Electric and magnetic fields
Computers, mobile phones, television sets, radios, microwaves and hairdryers all make our day-to-day lives easier. However, they also generate electromagnetic fields. Elia’s high-voltage facilities generate extremely low frequency (ELF) electric and magnetic fields and as such are also governed by applicable legislation. As the system operator, one of Elia’s responsibilities is to address the potential impact of its infrastructure.
The issue of electric and fields has long prompted many questions. In this section we look at the main topics, but for a more in-depth examination take a look at our brochure entitled Electromagnetic fields and the high-voltage grid and the accompanying factsheets.
What are electric and magnetic fields?
An electric field is created naturally by electrical charges high up in the atmosphere. At ground level such fields are generally weak but in storms they become considerably stronger. Everybody is familiar with the magnetic field of the Earth thanks to compasses, in which a magnet points to the north under the influence of the geomagnetic field.
The term ‘field’ is used in physics to describe the impact of an object on its environment. An electric field is the force of attraction or repulsion that is exercised by electric charges on each other. A magnetic field is the force exercised by a moving electric charge.
When a light is connected to the power grid, there is always an electric field, even when the light switch is off and no power is being supplied to the light. When the light is on, i.e. when power is flowing through the cable, this generates a magnetic field as well as an electric field.
The geomagnetic field changes very slowly and so is considered a constant field. However most electric and magnetic fields, including natural ones, fluctuate quickly and on a regular basis. They are known as alternating fields and have a specific field strength and a frequency.
The power grid, from the high-voltage and distribution networks to the low voltage in our homes, generates fields with a frequency of 50 Hz (50 cycles per second) and is classified as ‘extremely low-frequency’ (ELF). By way of comparison, mobile-phone frequencies range between 900 MHz and 1,900 MHz (million hertz).
Electric and magnetic field strength through the high-voltage grid
The electric field changes with the voltage (V). The higher the voltage, the stronger the electric field it generates. The electric field strength is expressed in volts per metre (V/m).
The magnetic field changes with the current (A). The greater the current, the stronger the magnetic field that is generated with it. The units for measuring magnetic fields are amperes per metre (A/m) but we generally use teslas (T), i.e. the unit for expressing magnetic flux density. The magnetic fields we usually measure are expressed in microteslas (μT). A microtesla is one millionth of a tesla.
The strength of the electric field along a high-voltage line depends on the voltage and the distance from the line. Under a 380,000 V line the average electric field 1.5 metres above the ground will be 4 kV/m. This field quickly weakens as you move away from the line. Buildings and vegetation can weaken an electrical field substantially: inside a home, it can be between ten and 100 times weaker than outside. For lines with a lower voltage (220, 150 and 70 kV), the electric field will be considerably weaker.
The magnetic field generated by a high-voltage line will depend on the amount of the electricity (i.e. the capacity) flowing through the line. Likewise, in the same way as an electric field, the magnetic field is also affected by the location of the conductors and the distance from the line.
The figure above plots the typical values for a magnetic field beneath idle 380-kV high-voltage lines and shows clearly that the strength of the field is directly proportional to its distance from the line. The field’s exposure value varies by location and time depending on the voltage, the electricity passing through, and the height of the line.
Since the magnetic field is not determined by voltage, a high-voltage connection with a higher voltage will not necessarily generate a stronger magnetic field. However, in practice the strongest magnetic fields are recorded in the vicinity of 380-kV lines. After all, the higher the voltage, the greater the line’s transmission capacity and thus the greater the volume of electricity being carried through it. Magnetic fields beneath high-voltage lines with a voltage of less than 380 kV are generally no greater than 4 μT and become steadily weaker the further away from the line they are measured. In the case of 150-kV high-voltage lines, the average magnetic field is a little lower at approximately 1.5 μT. At a distance of 30 metres from the line, the magnetic field will still measure approximately 0.2 μT. Finally, beneath 70-kV high-voltage lines, 70 kV being the lowest voltage of any of Elia’s lines, the magnetic field is ≤ 1 μT. At a distance of 20 metres away, the magnetic field barely registers at all (< 0.1 μT).
Underground cables do not produce electric fields since the latter are blocked by metal cladding around the conductors.
