By contrast with electric fields, it is very difficult to screen magnetic fields. Careful arrangement of the conductors and optimization of the order of the phases are the best options for limiting their reach. The alternating currents in each conductor are shifted in time with respect to each other – they are said to have different phase angles. Depending on the way in which the three phases are connected to the conductors at the ends of the transmission line, the magnetic field will be of smaller or larger spatial extension. Optimization of the order of the phases means connecting the conductors in such a manner that the spatial extension of the magnetic field is minimized. For this purpose, simulation programs are used that calculate the most suitable order of the phases based on the given conductor arrangement and predominant power flow directions.
With a favorable arrangement of the conductors and by optimizing the order of the phases, it is possible to significantly reduce the spatial extension of the magnetic field.
The magnetic field depends on the current, and thus on the corresponding electricity consumption in households, industry, etc. The time profile of magnetic field exposure in the vicinity of high-voltage power lines thus reflects the fluctuating electricity consumption, depending on time of day and season. Unlike current, the voltage remains practically constant, and this also applies to the electric field of high-voltage power lines, which stays proportional to the voltage.
Electric fields from overhead power lines
The strength of electric fields is measured in volts per meter (V/m). It largely depends on the voltage and the distance from the conductor. Directly beneath a 380-kV high-voltage power line, the electric field strength close to the ground can reach 5,000 V/m. The lower the voltage, the less intense the electric field. For example, beneath a 220-kV line the strength is up to 3,000 V/m, for 110-kV lines it is a maximum of 700 V/m and for lines below 50-kV it is up to 400 V/m. As the diagram shows, the field strength weakens with increasing distance from the conductors. Electric fields can be distorted and weakened by low-conductive materials such as trees, bushes and buildings. The conductivity of building materials usually suffices to reduce an external electric field by 90 percent or more inside the building.