Electricity networks generate electric and magnetic fields
Electricity is transmitted from power plants to consumers through the electricity networks. They consist of various electrical lines and cables, substations and distribution substations, and switchboards. Electricity is transmitted over long distances at high voltages and currents in the main grid and regional networks. Distribution takes place at lower voltages and currents in local distribution networks. Transmission and distribution generate electric and magnetic fields. Since the electric field depends on voltage and the magnetic field depends on current the main grid and regional networks generate higher fields than the distribution networks.
Finland's electricity networks are part of the Nordic electricity system. There are two direct current and alternating current links to Sweden, an alternating current link to Norway and two direct current links to Estonia. Electricity imported from neighbouring countries and generated by power plants is transmitted to throughout Finland in the main grid managed by Fingrid Oyj and in regional networks managed by local electricity network companies. Electricity is transmitted over overhead lines with a voltage of more than 100-kilovolt (1 kilovolt (kV) = 1,000 volts (V)), i.e., power lines, because energy losses in transmission are less at high voltages than at low voltages. In addition to power lines, electricity transmission networks have substations that distribute the transmission over the different lines and can be used also to convert the voltage between the different parts of the electricity network.
The high voltage of power lines is converted at local substations to the 20 kV medium voltage typically used in local distribution networks. The medium voltage is further converted to a low voltage (400 V) suitable for electrical equipment at distribution transformers located near consumers. The low voltage is distributed through the switchboards to the buildings' electricity networks.
Mostly overhead power lines are used in the main grid and regional networks, and as large structures, these power lines are the most visible part of the electricity network in the environment. At the end of 2021, there were approximately 5,200 km of 400 kV overhead lines, approximately 1,400 km of 220 kV overhead lines and approximately 14,750 km of 110 kV overhead lines. In addition, there were approximately 330 km of 110 kV underground cables, which are used in urban areas instead of overhead lines. In the near future, 400 kV underground cables will also be installed in urban areas. The direct current links from Sweden and Estonia have been established with submarine cables and with 400, 450 and 500 kV overhead lines of total length of approximately 60 km in Porvoo and Rauma.
In the distribution networks overhead lines are used in sparsely populated areas and underground cables in urban areas. At the end of 2021, there were approximately 89,000 km of medium-voltage overhead lines and approximately 66,000 km of underground cables. There were 112,000 km of low-voltage overhead lines and 144,000 km of underground cables. This means that the share of underground cabling was 42% in the medium-voltage networks and 56% in the low-voltage networks. To reduce power cuts caused by storms, underground cables are increasingly used in the distribution networks also outside urban areas.
An overhead line generates an electric and magnetic field in its vicinity, but an underground cable generates only a magnetic field. The highest electric and magnetic fields in overhead lines, approximately 10,000 volts per metre (V/m) and 10 microtesla (µT), are under 400 kV power lines. The fields decrease rapidly as the distance from the conductors increases. The electric field is attenuated in trees and bushes and in the structures of houses and does not penetrate into buildings as the magnetic field does. An underground cable generates a higher magnetic field above the ground than an equivalent overhead line but it decreases much faster than the magnetic field of an overhead line as the distance increases. The magnetic field of an underground cable decreases but the magnetic field of an overhead line increases as the height above the ground increases.
Power lines are overhead lines with an alternating voltage of 110, 220 or 400 kilovolts. Power lines appear in the landscape as large structures and require a wide corridor free of trees or buildings.
Power lines are best distinguished from lower-voltage lines by their poles and insulators. The higher the voltage, the taller are the poles and the longer are the insulators. The poles are always made of steel for 400 kilovolt (kV) lines. For other power lines, they are usually wooden, but steel where wooden poles are not durable enough, for example, as corner poles and free-standing tall poles. The easiest way to identify the voltage on a power line is to look at the length of the insulator chain suspending on the cross arm of the pole and the number of insulating discs in the chain. There are less than 10 discs in a 110 kV line, approximately 10 discs in a 220 kV line and approximately 20 discs in a 400 kV line. In distribution networks the insulators on medium-voltage lines are the size of a jug and those on low-voltage lines the size of a coffee mug. They are not suspended on the cross arm but are attached to it or the pole.
A variety of pole and conductor structures is used for power lines. The most common type of pole is the so-called portal pole which has a cross arm and is supported by steel cables. The conductors are at the same height from the ground. Free-standing portal poles without supporting cables are used in the fields which facilitates field work in the vicinity of the poles. Free-standing steel poles are also used to save space in urban areas. These are often double-line poles with two current circuits, in other words, six current conductors. The three conductors of the same circuit are at different heights from the ground. When high power is transmitted, two or three sub-conductors are used for each conductor. At the top of the poles, there are two currentless conductors, the so-called thunder ropes, to protect the power line from lightning strikes.
