Undergrounding is the replacement of overhead cables providing electrical power or telecommunications, with underground cables. This is typically performed for aesthetic purposes, but also serves the additional significant purpose of making the power lines less susceptible to outages during high wind thunderstorms or heavy snow or ice storms. Undergrounding can increase the initial costs of electric power transmission and distribution but may decrease operational costs over the lifetime of the cables.
Telegraph cable undergrounding was considered in Northern Germany as early as 1870.
The 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 or areas where land is costly or environmentally or esthetically sensitive. Underground and underwater crossings may be a practical alternative for crossing rivers.
Other advantages include:
- Less subject to damage from severe weather conditions (mainly lightning, wind and freezing)
- 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.  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.
- Much less subject to conductor theft, illegal connections, sabotage, and damage from armed conflict.
- 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 foot and underground lines cost in the range of $20 to $40 per foot. In highly urbanized areas the cost of underground transmission can be 10-14 times as expensive as overhead.
- 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 the high reactive power of underground cables produces large charging currents and so makes voltage control more difficult.
- 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.
The advantages can in some cases outweigh the disadvantages of the higher investment cost, and more expensive maintenance and management.
Most electrical power in Japan is still provided by aerial cables. In Tokyo's 23 wards, according to Japan's Construction and Transport Ministry, just 7.3 percent of cables were laid underground as of March 2005.
The UK regulator Office of Gas and Electricity Markets (OFGEM) permits transmission companies to recoup the cost of some undergrounding in their prices to consumers. The undergrounding must be in National Parks or designated Areas of Outstanding Natural Beauty (AONB) to qualify. The most visually intrusive overhead cables of the core transmission network are excluded from the scheme. Some undergrounding projects are funded by the proceeds of the national lottery.
All low and medium voltage electrical power (<50 kV) in the Netherlands is now supplied underground. Other EU countries such as the UK and Germany are undergrounding a portion of their cables each year.
In the United States, the California Public Utilities Commission (CPUC) Rule 20 permits the undergrounding of electrical power cables under certain situations. Rule 20A projects are paid for by all customers of the utility companies. Rule 20B projects are partially funded this way and cover the cost of an equivalent overhead system. Rule 20C projects enable property owners to fund the undergrounding.
A compromise between undergrounding and using overhead lines is installing air cables. Aerial cables are insulated cables spun between poles and used for power transmission or telecommunication services. An advantage of aerial cables is that their insulation removes the danger of electric shock (unless the cables are damaged). Another advantage is that they forgo the costs - particularly high in rocky areas - of burying. The disadvantages of aerial cables are that they have the same aesthetic issues as standard overhead lines and that they can be affected by storms. However if the insulation is not destroyed during pylon failure or when hit by a tree, there is no interruption of service. Electrical hazards are minimised and re-hanging the cables may be possible without power interruption.
- page 42 édition de: 1870/02/25 (VOL1,N4) Titre : Journal télégraphique Éditeur : [s.n.?] (Berne) Date d'édition : 1870 Type : texte,publication en série imprimée Identifiant : ark:/12148/cb32802376k/date Source : Bibliothèque nationale de France, département Sciences et techniques, 4-V-1216 Relation : http://catalogue.bnf.fr/ark:/12148/cb32802376k Provenance : bnf.fr Date de mise en ligne : 25/05/2009 http://gallica.bnf.fr/ark:/12148/bpt6k5650101k
- "To curb power theft, Maharashtra explores underground supply network across state". DNA India. Retrieved 20 July 2015.
- Edison Electric Institute - Underground Vs. Overhead Distribution Wires: Issues to Consider
- Los Angeles Department of Water & Power System Development Division cost estimate data
- Should Power Lines be Underground?
- City of San Diego Undergrounding Program
- San Francisco Utility Undergrounding
- Wellington New Zealand - Undergrounding
- London England - Undergrounding for 2012 Olympics
- US lobby group - Scenic America
- UK lobby group - "REVOLT"