Fiber cable fundamentals

Companies that deploy fiber optic cable for long-haul, distribution or campus environments must weigh often-overlooked factors such as proper grounding and contractor dig-ups when they choose a cable design. Two design options-metallic-armored or all-dielectric-have cost/performance tradeoffs.

Aerial, buried and underground cables share risks from both man-made and environmentally caused outages. The highest risk by far is man-made, and dig-ups represent the greatest threat to buried and underground installations. The challenge is to select a cable design that will provide the most reliable and cost-effective solution.

First, it's necessary to understand cable installations. Aerial installation usually offers the lowest cost for new cable placement, particularly when the pole route exists with available support capacity. Direct-buried is good for long-haul routes when companies can plow and trench in favorable right-of-way conditions. In new suburban areas, buried cable construction is often required to eliminate pole structures that compromise aesthetic appeal. Underground construction is generally defined as cable placement in prebuilt duct structures buried under city streets. It is the most expensive installation method.

Cable structure is another important factor. The structure-with integral strength members, outer sheath and water-blocking filling-protects the optical fibers. Without it, the fibers' long-term survivability is jeopardized.

Cable selection should be made with the relevant environmental, installation and maintenance factors in mind so that the optimum cost/performance tradeoff is made. This will ensure the maximum operational life of the cable system with minimum maintenance costs.

Once the fiber type and fiber count are decided, the cable filling, armoring and sheath design are next on the list (Table 1).

An inside look Aircore cable, which contains no water-blocking mechanisms, is generally preferred for intrabuilding applications where ease of handling and connectorization are the key issues. For outside applications, a water-blocked cable is a better choice.

Wax or petroleum-based jelly filling compounds have been in use for years, starting with plastic insulated copper cable and continuing with fiber optic cables. These compounds are applied to the fiber tubes and surrounding cable elements to fill voids, eliminating water paths inside the cable structure.

The jelly-filling compounds have been greatly improved over the years with excellent temperature performance and long-term compatibility with cable components, including the optical fibers and fiber coatings.

The downside to filling compounds is handling. Splicing and connectorization require careful cleaning and handling of the fibers to remove the filling compounds.

During the last few years, dry powder water-swellable materials have been introduced in place of the jelly filling compounds. The powder materials are easier to handle and reduce splicing time. But as the term swellable implies, the compound must get wet at the water entry point before swelling and blocking occurs. This is claimed to be limited to several inches with no chemical or mechanical detrimental effects to the cable or fibers.

Cable sheaths come in metallic-armored, all-dielectric and all-dielectric with glass reinforced plastic armored designs. They share a common polyethylene outer jacket that is UV- and abrasion-resistant and has excellent electrical insulating properties.

Metallic-armored cables have steel wire armor, tape or both under the outer jacket (Figure 1). Armored all-dielectric cables have a layer of glass reinforced plastic strength members or similar non-conducting strength elements stranded under the jacket.

The non-armored, all-dielectric cables may have aramid yarn elements layered under the outer jacket to give good pull-in strength (Figure 2). However, these strength elements do not provide significant abrasion, cut-through or gnawing protection.

Gophers, squirrels, rats and other gnawing animals can cause considerable damage to fiber optic cables. Years of study and cable testing have demonstrated that a layer of steel armoring is the most cost-effective way to protect exposed cable.

All-dielectric cables can also be armored by stranding a layer of glass reinforced plastic rods of 1 to 2 mm in diameter over the cable core before applyin g the plastic jacket. However, the glass reinforced plastic armor is considerably more expensive than steel armor and makes the cable stiff and somewhat difficult to handle.

Another option is to install the cable in a plastic overduct of 2 inches or more outer diameter. The larger diameter makes it difficult for the rodent to bite into the duct. This method incurs the additional cost of the overduct, however.

Humans, and the dig-ups they cause, remain the greatest cause of major outages for buried cable. Dig-ups account for nearly 60% of failures, according to a report by the FCC's Network Reliability Council. Call-before-you-dig programs are being strongly promoted as a result.

Monitoring defenses New toning and cable locating systems that reach up to 50 miles also have emerged. These systems include a rack-mounted generator that users can dial up and control remotely, along with highly sensitive and selective hand-held receivers that provide precise cable location and depth information on digital displays.

