Corona discharges and treeing may lead to premature cable failure. The individual molecules of polyethylene join together during a curing process to form an interconnected network. While the use of peroxide as the crosslinking agent means that only density polyethylene or EVA can be crosslinked, the introduction of silane crosslinking technology allows the crosslinking of all densities of polyethylene. Cable produced with crosslinked polyethylene can operate at higher temperatures than cables produced with thermoplastic or noncrosslinked polyethylene. Crosslinking also significantly improves the physical properties of the polyethylene.
Additives tend to reduce the electrical properties of the insulation.
This is the reason that the EVA copolymer is used only for low voltage applications. For medium voltage applications crosslinked polyethylene fares well because the dielectric strength of the unfilled crosslinked polyethylene is about the same as that of thermoplastic polyethylene. It also provides the necessary ultraviolent protection for outdoor applications without the use of jacket. XLPE has good low temperature properties, shows increased resistance to corona when compared with thermoplastic polyethylene, and has good impact, abrasion, and environmental stress crack resistance. Recent technology has resulted in XLPE insulation compounds that are resistant to degradation from treeing.