The power line area includes the right-of-way and border zones on both sides of it. The owner of the power line has a limited right to use the power line area. The owner may clear the right-of-way, restrict construction in the power line area and limit the growth of trees in the border zones. Heated buildings must not be built in the right-of-way. The photo below shows the typical widths of the right-of-way and border zones.
The power line area includes the right-of-way and border zones on both sides of it. The width of the border zone is usually 10 m. The width of the right-of-way is 26–30 m for a 110 kV power line, 32–38 m for a 220 kV power line and 36–42 m for a 400 kV power line.
The substation distributes the transmission of electricity to the different lines and can also convert the voltage between the parts of the electricity network. Due to the high voltages at the substation, a person can be fatally electrocuted by a live part several metres away. Therefore, the substation is surrounded by a fence to ensure that only well-trained and properly equipped electrical workers have access. Outside the fence surrounding the substation, the electric and magnetic fields are not generated by the substation equipment but by the power lines entering the substation.
Transmission line area: https://www.fingrid.fi/en/grid/maintenance/transmission-lines/transmission-line-area/
Power lines generate the highest electric and magnetic fields of overhead lines. The maximum electric field can be about 10 000 V/m and the magnetic field about 10 microtesla (µT) under a 400 kV power line. The fields decrease rapidly with increasing distance from the power line. Trees, bushes and building structures attenuate the electric field but not the magnetic field. Thus, the electric field does not penetrate houses like the magnetic field does. At a distance of about 100 metres from the power line, the magnetic field is already lower than the magnetic field of about 0.1 µT generated by the house's electrical wiring and equipment. In addition to the voltage and current of the power line, the location and phasing of the current conductors will affect how strong the electric and magnetic fields are under the power line and how quickly they decrease as the distance from the power line increases.
Under a 400 kV power line, the electric field can have a direct effect on the human body, which can be felt as a vibration of the hairs on the skin caused by the surface electric charge. An indirect effect is a spark when a metallic object insulated from the ground, such as a car body, is touched under the power line. In addition, vibration may be felt when metal parts such as an umbrella or bicycle are touched under the power line. These effects can be annoying but not dangerous. Pacemakers and other active medical implants may be disturbed under the power line. Thus, it would be advisable to go under power lines near poles, where fields are at their lowest. Long-term exposure to the electric field of power lines has not been shown to cause any adverse health effects.
The magnetic field causes no direct effects on the human body under or near a power line. On the other hand, long-term exposure to magnetic fields has been suspected to have the potential to cause adverse health effects. Over the past few decades, dozens of population studies have been conducted on people living near power lines. Some of these studies have shown that the risk of leukaemia in children is slightly increased after long-term exposure to magnetic fields with an average greater than 0.4 µT. Even these studies have not shown that leukaemia was a consequence of exposure to magnetic fields. Cellular and animal studies have not provided results to support this finding, and no mechanism is known by which such a low magnetic field would cause leukaemia or other cancers and diseases. Recent population studies have not provided significant new information on the previously observed risk, so there is still no certainty about the long-term effects of magnetic fields.
Due to scientific uncertainty, the Radiation and Nuclear Safety Authority recommends that new power lines or residential buildings should be built so that the long-term average magnetic field from power lines would not exceed 0.4 µT in areas where children are permanently present. In view of the concerns raised by the uncertainty, it would also be advisable to avoid locating schools and kindergartens near power lines.
Electricity consumption is increasing, which is why it is necessary to strengthen the transmission network by building new power lines. The aim is to place them in old line corridors in order to protect the environment. When planning new power line corridors, urban areas will be taken into account and attempts will be made to circumvent them in order to minimize the number of residential buildings that are located in the power line area and thus must be redeemed.
Even under the largest power lines and in their vicinity, there exist no such electric and magnetic fields to which exposure is restricted by legislation. Therefore, the legislation does not prevent the construction of new power lines near residential buildings in terms of these fields. The legislation protects people from the direct effects of the fields. Some epidemiological studies have suggested that long-term exposure to the magnetic field would cause possible harm to health at a much lower level than restricted by the legislation for the exposure of general public. Because the long-term effects of the magnetic field are not known, the Radiation and Nuclear Safety Authority (STUK) recommends the construction of new power lines so that the long-term average of the magnetic field is not more than 0.4 µT in residential buildings near the power line if it is possible with reasonable measures.
When new power lines are built in the old line corridors, the corridors will become wider and the new lines will be closer to residential buildings than the existing lines. As electricity consumption increases, so do conductor currents, and residential buildings are exposed to higher magnetic fields. In practice, it can sometimes be very difficult or even impossible to comply with STUK's recommendation with reasonable measures. As there is no scientifically confirmed evidence of the harmful health effects caused by the long-term exposure to the magnetic field, new power lines can be built without following STUK's recommendation.
Unlike power lines high-voltage underground cables can be installed even at a distance of a few metres from residential buildings in accordance with STUK's recommendation. The magnetic field on the ground level generated by an underground cable is at its highest directly above the cable and decreases to the side much faster than the magnetic field of a power line. In addition, it decreases but the magnetic field of the power line increases upwards from the ground.