These cable tracing systems-the most effective way to locate and mark buried and underground cable-require a metallic conductor. A metallic armor is well-suited for this purpose.

Also available are proactive systems that monitor the cable sheath and splice closures for damage, as well as protect against vandalism and intrusion. These systems, which warn of developing problems long before outages occur, are easy to install and can be integrated into supervisory network alarm systems.

Used with a preventive maintenance plan, these new tools can reduce outages from dig-ups, provide considerable savings and increase cable reliability.

Monitoring and locating instruments use electric and electromagnetic fields. Cable-locate tones and cable monitoring signals cannot be applied to all-dielectric cables. Where all-dielectric cables are used below grade, an additional large gauge-insulated conductor must be placed with the cable to provide a means of tone locating (Figure 3).

In high lightning environments, all-dielectric cables, which are non-conductive, have a clear advantage over cables containing metallic elements. Yet lightning damage caused fewer than 2% of outages for buried cable, according to a 1993 report by the Network Reliability Council.

The low exposure to severe lightning damage is particularly true of modern fiber optic cables, which have an overall jacket of high-grade, UV-resistant, high dielectric strength polyethylene.

Early designs included steel central strength members, which could attract lightning into the core of the cable. But most new designs use all-dielectric-strength elements in the cable core area and metallic armoring only in the outermost layers.

The current generation of armored fiber optic cables is robust and can withstand high surge currents and electric fields with little or no damage to the sheath. Studies have shown that poor grounding at splice and repeater sites is a root cause of damage to metallic-armored fiber optic cables.

Grounding is particularly important when deploying metallic-armored fiber optic cable. Lack of understanding and poor installation practices contribute greatly to the problems.

Optical repeater sites are often placed in remote locations, and the power feeds to these locations are typically end-of-line feeds isolated from the main distribution line. At the last pole or service entrance, a ground rod is usually driven to provide neutral grounding. It is important that this power ground is of a low resistance and not in the "zone of influence" of the building ground. The zone of influence is the area of surrounding earth, approximately 300 feet, that extends from a central grounding point. At the repeater sites, it is the power ground.

If both the building and power ground resistance are not sufficiently low, the grounds become electrically coupled. When the grounds are coupled at a building site, a nearby lightning strike to the power conductors will drain into the grounds and cause the entire site, including the building grounds, to reach a high voltage relative to a remote ground. During conditions of a ground voltage rise, surge currents can travel out over the cable armor and arc to a remote ground, damaging the sheath.

The same holds true for splice grounds. A high-resistance ground at a splice becomes a connection to earth through which a nearby ground strike can find an easy path into the cable. This is evidenced by lightning damage for 100 feet or more on either side of a splice location.

Good low-resistance grounds at both building and splice locations will minimize fiber optic cable exposure to lightning and other foreign surge currents and provide proper safety grounding.

Power substation cable installations are one circumstance in which all-dielectric cables offer the only practical solution to a difficult problem. During ground fault conditions, the rise in ground potential in the immediate vicinity of a substation can reach several thousand volts. All-dielectric cables are immune to ground potential rise and provide a safe communication and control link.

Water woes Water in cables and splice closures has long been a concern. With copper cables, the effects are immediate and usually catastrophic with sudden loss of service. The introduction of fiber optic cables and glass as the transmission media were expected to bring an end to concerns for water in cables. This turned out to be a false hope.

Although the effects of water in fiber optic cables and splice closures is not immediate, degradation caused by long-term exposure of the cable elements, splice-closure components and the glass fiber and coatings is well-documented.

A sometimes forgotten fact is that in molecular form, water will migrate through plastic. The rate of migration depends on the plastic's thickness and molecular density and the relative humidity on either side, among other factors.

The food industry leverages this phenomena with packaging designed to keep food fresh during a long shelf life. Metals form an absolute barrier to moisture permeation. Thin film metal coating on plastic foil packaging is often used to keep foods dry and fresh.

>From the beginning, telephone cable manufacturers have used metals in the protective cable sheath to block water permeation into the cable core. All-dielectric cables do not have this blocking mechanism.

A plastic-jacketed fiber optic cable with an outer diameter of about an inch and a nominal jacket thickness of 0.06 inches placed in a wet environment will only accumulate about 1 cc of water per foot of cable in 10 years. However, this should be considered if both an all-dielectric cable sheath and water-swellable filling is deployed.