Overhead systems are out in the open, so it is easy to detect
and fix design and installation problems. Underground problems,
however, are out of sight and out of mind, at least until cables
start failing. Although utilities design their underground
circuits for a 30-year life, improper installations often can
lead to premature field failures. Unless you lay your cables to
rest properly, they may come back to haunt you. Here's a brief
example. A wind-generating farm was installed with underground
cables tied directly to a main feeder cable. Unfortunately, the
cables were simply placed in a trench using native soil backfill
with minimal soil compaction. Ampacity calculations were
performed using typical soil values, but thermal properties were
not measured. Since wind turbines operate almost continuously,
the feeder cable often ran at maximum capacity. The heat
generated from the feeder cable dried out the surrounding soil
completely. Because the native soil was poorly compacted fine
silt, it acted like an ins