausto Nayon Kontrol Systems 354 (44) Quezon Avenue Quezon City, Philippines Tel. +63 2 4485074 Fax. +63 2 4485077

 

Start by estimating the force that the rushing water (all 10km long) will have as it pulls the fiber optic cable along its entire length at worst case flow conditions. Then compare this force to how much force it will take to damage the cable and/or its connectors. My guess is that you will decide to install it inside some kind of raceway just to protect it. A half inch conduit secured every few feet to the top of the tunnel probably would work, BUT I would be inclined to have this idea reviewed by a structural/civil engineer. The force on the conduit might turn out to be a problem as well. 10 km is a LONG way. Think of a tug of war with a rope 10 km long, and people pulling on it all the way along the length of the rope. The force on the rope (conduit or cable in this case) has to be seriously considered. You may also have to look at some way of allowing for thermal stress relief along the length of the conduit. Even a few degrees change in water temperature could make a big difference in the length of the conduit over a 10 km distance.

 

Optical fiber cables are communication cables and are treated as such. See US.NEC1999 Code Art 700. We tried not to mix different type of energy media together, if there is other logical and safe way to do it. Is this a temporary installation? The best approach it to treat is a typical power cable that needs protection from the environment hazard and security threat. Using a separate duct is the best approach, but if it is really necessary to run it inside a water intake tube as you described, ensure that the installation will not create turbulence or affect the flow of the water inside, and conversely ensure that the turbulence inside will not affect the F.O. installation in worse case scenario. 10km is it the limit of FDDI? unless you use a repeater. Again depending the type of F.O. as per NEC1999, consider the effects of the installation of: pull boxes, conduits, or junction boxes, hangers etc. that will accumulate debris in the long run. Again, specification depends on the mission. If it is critical, redundancy is a good policy. But the bottom line it's the budget that sets the target. Vic Ellescas Controls Engineer NASA Ames Res. Cntr Mountain View CA