Tight-buffered cable holds up well in hostile environments

Recently, engineers considered using tight-buffered indoor/ outdoor fiber-optic cable for a cabling project in New York City that called for cables to run through a tunnel and across a bridge. However, they ultimately chose a loose-tube, gel-filled cable for the project. Although they were impressed with the tight-buffered cable`s sturdy construction, they considered the product unproved and were not confident it could withstand the rigors of the environment in which it would be placed. Their th

Apr 1st, 1998

Barbara Turner,

Optical Cable Corp.

Recently, engineers considered using tight-buffered indoor/ outdoor fiber-optic cable for a cabling project in New York City that called for cables to run through a tunnel and across a bridge. However, they ultimately chose a loose-tube, gel-filled cable for the project. Although they were impressed with the tight-buffered cable`s sturdy construction, they considered the product unproved and were not confident it could withstand the rigors of the environment in which it would be placed. Their thinking was flawed; tight-buffered cable is not a new technology, and the design and materials are well-proven.

Tight-buffered cable has been in use much longer than loose-tube, gel-filled products. It was used in the first Rural Electrification Administration-funded field trial for the Commonwealth of Pennsylvania Telephone Co. in 1978. That first run functioned into the early 1990s, when the network`s electronics system became obsolete and the communications company chose to discontinue maintaining it.

Loose-tube products have gone through metamorphoses since they were introduced; first there was one fiber per tube, then several fibers per tube, then gel was added to the tubes to keep the multiple fibers separated, and strength members were added. So no specific cable of loose-tube, gel-filled construction has been in constant, successful use for as long as the tight-buffered cable that was used in the Pennsylvania Telephone project.

Optical Cable Corp. uses outdoor-quality polyvinyl chloride (pvc) with ultraviolet inhibitors and fungicides for the outer jacket of its tight-buffered indoor/outdoor fiber-optic cables. There has been confusion about whether or not pvc is suitable for outdoor use. Not all pvc materials can withstand both indoor and outdoor environments, causing some consumers to mistakenly believe that no pvc materials are suitable for outdoor use, and that a fiber-optic cable with a pvc jacket and buffers should only be used indoors, or that it performs best indoors. In designing its indoor/outdoor cable, the company first designed a pvc cable suitable for outdoor use and then modified the cable so that it also met requirements for indoor use.

Many electrical-power cables have pvc jackets and perform for 30 or more years in outside-plant applications. Conversely, loose-tube, gel-filled products are jacketed with polyethylene. This material has commonly been used for fiber, but not for other outside-plant products such as electrical wires, because of its substandard insulating properties.

The optical fibers inside a tube filled with petroleum-based gel are protected from the corrosive effects of salt air only if the tube`s polyethylene-based jacket is not breached. If the tube is ever breached, causing the gel to leak, the fibers are in jeopardy of damage from salt air, which can cause them to become brittle and fail. Unfortunately, the 250-micron acrylate coating applied to the fiber when it is drawn will not withstand the rigors of such a harsh environment. The fibers in Optical Cable`s tight-buffered cables are buffered by either a pvc or an elastomeric buffering to 900 microns.

A loose-tube, gel-filled cable is not suitable for riser applications. Because the gel is flammable, the cable will not meet regulations for indoor use. Also, in a riser application, the gel will seek its level (since it is impossible to fill the tube completely with a liquid or a semi-liquid) and consequently expose portions of the fiber. Inclines, trusses, and expansions in tunnels and bridges have the same structural characteristics as risers. Also, cable is often installed vertically or in complex conduit systems in quays and docks. In these applications, a riser-rated tight-buffered cable will not expose its fibers or allow them to undergo axial migration (microbend increase).

In applications where vibrations in the structure or edifice are frequent, the connectorized ends of loose-tube, gel-filled cables are more prone to failure than are the connectorized ends of tight-buffered cables. This is because tight-buffered cables are directly terminated, and loose-tube, gel-filled cables are not.

Although loose-tube, gel-filled fiber-optic cables are used in long-haul outdoor applications, they have limitations in applications in which the cable will be exposed to a salt-air environment and vibrations.

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