Selecting a Route for OSP Installations

Selecting a Route for OSP Installations

Knowing the various types of pathways and spaces available to get from point A to B is crucial to a successful project. Several of our daily activities involve getting from point A to point B. For instance, most of us climb into our cars in the morning (point A) and drive to work (point B). Once at work, we determine our tasks for the day (point A), develop a plan to accomplish them, and then check

Knowing the various types of pathways and spaces available to get from point A to B is crucial to a successful project.

Several of our daily activities involve getting from point A to point B. For instance, most of us climb into our cars in the morning (point A) and drive to work (point B). Once at work, we determine our tasks for the day (point A), develop a plan to accomplish them, and then check them off one-by-one as we finish them (point B). At the end of the day, we all climb back into our cars (point A) and head home (point B).

This same thinking also applies to customer-owned outside plant (CO-OSP) cabling systems. Determining the most efficient and least costly way to get from point A to point B, and even points C and D, is critical in the design and installation of these important telecommunications systems.

In the first installment of this 12-part series on the Second Edition of BICSI's Customer-Owned Plant Design Manual, we introduced the three methods of installing OSP cable: aerial, direct-buried, and underground conduit pathways. This month, we take a more detailed look at these three options and discuss the advantages and disadvantages of each.

Preliminary investigations, field surveys, and permits. First, consult available records and contact local utilities and governmental agencies to pinpoint existing or proposed facilities such as electric, oil, gas, sewer, water, telephone, and cable systems along your proposed route. In general, you should try and avoid these systems as best you can when selecting a final route.

Many times, to reduce the cost of multiple trenches and minimize the potential for damage to existing facilities, the telephone company, CATV provider, and electric utility choose to dig one trench and share it with one or several parties. If you plan to joint trench, make sure you refer to the National Electrical Safety Code (NESC) Part 3 for rules on separation of cables.

After you've completed the design, but before beginning construction, obtain all required permits from the AHJ. If construction is planned on a public right of way, obtain permits from state, county, and city agencies. If you're working on private right of way, negotiate rights with the landowner.

Underground conduit pathways. Underground conduit construction may be your best option when a new overhead pole line installation isn't practical, an existing pole line can't support additional cable placements, or an area is too congested to allow for a direct-buried installation.

A primary objective of conduit system design is to make your conduit section lengths as long and straight as possible, with a minimum number of horizontal and vertical directional changes (see photo above). This reduces the number of maintenance holes (MHs), hand holes (HHs), cable splices, and associated cable setups required during the cable-pulling stage of the project. Installing underground conduit so a slope exists at all points of the run allows drainage and prevents the accumulation of water. A drain slope of no less than 1% grade toward the MH is desirable.

Conduit that breaks away from the main duct run and connects to structures such as buildings, pedestals, cabinets, or poles is known as branch conduit, of which there are two types: lateral and subsidiary. Lateral ducts typically run from the sidewall of an MH to the structure. Subsidiary ducts extend from the end wall of an MH, run along the main conduit run for some distance, and then turn to feed a structure (See Fig. 1 above).

If constructed at the same time as the main conduit run, subsidiary ducts should be placed on top of the main run; this is the most economical means of placement. It also affords some top protection for the main run duct bank. However, make sure these individual ducts enter the sidewall corner of the MH. Note, however, that this placement reduces racking space in the MH. When this duct is part of a multiple-duct structure, you should design the upper tiers of the structure (preferably the corner ducts) for subsidiary use because they are more readily accessible. Generally, 2-, 3-, or 4-wide arrangements are preferred for single- or double-wall racking.

The advantages of constructing formations using individual conduit is that the material is lightweight; provides good joint integrity; produces a strong, stable structure if concrete-encased; can be easily rearranged to avoid obstacles; can be pneumatically rodded; and is available with bell ends to allow ease of joint connection. In some situations, you may find it more advantageous to use multiple-bore conduit because it doesn't require long trench openings, select backfill, or ready-mix concrete.

In some special applications, you may need to place conduit in large steel tubes (casings) to protect it or to facilitate a crossing where an open trench cannot be provided. This may occur at railroads, major state highways or freeways, or at river or stream crossings. After the conduit installation is complete, you must fill the casings with fine sand (blown in under air pressure) and seal both ends with a 3-in. concrete wall.

