In outside-plant installations, conduit is generally installed underground to protect cables from damage as well as facilitate cable placement for fast and future needs. You can also install Conduit Fittings inside buildings to facilitate pulling cable between two points including in the telecommunications closet (TC) to function-area outlets, or from an equipment room to a TC. To protect, isolate, and identify the cables, innerduct–also referred to as subduct–may be installed inside existing larger-diameter conduit.
Conduit is described as a rigid or flexible metal or nonmetallic raceway by which cables can be pulled. Additionally, although conduit can be used to house various types of cable, the National Electrical Code (NEC) uses the word “optical fiber raceway” in Article 770 to explain conduit, or raceways, for optical-fiber cable. Several kinds of conduit are available, such as electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and versatile conduit. For premises installations, how-ever, metal flexible conduit will not be recommended as a consequence of potential abrasion injury to the cable jacketing.
Metal conduit, which typically comes in 10-foot lengths, is pretty rigid and needs special tooling and accessories to join it. Nonmetallic conduit is offered on reels in longer, continuous lengths which do not really need to be joined as much.
“The only problem with installing EMT conduit is that it needs a special skill set and training, together with a lot of practice–or you wind up making swing sets,” explains Kevin Smith, project manager at MTS Services (Bedford, NH). “Metal conduit comes in 10-foot lengths so you should do any nonstandard bends by hand, and that`s where technician`s special skill is needed.”
Arnco Corp. (Elyria, OH) sells innerduct towards the cable-TV, telecommunications, and electric utility markets, says Tom Stewart, electrical products sales manager. “In the building, various kinds duct are being used–for example, riser- and plenum-rated–but all our products are produced from thermoplastic materials, like polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are easier to install than metal.”
You will find three different types (or ratings) of innerduct: outdoor, riser-rated, and plenum-rated. Robert Jensen, engineering manager at Endot Industries Inc. (Rockaway, NJ), explains: “Outdoor is normally polyethylene and it`s certainly not rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], which is generally a thermoplastic material like polyethylene or PVC with fire-retardant chemicals included with it. And the third kind of duct is UL plenum-rated, generally a pvdf product, which can be fire-retardant and smoke-resistant,” says Jensen.
In accordance with Mike D`Errico, regional director of sales at Pyramid Industries (Erie, PA), most products which conduit and innerduct manufacturers make is designed for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “very often incorporating some type of silicon,” he says. “For premises cabling, Pyramid delivers a plenum raceway (tested to UL-910) as well as a riser raceway (UL-1666) for installation in vertical shafts.” Furthermore, the riser product is halogen-free which is often used for military, shipboard, or tunnel applications, depending upon the specifications.
Obviously contractors install conduit where building codes require it, and also where cabling system needs physical protection or protection from unauthorized access.
“We use conduit in riser and backbone systems from the building entrance for the main distribution frame,” says Karl Clawson, senior vice president and partner, Clawson Communications (Greenwood, IN). “So we also set it up for horizontal cabling, particularly in university campuses. Inside the living quarters, we install cable in conduit since it gives the cable extra protection, and hopefully, keeps it all out of students` reach,” he says.
Some cabling contractors want to have other trades install conduit; by way of example, electricians who definitely have more experience with performing this task. “Generally, the sole time we use Flexible Plastic Conduit for Cables takes place when we`re creating a riser or penetrating a fire wall,” says Smith. “Typically, we may not install conduit from the wiring closet on the workstation outlet. For short distances, around 100 feet, we would install conduit between buildings depending on the existing infrastructure.
Along with the traditional smooth-bore type, innerduct is available having a ribbed inner wall to minimize friction between your cable sheath and also the innerduct wall. “A wave-rib within the duct reduces surface contact in between the cable and also the wall of your duct, thus lowering the coefficient of friction and allowing you to pull cable over longer distances,” says Stewart.
Another variation will be the multicelled conduit system, which provides outerducts with pre-installed innerducts. Clawson says that, due to its cost, his company is not going to use conduit with pre- installed innerduct. “We keep leftover conduit in store to make use of on other jobs,” he says. “But pre-installed conduit is a special application, so overages and underages are kind of costly to manage.”
For premises applications, Dura-line (Knoxville, TN) has created a conduit, referred to as Hex-line, for multiple-duct applications between buildings. “When you pull the ducts away from the reel (two to every reel), they go deep into a collector, which Dura-line supplies cost-free,” says Ray McLeary, vice president of sales. “Each duct has a male and female part, which can be snapped together, creating a multiple duct system. This saves time, space, and funds, but the most significant savings is space.” He explains: “Normally, you may put three 1-inch innerducts right into a 4-inch conduit. With this system, it is possible to fit four 11/4-inch or six 1-inch innerducts into the conduit.”
