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Exposure to electric and magnetic fields from high-voltage lines: overhead lines & underground cables
Whether in the household, at work or on-the-go - wherever electricity is generated, transmitted or used, we may be exposed to electric and magnetic fields. High- and extra-high voltage lines, which serve to transport and distribute electricity, contribute to electromagnetic field exposure.
Electric and magnetic fields may occur in the vicinity of direct and alternating current lines. Usually, however, most electromagnetic field exposure comes from house wiring and electrical appliances running on low voltages. This is important: the greater the distance from high- or extra-high voltage lines, electrical appliances and house wiring, the less they contribute to total exposure.
Electric fields are shielded well by soil and ordinary building materials. For this reason, they are irrelevant for underground cables but do occur in the vicinity of overhead lines.
The electrical field strength depends especially on the operating voltage of the line. Field strengths which exceed the limit value for low-frequency electric fields may occur under 380 kV AC-overhead lines (extra-high voltage lines). This limit value is only obligatory for places where people may stay for extended periods such as residential estates or school yards. The intended use of a place is decisive in this regard. In the case of high- and medium-voltage lines, the limit value is usually complied with even directly below the lines. The limit value does not apply to low-voltage lines; the electric field strengths, however, are low because of the low voltage.
Direct current lines produce static electric fields. Unlike the low-frequency fields produced by alternating current, they are approximately constant in time and do not alternate direction continually. Long high voltage direct current installations are still in the planning stage in Germany. Measurement values for the vicinity of the lines are not available yet.
Magnetic fields occur around overhead lines and underground cables. They are not shielded by soil or building materials and therefore penetrate buildings and also the human body.
Magnetic fields are produced when current flows. As the magnetic field strength depends on the current, the field strengths vary with the currents in the lines. That is why the magnetic field around a line is stronger when – at certain times of day - a lot of current is used or transferred. The highest field strengths can be found under overhead lines and above underground cables. They decrease significantly with lateral distance from a transmission line route.
With overhead lines the field distribution depends mainly on the tower height as well as on the sag and the arrangement of the conductors. The sag of the conductors is, among other things, determined by the distance to neighbouring towers along the transmission line (span length) and by the quantity of power transmitted: The more current flows, the warmer the lines get. In the process, they expand and their sag increases. The same effect occurs in summer with high temperatures. In winter, ice on the lines may cause an increase in sag. The reduced distance to the ground may then result in increased field strength values.
Direct current lines produce static magnetic fields. Unlike the low-frequency fields produced by alternating current, they are approximately constant in time and do not alternate direction continually.
Study: exposure to magnetic fields
In a study on the "Assessment of the Bavarian citizens' exposure to low frequency magnetic fields", the Federal Office for Radiation Protection (BfS) has found that individuals who - according to their own information - live within 100 metres of a high-voltage line were only exposed to slightly (about 10 per cent) higher fields than the other study participants. In the process, the exposures were recorded and averaged over 24 hours.
Comparison of electric and magnetic fields from overhead lines and underground cables
In a study completed in 2009, the BfS commissioned the measurement of field intensities in the vicinity of alternating current overhead lines and underground cables at high- and extra-high voltage level. The highest magnetic field strengths were measured under 380 kV overhead lines and above 380 kV underground cables. At 1,0 metre above ground level the field strengths were 4.8 (overhead line) and 3.5 (underground cable) microteslas (µT).
Data on the current flow at the time of measurement were gathered from the transmission grid operators and the measured field strengths were additionally extrapolated to the situation that may occur during maximum current flow. For the examined installations, the limit value of 100 microteslas measured at a height of 1 metre above ground level was also complied with under the aforementioned condition.
Compared to overhead line routes, the magnetic fields from underground cables drop off significantly earlier and more rapidly with increasing distance from the centre line of the transmission line route as the adjacent figure shows
Long high voltage direct current lines are still in the planning stage in Germany. Results of measurements taken in the vicinity of the lines at places, which are accessible to the general public, are not available yet but are expected to be well below the German limit of 500 microteslas. The limits for magnetic fields of high voltage direct and alternating current lines deviate from one another because of the different effects of static and low-frequency fields.
State of 2017.10.23