The principles of fiber-optic cable installation
Following appropriate steps will ensure installers achieve the four goals of cable installation.
From the February, 2012 Issue of Cabling Installation & Maintenance Magazine
Following appropriate steps will ensure installers achieve the four goals of cable installation.
By Eric R. Pearson, Pearson Technologies Inc.
When examining what makes a fiber-optic network successful from the standpoints of installation and performance, the characteristics can be organized into groups of four. A successful fiber-optic network installation will have four characteristics: 1) low optical power loss, 2) low installation cost, 3) low time of installation, and 4) high reliability. Likewise, there are four goals of fiber-optic cable installation: 1) avoid breakage, 2) avoid reduced power at the receiver, 3) avoid reductions in reliability, and 4) proceed in a safe manner. To meet these four goals, the installer must recognize and respect certain principles. In this article, I organize these principles into four groups: 1) environmental limits, 2) installation limits, 3) National Electrical Code compliance, and 4) end preparation.
The network designer is responsible for specification of a cable proper for the intended environment. With this definition of responsibility, the installer might not need to be concerned with environment limitations. However, network designers may not recognize all relevant environmental conditions. In order that the installer recognize and avoid common field problems, we provide this first principle: Respect environmental limits
The installer installs the cable in environmental conditions that are within the limits. Should the cable be exposed to conditions in excess of its limits, the cable can fail to protect the fibers. Such failure can lead to fiber breakage and excessive attenuation rate.
While not exhaustive, this list of five environmental conditions represents most of the conditions that have resulted in installation problems: moisture, operating temperature range, bend radii, crush load and use load along with vertical rise distance.
Moisture. An installer can install a cable that is not moisture-resistant in an environment that contains moisture. In such a case, the cable can channel moisture into electronics, develop increased attenuation rate and fiber breakage due to frozen water, and/or experience reduced fiber strength and breakage due to attack from the chemicals in groundwater.
Operating temperature range. If an installer installs cable in an environment with temperatures outside of the cable’s operating range, the cable can exhibit increased attenuation and/or degradation of cable materials. In the former case, there may be insufficient power at the receiver for proper link operation. In the latter case, the fibers may break.
Bend radii. During installation, the installer can bend a cable to less than either the short-term bend radius (20x cable diameter) or the long-term bend radius (10x cable diameter). In these cases, the cable can develop an increased attenuation rate, fiber breakage or reduction in fiber strength. Such a situation can occur in an underground conduit path, in which the conduit sweep, or elbow, has a radius less than either bend radius (20x or 10x) of the cable. In this situation the cable path—its environment—is forcing a violation of the bend radius.
Crush loads. If an installer installs a cable in an environment that imposes either a long- or short-term crush load in excess of the cable’s limit, the cable can experience increased attenuation rate and fiber breakage. Such a situation can occur when an indoor, tight-tube cable is directly buried.
Use load. An installer can install a cable with long-term, or use, load on the cable in excess of the cable’s rating. Such a condition can occur when the cable is installed between widely spaced buildings, widely spaced telephone poles, widely spaced power transmission towers, and up a long vertical rise. Two problems can occur—increased attenuation rate and fiber breakage.
An installer should limit the long-term load to a value less than or equal to the cable’s rating. To do so, the installer obtains the use load from the manufacturer, typically from a data sheet or from the manufacturer’s website.
The installer also should limit vertical rise distance. The use load and the vertical rise distance are different statements of the same characteristic. The vertical rise distance is the distance to which the cable can be installed vertically without support. The installer limits the cable to a vertical distance less than the vertical rise limit. To do so, the installer obtains the vertical rise limit from a data sheet. The installer can achieve a total vertical rise distance in excess of this limit by supporting the cable at a separation no greater than this limit.
Indoor loose-tube cables can allow the fibers to slide out of the cable. Thus the installer installs service loops in vertical loose-tube cable.
There are two types of cable installation: 1) pulling the cable into its path, and 2) placing the cable in its location. The installer installs the cable with conditions that are within the limits of the cable. The overall principle is to respect installation limits.
During the cable pull, the installer respects five installation limits: 1) no twisting, 2) installation load, 3) installation bend radius, 4) installation temperature range, 5) storage temperature range.