For the safety of workers and the protection of telecommunications equipment, the NESC requires that you abide by a minimum set of clearances (Table right) when installing underground conduit pathways. In addition to these safety requirements, you should also install conduit at a sufficient depth, normally 24 in. to 30 in. below surface grade so external bearing loads don't damage the conduit structure.

How do you go about sizing the conduit? As a rule of thumb, make sure the diameter of your duct is at least 1.15 times greater than the diameter of your cable, or one-half trade size larger in diameter than the diameter of the largest anticipated cable you plan to install. It's often the diameter of the pulling eye that is most critical to sizing conduit. Except when working with small cables, you can calculate the diameter of the pulling eye (de) as follows:

de < 1.1 x dc

where dc is the cable diameter. ANSI/TIA/EIA-758, Customer-Owned Outside Plant Telecommunications Cabling Standard, requires you to use a minimum 4-in. conduit.

Tunnels. Sometimes, a new or existing tunnel may be the only pathway available to provide service into a building. Aside from being a convenient option, tunnels offer the following advantages:

  • Reduced street maintenance.

  • Decreased chance of accidental digups.

  • Reduced ground corrosion factors.

  • Continuous inspection path for all facilities.

  • Permanent space allocation.

There are three basic types of tunnels: utility, pedestrian, and vehicular.

Utility tunnels house various utilities such as steam, power, gas, water, and sanitary sewer. Although these ready-made pathways offer a nice alternative to installing an underground conduit bank, there are some concerns you need to be aware of:

  • Power cables can produce electromagnetic interference (EMI).

  • Gas lines can produce a hazardous atmosphere.

  • If placed too close together, steam lines can damage telecommunications plant.

  • Water (and steam) lines may create a humid atmosphere.

  • Sanitary sewer lines can produce a biological hazard if ruptured.

  • Storm drains that feed into the tunnels can cause flooding.

Addressing these issues on the front end of the project can save you headaches down the line.

Pedestrian tunnels allow passage of personnel from one part of the campus to another, usually under streets, railways, or other thoroughfares. These tunnels are usually environmentally conditioned and contain spaces that can efficiently house pathways for telecommunications cabling and equipment.

Vehicular tunnels provide passage for vehicles from one part of the campus to another, but are not typically environmentally controlled. In most instances, you can cost-effectively install cabling in these pathways with relative ease.

Direct-buried pathways. If you don't have access to a pre-existing tunnel, you can install the cable directly in the ground and forgo the added time and expense of designing and constructing a conduit infrastructure. A direct-buried installation offers many benefits over an underground conduit system installation, but it does have its drawbacks, the most obvious being the chance for accidental dig ups. When crossing under roads, railroads, and waterways, you should place the cable inside either a metal or rigid plastic duct for added protection.

At a minimum, the NESC requires you to place direct-buried cable 24 in. below finished grade. However, when installing optical fiber cable it's generally recommended that you place the cable at a minimum depth of 3 ft below grade. The added depth takes into account the added circuit — carrying capabilities of the optical fiber cable. In areas of the country where the ground freezes, you should place the cable below the frost line because frost uplift can damage cables.

Aerial pathways. You may find aerial construction or a combination of aerial and direct-buried construction to be less expensive in heavily developed rocky areas, especially if a pre-existing pole line (partial or complete) can be used. The downside of aerial plant systems is the possible damage of poles and equipment due to vehicular traffic and the risk of damage to conductors or pole structures from falling tree limbs, high winds, ice loading, and other environmental factors.

When designing an aerial system, you'll have to consider three types of loading.

  • Transverse loading is the force exerted on the pole and its attachments by wind blowing at a right angle to the length of the line.

  • Vertical loading is the downward force on the pole produced by the guy wires and weight of the cable, attachments, and any other equipment mounted on the pole.

  • Bending moments are the forces produced by objects like transformers or by unbalanced tensions at corner structures or dead-end poles.

The NESC sets standards for determining a pole line's storm load requirements by dividing the United States into three geographical areas based on the severity, frequency, and damaging effects of wind and ice storms. The divisions reflect the forces exerted on overhead lines by the combination of wind, ice load, and snow load.

