Transmission lines are used to transport electricity from places where it is generated to places where it is used. Almost all electricity in North America is generated in fossil-fueled, nuclear-fueled, or hydroelectric generating stations. These are located some distance away from the factories, businesses, institutions, and homes where the electricity is actually used, in some cases hundreds of miles away, so that the electricity must be transmitted from the generating stations to these diverse locations.
Transmission lines are strung between tall, well-spaced towers and are linear features that appropriate long, narrow areas of land. Most transmission lines carry a high voltage of alternating current, typically ranging from about 44 kilovolts (kV) to as high as 750 or more kV (some transmission lines carry a direct current, but this is uncommon). Transmission lines typically feed into lower-voltage distribution lines, which typically have voltage levels less than about 35 kV and an alternating current (in North America) of 60 Hertz (Hz; this is equivalent to 60 cycles of positive to negative per second), and is usually 50 Hz in Europe.
Electrical fields are generated by transmission lines (and by all electrical appliances), with the strength of the field being a function of the voltage level of the current being carried by the powerline. The flow of electricity in transmission lines also generates a magnetic field. Electric fields are strongly distorted by conducting objects (including the human body), but magnetic fields are little affected and freely pass through biomass and most structures. Electric and magnetic fields both induce extremely weak electrical currents in the bodies of humans and other animals. These electrical currents are, however, several million times weaker than those induced by the normal functions of certain cells in the human body.
Transmission lines are controversial for various reasons. These include their poor aesthetics, the fact that they can destroy and fragment large areas of natural lands or take large areas out of other economically productive land-uses, and the belief of many people that low-level health risks are associated with living in the vicinity of these structures.
Aesthetics of transmission lines
Transmission lines are very long, tall, extremely prominent linear features. Transmission lines have an unnatural appearance and their very presence disrupts the visual aesthetics of natural landscapes, as viewed from the ground or the air. As such, transmission lines represent a type of "visual pollution" that detracts from otherwise pleasing natural or pastoral landscapes. These aesthetic damages are an important environmental impact of almost all rural transmission lines. Similarly, above-ground transmission lines in urban and suburban areas are not regarded as having good aesthetics.
Damages to natural values
Apart from difficult crossings of major rivers and mountainous areas, transmission lines tend to follow the shortest routes between their origin and destination. Often, this means that intervening natural areas must be partially cleared to develop the right-of-way for the transmission line. This can result in permanent losses of natural habitat , as typically happens when forests are cleared to develop a powerline right-of-way. Moreover, it is not feasible to allow trees to regenerate beneath a transmission line because they can interfere with the operation and servicing of the powerline.
For these reasons, vegetation is cleared beneath and to the sides of transmission lines (for a width of about one tree-height). This can be done by periodically cutting shrubby vegetation and young trees or by the careful use of herbicides, which can kill shrubs and trees while allowing the growth of grasses and other herbaceous plants.
These sorts of management practices result in the conversion of any original, natural habitats along a transmission right-of-way into artificial habitats. The ecological effects include a net loss of natural habitats and fragmentation of the remainder into smaller blocks. In addition, roads associated with the construction and maintenance of transmission lines may provide relatively easy access for hunters, anglers, and other outdoor recreationalists to previously remote and isolated natural areas. This can result in increased stress for certain wild species , especially hunted ones, as well as other ecological damages.
Some studies of transmission lines have found that hypothesized ecological damages did not occur or were unimportant. For example, naturalists suggested that the construction of extensive hydroelectric transmission lines in the boreal forest of northern Quebec would impede the movements of woodland caribou. In fact, this did not occur, and caribou were sometimes observed to use the relatively open transmission corridors during their migrations and as resting places. Similar observations have been made elsewhere for moose, white-tailed deer, and other ungulates. Predators of these animals, such as wolves and coyotes, will also freely move along transmission corridors, unless they are frequently disturbed by hunters, recreationalists, or maintenance crews.
Certain birds of prey, particularly osprey, may use transmission poles or pylons as a platform upon which to build their bulky nests, which may be used for many years. In some cases the birds are considered a management problem, especially if their nests get large enough to potentially short out the powerlines or if the parent birds aggressively defend their nests against linesmen attempting to repair or maintain the transmission line. Fortunately, it is relatively easy to move the nests during the nonbreeding season and to place them on a "dummy" pole located for the purpose beside the transmission line. In most cases, the ospreys will readily use the relocated nest in subsequent years.
Transmission lines also pose lethal risks for certain kinds of birds, especially larger species that may inadvertently collide with wires, severely injuring themselves. There have also been cases of raptors and other large birds being electrocuted by settling on transmission lines, particularly if they somehow span adjacent wires with their wings.
Disruption of land-use
Some economically important land-uses can occur beneath transmission lines, for example, forestry and some types of agriculture. Other land-uses, however, are not compatible with the immediate proximity of high-voltage transmission lines, particularly residential land-uses. In cases of land-use conflicts, opportunities are lost to engage in certain economically productive uses of the land, a context that detracts from benefits that are associated with the construction and operation of the transmission line.
Many people, including some highly qualified scientists, believe that low-level health risks may be associated with longer-term exposures to the electric and magnetic fields that are generated by transmission lines. These increased risks mean that people living or working in the vicinity of these industrial structures may have an increased risk of developing certain diseases or of suffering other damages to their health.
Although people are routinely exposed to electromagnetic fields through the operation of electrical appliances in the home or at work, those exposures are typically intermittent. In contrast, continuous electromagnetic fields are generated by transmission lines, so longer-term exposures can be relatively high. It must be understood, however, that the scientific knowledge in support of the low-level health risks associated with transmission lines is incomplete and equivocal and therefore highly controversial.
In particular, some studies have suggested that long-term exposure to electromagnetic fields generated by high-voltage transmission lines may be associated with an elevated incidence of certain types of cancers. The strongest suggestions have been for increased risks of childhood leukemia . There is weaker evidence of increased risks of cancers of the lymphatic and nervous systems and of adult leukemia. It must be remembered, however, that not all epidemiological studies of transmission lines have reported these statistical relationships and that the increased risks are rather small when they are found. Studies have also been made of possible increases in the incidences of migraine headaches, mental depression, and reproductive problems associated with longer-term exposures to electromagnetic fields near transmission lines. The results of these studies are inconsistent and equivocal.
Some researchers who have assessed the medical problems potentially associated with high-voltage transmission lines have concluded that it would be prudent to not have people living in close proximity to these industrial structures. Even though there is no strong and compelling evidence that medical problems are actually occurring, the precautionary approach to environmental management dictates that the potential risks should be avoided to the degree possible. It would therefore be sensible for people to avoid living within about 54.5 yd (50 m) or so of a high-voltage transmission line, and they should avoid frequently using the right-ofway as travel corridors.
[Bill Freedman Ph.D. ]
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