Electric and magnetic fields (EMFs) are produced wherever electricity is used. A key characteristic of EMFs is their frequency. They always have the same frequency as the electricity that produced them. Most electricity supply in the UK is alternating current (AC) with a frequency of 50 cycles per second or 50 hertz (Hz). So, the EMFs produced also alternate, or change direction, with a frequency of 50 Hz.

Offshore high voltage direct current (HVDC) transmission links are different as they use DC, so they produce EMFs that always point in the same direction.

Electric fields are created by voltage, and magnetic fields by the flow of current. All electricity assets operate at a fixed voltage, so the electric field produced by each asset is practically always the same. For example, an overhead line operating at 400 kilovolts (kV) will typically produce an electric field of approximately 5 kV/m, and the electric field will remain around this level 24-hours a day, every day of the year. 

Magnetic fields however, go up and down depending on the current flowing through the overhead line. This varies depending on the electricity consumption at any given time. Transmission voltage overhead lines tend to have relatively stable current flows, but cables and overhead lines that supply electricity to our homes can vary depending on how much electricity we are consuming.
 

How do AC and DC EMFs differ?

DC fields occur naturally as well as being human-made. The Earth has its own natural DC magnetic field which we are surrounded by all the time. AC fields are human-made, and 50 Hz fields are produced by household wiring and appliances, as well as power lines and cables.

The EMFs that this website covers are those produced by the electricity system and by the electrical devices we use. Other EMFs at higher frequencies do exist such as those produced by mobile phones, radios, and satellites. Those are different to the low frequencies covered on this website, and the information provided is not applicable to them. Further information regarding other frequencies can be found here.