When installing optical cable, what you don't know could really hurt you or damage your cable. Familiarizing yourselfwith Standard 301 is a smart step to avoid such problems.
Up until a little over a year ago an installation standard for optical cable didn't exist. Yes, the NEC has covered optical cable for longer than that, but these requirements focus on fire safety and installation with electrical conductors; and not a whole lot more. That's all changed now. As of late 1998, a joint venture of the National Electrical Contractors Association (NECA) and the Fiber Optic Association approved and published ANSI/NEIS Standard 301. This standard is part of the American National Standards Institute (ANSI) National Electrical Installation Standards program.
If you're company installs optical cable, you'll need to become very familiar with the guidelines outlined in this standard. We'll go through some of the more important sections now to bring you up to speed.
General section. This section describes "procedures for installing, testing, and commissioning of systems that use fiber-optic cables and related components to carry signals for telecommunications, control, and similar purposes." The intent of the standard is to define "a minimum level of quality for fiber-optic cable installations."
Cable installation. These sections outline proper installation of optical cable. The first requirement is "all fiber-optic cables should be installed in accordance with their listings and manufacturers' instructions." Although this seems obvious, one section details how cables must be supported:
All optical cables shall be securely supported, and shall have the supports spaced closely enough that there will be no excessive force placed on the cable. In general, horizontal indoor cables shall be supported at intervals not exceeding 3 ft (91 cm). Supports may be placed up to 5 ft (1.52 m) apart for armored cables, or cables over 1/2 in. (1.3 cm) in diameter. Cables directly buried require no additional support. Cables in raceways are considered to be adequately supported by the raceway.
Another section addresses the tightness of supports. The section reads: All straps or supports placed on fiber-optic cables shall be tight enough to hold the cable securely, but shall not be tight enough to substantially deform the shape of the cable. When optical cables are squeezed out of shape, unacceptable forces are placed on the fibers, which frequently result in microbends, microcracking, or even broken fibers. Where possible, rounded or padded supports shall be used. Cable ties shall not be cinched too tightly, and shall have the free tab cut off, to prevent overtightening in the future.
Notice that with power wiring we are seldom concerned with supports being overtightened. But with data cables (not only fiber), overtightening is a frequent source of cable damage you must avoid.
The standard also lists general rules to be followed when pulling cables into place. After mentioning that manufacturer's instructions take pre-eminence, the following rules are given:
• When pulling optical cables into conduits, cable trays, or raceways, the strength member(s) of the cable shall bear all or nearly all of the pulling force. Cable jackets shall not be directly pulled unless designed for the purpose, or unless the run is very short and requires a minimal pulling force. Optical cables shall not be pulled into place by applying tension directly to the fibers (pulling the fibers).
• Optical cables shall be attached to a pulling line only by methods recommended by the manufacturer of the cable.
• Unless stated otherwise by the cable manufacturer, the maximum pulling tensions used for optical cables shall be 300 lb. (136 kG) for multi-fiber indoor cable and 600 lb. (273 kG) for outdoor cable. The pulling force shall be uniform and consistent; cables shall not be jerked.
• Cable pulling shall be done by hand, except when tension meters, tension-controlled, or breakaway swivels are employed.
• When powered pulling equipment is used to install optical cable, tension monitoring equipment or breakaway swivels shall be used. Swivels shall be used when pulling optical cables intoconduits. Exceptions shall be made to this requirement only for very short runs, which require a minimum pulling force.
• Continuous cable pulls shall be used whenever possible, avoiding splice points.
• Boxes used with optical cables shall be designed for the purpose, and shall be equipped with cable supports. Pull boxes shall be sized so that no cables in the box shall be tightly bent.
• A length of free cable shall be provided at each end of a cable pull. Loops of cable (commonly called service loops) shall be provided at all intermediate pulling points, such as in manholes and pull boxes. The cables' minimum bending radii shall not be violated.
• When pulls are accomplished in two or more stages, and spare cable must be unreeled, it shall be configured in large figure-eights on a safe, flat surface, such as the ground or a clean floor.
