ADSS fiber optic cables-all-dielectric self-supporting cables-are widely used worldwide. Since their implementation, significant improvements have been made to the "arc-resistant" sheath. The sheath material improves cable performance and extends their lifespan. However, even when a certain level of risk can be defined, it is impossible to predict the expected lifespan of a cable in any given environment. Furthermore, some large numbers of installed cables are at risk, but such issues are often unreported due to commercial sensitivity.
Currents along the ADSS cable act as an aging factor for the sheath material. These currents along the cable are expected due to potential gradients from three sources: capacitive coupling between the conductor, the ADSS cable, and ground; the voltage difference between the clamp and the midspan location; and the conductivity of the cable surface. Typically, the potential is highest in the midspan, where the ADSS cable is positioned higher than the conductor (cable sag), and the grounding potential is forced through the conductor and the grounding metal clamp of the ADSS cable. The conductivity is affected by cable contamination and humidity. As the sheath material ages, it becomes hydrophilic, and its resistance per unit length can decrease to very low values, such as only a few hundred kilohms per meter, while the current increases to a few milliamps. For a 150kV laboratory test line, results showed limits of 1 mA and 1.5 mA for polyethylene.
Surface current and discharge can cause Joule heating of the moisture and an increase in the surface temperature of the optical cable. Following this heating effect, dry strips can therefore occur on the surface of the optical cable. Dry strips are most likely to occur near towers where the current is typically highest. Dry strips have a higher linear impedance than other parts of the optical cable surface. This high impedance characteristic leads to a large voltage drop in short sections of dry cable and during arc discharge. When the current is high enough to sustain the arc, unstable discharges occur on the dry strips. The non-uniform conductivity on the optical cable surface caused by dry strips increases the local electric field and leads to corona discharges that generate ultraviolet radiation and ozone, both of which damage the optical cable surface.