1. High Pressure Fluid Filled pipe type (HPFF)- The conductor and its wrappings are often referred to as “cables.” The cables are surrounded by a dielectric oil that is maintained at 200 pounds per square inch (psi). This fluid acts as an insulator
2. High Pressure Gas Filled pipe type (HPGF)- Instead of a dielectric fluid, pressurized nitrogen gas insulates the conductors. Nitrogen gas is less effective than dielectric fluids.
3. Self Contained Fluid Filled (SCFF)- The conductors are hollow and filled with an insulating fluid that is pressurized to 25 to 50 psi.
4. Cross Linked Polyethylene (XLPE) - The solid dielectric material replaces the pressurized liquid or gas of other types of insulation. This type of construction has three independent cables. They are not housed together in a pipe, but are set in concrete ducts or buried side-by-side.