• Cables that are buried underground need insulation and protective coverings. This is to protect the wire, as well as to prevent electrical current from harming passers-by. The insulation used will vary depending on the amount of voltage the wire will carry. Three common types of cable are Self-Contained Liquid-Filled Cable (SCLF), High Pressure Liquid Filled Pipe-Type Cable (HPLF) and Solid Dielectric Cable (SDC). These types are named based on the type of insulation used.
  
  

SCLF

• Paper insulation was originally used in many cables. It was quite effective due to its combination of electrical and mechanical characteristics, and was used for nearly 100 years. These paper systems left small gaps in the insulation process due to air voids that formed under electrical stress. In 1924 an oil-filled cable was introduced. The liquid filled any gaps and prevented any voids from forming in the insulation. The insulated cable is filled by completely immersing the cable in the insulating liquid. Once this process is completed and the cable is cool enough, a metallic sheath is applied, usually constructed of either a lead alloy or aluminum.
  
  

HPLF

• HPLF typically consists of three SCLF cables, minus the metallic sheath, inserted into one steel pipe. The pipe is then filled with liquid. The pipe provides the same protection the metal sheath normally would. Once the cable is prepared, it is wound onto a special reel and covered with a blanket of nitrogen until it is installed. The pipe is typically in 40- to 50-foot lengths and approximately one-quarter of an inch thick. The diameter of the pipe is generally 2.5 to 2.8 times larger than the space required by the three cable. This allows plenty of installation room without jarring the cables. The diameter of the pipe determines how must voltage it is rated for. This ranges from 72 KV to 345 KV (kilovolts)
  
  

SDC

• Solid Dielectric Cables typically use polythethylene or ethylene-propylene rubber for insulation purposes, as well as a copper compacted single conductor. These cables are typically rated for an electrical load of 138 KV. High-voltage cables, such as those used underground, must use a lead-alloy or aluminum covering.