Underground Distribution Systems
Foundation
Underground systems trade visual and weather advantages for higher cost, harder fault location, and thermal limits. Know the components: conduit and duct banks, manholes and vaults, direct-buried cable, splices, terminators, and pad-mounted equipment. Understand cable construction - conductor, insulation, insulation shield, metallic shield, jacket - and why the insulation shield must be terminated properly at each end.
Concepts to master
Ampacity of buried cable depends on soil thermal resistivity, burial depth, and ductbank grouping; cables in a duct bank run hotter than a single direct-buried cable. Understand the difference between a cable that is shielded and one that is not, and why shield grounding matters. Know the purpose of a faulted-circuit indicator and the role of a sectionalizing cabinet. Recognize common failure causes: water treeing, splice defects, and mechanical damage during excavation. Understand cable pulling tension limits and sidewall pressure.
Traps
Questions often assume deeper burial always improves ampacity, or ignore the effect of adjacent cables heating each other. Another trap is treating the metallic shield as optional at terminations.
Rule of thumb
Cables grouped closely or in ducts carry less current than the same cable isolated in soil, because heat cannot escape as readily.
Worked example
Adding a fourth cable to a duct bank reduces the ampacity of the existing three because mutual heating raises the ambient temperature each cable sees.