For existing power lines, ADSS optical cables serve as an “addition.” Consequently, ADSS cables must be designed to adapt as closely as possible to the original line conditions. These conditions include (but are not limited to) meteorological loads, pole and tower strength and geometry, the phase arrangement and diameter of existing conductors, sag and tension parameters, span lengths, and safety clearances. Although the external appearance of an ADSS cable may resemble that of ordinary “all-plastic” or “non-metallic” optical cables, they are, in fact, two entirely different types of products.
I. Representative Structures
Currently, two primary types of ADSS optical cable structures are prevalent both domestically and internationally:
1. Central Tube Structure:
Optical fibers, incorporating a specific amount of excess length, are housed within a PBT (or other suitable material) tube filled with water-blocking gel. A layer of suitable aramid yarn is then wrapped around the tube to provide the required tensile strength, followed by the extrusion of an outer sheath—either PE (for electric field strengths ≤ 12 kV) or AT (for electric field strengths ≤ 20 kV).
The central tube structure facilitates the production of cables with smaller diameters, resulting in reduced ice and wind loads. It is also relatively lightweight; however, the available excess fiber length is limited.
2. Layer-Stranded Structure:
Loose tubes containing the optical fibers are helically stranded around a central strength member (typically FRP) with a specific lay pitch. An inner sheath is then extruded over this assembly (this inner sheath may be omitted in applications involving low tension and short spans). Subsequently, a layer of suitable aramid yarn is wrapped around the cable core to provide the required tensile strength, followed by the extrusion of a PE or AT outer sheath. The cable core may be filled with gel; however, when an ADSS cable operates over long spans with significant sag, the low internal friction of the gel can cause the cable core to “slide,” potentially altering the lay pitch of the loose tubes. This issue can be mitigated by employing appropriate methods to secure the loose tubes to the central strength member or by utilizing a “dry-core” design, though these solutions present certain manufacturing complexities.
The layer-stranded structure facilitates the achievement of a reliable excess fiber length. Although the cable’s diameter and weight are slightly larger compared to the central tube structure, this design offers distinct advantages in medium-to-long span applications. II. Key Technical Parameters
ADSS optical cables operate in an overhead configuration characterized by long-span, two-point support (typically spanning hundreds of meters, or even exceeding 1 kilometer). This differs fundamentally from the traditional concept of “overhead cabling” (where, according to postal and telecommunications standards, an overhead messenger wire suspension system provides a support point for the optical cable approximately every 0.4 meters). Consequently, the key technical parameters for ADSS optical cables are aligned with the regulations governing overhead power transmission lines.
1. Maximum Allowable Tension (MAT / MOTS)
This refers to the tension experienced by the optical cable under specific design meteorological conditions, based on the theoretically calculated total load. Under this tension, the strain on the optical fibers must remain ≤ 0.05% (for stranded-layer designs) or ≤ 0.1% (for central-tube designs), with no resulting increase in optical attenuation. Simply put, this signifies the point at which the excess fiber length (slack) within the cable is completely “taken up” at this specific control value. Based on this parameter, along with the prevailing meteorological conditions and the controlled sag requirements, the maximum allowable span length for the optical cable under these specific conditions can be calculated. Therefore, the MAT serves as a critical basis for sag-tension-span calculations and acts as a key indicator of the stress-strain characteristics of the ADSS optical cable.
2. Rated Tensile Strength (UTS / RTS)
Also referred to as Ultimate Tensile Strength or Breaking Force, this parameter represents the calculated sum of the tensile strengths of the cable’s load-bearing cross-sections (primarily comprising aramid fibers). The actual measured breaking force must be ≥ 95% of the calculated value (where the fracture of any single component within the cable is deemed to constitute a failure of the entire cable). This parameter is by no means optional; rather, numerous critical design values—such as tower/pole strength requirements, tension hardware specifications, and vibration mitigation measures—are directly contingent upon it. From the perspective of optical cable engineering, if the ratio of RTS to MAT (which is analogous to the “safety factor K” used in overhead power line engineering) is inappropriate—meaning that despite the inclusion of a large quantity of aramid fibers, the available strain margin for the optical fibers remains excessively narrow—the resulting cost-performance ratio will be poor. Consequently, the author strongly recommends that industry professionals pay close attention to this specific parameter. Typically, the MAT is approximately equivalent to 40% of the RTS. 3. Everyday Stress (EDS)
Sometimes referred to as “daily average stress,” this parameter denotes the theoretical tensile force exerted on the optical cable under conditions of zero wind, zero ice, and the annual average temperature. It can be regarded as the average tension (or stress) experienced by the ADSS cable during its long-term operation. Typically, the EDS falls within the range of (16–25)% of the cable’s Rated Tensile Strength (RTS). At this specific tension level, the optical fibers within the cable should exhibit no strain and incur no additional attenuation, thereby ensuring a state of high stability. Furthermore, the EDS serves as a critical parameter for assessing the cable’s fatigue aging characteristics, and it forms the basis for determining the cable’s vibration-damping design requirements.
4. Ultimate Everyday Stress (UES)
Also known as “special usage tension,” this parameter refers to the maximum tensile force that the optical cable may potentially encounter—exceeding its standard design load—at any point during its effective service life. It implies that the cable is permitted to withstand short-duration overloads, during which the optical fibers may undergo strain within a limited, permissible range; typically, the UES is required to be greater than 60% of the RTS. Under this specific tension, the strain on the optical fibers should remain below 0.5% (for cables with a central tube structure) or below 0.35% (for cables with a layer-stranded structure). While the fibers may exhibit some additional attenuation under these conditions, they are expected to fully recover to their normal state once this extreme tension is relieved. This parameter serves to guarantee the reliable operation of the ADSS optical cable throughout its entire service life.
III. Compatibility Between Hardware Fittings and Optical Cables
“Hardware fittings” (or “line fittings”) refer collectively to the various mechanical components and accessories utilized for the installation and mounting of optical cables.
1. Tension Clamps
Although referred to as “clamps,” the preferred design for this application—with the exception of installations involving low tension or short spans—is the helical pre-formed rod type. These fittings are sometimes alternatively termed “terminal fittings” or “dead-end fittings.” The selection of appropriate tension clamps is based primarily on the optical cable’s outer diameter and its RTS; generally, the required gripping strength of the clamp must be equal to or greater than 95% of the cable’s RTS. When deemed necessary, compatibility tests should be conducted to verify the proper interaction between the specific hardware fittings and the optical cable.
2. Suspension Clamps
Similar to tension clamps, the helical pre-formed rod type is generally the preferred design for suspension clamps—again, with the exception of installations involving low tension or short spans. These fittings are sometimes referred to as “intermediate fittings” or “suspension fittings.” Typically, the required gripping strength of the suspension clamp is specified to be within the range of (10–20)% of the cable’s RTS.
3. Vibration Dampers
For ADSS optical cables, helical vibration dampers (SVDs) are the most commonly adopted solution. If the Everyday Stress (EDS) of the cable is equal to or less than 16% of its RTS, vibration damping measures may be deemed unnecessary; however, when the EDS falls within the range of (16–25)% of the RTS, the implementation of appropriate vibration damping measures is required. If optical cables are installed in areas prone to vibration, anti-vibration measures should be determined through testing, if necessary.
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