/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgHelical Tape MV Shielding - Overlap Makes It BetterHelicalTapeMVShieldingMakesItBetter.htmHelically wrapped copper tape shields have been a medium-voltage standard for years. Wrapping design can improve performance. When an industry standard product gets better, it’s like no-calorie pie. What’s not to like? Helical tape shielding in medium-v
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Helical Tape MV Shielding - Overlap Makes It Better

Helically wrapped copper tape shields have been a medium-voltage standard for years. Wrapping design can improve performance.

When an industry standard product gets better, it’s like no-calorie pie. What’s not to like? Helical tape shielding in medium-voltage (MV) power cables is a case in point.

“Tape shields over ethylene propylene rubber (EPR) insulation have been a favored power cable construction for years,” says David Cooper, Southwire senior product engineer. “But the way the tape is wrapped can deliver significant reliability and performance benefits.”

In particular, the overlap of the tape windings is a key design feature in helical-tape construction. Although the Insulated Cable Engineers Association (ICEA) recommends a minimum tape overlap of 10 percent, all of Southwire’s tape-shielded power cables use an overlap of 25 percent. Extra overlap delivers increased short-circuit capacity and better mechanical reliability.

More overlap carries more current

Southwire tape shields with 25 percent overlap allow for higher short-circuit currents. Here’s an example. A 15kV cable with 500kcmil conductor, EPR insulation and PVC jacket. For a short-circuit duration of 30 cycles, here are current capacities for two different overlaps:

12.5% Tape Overlap:                  1893 Amps

25% Tape Overlap:                   2045 Amps

That’s an increase in short-circuit protection of more than 10 percent.

More overlap provides better mechanical protection

The 25 percent tape overlap also reduces the risk of hidden mechanical damage during installation – especially pulling through conduit. Here’s why:

If a tape-shielded cable is bent too sharply or pulled around bends with too much tension, the tape windings may separate and the tape can buckle when the cable straightens out. The buckling can damage the underlying insulation shield. In a conduit, the damage is invisible, and even in a cable tray the cable jacket may conceal the condition. What’s worse, most field electrical proof tests won’t reveal this kind of damage.

“Damage from buckled tape can be critical,” says Cooper, “because it may get worse over time, as the cable expands and contracts during service. The result can be eventual premature cable failure. Southwire's 25 percent overlap helps prevent the problem from occurring at all. If you’re specifying tape shielding, be sure to check the overlap.”