Each of three pathways for outside plant cabling has its own design requirements.
Many designers, installers, and maintainers of telecom networks are involved not only with the wiring inside a building, but also with connecting the structured wiring system to two or more separate buildings on a site. Any wiring external to the building is called outside plant cabling. A customer-owned outside plant cabling system can be defined as a telecommunications facility on the premises of some type of contiguous property that connects to the information and/or datacom system. This is a requirement for many large scale facilities, such as hospitals, universities, research centers and industrial plants. Outside plant wiring systems can support a wide variety of communication services. These include telephone and data transfer, live video, security, building automation control systems and any other low voltage circuitry.
You must install the outside plant cabling in compliance with the National Electrical Code (NEC), the National Electrical Safety Code (NESC), utility franchise regulations and local building codes.
You should base any project of this type should on a 10-year planning cycle. The size, or capacity, of each cable run and the precise pathways you use are extremely important in determining the success of the project.
For example, in doing a layout of a campus outside plant system, consider the fact that, in the future, owners may sell portions of the property. Therefore, it may be necessary to obtain property easements. In addition, any site survey should observe if the cabling layout is going to cross a railroad track, any other utility company right-of-way, or some natural occurrence, such as a stream. Additionally, during the design stage, you may need to divide the property into sections, so you can select the proper wiring media for each section. Creating subdivisions and specifying certain ancillary hardware devices for each section (such as satellite dishes or antenna towers) may be necessary.
Other design factors include: maintaining the security of cable runs, providing alternate routing in case of disaster recovery, the location of local exchange carrier facilities, and the physical terrain of the campus. In many cases, extra pathways should be planned for maintenance purposes. It's usually necessary to check with local exchange carriers regarding their facilities within, or adjacent, to the campus.
Three pathways used in outside plant construction are aerial, underground conduit, and direct buried; of course, they may be used in any combination.
Aerial cabling systems. This method has the highest level of risk regarding natural disasters, or accidents, such as the possibility of lightning strikes, falling tree limbs, or accidental vehicle damage to utility poles. Consider that an aerial run detracts from the aesthetic appearance of the property, and the installation of hardware can damage the exterior of a building. The advantages are fast installation and readily accessible for maintenance and for any future alteration of the aerial runs. However, some local ordinances may restrict aerial construction.
You must plan a route that provides enough stabilized ground so that a line or splicing truck can be supported during installation and later, when maintenance of the aerial run is required.
Pole placement should take into account future cable capacity needs, the classification of pole type, storm loading requirement, optimum span lengths and minimum height clearance requirements. The installed cable must maintain a specific distance known as sag clearance; from the ground and from other utilities. These distances vary, based on what's traversed (see NRSC for clearances). As more cable runs are added to the pole, line clearances must continue to be observed.
A class number indicates pole strength (the strongest is Class 1 and the weakest is Class 10). Marked poles indicate their class, species of timber, preservative treatment, and footage. If you set a pole in sloping ground, the depth of the pole set must be increased over the depth used for flat ground installation. A designer should be aware of three definitions of pole loading: transverse storm loading, vertical loading and bending moments.
You must use suspension strand, which is available in two types (Class A for general use and Class C for corrosion prone areas) to support cable between poles. The selection of span length should follow these guidelines: * Strand tension should not exceed 60% of breaking strength under storm loading conditions. * Strand tension should not exceed 70% of breaking strength with cable in place and a 300 lb load concentrated at midspan. * Sag should not exceed 10 ft at a 60 DegrF temperature, without any wind loading.
For self-supporting cable, span length is limited by the simultaneous application of the two previous factors. Self-supporting cable is a special construction in which the sheath covers both the support strand and the telecom conductors. When tensioning a self-supporting cable, special clamping devices are required for the come-along (a small chain tackle that is tied to a pole and clamps to the strand) to avoid cutting the polyethylene strand covering.
In many cases, a pole will require some type of guying hardware, which is available as side, head, anchor, pole-to-pole, or pole-to-stub guy.
