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Berlin Environmental Atlas

08.05 Electromagnetic fields

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Natural Fields

Static electric and magnetic fields (constant fields) of a significant field strength have always existed on this planet.

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Fig. 4: Natural Electric (Direct) and Magnetic (Constant) Fields

The movement of air in the atmosphere and the ionising effect of cosmic radiation in the higher regions, the ionosphere, create a field of direct electric current between the surface of the Earth and the ionosphere. Under normal weather conditions, the field strength near the ground is around 100-500 V/m, whereas it can rise to 20,000 V/m (20 kV/m) during storms. Alternating currents at frequencies used in energy supply are practically non-existent. The natural background field strength at 50 Hz is only 0.1 mV/m.

The static magnetic field is familiar because of the way it affects a compass needle. It is almost constant over time and measures about 42 µT in Germany. This constant field is created by circular action in the Earth's core. Extremely high field strengths can occur in the vicinity of lightning (up to 1 T, which can cause heart failure in humans). Small variations in flux density are induced by the solar wind, which distorts the earth's magnetic field due to its streams of charged particles. Furthermore, global storm activity also results in high-frequency components within the magnetic field. However, these are so small that at 50 Hz the alternating field component is merely 10-6 µT (WHO 1984).

Technical Fields

At low frequencies technical field sources tend to be much stronger than naturally occurring fields. Most of these are either power supply installations, which generate and distribute electricity, or the technical entities which consume that energy. This includes industrial plants, private installations and consumer devices (e.g. household gadgets) and public transport systems (e.g. underground and railroad).

In addition to field emissions from large-scale technical plants, people are surrounded today, both at home and at work, by a multitude of sources of electric and magnetic fields which, if taken together, may be generating cumulated field strengths greater than those of the aforementioned technical plants. Field strength will depend primarily on the distance from the device in question and on its technical make-up, which accounts for a strong scattering of values for individual types of apparatus. The legend to the map lists the field strengths of electrical devices at normal usage distance ("Typical values for the magnetic flux densities of household devices at varying distances"). Comparison with the field strengths of high overhead voltage lines, also included in the map, shows that the field strengths of ordinary household gadgets are, indeed, often higher.

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