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Engineering & Design – The Basics of Fiber Optic Cable Design

The most recent advances in cable jacket designs ensure the longevity and reliability of a fiber-optic-based network.


March 1, 2000  


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Optical fibers have advanced the speed of communications to the point where billions of bits of information can be transmitted over a single glass strand in a fraction of a second. Fiber optic cables, smaller than one inch in diameter, are often entrusted with the task of carrying all the video, voice and data traffic of entire office complexes, educational institutions, or manufacturing plants.

Optical fibers are inherently very robust — typically withstanding pulling forces in excess of 400,000 psi. Yet, due to their small size and to the tendency of the glass to lose mechanical strength when exposed to moisture, fibers must be encased in several layers of protection to provide long-term reliability and optimal performance.

Protecting the fibers becomes increasingly critical as the industry reliance on this technology continues to grow. The single most important component in protecting the cable’s fibers from external stresses is the outer jacket. The jacket works in conjunction with the other cable elements to isolate the fibers from environmental and mechanical strains. The jacket type will also determine which industry standards for flame retardancy, smoke emission and toxicity the cables will pass, determining their suitability for use in enclosed spaces.

OUTER JACKET SELECTION

More than half of a cable’s total weight is typically represented by the jacket or by the series of jackets used. The material selection becomes very critical in determining the flammability, toxicity and flexibility of the total cable. Table 1 illustrates a number of jacketing materials typically used to manufacture fiber optic cables.

INDOOR APPLICATIONS

Cables installed indoors will not be exposed to the same temperature extremes of outdoor environments nor will they experience the same degree of mechanical stresses. However, these cables have to offer a certain degree of flame resistance and, in some cases, low smoke emission in a fire. Additionally, indoor cables must be flexible for ease of routing under raised floors, above ceilings, in riser shafts, and in termination cabinets.

The National Electric Code Article 770 addresses the flammability and smoke emission requirements of indoor fiber optic cables. The requirements vary, based on the installation location. The cables are tested and listed by Underwriters Laboratories (UL) and by Intertek Testing Services (formerly Environmental Testing Laboratories — ETL) to insure compliance to the NEC Article 770 requirements. The three NEC classifications are as follows:

1. GENERAL PURPOSE

Applications: Cables installed indoors under raised floors, connecting devices to wall outlets, and used within interconnection cabinets.

Typical jacket materials: PVC, Low smoke, zero halogen Polyolefin.

Test method: UL-1581 vertical tray fire test. Flame spread up an eight-foot tray filled with cable is measured.

Cable marking: OFN (Optical Fiber, Non conductive) or OFC (Optical Fiber, Conductive) for those designs containing metal cable components.

2. RISER

Applications: Backbone installations in the riser shafts of buildings. These cables can also be used for all general purpose applications.

Typical jacket materials: PVC, low smoke, zero halogen Polyolefin.

Test method: UL-1666 riser fire test. This test set up duplicates the riser shaft of a building. Failure occurs when the flames propagate up the cables and reach the next floor.

Cable marking: OFNR or OFCR for those designs containing metal cable components.

3. PLENUM

Applications: Installations in the air handling spaces of buildings and in the plenum ceiling spaces. These cables can also be used for riser and general purpose applications.

Typical jacket materials: Modified PVC, ETFE or other similar Fluoropolymers.

Test method: UL-910 Steiner Tunnel test. The test duplicates an air handling duct. Flame spread and smoke emission are measured.

Cable marking: OFNP or OFCP for those designs containing metal cable components.

General purpose and riser listed cables cannot be installed in the plenum space of a building unless they are encased in Electrical Metallic Tubing (EMT). The tubing protects the cables from mechanical damage and prevents ignition of the cables in case of a fire.

INDUSTRIAL ENVIRONMENTS

Cables that will be installed in a factory or industrial environment and placed in open cable trays do not have to comply with the NEC guidelines. However, they do have to pass the IEEE-383, 1974 vertical tray flame test. This test is similar to the UL-1581 test for general purpose applications, where cables are placed in a tray 12 inches wide and eight feet high. A 70,000 BTU flame is applied 18 inches above the bottom of the tray for 20 minutes. After removal of the flame, the cable must self-extinguish and there must be no charring or blistering to the top of the tray.

The demanding environment of a tray installation requires the use of tough flame-retardant Polyethylene or cross-linked Polyolefin jackets.

OUTDOOR INSTALLATIONS

Cables used outdoors will be exposed to high pulling forces during installation and subsequently to wide temperature extremes, water, moisture, sunlight, crushing forces, and possibly rodents. These cables do not have to meet any flammability requirements because they are not placed in indoor environments. The National Electric Code dictates that outdoor cables without a UL or ETL listing (general purpose, riser, or plenum) cannot be routed from the outside to more than 50 feet into a building. A listed cable must be used past the 50-foot demarcation point. Polyethylene is the preferred jacket for outdoor applications.

Although PE offers excellent characteristics for outdoor installations, it will not pass any of the flame tests required by the NEC for indoor use. A sunlight-resistant and water-resistant PVC outer jacket over a loose tube cable design should be specified for those applications where an outdoor rated cable needs to be routed inside a building past the 50-foot demarcation point.

These indoor/outdoor rated cables are widely used to network college campuses or urban locations where several transitions between indoor and outdoor environments may take place.