• When pulling fiber-optic cable through nonmetallic raceways (or nonmetallic 90 degrees elbows) with rope, maximum speed through a duct shall be about 3 ft (91 cm) per second. If mule tape is used, the top speed shall be 9 ft (274 cm) per second. The intent of this requirement is that the nonmetallic conduit or elbows will not be cut or grooved by the pulling process.
• When underground raceways containing optical cables enter a structure, the raceway shall be sealed to prevent the entry of gasses into the structure. The entry of outside plant cables into a structure may require special fire safety considerations.
Safety precautions. I was quite pleased to find this section made it into the final standard without much revision. (I was involved with the production of this standard, and its final form was by no means certain until the standards committee took several votes.) With two primary safety hazards associated with optical fiber systems, the more commonly talked about hazard is retina damage; due to looking into the ends of live fibers. In real life, however, this is a rare situation.
Few fiber systems have power enough to cause eye damage (cable TV systems being the exception), and most broken fiber ends will diffuse the light passing through them anyway. Nonetheless, this is a legitimate concern.
The second concern, however, is far more serious. This is the hazard posed by broken pieces of fiber. These are essentially small glass needles, and can be quite dangerous: Painful when stuck in the skin as a splinter, they are potentially life-threatening if ingested. Hence the safety requirements of this section of the standard:
• Live optical fiber ends (live fibers are those with signals being sent through them) shall not be inspected by technicians; fibers shall be dark (no signal being transmitted) when inspected. Care shall be taken in verifying that the fibers are not live, since the light used in the majority of optical systems is not visible to the human eye.
• If there is a risk of fibers being inspected live; especially when the system light source is a laser, all technicians working on the system shall wear protective glasses, which have infrared filtering.
• Fiber-optic work areas shall be clean, organized, well lit, and shall be equipped with a bottle or other suitable container for broken or stray fiber pieces.
• No food, drink, or smoking shall be allowed in areas where fiber-optic cables are spliced or terminated, or in any area where bare fibers are being handled.
• Technicians making fiber terminations or splices or working with bare fiber shall be supplied with double-sided tape, or some other effective means, for picking up broken or stray pieces of fiber. All work areas where bare fiber may exist shall be repeatedly and consistently cleared of all bare fiber pieces. All bare fiber pieces shall be disposed of so that they cannot escape and cause a hazard. (For example, bare fibers should be sealed in some type of bottle or container before being dumped into a wastebasket.)
• All technicians working on bare fiber shall thoroughly wash their hands immediately when leaving the work area. They shall also check their clothing, and pat themselves with clean tape to remove any stray pieces of bare fiber.
Documentation. The installation of optical cable relies heavily on testing and documentation. For most electrical installations, the only testing done is flipping on the light switch or pushing a Wiggy into the outlets. But for data, every run of cable must be separately tested and the results permanently documented.
This is not especially difficult, but it does require time and effort. More importantly, it requires a change of mind-set for workers that cut their teeth on power wiring. The requirements of this section are as follows:
All optical test results shall be thoroughly and accurately documented, and copies of the test results saved in a permanent form (typically in both hard copy and on computer disk). In general, documentation of test results shall include the following information:
• Cable type and length
• Splice and termination points
• Fiber type and size
• Connector types
• Splice types
• Cable paths
• End-to-end losses of completed transmitter-to-receiver links
• (Optional) End-to-end losses of individual fiber links The following tests shall be performed on all fiber-optic cable installations:
• Continuity testing upon delivery of optical cables to the site of installation
• End-to-end loss testing for each complete transmitter-to-receiver link
• End-to-end loss testing on each individual link in the system, if transmitter-to-receiver loss is beyond acceptable limits.
In addition, other types of testing may be required for certain types of installations. Long runs shall be tested with an optical time domain reflectometer, and permanent copies shall be made of all OTDR traces. All such trace copies shall be clearly marked as to the date taken, and details of the cable and fiber runs.
Conclusion. Although I'm sure there will be changes to this document as time goes on, I think it provides an excellent starting point. In the future, I'd like to see the standard contain specific requirements for the amounts of free cable that must be accessible at outlet points. My recommendation is to allow 30 mm at every outlet box, and at least 3 m at every pulling point.
Copies of the standard are available from the National Association of Electrical Contractors (NECA), and perhaps from some of the standards organizations themselves.