"Triplex" or "Quadriplex" are generic terms to describe a wire assembly of 3 or 4 conductors. Underground Triplex or Quadriplex are called URD around here and all of the conductors are insulated and listed for direct burial. Overhead tri/quad plex has an uninsulated grounded conductor and is called ACSR which stands for Aluminum Cable Steel Reinforced and is not listed for direct burial. I didn't realize URD was a regional term. I believe the correct term for the insulation type which is listed for direct burial is USE.