When choosing innerduct, you also have to be worried about its tensile strength and crush resistance. “The thicker the wall material, the larger the tensile rating,” says Stewart. “If you`re planning to pull it over a long-distance, pick a wall thickness that lets you pull the duct over that distance. The crush-resistance feature helps to make sure that the innerduct won`t be damaged through the placing process–or perhaps you can`t pull in the cable,” he explains.
Because of the limited volume of tensile pull that you can exert around the cable, people seek out strategies to minimize the coefficient of friction inside of the conduit. “There are products in the marketplace like prelubricated conduit,” says Stewart. “And there`s a good different technology being used for placing cable, known as air-blown fiber (or ABF), where the fiber-optic cable is blown in to the conduit. We manufacture whatever we call the `air-trak` system–a conduit system with chambers–to be used in ABF installations.” [Air-blown fiber is available in america from Sumitomo Electric Lightwave Corp. (Research Triangle Park, NC).]
Conduit and innerduct have one thing in common: They facilitate pulling or replacing a cable for more capacity in the premises cabling system. However, every contractor is aware that for an installation grows, the amount of cables grows to fill all the space within the conduit. Therefore, choosing the correct trade dimensions are important, simply because you must leave sufficient clearance between the walls from the conduit along with other cables (view the eia/tia-569 standard). Typically, conduit trade sizes range from 1/2 to 6 inches in diameter. Minimum conduit size appropriate for backbone cables is 4 inches. Sufficient clearance should be available to allow pulling the cable without excessive friction or bending.
The NEC conduit-fill tables define the exact amount (like a percentage) of various kinds of cable you may use in the conduit. “The NEC typically covers power cables,” says Stewart. “Rich in-voltage cables, you must consider temperature and impedance, which really don`t apply when it comes to data cables in conduit. The real question for data cable is: Can you pull it into the dimensions of duct that you`ve selected?”
“The most significant decision when installing conduit is the dimensions of the conduit and clearance from the wall,” says Clawson. For external use, we use 4-inch PVC conduit, so we try and install all the conduit from the trenches when we can for future use.”
Cables are continually put into conduit systems that happen to be often filled to capacity with generations of older cable. When new cables are added, friction and pulling tension can damage existing cables inside of the conduit. A great way to offer future changes is always to subdivide larger conduits with innerducts, that are smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“Inside an existing structure, many installers will not would like to pull new cable across the cable already inside the conduit,” says Stewart, “since they risk damaging the existing cable. To optimize a greater conduit, they`ll install several smaller innerducts within it. They`ll pull a lesser fiber cable into one of many innerducts, after which have additional ducts to use for future cable placement.”
Innerducts are classified by outside diameter (OD) whereas trade-size conduits use inside diameter (ID). One-inch innerduct is usually used within buildings; however, 11/4-, 11/2-, and 2-inch innerducts are accessible for larger fiber cables. Although innerducts consume space inside a conduit, they give additional protection and suppleness in constantly changing cabling installations.
“Generally, if you`re installing a 4-inch conduit,” says Smith, “you`ll wind up setting up three 1-inch innerducts: one for fiber, one for data, and something spare. What you should do is pull the maximum amount of dexlpky51 it is possible to at installation time.”
Typically produced from thermoplastic materials, innerduct features a pull string already installed. It is available in ribbed-, corrugated-, and smooth-wall styles. Some types have prelubricated inside walls. These special coatings along with the physical properties of the inner wall of your innerduct ensure less friction and tension when pulling cable.
“Corrugated innerduct is commonly used in plenum and riser products,” says D`Errico. “And, when constructed from high-density polyethylene, it can be typically employed for short–1000 feet or less–installations.” Smooth wall is utilized for direct-buried, trenching, plowing, aerial, and directional boring applications. “The Flexible Metal Conduit Pipe is the cable jacket is “lifted” from and has a reduced area of contact with the pipe, lowering the coefficient of friction. Although the guideline is: the greater the hole, the better it`s will be to drag the cable,” he says.
Based on Clawson, “We use ribbed innerduct if we`re pulling one innerduct, because it`s easier to handle. If we`re pulling via a directional boring machine and it`s a multiple pull, we use smooth innerduct. It can be easier to pull smooth innerduct along with an even surface, and yes it doesn`t kink as easily as ribbed innerduct.”
When working with innerduct, it is essential to verify be it a plenum or non-plenum area as well as to install the innerduct using the appropriate support. In case the innerduct is secured with tie wraps within a plenum area, only take plenum-rated products.
Innerduct is generally offered in a single color–orange for your fiber-optic communications industry. Color can often be installation-specific; for example, one color for data cable, one for telephone, etc. “There exists a movement afoot to try and use color designations for various types of applications,” says Stewart. “Orange is generally communications, red will be for electric power, and yellow for gas.”