All Dielectric Self Supporting (ADSS) fiber optic cables are located in high electric fields. Their sheath can be subjected to dry band arcing and corona especially in highly polluted areas with little rainfall where there is regularly high conductivity precipitation on the cable and it is rarely cleaned by natural rainwater.cleaned by natural rainwater.
Corona or dry-band arcing puncture the cable sheath allowing water penetration leading to destruction of the aramide yarn and exposing the PBT tubes that contain the optical fibers. In more severe cases a massive reduction of mechanical strength can even lead to cable dropping.
Corona discharge is expected in locations with local high electrical fields caused by sharp edges, for example, at the tips of armor-rod assemblies of transmission lines conductors. Armor rod assemblies are attached to transmission line structures, support cables, provide mechanical strength protecting them from bending and also ground the cables. Depending on the placement of the cable with respect to the high voltage conductors, the conductor’s voltage and the pollution on the cable surface can induce current to flow along the cable. This gives rise to corona, micro-sparking and dry band arcing, especially near the tower where the e-field is greatest. The local partial discharge processes involves the emission of UV and the creation of ozone and other acids, reagents that erode the galvanized conductors and the ADSS sheath.
Even though ADSS cables are electrically non-conductive, contamination that accumulates on their outer sheath can turn them into being conductive. When the layer of pollution gets unevenly wet it becomes semi-conductive. Most cases of failing ADSS occurred in highly polluted areas or in coastal areas. The wind from the sea drives saltwater droplets onto the fiber-optic-cable surface, covering the cable with a thin layer of salt. Fog or dew wets the pollution layer and forms a conductive layer on the cable surface. Capacitive coupling between the phase conductors and fiber-optic cable induces current along the wet pollution layer. This current dries the layer and forms small dry bands. The dry band interrupts the current and generates high voltage across the band. This voltage produces arcs. The heating effect of the arc extends the dry-band length, which stops the arcing. However, condensation and wind-driven saltwater from the sea wet the cable and re-initiate the arcing. Dry-band arcing is a periodic phenomenon that occurs when the cable is simultaneously wet and polluted.
The higher the contamination and moisture, the higher the induced current thus, in subtropical areas, dry-band arcing as well as corona are especially harmful due to the specific climate conditions: an eight- to nine-month dry period followed by a shorter rainy period in winter. The cable surface pollution layer after the dry period is hard and adhesive, and particularly thick in the regions nearing clamps causing a serious corona problem.
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