Power lines produce electric and magnetic fields

Power lines produce electric and magnetic fields

A power line generates an electric field around it, the magnitude of which depends on the line voltage. The electric field weakens rapidly with distance. In addition, vegetation and building materials absorb it effectively. Thus, the electric field does not penetrate the walls of buildings. The electric current flowing through a power line creates a magnetic field around it, the magnitude of which depends on the load on the line, i.e. how much current flows through the transmission line at that moment. Vegetation and building materials do not significantly attenuate the magnetic field, which thus penetrates into buildings.

The unit of electric field strength is volt per metre (V/m), and in the case of transmission lines usually kilovolt per metre (kV/m). The strength of the electric field on the ground close to a power line depends, in addition to voltage, also on the mutual location and height from the ground of phase conductors. The closer to the ground the conductors are, the higher the electric field intensity at a given line voltage. Beneath a 400 kV power line, the electric field strength reaches a maximum of 10 kV/m and attenuates rapidly when moving away and staying always below 5 kV/m on a border zone. Beneath a 110 kV power line, the electric field strength reaches a maximum of approximately 3 kV/m. The line voltages and thus also the electric fields caused by them remain almost constant regardless of the load.

The unit of magnetic field strength is ampere per metre (A/m), but the quantity that is generally used instead of it is magnetic flux density, the unit of which is tesla (T). In practice, we are speaking here of millionths of the tesla, or microteslas (μT). The magnetic flux density depends on the current, the mutual location of phase conductors and the height from the ground. The highest magnetic flux densities are beneath a 400 kV power lines in the range of 10-20 µT and beneath a 110 kV power lines in the range of 5-8 µT. The magnetic flux density decreases rapidly when we move away from the transmission line and is less than 1 μT at a distance of 60 metres or more from a 400 kV line. The magnetic flux densities are generally much lower in the vicinity of transmission lines than the above ones calculated with maximum currents, because the currents of the lines vary a lot. Hence, the magnetic flux density may be less than 1 μT already at a distance of 30 metres from a 400 kV line.

The electric and magnetic fields of power lines are said to be low-frequency fields because their frequency is the same as the frequency of the alternating current (50 Hz) in the conductors. In general, low frequency fields refer to fields with a frequency of less than 100 kHz. Low frequency fields with a frequency below 300 Hz are called extremely low frequency fields.

To prevent harmful effects, the exposure limit values and action levels for low frequency fields have been confirmed in the Decree of the Ministry of Social Affairs and Health (1045/2018). The limit values are given as internal quantities that cannot be measured. On the contrary, the action levels have been given as external field quantities that can be measured: as the electric field strength and the magnetic flux density. At the operating frequency of 50 Hz of electrical devices and electric wires, the action levels are 5 kV/m for an electric field and 200 µT for a magnetic field. However, the Decree is not applied to the electric field of power lines, because the requirements of the Finnish Electrical Safety Act (1135/2016) limit their electric field on a safe level.

Generally, low frequency electrical and magnetic fields in Finnish homes and in most workplaces are well below the action levels of the fields concerning the exposure of general public. Electric and magnetic fields in homes and workplaces do not only result from power lines but also from other electrical appliances and wires. Based on measurements, the average magnetic field strength in homes is slightly below 0.1 μT and in workplaces slightly above 0.1 μT, which is one two-thousandth of the action level concerning the exposure of general public.

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