/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgCold-Weather Cure for Jacket Shrink-BackColdWeatherCureforJktShrinkBack.htmGood installation practices control jacket shrinkage caused by extreme temperatures.
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Cold-Weather Cure for Jacket Shrink-Back

Good installation practices control jacket shrinkage caused by extreme temperatures.

They say that in western Wyoming in the winter, there’s nothing between you and the North Pole but a barbed-wire fence – and that may be down in a few places. Cold weather gets serious there, and thermal expansion and contraction takes on a whole new meaning. Cable jackets exposed to that weather will shrink, if you let them.

“We got a call from a Wyoming customer who saw a jacket shrink-back problem in the winter,” says Paul White, Southwire applications engineer. “The application was a 200-foot run of medium-voltage cable installed outdoors in a cable tray. It turned out that several factors were involved, but the problem was both preventable and curable.”

Wide temperature swings encourage shrinkage

The circuit was composed of multiple 750kcmil single-conductor cables with PVC jackets. They ran in an outdoor bus-duct, separated from each other by a few inches to deliver maximum ampacity. The cable tray was completely exposed to the ambient air.

The cables were installed in September, during a fall season with record-setting high temperatures. Then the cables were left un-energized through a winter that saw outdoor temperatures drop to -40° F (-40° C). With no current flowing, there was no internal heating in the cable. By February, technicians observed that the cable jackets had shrunk back from the termination sleeves by several inches.

When the weather warmed up in the spring, the jackets partially returned to position, but not completely. During this period, portions of the same cable runs that were inside a building and saw much less change in temperature had no shrink-back problems.

“This is a known condition with a known cure,” says White. “Cable jackets do try to expand and contract with extreme temperature, but if they’re held in place mechanically, they won’t be able to move back from the termination.”

Clamps can keep jackets in place

To begin with, there were no clamps on the cable at the terminations. Jacket shrink-back needs to be controlled during extreme temperature swings. IEEE Standard 532, IEEE Guide for Selecting and Testing Jackets for Underground Cables deals with exactly this issue. Section 9.6 of that standard calls for clamps to restrain jacket movement in extreme temperature changes.

In addition, the cable termination kits used heat-shrink sleeves. These maintain a fixed size after they’re installed. The sleeves were installed in 90° F (32° C) weather, when the cable had maximum diameter due to heat expansion. At -40° F (-40° C), the cable shrank inside the sleeve and there was less pressure on the jacket to hold it in place. Termination kits using cold-shrink sleeves will continue to keep pressure on the jacket as the cable expands and contracts with extreme temperatures.

“The cure for this situation was to install a jacket repair sleeve bonded to the original cable jacket with a heat-activated adhesive,” White says. “Tests at Southwire showed this sleeve holding its position from -40° C to 90° C. To prevent this problem, follow IEEE guidelines.”