ELF sources are mainly low, medium and high voltage overhead or underground power lines, power distribution lines, transformer substations, electrical equipment for industrial and domestic use, etc. These are to be considered unintentional and undesirable electric and magnetic field sources because their function is to conduct energy from the generator to the end user, and the fields are a side effect.
In the range of these frequencies, magnetic and electric field values are measured or calculated in separate ways, as they are considered distinct entities: the electric field is generated by voltage or electric charges while the magnetic field is generated by electric currents.
In some cases, the issue of ELF magnetic fields is disregarded while in others it is underestimated.
While overhead power lines are visible to all, those buried underground — in vertical shafts or in electrical transformer substations, located in the lower floors of residential buildings — are not perceptible except by experts.
Within residential buildings, rooms are increasingly being set up to house the transformer substations operated by electricity production and distribution companies.
More specifically, electrical transformer substations located in rooms adjacent to, below or above apartments, shops or professional offices will need to be evaluated.
Operation of the components inside transformer substations emit magnetic and electric fields.
When the limits defined by legislation — to safeguard the general population and workers or to protect sensitive electronic equipment (e.g., electron microscopes) from disturbance — are exceeded, shielding systems must be implemented to return the electric and magnetic fields to below certain levels.