An underground power line provides electrical power with underground cables. Compared to overhead power lines, underground lines have lower risk of starting a wildfire and reduce the risk of the electrical supply being interrupted by outages during high winds, thunderstorms or heavy snow or ice storms. An added benefit of undergrounding is the aesthetic quality of the landscape without the powerlines. Undergrounding can increase the capital cost of electric power transmission and distribution but may decrease operating costs over the lifetime of the cables.
History Early undergrounding had a basis in the detonation of mining explosives and in undersea telegraph cables. Electric cables were used in Russia to detonate mining explosives in 1812, and to carry telegraph signals across the English Channel in 1850. With the spread of early electrical power systems, undergrounding began to increase as well. Thomas Edison used underground DC “street pipes” in his early electric power distribution networks; they were insulated first with jute in 1880, and progressed to rubber insulation in 1882. Subsequent developments occurred in both insulation and fabrication techniques:
1925: Pressurized paper insulation used on cables 1930: PVC insulation used on cables 1942: Polyethylene insulation first used on cables 1962: Ethylene propylene rubber-insulated cables become commercially available 1963: Preformed cable accessories become available 1970s: Shrinkable cable accessories become available During the 20th century direct-buried cable became commonplace.
Comparison Aerial cables that carry high-voltage electricity and are supported by large pylons are generally considered an unattractive feature of the countryside. Underground cables can transmit power across densely populated areas or areas where land is costly, environmentally sensitive, or aesthetically sensitive. Underground and underwater crossings may be a practical alternative to crossing rivers. The capital costs of underground power lines are typically significantly higher than for a suspended line along the same route. As an example, in 2024 the Public Service Commission of Wisconsin determined that the installation cost of a 69-kilovolt underground costs $1.5 million per mile, compared to only $284,000 per mile for an equivalent above-ground line. As ratepayers ultimately bear these costs, utilities exercise discretion in selecting which lines to bury.
Advantages Less 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 a much-reduced risk of damage caused by human activity such as theft, illegal connections, sabotage, and damage from accidents. Burying utility lines makes room for more large trees on sidewalks, for environmental benefits and increase of property values.
Disadvantages
Undergrounding is more expensive, since the cost of burying cables at transmission voltages is several times greater than overhead power lines, and the life-cycle cost of an underground power cable is two to four times the cost of an overhead power line. Above-ground lines cost around $10 per 1-foot (0.30 m) and underground lines cost in the range of $20 to $40 per 1-foot (0.30 m). In highly urbanized areas, the cost of underground transmission can be 10–14 times as expensive as overhead. However, these calculations may neglect the cost of power interruptions. The lifetime cost difference is smaller for lower-voltage distribution networks, in the range of 12–28% higher than overhead lines of equivalent voltage. Whereas finding and repairing overhead wire breaks can be accomplished in hours, underground repairs can take days or weeks, and for this reason redundant lines are run. Underground cable locations are not always obvious, which can lead to unwary diggers damaging cables or being electrocuted. Operations are more difficult since underground cables' high reactive power produces large charging currents, making voltage control more difficult. Large charging currents arise due to the higher capacitance from underground power lines and thus limit how long an AC line can be. To avoid capacitance issues when undergrounding long-distance transmission lines, HVDC lines can be used as they do not suffer from the same issue. Whereas overhead lines can easily be uprated by modifying line clearances and power poles to carry more power, underground cables cannot be uprated and must be supplemented or replaced to increase capacity. Transmission and distribution companies generally future-proof underground lines by installing the highest-rated cables while being still cost-effective. Underground cables are more subject to damage by ground movement. The 2011 Christchurch earthquake in New Zealand caused damage to 360 kilometres (220 mi) of high voltage underground cables and subsequently cut power to large parts of Christchurch city, whereas only a few kilometres of overhead lines were damaged, largely due to pole foundations being compromised by liquefaction. As underground repair and check-ups require street digging, they create patches and potholes, leading to bumpy and unsafe rides for cars and bicycles. Utility work also increases lane closure, which leads to traffic jams and increases the cost of resurfacing work by the local government.
Methods
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