Article - Sept 2000
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BACnet Testing Laboratories

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Kent McCord
The Wiremold Company

 Kent McCord is a senior design engineer with The Wiremold Company, West Hartford, Conn.

The structured cabling industry is witnessing several major trends, including the increased use of accessible, flexible wire and cable management systems that facilitate moves, adds, and changes. Yet another trend is the use of optical fiber cable in horizontal applications, frequently called fiber-to-the-desk. Together these trends focus attention on the effect of bend radius on system performance - and the need to maintain a specified cable bend radius.

The TIA/EIA and the American National Standards Institute (ANSI) have recently approved a bend radius standard for fiber optic cable that replaces an often confusing set of "recommendations" and "rules of thumb" that evolved from cable manufacturers, industry standards bodies, and independent training organizations. Cabling system designers, installers, and building owners should carefully consider the practical implications of this new standard as it relates to the choice of cabling pathways.

Effects of bend radius on cable performance

When a fiber cable is bent excessively, the optical signal within the cable may refract and escape through the fiber cladding. Bending can also permanently damage the fiber by causing micro cracks, especially during cable installation when pulling forces are to be expected. The result is known as bend loss: a loss of signal strength that may compromise the integrity of the data transmission.

The new bend radius standard

Until recently the minimum bend radius allowable for a given fiber optic cable was not governed by an industry standard, but rather by the specifications of individual cable manufacturers. The new standard ANSI/TIA/EIA-568B.3: "Optical Fiber Cabling Components Standard" sets performance specifications, minimum bend radius standards, and maximum pulling tensions for 50/125-micron and 62.5/125-micron fiber optic cables. Of particular concern in selecting and installing cabling pathways is the new minimum bend radius standard for these cables:

For inside plant cable other than 2- and 4-fiber, the standard specifies:

A 2.0 inch bend radius is "future proof"

Many cabling pathways available in the industry offer only a 1.0" cable bend radius. While a 1.0" bend radius is sufficient for the lay-in of fiber optic cable, as well as the installation of most copper unshielded-twisted pair (UTP) cable, it is not sufficient for the installation of fiber optic cable under pull forces. System designers, installers, and owners can not be certain that the initial installation and any future changes will not require cable pulling. Thus, the installation and maintenance of cables may be severely limited by a pathway that does not maintain a 2.0" cable bend radius, since it will not be suitable for pulling fiber optic cable.

While ANSI/TIA/EIA-568B.3 sets standards for fiber optic cable - rather than cable pathways - wire and cable management systems with a 2.0" cable bend radius offer significant advantages over those with tighter bend radii. They do not limit contractors to a potentially impractical and costly no-load, lay-in installation. Similarly, any future changes can also be accomplished by pulling cable.

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