Magnetic fields from power lines should be assessed when planning new housing, schools or nurseries near power lines. Although there is no conclusive scientific evidence of the health effects of magnetic fields from power lines, the proximity of a power line can be a cause for concern. Therefore, as recommended by the Radiation and Nuclear Safety Authority (STUK), residential buildings should be sited so that the long-term average magnetic field generated by power lines does not exceed 0.4 microtesla.
The appearance of a power line does not indicate how quickly the magnetic field decreases with increasing distance. It depends on the currents and phases of the current conductors and the position of the conductors on the poles. At the design stage, it may be necessary to estimate the magnetic field generated by the power line in its vicinity by calculation. The calculation is based on technical information on the power lines obtained from the power line operators. If there is any doubt about the magnetic field, the municipality should ask STUK for a statement. This will include a calculation of the magnetic field generated by nearby power lines on planned residential buildings.
The electricity distribution network consists of medium-voltage overhead lines and underground cables, typically 20 000 volts (V), distribution transformers and low-voltage 400 V overhead and underground cables. The electric field is less than 100 volts per metre (V/m) under medium voltage lines and even less under low voltage overhead cables. The magnetic field is generally less than 0.1 microtesla (µT) and very rarely close to 1 µT under medium voltage lines. Under low voltage overhead cables, the magnetic field is less than 0.1 µT.
The magnetic fields generated by underground cables in the distribution network are small. The magnetic field generated by a distribution network underground cable installed in the ground at a depth of more than half a metre is less than 0.5 µT at ground level. It decreases rapidly with increasing distance from the cable.
The distribution transformer converts the medium voltage of the distribution network into a low voltage of 400 V for normal electricity users. There are about 130 000 distribution transformers in Finland. In rural and semi-rural areas, the transformer is mounted on a pole, while in urban areas it is mounted in concrete, brick or metal windowless cubicles in parks or alongside streets. In densely built-up urban areas, where there is no space for a transformer room, the distribution transformer is located in the basement or on the first floor of the building. There are about 9 000 such so-called 'real estate' transformers in Finland, of which about 2 800 are estimated to be located close to, below or next to residential dwellings.
The currents of the low-voltage conductors emitted by distribution transformers can be high, creating a relatively strong magnetic field in the vicinity of the conductors. However, at a distance of only five metres from the conductors, the magnetic field is reduced to about 0.1 µT. A transformer in a pole-mounted transformer located a few metres above the ground will therefore generate a magnetic field of about 0.1 µT at ground level. The magnetic field from the transformer mounted in a cubicle also decreases to this level at a distance of about 5 m from the wall of the transformer cubicle. In practice, the population can only be significantly exposed to the magnetic field of an old property transformer located below or adjacent to a residential room. In this case, a significant magnetic field may only be present in part of the room closest to the transformer.
Magnetic fields of a few tens of microteslas have been measured in residential rooms with an old property transformer underneath, with low-voltage busbars running close to the transformer room roof. There are no known cases where the legal limit of 200 µT has been exceeded. The old property transformers are reaching the end of their life cycle and will be replaced by new transformers that will create a much lower magnetic field in the space above or adjacent to them. The legislation provides for a transition period for property transformers until November 2033. At the latest by then, the magnetic field they generate in residential rooms must not exceed 200 µT.
The electrical network of a building consists of a main electrical switchboard, fuse boxes and electrical wiring. Their voltages and currents are low, and thus they generate only weak electric and magnetic fields. In some buildings in urban centres a distribution transformer, the so-called indoor transformer, may be located on the ground or basement floor of a building. In the vicinity of its low-voltage conductors, a relatively strong magnetic field may exist.
In general, the fields caused by the electrical network of the building are very low in Finnish homes. The background level of the electric field is approximately 10 volts per metre (V/m) and that of the magnetic field just under 0.1 microtesla (µT). A dwelling above an indoor transformer may typically have a magnetic field of a few microteslas at most, very rarely tens of microteslas. It is only present in part of the room and decreases rapidly from the floor upwards and from the maximum point to the side. A large main electrical switchboard, such as the main electrical switchboard of an apartment block, may generate a magnetic field of a few microteslas in its vicinity. However, this is reduced to a typical background level at a distance of only a few metres from the switchboard. The magnetic field from a single main electrical panel or fuse box in a single dwelling will be reduced to background levels at a distance of less than one metre.
Magnetic fields can also be higher than the background level of 0.1 µT in the vicinity of metal structures if the building has an old electrical installation. It uses only four conductors, a neutral and three phase conductors, so that all the electrical current entering the building returns to the supply point as stray currents through the neutral conductors and the metal structures connected to them. These currents thus create magnetic fields in the vicinity of the metal structures. In a modern electrical installation, a fifth conductor acts as a return conductor and eliminates the stray currents and the magnetic fields they generate.
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