The magnetic fields to which equipment is subjected may influence the reliable operation of equipment and systems.
The following tests are intended to demonstrate the immunity of equipment when subjected to power frequency magnetic fields related to the specific location and installation condition of the equipment (e.g. proximity of equipment to the disturbance source).
The power frequency magnetic field is generated by power frequency current in conductors or, more seldom, from other devices (e.g. Ieakage of transformers) in the proximity of equipment.
As for the influence of nearby conductors, one should differentiate between:
- the current under normal operating conditions, which produces a steady magnetic field, with a comparatively small magnitude;
- the current under fault conditions which can produce comparatively high magnetic fields but of short duration, until the protection devices operate (a few milliseconds with fuses, a few seconds for protection relays).
The test with a steady magnetic field may apply to all types of equipment intended for public or industrial low voltage distribution networks or for electrical plants.
The test with a short duration magnetic field related to fault conditions, requires test levels that differ from those for steady-state conditions; the highest values apply mainly to equipment to be installed in exposed places of electrical plants.
The test field waveform is that of power frequency.
In many cases (household areas, sub-stations and power plant under normal conditions), the magnetic field produced by harmonics is negligible. However, in very special cases like heavy industrial areas (large power convertors, etc.) they occur, and will be considered in a future revision of this standard.