Design considerations of medium voltage primary URD cables

The short service life of underground residential distribution (URD) cables has led to the development and evaluation of new insulators and cable designs. Some basic comparisons are made between the ethylene-propylene rubber (EPR) and cross-linked polyethylene (XLPE) families of primary cable insulation. Current insulation systems are briefly described, treeing and water-tree testing are discussed, and the performance of EPR and XLPE insulation is indicated. New design features, and their performance, are described. These features are: (1) improved insulation materials; (2) the use of 133% insulation levels in place of 100%; (3) jacketed constructions; (4) cable-in-conduit systems; (5) filled conductor strand; and (6) moisture barrier or moisture-blocked constructions