You can place an aerial cable from a stationary reel or a moving reel in either of two ways. The first way, temporary pulleys or "J" hooks are installed at each pole, then the truck winch is fed through the pulleys, or hooks, and attached to the end of the cable. The cable is then pulled into place.
The second way: the end of the cable is attached to the first pole and the rest of the reel is moved to the next pole, etc. Temporary pulleys or "J" hooks are not needed, because the cable is attached to each pole as it is unreeled.
Underground conduit system. Underground conduit has certain specific advantages over aerial cable; it's out of sight; adaptable to future cable placement and removal; secure from vandalism; and protected from most natural disasters; except flooding. While it's economical over its lifetime, underground conduit has a high initial cost compared to other systems.
Any underground conduit run needs careful planning, especially the location of pulling points, which, in turn, influence the location of maintenance holes (formerly called manholes), hand holes, and splice points. Follow the separation from other utility requirements.
Generally, underground ducts are made of polyvinyl chloride Types B, C and D multiple plastic duct, galvanized steel and fiberglass. If exposure to sunlight must be considered, a nonmetallic conduit should be either fiberglass or PVC Type D conduit. Any above-ground conduit run should have expansion joints at appropriate intervals, to account for flexing of the material. When fiber optic cable is the media to be placed, always consider the use of innerduct within the conduit.
A conduit bend should be a minimum of 102 the inner diameter of the conduit, and the total bend in any individual conduit section should not be more than 180 Degr. You should encase all bends and sweeps in a nonmetallic conduit in concrete, the exception being a bend that terminates at a pole. After installation, you should clean a conduit run of any possible obstruction. You should install a pull wire with a 200-lb pull strength, marked to show conduit length.
The burial depth of a conduit system must satisfy local code requirements, which is generally a minimum depth of 24-in. below final grade. The arrangement of ducts in a duct run should be consistent throughout all the maintenance holes in the system, and the ducts should align with any cable-racking within the maintenance hole.
A hand hole should be at least 6-ft wide, 8-ft long and 6-ft deep, and made of 3500 lb/sq in.-concrete, and the maximum distance between two of them should be 500 ft. It should be at least 4 ft in all dimensions, with pulling irons and bonding inserts. It should have a splay type opening (not a center-entrance type), to make cable pulling easier.
You should cap conduit at the ends to prevent infiltration of debris or water. A venting chamber, generally installed just outside a building, can be used to dispel underground gasses so they don't enter a building through the buried conduit.
Conduit sharing is sometimes considered for similar-type low-voltage systems. But, many consultants discourage this procedure for at least three reasons:
1. Possible damage to cable sheath from abrasion when two cable types are installed separately.
2. Jurisdiction disputes can arise about who is responsible for installation and maintenance, and the coordination of trades or jurisdictional work force can be a problem.
3. There is chance of electromagnetic interference from other circuit.
Direct burial system. A direct burial system, which is somewhat similar to a buried conduit system, also has many of the same advantages. But, the disadvantages of direct burial are that the capacity cannot be increased, and the telecom conductors do not have as much mechanical protection as they would in a buried conduit system.
Factors to consider are: type of soil and subsurface conditions, the possibility of joint trench use, and the backfilling method. The minimum depth of the trench should be 24-in. unless the local code requirement differs. Greater depth may be considered if there is a need for additional protection from someone accidentally digging up the cable. A warning tape or a mechanical barrier such as continuous plank should be used above the cable (placed 1 ft below grade level), but experience shows that this visual warning is often ignored, or not observed, by backhoe operators.
A specific recommendation is given for a direct-burial fiber optic cable: put down a copper cable or conductor along with the fiber optic cable, so that in the future, a cable locating instrument could be used to find the buried cable.
The same is true when installing power conductors should be followed when backfilling telecom cables: use clean backfill material, that is, the soil should not have any sharp objects or large rocks that could pierce the cable jacketing or cause other damage to the cable construction.
A joint trench holding more than one service should maintain the required distances between services.
A pedestal with cables entering from the bottom, is often used for splicing and terminating telecommunications cable. Such a closure, which should be installed according to the manufacturer's recommended specifications and clearances, should have an adequate lock for security.