1.01 This best practices procedure provides general information for the installation of fiber optic cables in direct buried applications. The methods described are intended for guideline use only, as it is impossible to cover all the various conditions that may arise during an installation. Individual company practices for placing fiber optic cable should supersede any conflicting instructions in this document when they do not exceed the cable’s optical and mechanical performance specifications.
1.02 Placement methods for direct buried fiber optic cable are essentially the same as those used for placing direct buried copper cable. However it must be kept in mind that fiber optic cable is a high capacity transmission medium which can have its transmission characteristics degraded when subjected to excessive pulling force, sharp bends, or crushing forces. These losses may not be revealed for up to two years after installation. For these reasons, extra care must be taken during the entire installation procedure.
1.03 Fiber optic cables are usually ordered in specific lengths as calculated by an OSP (Outside Plant) Engineer. The lengths are determined by measuring between splice locations then adding the amount required to reach the splicing vehicle (truck or trailer) and some minimum of excess cable.
1.04 It is assumed in this document that the reader is familiar with the direct burial procedure used for copper cable. Direct buried fiber optic cable installation practices are essentially the same as those used for placing copper cable. The following methods of direct burial of fiber optic cables will be addressed: plowing and trenching.
2.01 The following are some suggested precautions that should be observed when working with fiber optic cables. Before starting any buried cable installation, all personnel must be thoroughly familiar with Occupational Safety and Hazard Act (OSHA) regulations. Also, company safety precautions for direct buried cable operations should be reviewed before work begins and practiced during the entire installation process.
2.02 Before cable installation begins, the cable reels should be carefully inspected for any imperfections such as nails, broken flanges, cable crossovers or anything which might cause damage to the cable as it is paid out. Precautions should be taken to protect stored reels from possible damage by vandals or other sources when left unattended. The thermal protective covering provided with each reel of fiber optic cable should always remain in place when storing cable reels.
2.03 Whenever cable from the reel is placed on pavement or other surfaces, it should be protected with barricades or cones to prevent possible vehicular or pedestrian traffic damage. A “figure-eight” configuration should be used when the cable is removed from the reel and piled on the ground. This prevents kinking and twisting of the cable which could cause damage. Fiber optic cable should not be coiled in a continuous direction except for lengths of 30 meters (100 ft) or less. The preferred sized for the “figure-eight” is about 4.5 meters (15 ft) in length with each loop 1.5 meters (5 ft) to 2.4 meters (8 ft) in diameter.
Note: An alternative to the manual figure-eight is the “figure-eight” machine. This equipment will “figureeight” cable much faster than manual methods saving time and manpower. The machine winds any cable remaining on the reel on to the machine’s drum. Once the inside cable end is accessible, the machine is reversed and the cable is pulled from the machine’s drum through the duct. The machine’s drum and rollers are designed to keep the cable at a bend radius greater than the minimum bend radius of the cable.
2.04 Standard fiber optic cable has a maximum recommended pulling tension of 600 lbs. The maximum pulling tension is not to be exceeded. Please consult PANDUIT’S Best Practices for the proper installation and use of pulling grips. Cables may be ordered from the factory with pulling eyes already installed.
2.05 Fiber optic cables are more susceptible to performance degradation due to tight bending than are copper cables. The minimum bend radius of each cable is relative to the cable’s diameter. A general guideline is that a cable under tension should not be exposed to a bend radius less than 20 times the cable diameter and a cable with no tension should not be exposed to a bend radius less than 10 times the cable diameter.
3.01 A pre-survey of the fiber cable route is very important in planning for a direct buried optical fiber cable project. Each section of the route from splice location to splice location must be prepared properly before cable installation begins. It is very important to identify all conflicts and obstructions along the route before installation is under way. Conflicts and obstructions will influence the preliminary selection of splice locations and will have a direct effect on the overall transmission design of the route. Splices cases should not be located where the splicing vehicle will have to be parked in a hazardous area. This would include: over a hill top, around a sharp curve, near an intersection, too close to the road, a hidden area in an unsafe neighborhood, or anywhere the splicing vehicle cannot get safely and completely off the highway.
3.02 One of the objectives of the pre-survey is to determine where each reel of fiber optic cable is to be
placed. Slack locations and cable storage requirements must also be considered along with splice
locations. The pre-survey will verify construction methods, special tools required, or possibly require a
revision of preliminary splice locations.
3.03 The characteristic of the ground along the route needs to be investigated. If a subsurface investigation seems to be required, it should be conducted. This will clear up any concerns about underground conditions that may be encountered during plowing or trenching.
3.04 All road crossings, creek crossings, etc. need to be addressed so that preparatory work can begin before plowing or trenching. This will identify what casing is needed and what type of preparatory work will be required to negotiate these crossing points.
3.05 A good pre-survey and proper planning will preclude reel ends falling too close to roads, creeks, or any other undesirable locations.
4.0 LOCATION AND DEPTH
4.01 Buried fiber optic cable route locations are selected by the Outside Plant Engineering group. The cable route location should be one that is the least likely to be disturbed and that will have the least number of obstructions.
4.02 When burying fiber optic cables that must cross ditches, avoid crossing locations that might interfere with natural drainage. Also, avoid areas subject to surface drainage that may result in subsequent washing away of soil and exposing of the fiber optic cable.
4.03 Pipe or conduits are used at highway and railroad crossing.
4.04 The depth at which fiber optic cable can be buried will vary with local conditions according to freeze lines (depth to which the ground freezes in the winter). Under all conditions, the cable should be buried at a depth that will provide adequate protection. The depth may vary as conditions vary. In croplands and pastures, a cable depth of 36” is the minimum depth. The cable needs to be buried 12” deeper than the maximum depth reached by agricultural equipment.