Static fields

In the 19th century, at the dawn of the electric era, high voltage AC transmission system (50/60 Hz) was chosen at the expense of DC transmission (0 Hz) (see Electricity network). Nevertheless, high voltage DC is used to transmit electrical energy over long distances, including under the sea, and to interconnect different frequency networks (eg in Japan, for interconnection between 50 Hz network in the north and 60 Hz network in the south).

And the situation continues to evolve. Indeed, the need to interconnect countries to ensure trade in electricity (eg interconnection Nemo Link® in the North Sea and interconnection ALEGrO) and the relocation of electricity production plants (eg wind farm in the North Sea) involve the development of electrical connections on long distances, which are more effective in DC. Moreover, in Belgium for example, trains, excluding high speed train, also operate in DC.

Note: Detailed information on the challenges of transport in both AC and DC power are available at Electricity network.

Static fields

A static field is a field that does not change its sense, unlike fields of electricity network (50/60 Hz) and radiofrequency fields (GHz). This field is either electric (EF) or magnetic (MF).

The intensity of the static EF depends on the voltage, while the intensity of the static MF depends on the force of a magnet or the intensity of the flowing current.

Static electric field (or electrostatic)

There is a natural static electrical field at the surface of the earth. It is created by the potential difference between the upper atmosphere (the ionosphere, positively charged) and earth (negatively charged). In calm weather, this electric field is of the order of 100 to 150 V/m, but during storms it can reach 15 to 20 kV/m (15 000 to 20 000 V/m).

Static EF is also the basis of electrostatic discharges that we sometimes feel when getting out of a car by cold and dry weather conditions, for example. These discharges are due to the generation of an electric field between the car and the body, a hand for example. If the field is sufficiently high, i.e. if it exceeds the air-breakdown field (approximately 300 kV/m, depending on the humidity, pollution…), air becomes electrically conductive and the charges accumulated by the car will instantaneously flow to the ground. Static EF at the hand reaches values above the air-breakdown field.

Static EF that can be measured under DC transmission lines or railway lines are respectively close to 20 to 30 kV/m and 600 V/m. These are maximum values measured in the absence of any obstacle because EF is easily reduced.

In a DC train, a static EF close to 300 V/m can be measured (Source: ICNIRP).

Static magnetic field

Static MF are typically those measured between the two poles of a magnet. Among the magnets usually used, intensities are around 10 mT (10 000 µT).

Our earth is a big magnet that, thanks to the force lines of the static MF between its two poles, protects us from radiations coming from space. The geomagnetic field is about 45 µT in Belgium. Among medical devices, MRI uses a super magnet whose intensity can vary between 1.5 and 10 T (1 500 000 et 10 000 000 µT).

Nearby DC system, static MF of tens of µT can be measured under the electricity transmission lines and about 200 µT under a railway line. When we move 5 m away, lower values are measured: 10 µT for the transmission lines and about 100 µT for train lines (see our FAQ on static fields).

In a train, static MF is around 40 µT. Maximum values of 120 µT have been measured in DC powered locomotives (3 kV DC as in Belgium, measurements carried out in Russian and Italian trains).

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