Comprehensive document on types and locations of underground power lines, created for the Wisconsin Public Service Commission: electric11
From the document, on p. 21:
“The most non-debatable reason for choosing underground is in highly urban areas, where acquiring ROW [rights of way] that meets National Electrical Safety Code requirements is difficult or impossible. This makes the added cost of undergrounding acceptable to not being able to route the new line at all.”
In many jurisdictions the National Electrical Safety Code is adopted as law. It is recognized everywhere in the U.S. as setting minimally acceptable standards of safety. An engineer in Newcastle asserts that the widths within which PSE plans to construct and operate the proposed new poles and wires along either L or M are not wide and safe enough to comply with the National Electrical Safety Code.
Also, on pages 7-8 there is reference made to transition stations which some say would be required for putting lines underground for this PSE project, but the document says that would be required only for lines transmitting 345kV or higher. This project contemplates 230kV.