In Denmark, magnetic fields emanating from electricity supply installations change direction backwards and forwards 50 times per second. We say that the fields have a frequency of 50 Hz (hertz) or 50 cycles per second. The alternating current creating the fields has the same frequency, ie 50 Hz.

The Earth's magnetic field is static
The Earth also has a magnetic field. It is a static magnetic field, ie a field that does not change direction backwards and forwards. It has a frequency of 0 Hz. This is precisely what makes it possible for us to determine north and south by means of a compass. The Earth's magnetic field varies between 35 and 70 µT (microtesla) depending on where on the Earth we are. At our latitude it is approx. 50 µT.

Direct current produces static fields
Human-made static fields are present wherever direct current is used. Power lines, cables and appliances using direct current therefore have a static magnetic field.

Around the large direct-current cables, eg the submarine cables connecting Denmark with Norway, Sweden and Germany, there may be fields of the same magnitude as the Earth's magnetic field or a little larger. The same applies to the cables connecting submarine cables to converter substations onshore. Immediately close to the fence surrounding converter substations there will be direct-current static fields and 50 Hz alternating-current fields.

Fields in industry and health sector
The largest static fields are found in the vicinity of, for example, furnaces at aluminium-producing plants, special welding equipment and medical equipment, such as MRI scanners. In industry, static fields may be more than 1,000 times stronger than the Earth's magnetic field. Fields around scanners for clinical examinations may be between 0.2 and 3 T (tesla). 3 T is the equivalent of 3,000,000 µT.