Editing Electric power transmission (section)

=== Underground ===
{{Main|Undergrounding}}

Electric power can be transmitted by [[high-voltage cable|underground power cables]]. Underground cables take up no right-of-way, have lower visibility, and are less affected by weather. However, cables must be insulated. Cable and excavation costs are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair.

In some metropolitan areas, cables are enclosed by metal pipe and insulated with [[dielectric fluid]] (usually an oil) that is either static or circulated via pumps. If an electric fault damages the pipe and leaks dielectric, liquid nitrogen is used to freeze portions of the pipe to enable draining and repair. This extends the repair period and increases costs. The temperature of the pipe and surroundings are monitored throughout the repair period.<ref>{{cite news|url=https://www.nytimes.com/2001/09/16/us/after-attacks-workers-con-edison-crews-improvise-they-rewire-truncated-system.html|title=After the Attacks: The Workers; Con Edison Crews Improvise as They Rewire a Truncated System|first=Neela|last=Banerjee|newspaper=The New York Times |date=September 16, 2001}}</ref><ref>{{cite web|url=http://documents.dps.ny.gov/public/Common/ViewDoc.aspx?DocRefId={5B2369A6-97FC-4613-AD8B-91E23D41AC05} |title=Investigation of the September 2013 Electric Outage of a Portion of Metro-North Railroad's New Haven Line |publisher=documents.dps.ny.gov |date=2014 |access-date=2019-12-29}}</ref><ref>NYSPSC case no. 13-E-0529</ref>

Underground lines are limited by their thermal capacity, which permits less overload or re-rating lines. Long underground AC cables have significant [[capacitance]], which reduces their ability to provide useful power beyond {{convert|50|mi|abbr=off}}. DC cables are not limited in length by their capacitance.