/images/TDUpdate2.gif/images/southwire_100x70.jpgSlender Sheaths Put Underground HV on a Diet - 2006SlenderSheathsPutUndergroundHVOnADietJan2006.htmSlender Sheaths Put Underground HV on a Diet. Laminated sheaths cut costs by extending underground high-voltage runs.
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Slender Sheaths Put Underground HV on a Diet - 2006

Laminated sheaths cut costs by extending underground high-voltage runs.

If you're designing an underground high-voltage transmission cable system - or budgeting for one - you should consider laminated-sheath constructions. They can reduce both initial cable costs and overall installed systems costs.

"Laminated metallic sheaths are seeing growing use internationally in the 69kV - 400kV range because they help trim underground high-voltage budgets," says Thomas Wilki, engineering manager for Southwire HV SolutionsTM. "Laminated-sheath cables offer lower initial costs, and their smaller diameters can deliver project economies."

Bonded metal foils deliver slimmer cables

One budget factor driving the move to laminated constructions is the cost reduction of adding fewer manholes to a right-of-way. The smaller diameter and/or lighter weight of laminate-sheath cables allows up to 30 percent more cable on a reel. That means greater distances between manholes, which can be a significant saving.

Another factor that makes laminate sheaths attractive is the industry's growing appetite for ampacity. As conductor sizes increase, the overall cable diameter becomes a design factor for the installation. One example is the replacement of lead-sheathed cables, which are typically installed in 6-inch ducts. The lead-free laminate sheath allows conductors sizes as large or larger than lead-sheath designs and larger than with corrugated-sheath designs.

Bonded metallic tape seals and protects

Laminate sheaths use a heavy metallic tape (0.006 inch, 0.152 mm thick) coated on one side with a special ethylene copolymer. Copper tape is favored in the U.S., but aluminum is also available. The tape is applied longitudinally around the cable just before the outer protective jacket is extruded. High-strength bonding in the tape seam resists moisture, chemicals, and mechanical stress.

Under the laminated sheathing, two layers of semi-conductive water-swellable bedding tape surround the concentric neutral wires that carry fault currents. The bedding tapes protect the extruded layers and resist longitudinal moisture migration. This construction has become a preferred solution for many utilities.

As a manufacturing option, a thin semi-conductive layer can be extruded over the outer cable jacket to allow jacket integrity tests. Testers apply a voltage of about 20kV between the semi-conductive layer and the sheath to verify the integrity of the jacket dielectric. This test is useful in manufacturing, after pulling the cable, and after the installation is complete.

Cable construction illustration

Laminate sheaths reduce the overall diameter of cables for transmission applications.


1.  Conductor
2.  Super Smooth Conductor Shield
3.  Super Clean XLPE Insulation
4.  Insulation Shield
5.  Water-Swellable Semi-Conductive Tape
6.  Copper Concentric Wires
7.  Water-Swellable Semi-Conductive Tape
8.  Copper Laminated Sheath Jacket
9.  Semi-Conductive Jacket Layer


Choose from a complete line of high-voltage solutions

The only manufacturer of extruded dielectric high-voltage cables in the U.S., Southwire made significant investments in its Forte Power Systems plant in Heflin, Alabama to produce laminated-sheath products. "We are currently filling orders for laminated-sheath 161kV cables to replace an overhead line for a new convention center development in the Midwest and three additional cable projects in the Southwestern U.S.," says Wilki. "Southwire HV Solutions offers a complete line of HV cable, accessories, and turnkey installations from 69kV to 345kV in all conductor sizes."

Overhead Transmission and Distribution Products

  • Motion ResistantTM Oval Conductor
  • Covered Line Wire (Aluminum, Copper)
  • 5kV to 35kV Spacer Cable Tree Wire
  • Protected Ground Wire
  • Transformer Riser Wire
  • Solid and Stranded Bare Copper
  • Armor Wire, Tie & Ground Wire
  • HS285 High Strength Low Sag Transmission Conductor
  • Steel Strand & Guy Wire

Underground Transmission and Distribution Products

  • 15kV - 46kV Primary UD Cable
  • 15kV - 46kV Jacketed Primary UD Cable
  • 15kV - 46kV LCT Shielded Primary UD Cable
  • High Voltage Solutions (cable from 69 kV to 245kV) including design, installation, accessories and construction of high voltage cable systems
  • High Temperature Super Conductor

Secondary Cable Products

  • SureSealTM Self Healing 600v Cable
  • Single and Multiplex 600V Secondary UD
  • Single and Multiplex HI-SCORE 600V Secondary UD
  • AlumaFlexTM Extra Flexible Secondary UD
  • Multiplex Service Drop (Aluminum, Copper)
  • PAC and RTS Cable
  • Squirrel Shield

Power Generation Products

  • 600V Tray Cable
  • 600V Power Cable
  • 5kV - 15kV Medium Voltage Cables
  • SOLONON Excitation Cable (1550 kcmil)
  • Substation Control Cable