Transmission of electricity and power lines Transmission of electricity and power lines
Transmission of electricity and power lines

Distribution lines and substations

Distribution lines and substations

Distribution lines cause significantly smaller electric and magnetic fields than transmission lines.

Under the medium voltage lines (20 kV), the maximum electric field intensity is about 0.1 kV/m and under the low voltage lines (400 V), less than 0.1 kV/m. The maximum magnetic flux density under the medium voltage lines is about 1 μT and it decreases below 0.4 μT at about 5 metres from the centre line of the power line. Under low voltage lines, the magnetic flux density is less than 0.1 μT.

Distribution substations

The distribution substation (transformer) converts the 20 kV medium voltage of the distribution network down to 400 volts for consumers. There are approximately 130,000 distribution substations in Finland. Most of these are pole-mounted transformers in rural and sparsely populated areas. In urban areas, the substations are windowless small buildings made from concrete, masonry or metal. They are usually located in parks or other available spaces. In a tightly built area, where there is no room for a separate transformer building, the distribution substation is placed in the basement or ground floor of a block of flats. There are about 9,000 substations like this in Finland, about 2,800 of which are estimated to be located in the vicinity of an apartment, either below or adjacent to it.

Large currents flow through the distribution substations, which generate strong magnetic fields around them. However, at the distance of five metres, the magnetic field weakens to a level of about 0.1 µT. A pole-mounted transformer does not cause exposure to the general public as it is located at a height of a few metres. Neither does a substation built in a park cause exposure to a magnetic field at a distance exceeding five metres. In practice, the general public may be exposed to magnetic fields of distribution substations only in areas above or adjacent to substations located in blocks of flats.

Park transformer substation.

The magnetic field of a substation built in a park usually weakens to a level of 0.1 μT at a distance of 5 metres from the transformer.

The largest magnetic fields in residential premises, a few tens of microteslas, have been measured in apartments which have an old building transformer underneath them, with busbars or cables installed near the ceiling of the transformer room. The number of such building transformers is gradually decreasing as they are being replaced by transformers that cause a much weaker magnetic field in the area above them.

Transformer substation in an apartment building.A building transformer with busbars or cables running in the ceiling may cause a magnetic field larger than normal in the rooms above it. Other types of building transformers generate much weaker magnetic fields.

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