A Microstructural Study of the Damage Sustained by Copper Shield Used in an Underground Medium-Voltage Cable

We show that a 3-mm-thick copper tape acting as metallic shield in an underground medium-voltage cable has been prematurely damaged by overheating. Detailed microstructural characterization has shown that the damage is characterized by: (a) formation of CuO on the copper surface as well as localized melting and perforation, (b) distortion and cracking of the outer protection jacket of polyvinyl chloride allowing the copper tape to come into contact with high sulfate-containing soil leading to deposition of CuSO₄ on the copper surface, and (c) breakdown of the underlying layer of insulation shield. Most evidence suggests that overheating has been related to high return fault current exceeding the current-carrying capability of the copper tape used in the application. It is concluded that such a problem may be combated by using grounded objects with higher resistance to minimize the effect of return fault current and/or using copper shields with better current-carrying capability.