Poles subjected to heavy transverse loads tend to break at the ground level. Therefore, selecting poles with sufficient strength at ground level is an important design consideration. Each transverse load causes a moment on the pole that tends to move the pole in the direction of the applied load. The value of the moment (lb-ft) at any point is equal to the transverse load (lb) times the distance (ft) from the load point to the point where the moment is being considered. You must sum the moments of each transverse load to obtain the total load. The rated breaking strength of the pole is based on the resistant moment that the pole can withstand at ground level.

Vertical loads on poles may be caused by a combination of loading factors. These factors include anchor guying and the dead weight of wires, cables, and other attachments on the pole. As a general rule, conductors cause vertical loads on poles and pole attachments need not be considered in pole line design; however, these loads should be considered in the case of guyed poles. The most severe vertical load to which a guyed pole may be subjected is the vertical component of the tension in the guy or guys.

Bending moments on a pole are caused by an unbalanced tension in the line. You can calculate these loads at any point on the pole. The bending moment on an unguyed pole is equal to the sum of the total unbalance of the longitudinal tensions in the conductors. To find the longitudinal load on an unguyed pole, apply the following formula:

M = T x L

where M is the total bending moment (lb-ft) at the ground line caused by the longitudinal loading, T is total unbalanced force (lb) of conductor tensions, and L is the height (ft) of the wire or cable attachment above the ground.

You must also design the system to meet the minimum horizontal and vertical clearances as noted in the latest edition of the NESC.

Spaces. In OSP work, MHs, HHs, pedestals, and cabinets perform a variety of functions.Maintenance holes. Maintenance holes (MHs) are typically designed for main and branch conduit systems that require more than three, 4-trade-size ducts. Although they can serve multiple uses, MHs are typically used to aid in underground or direct-buried plant splicing operations. They can be constructed of either concrete or polyethylene and must be equipped with a sump, corrosion-resistant pulling irons, cable racks, and ladders.

There are two basic types of MHs: center conduit entry or splay conduit entry. A center entry MH is designed such that the main duct run enters at the center of the MH (see Fig. 2 right). This requires you to route the cables to the sidewall for splicing. On a splay entry design, the cables enter the MH near the sidewalls, allowing the incoming cable to more easily align with the cable racks. Where practical, when main conduit enters the sidewall of an MH, it should be splayed. The splaying of ducts usually results in a greater racking capacity and simplifies reinforcements.

You must seal all ducts entering an MH and building entrance point to prevent the intrusion of liquids and gases. This can be accomplished through the use of putty sealants, cementitious compounds, or hydraulic cement. Universal duct plugs are also available in a variety of sizes for use in unoccupied ducts.

The safety of personnel and the general public is a primary concern when selecting MH locations. A desirable location will:

  • Provide a safe work area.

  • Allow for adequate traffic control when the MH is open, such as traffic warning devices that alert motorists of construction.

  • Provide sufficient space for cable trailers, pulling trucks, and the like during construction.

  • Be located out of the roadway when possible.

MHs should not be located in or near an intersection or near a curve in the road.

Handholes. Handholes (HHs) are smaller than MHs, but their covers provide full access to the entire space inside the hole. They are typically manufactured of concrete, polyethylene, or composite materials. They can be placed in the same areas as MHs. When planned for traffic areas, they must be traffic-rated. HHs are typically used in an underground installation as pull-through points. However, they should never be used as splice points. HHs can also be used for storing excess optical fiber cable, optical fiber splices, CATV splices, or CATV taps.Pedestals and cabinets. Pedestals and cabinets are used in aerial, direct-buried, and underground plant design. You use pedestals and cabinets to store splice closures and terminals. They provide above-grade environmental protection, security, and quick access to splice closures, terminals, excess cables, and optical fiber equipment. You can place pedestals directly in the ground or mount them on concrete pads, floor stands, walls, or on poles. However, make sure you locate the pedestal away from traffic areas that could cause injury to maintenance personnel.

Environmentally controlled cabinets provide a suitable environment for electronic equipment. The cabinets typically provide air circulation with fans and thermostatically controlled heating and cooling. The air conditioning units may be internally rack-mounted or physically attached to the exterior of the cabinet.

Next month we'll take a look at your cabling options and discuss the basics of splicing.

The material for this article was excerpted with permission from BICSI's Customer-Owned Outside Plant Design Manual, Second Edition.

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