/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgShielding Systems Impact EPR PerformanceShieldingSysImpactEPRPerformances.htmSemiconducting shields affect installability – and reliability – of EPR cables.
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Shielding Systems Impact EPR Performance

Semiconducting shields affect installability – and reliability – of EPR cables.

In any medium voltage (MV) installation, labor costs are important – and so is long-term reliability. The insulation shielding system in the cable you choose has an impact on both.

“Installation expenses are a significant factor in MV cable applications,” says Doug Ramsey, Southwire’s national sales director for power generation. “A key step in applying MV terminations and splices is removing the insulation shield – and that step can affect labor costs.”

To minimize installation labor, the semiconducting insulation shield needs to come off cleanly and consistently. “Pick-offs” – bits of semiconducting material that cling to the insulation – are tedious and time-consuming to remove. Variations in stripping force, either between cable samples or on an individual cable, also slow down the installation and drive up labor costs. The question is how to optimize insulation shield removal without affecting long-term cable reliability.

Release agents may create contamination problems

One approach to MV shield strippability is to apply a release agent between the insulation and the semiconducting shield. When the cable uses EPR (ethylene propylene rubber) insulation and an EPR shield material, a release agent is mandatory. Without it, the two EPR layers bond together tightly and shield removal becomes difficult, if not impossible.

Further, release-agent cable constructions often show variations in stripping force because of uneven release agent application. Stripping force is also apt to vary with temperature. Both variations soak up installation time.

Compatible copolymers strip cleanly without contamination

“The more common approach in the industry for insulation shields in MV EPR cables is to use a compatible copolymer compound that gives efficient strippability without requiring a release agent,” says Ramsey. “That’s the approach that Southwire uses.”

A properly chosen copolymer shield material delivers optimum adhesion – not too much and not too little – and exactly matches the thermal expansion characteristics of EPR. Because copolymer shields need no release agent, this design delivers other benefits, too. First, stripping force is more consistent. In one study, samples of cable using release agents showed about 16 percent more variation in stripping force than cables without release agents. Copolymer shield materials also maintain more consistent stripping force over a very wide temperature range.

All EPRs are not the same

It’s important to know that not all EPR formulations are alike. The Southwire EPR insulation formulation is specifically selected for chemical compatibility with copolymer shields. The Southwire EPR formulation also delivers better tensile and elongation performance than EPR formulations that contain high proportions of clay fillers.

Southwire MV EPR power cable products have a substantial and growing record of successful operations in industrial and utility applications. They’re backed by rigorous AEIC qualification testing – the most severe universally recognized test for cable materials compatibility.

“Where other test conditions may give misleading results, the AEIC test suite represents practical real-world requirements,” says Ramsey. “And our MV extrusion lines are re-qualified monthly to assure our MV customers of consistent Southwire quality.”