LOW TOXICITY JACKETING MATERIALS

The NEC Article 770 classifies cables according to flammability and smoke emission. It does not address the toxicity of the byproducts of combustion. Typically, in order to achieve the desired degree of flame retardancy, chemicals from the halogen family, such as chlorine and fluorine, are added to the base compounds. For example, PVC materials (used to jacket general purpose and riser cables) contain chlorine, while Fluoropolymer jackets (widely used for plenum applications) contain significant amounts of fluorine.

These halogens can be extremely toxic and can be very damaging to electronic equipment when released in a fire. Using jacketing materials that achieve flame retardancy without the use of halogens solves the toxicity problem. In case of a fire, no halogens and very limited amounts of toxic fumes are emitted, decreasing the chance of harm to humans and to electronic machinery. Low smoke, zero halogen and low toxicity compounds (LSZH) are widely used by the European community, by the U.S. Navy for shipboard installations, and by the North American transit industry for installations in subway tunnels and subway cars.

LSZH cables are becoming increasingly popular in North America for installations in hospitals, schools and office environments. They are available with OFN listings for general purpose use or OFNR listings for installation in the riser portion of the building. Because of the severity of the Steiner Tunnel test, LSZH cables have not been able to obtain the OFNP listing and cannot be used in the plenum space of a building, unless they are encased in electrical metallic tubing.

THERMOSET CABLE JACKETS

Conventional thermoplastic jacketing systems such as Polyethylene, PVC, Fluoropolymers and CPE offer good protection in most situations. However, these jackets may not be sufficient
to insure the integrity of the optical fibers in conditions where the cables will experience high abrasion during installation, be immersed in chemicals, be flexed repeatedly, or placed near sources of heat and sparks.

Thermoset jacketing systems should be considered for the cable outer jacket in these more demanding applications. The molecules in a thermoset jacketing system are chemically bonded trough either a heat activated chemical process or through exposure to high energy electrons. It takes considerably more energy to separate these molecules once they are “cross linked”. This gives a thermoset much higher abrasion resistance, better low temperature flexibility, and greater resistance to oils and solvents. Table 2 illustrates the differences between a thermoplastic and a thermoset compound.

Thermoset jacketed cables are used extensively by the U.S. Navy for shipboard installations, due to the higher durability of the cable. They are also used by the transit industry to improve the resistance to sparks generated by the third rail and minimize the damage at installation. In addition, these cables are used by the petrochemical industry because of their high resistance to oils and chemicals.

The appropriate jacket will insure that the fiber optic cables are, at a minimum, as mechanically durable and reliable as similar sized copper cables. Fiber optic cables have often surpassed the strength of comparable copper cables, while offering lighter weight and greater bandwidth. Combined with the high information carrying capacity of the optical fibers, a well constructed fiber optic cable will guarantee over 20 years of reliable service, even in the most demanding environments.CS

Giovanni P. Tomasi is Director of Product Marketing at Chromatic Technologies, Inc., Franklin, MA, a member company of the Draka Holding Group in Amsterdam. He has over 17 years of experience in the fiber optic cable industry and has managed several development programs for military and commercial clients. He can be contacted at: gtomasi@drakausa.com.

TABLE 1 – JACKETING MATERIALS
Jacket material Properties
Polyvinyl Chloride (PVC) Provides good mechanical protection and flexibility, widely used for indoor applications. Flame retardant. Used for outdoor applications with the addition of UV light inhibitors.
Polyethylene (PE) Excellent UV and water resistance. Excellent low temperature flexibility, resistance to abrasion and scrapes. Used extensively for outdoor applications. Since it burns, it is not suited for indoor use.
Polyurethane Excellent abrasion resistance, UV resistance, and low temperature flexibility. Widely used for military field communication cables and outdoor deployable systems.
ETFE (Tefzel) High temperature jacketing system (150C) providing high abrasion resistance and excellent resistance to flames. Considerably stiffer and more expensive than PVC, it is used only in applications where its properties are required.
Flame Retardant By adding flame retardants to Polyethylene, a durable,
Polyethylene highly abrasion resistant material is produced. Widely used for indoor/outdoor applications in industrial and petrochemical environments.
Low Smoke, This family of compounds will not emit toxic fumes, smoke
Zero Halogen Polyolefin or acid gases in case of a fire, while providing mechanical performance comparable to a PVC or a flame-retardant
PE jacket.
Cross-Linked Polyolefin The cross-linking process “bonds” the molecules together, creating a material very resistant to abrasion, cut-through, solvents, ozone and stress cracking. Widely used for indoor/
outdoor installations in highly demanding environments.

TABLE 2 – THERMOPLASTIC VS. THERMOSET
Thermoplastic
Materials are composed by molecules held together by weak attractive forces.
Material softens and melts when heated; hardens when cooled.
Material flows and drips when exposed to a flame.
Examples: PVC, PE, Fluoropolymers, Polyurethane.
Thermoset
Materials are composed by chains of molecules chemically bonded (cross-linked) together. (It can be considered one giant molecule).
Material “sets” when exposed to the cross-linking process. Once it is set, it will not melt and flow when exposed to heat.
Material chars and remains around cable core when exposed to a flame.
Examples: Cross-linked PE, Neoprene, Cross-linked Polyolefin.

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