No twisting. Avoidance of fiber breakage requires that fiber cables be installed without twisting. In order to avoid twisting, the installer uses a pulling swivel between the pulling rope and the cable. The installer attaches the pull rope and the cable to the pulling eye.
In order to comply with the installation load limit, the installer must know the load limit and have a method for limiting the load applied to the cable. The installer will learn the limit from the data sheet for the cable, creating two principles: 1) Know the installation load limit; 2) Limit the installation load.
There are three methods by which an installer can limit the short-term, or installation, load applied to a cable. In addition, there are two methods for reducing the applied installation load.
The three methods have the advantage of providing concrete evidence that the installation load has not been exceeded. These methods are the use of 1) a pulling eye with a swivel with a shear pin, 2) a pulling device with a slip clutch, and 3) a pulling device with a load gage.
The eye has a shear pin, which is rated at a load less than the maximum installation load of the cable. For example, a cable with a rating of 600 pounds-force would require a shear pin rated 500-500 pounds-force. If the installer exceeds the rating, the shear pin (not the cable) breaks.
The installer can use a pulling eye with a swivel without a shear pin if the installer has some other method of limiting the load applied to the cable. Pullers have two methods to provide such limitation. These methods are a slip clutch and a load gage.
An installer can set the slip clutch of a puller to a level less than the installation load of the cable. Should the applied load exceed the level set, the clutch clips, eliminating fiber damage.
An installer can set the load gage of a puller to a level less than the installation load of the cable. Should the applied load exceed the level set, the load gage stops the pulling motor without fiber breakage or damage. The load gage has an additional advantage; it allows attachment of a chart recorder, which provides proof that the installer did not load the cable in excess of its rating.
Reducing the load
These three methods limit the load but do nothing to reduce the load. There are two methods for reducing the load applied to the cable: lubricant and a “Figure-8” installation.
The installer uses a cable lubricant to reduce friction and load on the cable. While copper-cable lubricants exist, the installer uses a fiber-cable lubricant, as such a lubricant is matched to the jacket of the cable.
The second method for reducing the load is pulling the cable by hand in multiple, reduced-length pulls. Breaking a long pull into multiple pulls of reduced length reduces the load on the cable. If the installer installs the cable in multiple pulls, he or she will store the cable at intermediate locations in a “Figure 8” pattern. The advantage of this method is reduced load. The disadvantage is increased labor cost.
During a “Figure 8” pull, the installer pulls the cable into the first manhole and out of a subsequent manhole. The subsequent manhole may be the next manhole along the cable path, or the Nth manhole along the cable path. The installer determines this manhole by the load the installer is willing to impose on the cable.
As the installer pulls the cable from the manhole, he or she places the cable on the ground in a “Figure 8” pattern. This pattern can be 12 feet long and 4 to 6 feet wide. For practical reasons, the pattern is rarely stacked more than 24 inches high.
When the installer has pulled the cable out of the manhole, the installer and several helpers pick up the “Figure 8” and flip it over so the cable end is on top. The installer puts the cable back into the same manhole along the cable path. At this manhole, the installer repeats the Figure 8 pattern on the ground. The installer can repeat this process as many times as desired, until the cable is installed along the entire path.
The installer can use the Figure 8 method for both unidirectional pulls, and mid pulls. In a mid pull, the installer pulls the cable in one direction. The installer places the remaining cable in a Figure 8 pattern on the ground. Finally, the installer pulls the cable from the Figure 8 pattern in the opposite direction.
Some professional installers use a fourth method for limiting the load they apply during installation. This method requires pulling a 600 pound-force cable by hand. The assumption is that it is essentially impossible to create a 600-pound load by hand pulling in a horizontal axis. This method does not have the advantage of providing concrete evidence that the installers have not exceeded the installation load.
All four methods require attachment of a pull rope to the cable. The installer attaches a pull rope to a cable so that the cable strength members support the load and no load is imposed on the fibers.
The installer has at least the following five attachment possibilities.
