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Reprint from: America's Network Magazine

Fiber's Fragile When it Freezes

When the weather outside is frightful, there are ways to protect your fiber cable. Now is the best time to ensure that the outside plant fiber optic cable (and the millions of dollars you've invested in it) is ready for next winter's weather.

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When it's freezing cold outside, fiber optic cable enclosed in innerduct systems that are not buried beneath the frost line--for example, innerduct strung under bridges--is at risk. Because it is made of glass, fiber cable resting in exposed innerduct--in which water has collected--can perform poorly or be damaged when that water turns to ice. As ice crystallizes, it can exert crushing pressure on the fiber cable inside the conduit. That pressure can cause microbends in the fiber and degrade the signal, especially as higher speeds are reached; or it can break the fiber and kill the signal, according to Win Miller, Vice President of Operations for American Polywater Corp., a Stillwater, MN-based specialty chemical manufacturer.

"The freezing phenomenon was discovered several years ago by a large telephone company, which noticed that its fiber optic network lost signal attenuation on cold nights in certain parts of its network," Miller says. "As it warmed up, signal clarity would come back."

For years, the carrier blamed everything on network electronics. Eventually, technicians traced the problem to a site where the cable was encased in conduit that ran across a bridge. Technicians discovered ice had formed inside the innerduct. As they removed the fiber cable from the innerduct, they found that the cable's plastic jacket had scarred. Solving the attenuation problem meant finding a way to remove the water and keep it out of the exposed innerduct system.

Nix Icky-Pick

Copper and fiber cable are protected from water damage by a substance known as "ickypic," which resides between a cable's plastic jacket and its "strength jacket." However, ickypic does not protect fiber cable from ice formation.

Rather than reroute the exposed cable--an expensive solution--technicians tried blowing out the water and sealing the innerduct system with foam, according to Ben Osborne, director of operations for C&S Contract Services, Inc., (Martinsville, IN). When that didn't work, C&S tried installing French drains at each end of the bridge where the innerduct system plunged below the frost line--again, to no avail, he says.

American Polywater had experience dealing with cold weather problems in Canada and Northern Europe. The contractor and vendor developed IceFree™ Antifreeze Gel, which could be pumped into the space between the fiber and the innerduct to force water out and form a protective barrier between the fiber and any ice that might form in the conduit. Since then, service providers around the world have pumped the patented gel into vulnerable innerduct systems, Miller says.

Although the fiber optic outside plant that is prone to ice formation is a tiny percentage of most carriers' networks, it is just such a weak link that can cost millions of dollars in lost revenues from a network outage (not to mention the trust of their biggest customers). "People are surprised about the amount of water in their innerduct systems, but the majority of them do hold water," Miller notes.

The Fix

According to Miller, two-and-a-half gallons of IceFree Gel will pack 100 feet of 1-inch innerduct containing 5/8-inch of fiber cable (see Figure 1). A 55-gallon drum of IceFree Gel costs approximately $1,000. Miller recommends preparing for winter early; once cold weather returns, costs for labor and equipment to install the gel will increase, because any ice in the innerduct has to be melted before the gel can be injected.

Gallons of IceFree™ Gel per 100 Feet
Duct Size Cable Size
1/2" 5/8" 3/4" 1"
1" 3.02.5 1.8 N/A
1¼" 5.34.8 4.0 2.3
2" 15.314.7 14.0 12.2
3" 35.735.1 34.4 32.6
4" 64.263.6 62.9 61.2

Figure 1. IceFree Gel -- Volume Estimates

A pump, air compressor, duct block hose and gel-insertion hose are used to tackle the job in 180-foot segments. After carefully opening the conduit and the innerduct, the duct block hose is inserted into the innerduct and used to place IceFree Chemical Duct Block at the far end of the system. The duct block, which expands on contact with water, can hold the IceFree Gel in place in each segment.

"You want to go at least 20 feet to 30 feet inside the headwall of the bridge and past the frost line," Miller advises.

Once the block is set, the gel-insertion hose is pushed all the way into the innerduct. As the gel is pumped into the system, the hose and any water inside are pushed out. Duct block is inserted at the open end of the system once the segment has been filled with gel, and the innerduct and conduit are closed in a manner that allows re-entry.

The non-toxic gel is dyed red for easier leak detection, Miller says. The substance is applied as a gel to make it less likely to seep out of any cracks or holes in the innerduct. The gel protects the cable from ice formation to temperatures of at least -45°F. The gel reportedly does not damage cable jackets, and innerduct systems that have been filled with the gel can be re-entered for cable removal and reinsertion.

"To date, everything we have treated has not experienced any further freezing problems," Osborne says.

Although the process has become a standard operating procedure for some telephone companies, the Rural Electrification Association rejected a proposal to require telcos to adopt the solution, Osborne notes.

The frost line was higher than usual this past winter in many areas because of heavy snow. The frost line can vary from about 6 to 7 feet below the ground in such northern states as Minnesota, to just about 1 foot below ground in the mid-central part of the United States. But ice-related fiber cable problems are not limited to the northern states; sites in Florida, Georgia, Texas and Mississippi--anywhere freezing temperatures are recorded--are at risk, Miller says.

"Lots of those southern states have a preponderance of water, especially where fiber in innerduct systems is hung on bridges and where aerial fiber that transitions to buried fiber via innerduct systems is mounted on poles," Miller notes.

Floods that have ravaged many areas this winter are bringing water into contact with fiber cable that had never before been exposed to large volumes of water.


Companies seeking an alternative to using the gel may be able to drill "weep holes" into the conduit or use high-maintenance air pressurized cable systems.

Weep holes are teardrop-shaped drainage holes that are drilled into exposed conduit. Although weep holes are used by many major telephone companies, Miller says he does not consider them acceptable over the long term. "The whole concept of fiber optic cable is a closed system, so weep holes are counterintuitive," he says. "They tend to get plugged up by debris and insects, so they often stop working."

"The goal is for the gel to last as long as the fiber optic cable, which is about 25 years," Miller says. "The thing that breaks [the gel] down most readily is sunlight, and there isn't any sunlight inside an innerduct. Therefore we expect it will have a very long life." Although the IceFree Gel liquifies as it breaks down, the gel retains its antifreeze capability, he maintains.

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Important Notice: The statements and information here are made in good faith based on tests and observations we believe to be reliable. However, the completeness and accuracy of the information is not guaranteed. Before using, the end-user should conduct whatever evaluations are necessary to determine that the product is suitable for the intended use. The user assumes all risks and liability in connection with such use. The statements contained herein are made in lieu of all warranties, express or implied, including, but not limited to, implied warranties of merchantability and fitness for a particular purpose, which warranties are hereby expressly disclaimed. American Polywater's only obligation shall be to replace such quantity of the product proven to be defective. Except for the replacement remedy, American Polywater shall not be liable for any loss, injury or damage, direct or indirect, arising from the use or the failure to properly use these products, regardless of the legal theory asserted. The foregoing may not be altered except by a written agreement by the officers of American Polywater Corporation.

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