When low-voltage lines are underground but transformers and medium-voltage lines are overhead, this is called a hybrid overhead/underground distribution system. When the system is entirely underground, the medium-voltage lines are also buried, while transformers are mounted on ground-level pads or in underground vaults. The latter system is more common in densely populated areas and in certain housing developments.Underground distribution is gaining popularity in North America. Most new residential developments have an underground distribution network. In many jurisdictions, wirefree environments are becoming a feature of the urban landscape. More and more towns and cities are choosing to bury distribution lines in new neighborhoods.Underground distribution can transmit power across densely populated areas or areas where land is costly or environmentally or aesthetically sensitive. Underground and underwater crossings may be a practical alternative for crossing rivers.AdvantagesLess subject to damage from severe weather conditions (mainly lightning, hurricanes/cyclones/typhoons, tornados, other winds, and freezing)Decreased risk of fire. Overhead power lines can draw high fault currents from vegetation-to-conductor, conductor-to-conductor, or conductor-to-ground contact, which result in large, hot arcs.Reduced range of electromagnetic fields (EMF) emission, into the surrounding area. However, depending on the depth of the underground cable; greater EMF may be experienced on the surface. The electric current in the cable conductor produces a magnetic field, but the closer grouping of underground power cables reduces the resultant external magnetic field and further magnetic shielding may be provided. See Electromagnetic radiation and health.Underground cables need a narrower surrounding strip of about 1–10 meters to install (up to 30 m for 400 kV cables during construction), whereas an overhead line requires a surrounding strip of about 20–200 meters wide to be kept permanently clear for safety, maintenance and repair.Underground cables pose no hazard to low-flying aircraft or to wildlife.Underground cables have much less danger of conductor theft, illegal connections, sabotage, and damage from armed conflict.Burying utility lines makes room for more large trees on sidewalks, the trees conveying environmental benefits and increase of property values.
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