However, laying cables underground does not weaken magnetic fields since underground cables generate magnetic fields which can be considerably stronger than those beneath overhead lines. However, their strength tails off more quickly the further away they are located.
The figure above shows the difference between magnetic fields beneath an active 150-kV line (1.5 m above ground) and a 150-kV underground cable (ground level). The strongest field recorded just above an underground cable may be double the strength of one beneath an overhead line, however, it will tail off much more quickly. Accordingly, the field barely registers at all at a distance of 10 metres from the cable route.
After over 30 years of research, scientists have yet to find formal evidence that exposure to ELF magnetic fields poses a health risk. However, neither have researchers been able to rule out any such risk. As such, many scientists are still working to determine whether exposure to magnetic fields is harmful in either the short or long terms.
Epidemiological studies have long suggests that there is a small – but nonetheless not insignificant – statistical correlation between long-term exposure to low-frequency magnetic fields on the high-voltage grid and an increased risk of leukaemia in children. We are talking here about exposure in the home over a long period to magnetic field strengths averaging in excess of between 0.3 and 0.4 μT.
Extensive testing of animal and cell cultures since the 1980s has been unable to confirm this theory and as such no causal link has yet been established between exposure to magnetic fields and an increased risk of leukaemia in children. However, in the absence of any explanation for the statistical link highlighted by epidemiological studies, neither has any study yet been able to rule it out.
In June 2001 the International Agency for Research on Cancer (IARC) classified low-frequency magnetic fields as “possibly carcinogenic” (Group 2B). This classification was based on epidemiological analyses that showed a statistical relationship between leukaemia in children and exposure to high average magnetic-field strengths.
The International Commission on Non-Ionising Radiation Protection (ICNIRP) is an internationally recognised body that draws up recommendations to protect workers and the public from the harmful effects of non-ionising radiation. In 1998 the ICNIRP published its guidelines on electric and magnetic fields, which – based on proven acute effects – introduced a maximum of 100 µT for public exposure to low-frequency magnetic fields. When these recommendations were updated in 2010, the relevant reference level was increased to 200 µT. As far as long-term effects such as childhood leukaemia are concerned, the ICNIRP states that in spite of the statistical relationships that have been established they are not included in the recommendations because no experimental studies had as yet corroborated this link or shown any causal link.
Standardisation and recommendations
In 1999 the European Union published a Recommendation on the limitation of exposure of the general public to electromagnetic fields. Like the ICNIRP, it recommended a limit of 100 μT for public exposure. Following the IARC classification of low-frequency magnetic fields as possibly carcinogenic in 2001, the scientific state of the art was re-assessed. However, no reason was found to revise the 1999 recommendations.
Belgium has no federal (i.e. national) legislation on very low-frequency magnetic fields. This means that the benchmark is provided by the Recommendation issued by the Council of the European Union, i.e. a maximum exposure limit of 100 μT. For electric fields the General Regulations on Electrical Equipment (AREI/RGIE) sets maximum levels between 5 kV/m (residential areas) and 10 kV/m.
Based on the sectoral conditions in the Walloon and Brussels-Capital Regions, the same 100 µT limit applies when operating power transformers.
In the Flemish Region, the Decree establishing measures to combat the health risks caused by pollution of building interiors has been in force since 2004. Under the Decree all those who are responsible for building, maintaining or fitting out homes or buildings that are open to the public must do everything they can to minimise the health risks affecting the interior of these buildings for the sake of the residents or users. To this end, target values and intervention values are given for 26 chemical and physical factors (benzene, particulate matter, temperature, etc.), including low-frequency magnetic fields. In the case of magnetic fields, the target value is 0.2 µT and the intervention value 10 µT.
Elia has been making an active contribution to broadening scientific knowledge for many years. It also supports a number of research centres and universities within the Belgian BioElectroMagnetics Group (BBEMG) whose scientific independence is enshrined in a cooperation agreement.
Elia has also concluded a research contract with the Electric Power Research Institute (EPRI), an agreement granting Elia access to the results of international research studies in this field.
Elia’s top priority is transparency and if requested to do so by a local community, it will arrange to take readings of electromagnetic fields free of charge. To schedule measurements in the vicinity of our facilities, please contact the Technical Department at firstname.lastname@example.org.