A recent survey of 25 U.S. utilities, each installing more than 1 million ft (305 km) of underground secondary cable per year, showed that the average cost of replacing a secondary failure was US$700. Secondary cable failures occur for several reasons, including shovel dig-ins, damage from contractors, damage from removing reel wrap and backfilling trenches with rocks. The typical failure mode for aluminum secondary cables is an open circuit caused by the formation of aluminum hydroxide.

Pirelli (Milan, Italy) has developed a solution for such secondary cable failures. Superseal cable is an insulation system that actively repairs itself when the insulation has been breached. Superseal consists of a standard aluminum conductor, a separator tape, a mobile self-repairing compound (sealant) and an overall high-density polyethylene jacket. When the insulation system of the cable is damaged, the mobile compound is forced into the exposed area via inherent pressure. This process re-insulates the conductor with a high dielectric strength material, restoring the cable to its original operating condition. Superseal is revolutionary in its design and function. Not only is it a ruggedized cable that passes ICEA S-81-570 and possesses self-repairing capabilities, it does so while requiring no special installation practices. Installers can use the same tools, techniques and accessories — no special tapes or mastics are needed at the cable ends to contain the sealant.

Recently, Pirelli partnered with Mid-Carolina Electric Cooperative (Lexington, South Carolina, U.S.) to perform a trial installation and analysis of Superseal cable.

A ruggedized cable was the control sample, while Superseal was used as the test cable. The cables were damaged in several places using knives and shovels, and were subsequently direct-buried in a trench. Fig. 1. shows an example of the severe knife damage on the two types of cables. The cables were terminated, and 120 V was applied. The leakage current was monitored for about two months. Results showed the leakage current on the control sample continuously increased, while the leakage current on the Superseal sample remained constant during the installation. The cables were excavated, dissected and analyzed. The excavation showed the Superseal sealant had re-insulated the cable (Fig. 2a) but that aluminum hydroxide had been forming on the ruggedized control cable (Fig 3a). Further analysis revealed the damage did not affect the conductor in the Superseal cable (Fig. 2b), and the conductor of the control cable had already suffered deterioration (Fig. 3b).

Russ Dantzler, manager of engineering and operations for Mid-Carolina Electric Cooperative, says, “Considering the extent of the damage the insulation was subjected to, I was quite impressed with the self-repairing capabilities of the cable and look forward to having a lower cost alternative to installing cable in duct.”
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