Static electric and magnetic fields do not change over time and are created by stationary charges and currents. Hence, they have a frequency of 0 Hz. There are both natural and man-made sources of static fields.
Static electric fields, also known as electrostatic fields, are created by charges that are fixed in space. The best known and most powerful display of electric static fields occur in nature as lightning. Static shocks, such as those created due to friction with clothes and other materials, are also common and trivial events caused by electrostatic fields.
Static magnetic fields are created by magnets or charges that move at a steady flow (current), such as in direct current (DC) electricity. They exert an attracting force on metallic objects, and so magnets are commonly used for this purpose. In nature, the geomagnetic field of the Earth exerts a force from south to north that allows, for example, the operation of a compass or the location of the Earth’s magnetic Poles. Much stronger fields are generated by some types of industrial and medical equipment, such as magnetic resonance imaging (MRI) devices, used for medical diagnosis. This table describes the field intensity or level associated with some common sources of static electric and magnetic fields.
Field type: Electric
| Source | Field Level | Explanation/Examples |
|---|---|---|
|
Thunderclouds/Lighting |
Up to 3 kV/m | Distribution of charges within stormy clouds |
|
Non-conductive materials |
Up to 500 kV/m | Friction (e.g., clothes) |
|
Direct current (DC) power cables |
Up to 20 kV/m | Charges moving at a steady flow |
|
DC rail systems |
Up to 300 V/m | Inside the train |
Field Type : Magnetic
| Source | Field Level | Explanation/Examples |
|---|---|---|
|
Geomagnetic field |
30 to 70 μT | Molten iron & Earth rotation |
|
DC rail systems |
Up to 2 mT | DC power electricity |
|
Household magnets |
Around 10 mT | Naturally occurring different distribution of charges |
| Magnetic resonance imaging (MRI)* |
5-10 T |
Use of electromagnets and superconducting materials |
Note: Electric and magnetic field estimates from GreenFacts. The current reference level for public exposure to static magnetic fields is 40 mT. There is no specific reference level for public exposure to static electric fields. However, the EC Recommendation 1999 states that “for most people, the annoying perception of surface electric charges will not occur at field strengths less than 25 kV/m.”
*The ICNIRP guidelines for static magnetic fields (ICNIRP, 1994) state that "For specific work applications, exposure up to 8 T can be justified, if the environment is controlled and appropriate work practices are implemented to control movement-induced effects."
