Electromagnetic-environment: Phenomena

The general interference phenomena were noted earlier. This section will look extensively at the type of source that will produce these phenomena, and the apparatus that may be affected by them, and then goes on to discuss the modes of coupling. The subject of mains harmonics is covered in a separate section. As an end piece, the interrelationship with safety issues is also covered.

In the context of EMC legislation, it’s usual to refer to a piece of equipment in relation to its environment. The environment contains all other sources of disturbances, and also contains all other apparatus which might be affected by the source equipment’s own disturbances. Control may therefore be exercised either on the equipment's ability to create disturbances, or on its immunity from external disturbances, or both; the coupling paths are defined as the totality of the equipment's interaction with its environment.

Electromagnetic-environment: Phenomena

The source document for environmental electromagnetic phenomena is IEC 61000-2-5. The tables are based on that standard. They are classified into continuous phenomena and transient phenomena. Several types or higher levels of transient phenomena may occur only occasionally

A great deal of research has been conducted into the prevalence of these phenomena, which has been distilled into IEC 61000-2-5. Most of the basic immunity test standards in the IEC 61000-4-X series include a description of the sources of the phenomenon they are dealing with, and IEC 61000-4-1 is itself an overview of the immunity test methods.

Not all of these phenomena are covered by the product or generic standards which are applied under the EMC Directive: in fact, it's probably fair to say that the majority are not. This can be a blessing, if you are a product manufacturer faced with days of EMC compliance testing, or it can be problematic, if you are a user who needs to have confidence in the performance of an item of equipment when faced with a particular electromagnetic threat. In the latter circumstance it’s usually necessary to discuss the detailed performance with the equipment supplier. The following tables, and the standards they refer to, may offer a helpful framework for such discussions.

Examples of radiated field threats

Examples of LF magnetic fields:

100A/m has been seen 10m from steel rolling mill DC drive cables (+8kA), and > 1000A/m at < l m.

++ A 1.1 kHz 800kW steel billet induction heater has been seen to emit 100A/m at 1m.

++ A 230kHz 400kW steel tube welder with a coil diameter of 50mm can create 40A/m at 0.25m from its coil.

++ A 50Hz 6MW copper billet heater generated 430A/m at a distance of 1 m.

++ A 700Vdc 60kA electrolytic process can create 15kA/m at operator position.

++ At ground level under an overhead 400kV line: 32A/m.

++ At ground level above an underground 400kV line: 160A/m.

++ Commercial premises under-floor heating can create 160A/m at floor level, 16A/m at lm height above floor.

++ 8A/m has been seen at floor level in a multi-storey office, above a sub-floor carrying cables from distribution transformer to switch-room, and up to 2A/m at desk height.

++ A 1kW water pump has been seen to emit 800A/m at 10mm distance, and 3A/m at 400mm, whereas an 18kW motor emitted 6A/m at 200mm.

++ A TIG welder has been seen to emit 800A/m at the surface of the welding cable and surface of its power supply, and < 160A/m at the operator's position.

Examples of LF electric fields: A 490kHz 8kW steel tube heater with a coil diameter of 60mm has been seen to emit 100V/m at 0.3m from its coil.

++ A 20kHz 1.5kW induction cooker hob generated 28V/m at 250mm.

++ 1kV/m = outdoors under 30kV lines, or indoors under 765kV lines.

++ 10kV/m = outdoors under 400kV lines.

++ 20kV/m = outdoors under 765kV lines.

Examples of RF fields: MHz-operation dielectric heaters of 3 to 15kW have been known to create 300V/m at the operator's position (this is not regarded as safe!). Hand-held walkie-talkies and cellphones can generate 30 V/m field strengths at distances of 400mm and 250mm, respectively (greater fields at smaller distances). A 1200kW medium-wave broadcasting station generated 32V/m at 0.5km.

A wire-type spark erosion machine generated the equivalent of 0.02V/m field at 1m.

Continuous radiated threats from radio transmitters

The distances given below assume free-space radiation and a fall off with distance proportional to l/r. Local field strengths can be doubled by reflections and resonances in nearby metal structures. In practice, near-field effects and scattering or absorption by objects along the path mean that actual fall-off with distance is proportional to l/m, where n varies between 1.3 for open country and 2.8 for heavily built-up urban areas (EN 55011 suggests an average value of n = 2.2).

General background EM radiation at a representative site has been found to be between 0.1 and 1 V/m, dominated by long, medium and short wave signals.

-----: Distances versus radiated field strengths Total emitted RF power, and type of radio transmitter 0.8W typical (2W maximum) hand-held GSM cellphone, and l W leakage from domestic microwave oven 4W private mobile radio (hand-held) (e.g. typical VHF or UHF walkie-talkies, TETRA mobiles) 10W emergency services walkie-talkies, and CB radio 20W car mobile cellphone, also aircraft, helicopter, and marine VHF radio-communications 100W land mobile (taxis, emergency services, amateur)' paging, cellphone, private mobile radio base stations 1.0kW DME on aircraft and at airfields; 1.5kW land mobile transmitters (e.g. some CBs) 25kW pulsed marine radars (both fixed and ship-borne) 100kW long wave, medium wave, and FM radio broadcast (Droitwich is 400kW) 300kW VLF/ELF communications, navigation aids 5MW UHF TV broadcast transmitters 100MW pulsed -- ship harbor radars; Proximity for 1Vim 5 (7.a)m, 11m, 16m, 25m, 54m, 210m, 850m, 1.7km; Proximity for 3 V/m, 1.6 (2.5)m, 3.6m, 5.0m, 8m, 18m, 70m, 290m, 580m, Proximity for 10 V/m; 0.5 (0.8)m, 1.1m, 1.6m, 2.5m, 5.4m, 21m, 89m, 170m, Proximity for 30 V/m; 0.16 (0.25)m, 0.36m, 0.5m; 0.8m, 1.8m, 7m, 29m 58m, 3km, 1km, 300m, 100m, 12km, 4km, 1.2km, 400m, 55km 18km 5.5km 1.8km 1GW pulsed -- air traffic control and weather radars 170km 60m 17km 6km 10GW pulsed -- some military radars 550km, 180km, 55km, 18km.

Attenuation of field strength by buildings

The attenuation of a double-brick wall may be assumed to be one-third (10dB) on average, but can be zero at some (unpredictable) frequencies. The attenuation of a typical steel-framed building can be much better than this except in the region 50 to 200MHz, depending on position within the building. --- gives attenuation values for seven different types of building common to the United States.

A note on radars

The peak value of emitted power depends on the type of radar and its pulse characteristics. Radar fields are line-of-sight, and the very high powers of ground-based radars are considerably attenuated by geographical features such as hills or the curvature of the ground/earth. Fixed radars are normally aligned so as not to include people or buildings in their main beam.

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