- Attachment of the pull rope around the outside of the cable jacket
- Attachment of the pull rope to a Kellems grip that grips the cable through the jacket
- Attachment of the pull rope to central strength members
- Attachment of the pull rope to strength members outside the loose buffer tubes
- Attachment of the pull rope to strength members between multiple jackets
With all these possibilities, the installer needs another principle. The best method of attachment is the method recommended by the cable manufacturer.
The manufacturer has designed the cable so that it can be installed without damage, and has done so by assuming that the installer will attach the pull rope to specific strength members. Attachment of a pull rope to any cable structural element other than these strength members may result in a load imposed on the fibers. Such a load can result in fiber breakage.
There is one final method that some installers follow. The installer uses a loose tube cable design for installations in which the installation load will be high. The excess fiber in this type of cable allows the fiber to move to reduce the stress on it. This movement can be for hundreds of feet from the high-stress area.
Bend radius. In order to avoid breakage, the installer limits the bend radius of the cable to above the minimum value. This limitation means that each deviation from a straight path requires some form of control. A pulley or sheave provides such control. To ensure compliance with these limits, the installer obtains the values from the cable data sheet. A cable that complies with TIA/EIA-568-C meets the requirement to limit a short-term bend radius to 20x the cable diameter. Some cables will withstand bend radii tighter than 20x.
Supply reel. The installer monitors the supply reel to avoid bend radius violations due to improper winding of cable on the reel, loosening of cable, cable wrapping around the shaft that supports the reel, or back wrapping when the pull stops. If the cable is improperly wound on the reel, the cable will attempt to pull a lower layer from under an upper layer. This situation results in the cable making a right angle as it leaves the reel—an obvious bend-radius violation.
If the cable loosens during the pulling process, it can ride over the top of the flange of the reel and wrap around the shaft that supports the reel. This situation is another example of a bend-radius violation.
If the installer does not stop the reel from rotating at the end of the pull, the cable can wrap backwards around the reel. Back wrapping provides a third example of a bend-radius violation.
Communication during the pulling process is essential. The installer communicates and coordinates the pulling actions. In advance of a stop, the installer in charge of the pulling equipment informs the installer at the supply reel of his or her intent. With this advance notice, the installer at the supply reel can don the heavy work gloves needed to avoid splinters while grabbing the rapidly spinning supply reel flange in order to stop the reel.
The installer in charge of the pulling equipment may forget to alert the installer at the cable supply reel. Because of this possibility, each installation requires at least three installers. The third installer coordinates the activity of the other two.
An installer monitors each pulley location. This monitor ensures that the cable does not “cable jump” from the pulley. Should the cable jump, the installer can have two problems: fiber damage and strength-member damage. Should the cable jump from a pulley, the bend radius is no longer under control. In addition, the cable may scrape against a sharp edge at the entrance of the conduit. Should this sharp edge cut through the jacket, it may damage strength members under the jacket. Such damage will reduce the installation load capability of the cable, increasing the likelihood of fiber damage.
During installation, the installer pulls, does not push, the cable. Pushing can cause a violation of the bend radius.
The installer installs the cable at a temperature within the range specified on the data sheet. At excessively low temperatures, the cable materials may be brittle enough to crack. At excessively high temperatures, the cable materials may stretch excessively, resulting in numerous problems.
The installer stores the cable at a temperature within the storage temperature range specified on the data sheet. Storage of a cable at a temperature outside of its storage range will cause the material problems.
The term “cable placement,” as used in this article, means cable installation without pulling. Such placement occurs in cable trays, troughs and raceways. Once again, bend radius comes into play. Once a pull is completed, the installer limits the long-term bend radius to at or above the minimum value of 10x the cable diameter.
The installer bundles cables that run along the same path. Bundling reduces the risk of bend-radius violation. Additionally, bundling simplifies circuit tracing.
Cable ties, if they are used, are tightened by hand, not with a cable-tie tool. Excessively tight cable ties can deform the cable jacket, resulting in a localized bend-radius violation and excess power loss. The use of hook-and-loop bands to bundle cables minimizes the risk of bend-radius violation.
Segregate fiber cables. Whenever copper and fiber cables reside in the same tray, the installer places the fiber cables in innerduct. By segregating the two types of cables, the installer reduces the risk of bend radius and crush-load violations of the fiber cables.
