Corrosion of Buried Concentric Neutrals

The effect of alternating current on the corrosion of buried structures such as USD cable with bare concentric neutrals has become of great concern to industry, particularly power distribution companies. Many authors 1-13 have discussed the subject and at one time there was a consensus of opinion that a.c. did not contribute significantly to corrosion. With the rapidly increasing number of miles of buried cable and the increasing tendency of power distribution systems to use the parallel earth return circuit for carrying part of its current many people are having second thoughts about the subject. Holley 5 has shown the percentage of load current in the neutral return, for various ratios of conductor to neutral cross-section and diameter of cable in soils of various resistivities. His studies indicated that, in some situations, about twenty five percent of the a.c. current left the bare neutrals near the load, followed the parallel path through the earth and reentered the neutrals near the source transformer. His work suggested that for all practical purposes, more than two percent of the load current would always be found passing between the neutral and earth at two places on the cable. He did not indicate the current densities at any point along the concentric neutral.