Q: I have to run some fiber from one building to another. The fiber will be buried in a 4-inch conduit except for about 80 feet, which, enclosed in conduit, will cross a drainage overpass. Will I need to be concerned about temperature changes from the buried part of the conduit to the exposed conduit and back to the buried?
Huntsville Hospital System
A: The mechanical and environmental specifications for outdoor optical-fiber cable are spelled out in the ansi/icea s-83-640 standard of the American National Standards Institute (ansi--New York City) and the Insulated Cable Engineers Association (icea--South Yarmouth, MA). If you are installing an optical-fiber cable designed to be installed as part of an outside-plant cabling system, you will not have a problem--at least, not if you are using the correct cable. Loose- or tight-buffered optical-fiber cable can be placed in conduits below the frost line. But, when an optical-fiber cable is exposed to different environments in the same run, as in your case, a loose-buffered cable should be used. Tight-buffered cables installed above the frost line are subject to damage from frost heave.
Recently, loose-buffered cables that meet the optical-fiber nonconductive riser (ofnr) requirements of the National Electrical Code have made their appearance in the market. Therefore, it`s possible to place an indoor/outdoor-rated cable in outdoor environments without having to choose between placing tight-buffered cable or splicing at the buildings. But all this design flexibility is not free--these cables cost more per foot than their outdoor-only cousins. If the distance between the buildings is significant, the splices may prove to be more economical in dollars but will also be more expensive in power budget loss.