Application Report
Fiber Drops in Rural Applications
Due to their light weight, low cost and flexible attachment methods, CommScope fiber drops have been chosen as a
preferred cable for many rural applications.
Unfortunately, these rural applications often include very long spans — averaging approximately 300 feet. This application report will discuss options for successfully deploying low-cost fiber solutions in long-span rural environments.
Sag and Tension Basics
Sag and tension calculations are performed as a means to ensure that installed aerial cable of every sort meets required clearances
in all loading conditions. This analysis is grounded in the common assumption that cable span, sag and tension are well understood
and mathematically related. Programs like CommScope’s SpanMaster® are often used to make these calculations, which can be
complex. Fundamentally, however, to maintain a shallow sag and the associated high clearance (see diagram) — high tension may
be required at the attachment hardware.
Here lies the issue with long drop fiber designs. Long, unsupported spans can generate very high tension, particularly when loaded
with ice. These high tensions may quickly exceed the maximum allowable loading for the cable as well as tensile capabilities of
the attachment hardware typically associated with cables like CommScope’s Flat Drop designs. In one example, in heavy loading
conditions* a simple Flat Drop wedge clamp capable of handling 500 pounds of tension might potentially fail once faced with a
span of only 175 feet.
CommScope’s Flat Drop is typically recommended for single span installations only. In cases where multiple spans are unavoidable,
or spans are long, different hardware and methods are required. More robust hardware, like that associated with carrier strands, is
recommended for spans longer than 300 feet. How can an inexpensive Flat Drop be deployed in this environment?
A drop in rural fiber-to-the-home (FTTH) applications may run several spans from the tap to the home. Typically, these first spans
are parallel to the distribution fiber, which is lashed. In these span areas, the drop fibers should also be lashed to the strand and
enhanced hardware for support. More robust self support cables, such as the M-xxx-LN-188 or 250 can also be deployed in these
longer span areas; but, in any case, if high clearance is required and spans are very long, strand and lash is the best choice.
Once the drop departs from the pole line, it may span an additional set of poles spaced 300 feet apart. In these areas, an additional
strand is recommended. The last pole, rider pole or home attachment may be significantly shorter. In instances where final spans are
less than 100 feet apart, simple wedge clamps and P or Q hooks may be used in any loading area without concern.
* Loading conditions (light, medium, heavy) are defined by the National Electrical Safety Code, a publication of the IEEE.
Application Report
Fiber Drops in Rural Applications
Clearance Considerations
In any installation, clearance must be taken into account. The following table shows calculated span lengths that must not be
exceeded in order to maintain clearance in pedestrian and vehicular areas. In a typical installation, with 18' attachment height,
cable in a vehicular area is limited to 3' of sag to maintain 15' of clearance per NESC requirements. In pedestrian areas, where
9' clearance is allowed, deeper sag and the associated longer spans are possible. These guidelines are examples using a typical
set of installation conditions. CommScope recommends each span situation be reviewed by a qualified engineer.
18-foot attachment
3-foot sag vehicular areas
9-foot sag pedestrian areas
95
115
M-xxx-MN-109 / .109 Messenger
110
250
M-xxx-LN-188 / .188 Messenger
125
295
¼” EHS D-xxx-LN / Strand and Lash
400
400
O-xxx-DF / Flat Drop
Maximum recommended span to maintain proper clearance in aerial plant (feet), heavy loading.
Central Tube Considerations
A final topic which must be considered is the use of central tube cables over long spans. Central tube designs do not couple mechanically like stranded loose tube cables do, so in long length deployments pistoning or withdrawal of the tube or fibers is possible.
This issue can be controlled by forming loops using the excess cable recommended for storage at each splice location. Forming a
series of loops at the minimum bend radius of the cable has been shown to control tube withdrawal and fiber pistoning due to the
expansion and contraction of long central tube cable spans.
For additional information, refer to CommScope’s Fiber Optic Cable Construction Manual.
LEGAL DISCLAIMER
THESE CALCULATIONS ARE PROVIDED FOR GUIDANCE PURPOSES ONLY AND SHOULD NOT BE USED OR IN ANY WAY RELIED UPON WITHOUT CONSULTATION WITH
AND SUPERVISION OF EXPERIENCED NETWORK DESIGN SPECIALISTS. COMMSCOPE MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING ANY REPRESENTATION OR WARRANTY REGARDING THE QUALITY, CONTENT, COMPLETENESS, SUITABILITY, ADEQUACY OR ACCURACY OF THE DATA
CONTAINED HEREIN. COMMSCOPE IS UNDER NO OBLIGATION TO ISSUE ANY UPGRADES OR UPDATES OR NOTIFY CUSTOMERS/USERS OF THESE CALCULATIONS
THAT CHANGES HAVE BEEN MADE TO THE CALCULATIONS. THE USER OF THESE CALCULATIONS ASSUMES ALL RISKS ASSOCIATED WITH SUCH USE, AND COMMSCOPE
HEREBY DISCLAIMS ANY AND ALL LIABILITY FOR DAMAGES OF ANY KIND RESULTING FROM SUCH USE.
1100 CommScope Place, SE • P.O. Box 1729 • Hickory, North Carolina 28603
Tel 1.800.982.1708 or 828.324.2200 • Fax 828.328.3400 • www.commscope.com
AN-107068-EN • 7.13