What Are Underground Cables?

Underground cables are insulated electrical conductors installed below the surface of the ground for the transmission and distribution of electric power or communication signals. Unlike overhead lines, which suspend bare or lightly insulated conductors on towers and poles, underground cables encase their conductors in multiple protective layers designed to withstand soil pressure, moisture, and chemical exposure over service lives measured in decades. The technology spans a wide voltage range, from low-voltage service drops feeding residential buildings to extra-high-voltage transmission links operating above 400 kV that carry bulk power between substations in dense urban environments.

Underground cabling draws on electrical engineering, materials science, and civil engineering. Decisions about whether to route power underground rather than overhead weigh higher installation cost against reduced visual impact, lower susceptibility to weather-related outages, and, in congested urban corridors, the absence of a viable overhead right-of-way. IEEE Std 525-2007 provides guidance on the application and installation of cables in electric power substations, addressing both metallic and optical cable systems.

Cable Construction and Insulation

The core of an underground cable is a stranded copper or aluminum conductor sized to carry the rated current without exceeding thermal limits. Around the conductor, one or more layers of insulation electrically isolate it from the grounded metallic shield and outer jacket. For medium- and high-voltage applications, cross-linked polyethylene (XLPE) is the dominant insulation material, valued for its ability to sustain continuous operation at conductor temperatures up to 90°C and withstand short-circuit temperatures up to 250°C without deforming. Older installations used paper impregnated with dielectric fluid (PILC cables), which remain in service in some utility networks but are no longer installed for new construction. Surrounding the insulation is a metallic screen, typically copper wires or tape, which confines the electric field and provides a return path for fault current. The outermost layer, usually high-density polyethylene, resists mechanical damage and moisture ingress.

Installation Methods

Underground cables reach their final positions through direct burial, duct-and-manhole systems, or tunnels. Direct burial places the cable in a trench backfilled with selected sand or controlled low-strength material, offering the lowest upfront cost for rural and suburban feeders but requiring excavation for any future repair. Duct-and-manhole systems draw cables through conduit between accessible vaults, making it possible to replace or add circuits without reopening the trench; this method is standard in urban distribution networks. For extra-high-voltage transmission, where a single cable circuit may carry hundreds of megawatts, purpose-built tunnels allow multiple cable systems to share a single excavation while remaining accessible for inspection and maintenance. Installation depth, backfill thermal resistivity, and conduit fill ratios all affect the cable's current-carrying capacity and must be calculated for each installation using thermal rating methods described in IEC 60287.

Fault Detection and Testing

Because underground cables are inaccessible once installed, locating a fault requires specialized diagnostic techniques rather than visual inspection. Time-domain reflectometry (TDR) sends a pulse along the cable and measures the transit time to a reflection from an impedance discontinuity, identifying the fault's distance from the test point. For high-resistance faults, arc reflection and decay voltage methods condition the fault before applying the TDR pulse. Partial discharge testing detects the small electrical discharges that precede insulation breakdown, allowing utilities to identify cables at risk before a failure occurs. NIST measurement research supports calibration of the instruments used in these diagnostics.

Applications

Underground cables have applications across a broad range of sectors, including:

• Urban power distribution networks where overhead lines are impractical
• High-voltage direct current (HVDC) links connecting offshore wind farms to onshore grids
• Railway electrification systems and transit authority networks
• Campus and industrial facility power distribution
• Telecommunication and data network infrastructure in dense urban areas