Such segregation avoids a bend-radius violation during installation of additional cables in the tray. If an installer is not aware of bend-radius concerns, he or she may use cables ties to attach new cables to those in place. In this situation, a violation of bend radius may result.
Segregation avoids a crush-load violation that can occur when heavy copper cables are installed on top of fiber cables in ladder trays. In this situation, the fiber cables can experience localized bend radius violations.
Service loops. The installer leaves service loops throughout the link. Service loops are lengths of excess cable that can be pulled into problem areas. Service loops are inexpensive insurance; they certainly are less expensive than replacing the entire segment.
For indoor links, a common practice is a 10- to 12-foot service loop near both ends of each link. This service loop coil may be in the back of an enclosure, on a cable tray, or above a suspended ceiling.
For outdoor links, a common service loop practice has the following three parts.
- A 100-foot service loop for each 1000 feet of cable
- A 100-foot service loop for each street crossing
- No more than 200 feet of service loop per 1000 feet of cable
Finally, there will be a service loop of at least 10 to 12 feet at each cable end. Often this service loop is 50 feet, and is in addition to the length of cable that will be stripped for termination. For aerial cable systems, service loop holders hold the service loop.
In any location in which the cable can be access, the installer marks it as “Fiber Optic Cable.” There are two reasons for such marking. First, when the cables are so marked, electricians will not be tempted to cut, reroute and splice them with black tape. Second, installers will not be tempted to use such marked cables in a manhole as a ladder.
Installers install outdoor cables with sag to allow for thermal expansion and contraction. A 2.5-foot sag for 150-foot span is a common practice.
NEC compliance. Indoor cables must comply with the National Electrical Code (NEC) and local electrical codes. Horizontal cable runs require OFN or OFC-rated cables. Cable runs between floors require OFNR or OFNC-rated cables. Cable runs in air-handling plenums require OFNP or OFCP-rated cables. In addition, firewall penetrations must meet the requirements of the applicable fire code. To determine compliance, the installer reads the printed information on the cable, because the NEC requires that all cables be printed with their rating and test number.
After pulling and placing the cable, the installer prepares the cable ends for splicing or connector installation. The installer protects buffer tubes and fibers. This principle means that the jacket ends inside the enclosure, not outside the enclosure. The buffer tubes and fibers are not designed to withstand exposure to the working environment.
The installer seals the ends of grease-filled, gel-blocked cables with a fiber-optic cable sealant. Water-blocking compounds can and will flow from even a small vertical drop. Such flow will cause maintenance problems. An improper sealant can attack primary coating of the fiber.
It is this author’s understanding that some silicone sealants cure to produce acetic acid while others cure to produce water. Acetic acid can attack the primary coating of the fiber. Thus, the installer should use a fiber-optic cable sealant.
Some installers put heat-shrink tubing over the end of the jacket. This conceals any uneven jacket preparation, sealant, and cable materials not trimmed flush with the jacket.
Eric R. Pearson, CFOS is principal of Pearson Technologies Inc. (www.ptnowire.com). This article is excerpted from Pearson’s recently published book entitled “Professional Fiber Optic Installation – The Essentials For Success, Version 8.0.” The article is a derivation of the book’s Chapter 11, Cable Installation Principles. The book contains a total of 26 chapters plus appendices. Each chapter concludes with a set of review questions. The book can be ordered from the Pearson website.
Text book goes into great detail on fiber-optic installation
This article is derived from Chapter 11 of Eric R. Pearson’s book entitled “Professional Fiber Optic Installation - The Essentials for Success, Version 8.0.” The chapter is entitled “Cable Installation Principles.”
Each chapter of the book ends with a set of review questions. Chapter 11 includes 9 such questions. The first question reads as follows: “An installation team is planning to pull a cable through an underground conduit system. The system consists of conduits between manholes. There are no sweeps at locations at which the cable path changes directions or at which the conduit changes elevation. What must the installer do at these changes of direction?” Another question is: “Prior to preparing the end of a filled and blocked cable, an installer stops at Home Depot for silicone sealant. What should you say to him?”
The book includes a total of 26 chapters plus appendices. Among chapter titles are “Insertion Loss Principles and Methods,” “Connector Inspection,” and “Cable End Preparation.”
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