Power Magnetic Field

The magnetic fields to which equipment is subjected may influence the reliable operation of equipment and systems. The 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. leakage 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 and 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 protecion relays). The tests 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 tests 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 plants under normal conditions) the magnetic field produced by harmonics is negligible. However, in very special cases like heavy industrial areas they will occur.

When performing the tests it is required that the EUT and possible auxiliary equipment are exercising all normal modes of operation. The test results may be classified on the basis of the operating conditions and functional specifications of the EUT, according to the following performance criteria:

• (A) Normal performance within the specification limits;
• (B) Temporary degradation or loss of function or performance which is self-recoverable;
• (C) Temporary degradation or loss of function or performance which requires operator intervention or system reset;
• (D) Temporary degradation or loss of function or performance which is not recoverable, due to damage of equipment (components) or software, or loss of data.