Electromagnetic Fields 1996
Since its discovery, electricity has radically transformed people’s lives and has become an indispensable part of our civilisation. Electricity can be converted into any other kind of energy, such as mechanical work, heat or light, which means that its applications are universal. The use of electric energy inevitably entails the occurrence of electric and magnetic fields. These are almost always oscillating fields, as most technical devices are powered by alternating current or generate it themselves. Because the polarity of flux in an alternating current changes, field direction changes constantly, too. The number of cycles per second is known as the frequency, which is measured in Hertz (Hz). Fig. 1 summarises the spectrum of electromagnetic fields.
Strictly speaking, the term “electromagnetic field” only applies to high frequencies, where electric and magnetic fields are inextricably linked and can propagate freely in space as electromagnetic waves. At low frequencies, on the other hand, there are two independent fields, magnetic and electric. Electric field strength is described as E and measured in units of V/m or kV/m. The electric field is represented visually as field lines standing at right angles to the surface of the conducting object. Every geometry creates its own characteristic electric field. By way of example, Fig. 2 shows the field lines around a double-wire cable.
Magnetic field strength H is measured in amperes per metre (A/m), and magnetic flux density B in units of T (tesla). As magnetic flux densities are often very small, we usually refer below to a millionth of a tesla, or µT. Magnetic field lines run in circles around the conductor (cf. Fig. 3).
Electric and magnetic fields always spread out in space from a source. The electric field is a source field which occurs between separate charges (battery, mains socket). The magnetic field is a vortical field which only occurs when charges move, i.e. when a current flows. Any charged conductor has an electric field, whereas the magnetic field is only created when a flux begins, e.g. a lamp is switched on.
Field strengths decrease very rapidly as their distance from source increases.
Static electric and magnetic fields (constant fields) of a significant field strength have always existed on this planet.
The movement of air in the atmosphere and the ionising effect of cosmic radiation in the higher regions, the ionosphere, create a field of direct electric current between the surface of the Earth and the ionosphere. Under normal weather conditions, the field strength near the ground is around 100-500 V/m, whereas it can rise to 20,000 V/m (20 kV/m) during storms. Alternating currents at frequencies used in energy supply are practically non-existent. The natural background field strength at 50 Hz is only 0.1 mV/m.
The static magnetic field is familiar because of the way it affects a compass needle. It is almost constant over time and measures about 42 µT in Germany. This constant field is created by circular action in the Earth’s core. Extremely high field strengths can occur in the vicinity of lightning (up to 1 T, which can cause heart failure in humans). Small variations in flux density are induced by the solar wind, which distorts the earth’s magnetic field due to its streams of charged particles. Furthermore, global storm activity also results in high-frequency components within the magnetic field. However, these are so small that at 50 Hz the alternating field component is merely 10-6 µT (WHO 1984).
At low frequencies technical field sources tend to be much stronger than naturally occurring fields. Most of these are either power supply installations, which generate and distribute electricity, or the technical entities which consume that energy. This includes industrial plants, private installations and consumer devices (e.g. household gadgets) and public transport systems (e.g. underground and railroad).
In addition to field emissions from large-scale technical plants, people are surrounded today, both at home and at work, by a multitude of sources of electric and magnetic fields which, if taken together, may be generating cumulated field strengths greater than those of the aforementioned technical plants. Field strength will depend primarily on the distance from the device in question and on its technical make-up, which accounts for a strong scattering of values for individual types of apparatus. The legend to the map lists the field strengths of electrical devices at normal usage distance (“Typical values for the magnetic flux densities of household devices at varying distances”). Comparison with the field strengths of high overhead voltage lines, also included in the map, shows that the field strengths of ordinary household gadgets are, indeed, often higher.
“Electrosmog” is the buzzword which has directed public awareness towards technical field emissions in recent years. All over the world, numerous studies have been carried out on the possible effects of electromagnetic fields on humans, animals, plants and cell or tissue cultures, and a series of large-scale epidemiological surveys has also been conducted. The effects of electromagnetic fields generally depend on the frequency and intensity, but also on individual characteristics such as body size or angle towards the field.
Findings have been largely substantiated with regard to the effects of induced eddy currents at higher and medium-range field strengths (cf. Fig. 5), and these have formed the basis for the limit values in protective legislation.
An external magnetic field induces eddy currents in the human body on a circular plane perpendicular to the direction of the field. Similarly, an electric field creates a flow in the body which follows the same direction as the field: under high overhead voltage lines, for example, the flow would be from head to foot – and vice-versa (alternating field!). These field-induced flows are recognised as the predominant cause of biological effects at low-frequency fields. Above certain trigger values, the induction flows, just like direct body current, cause effects which can damage the organism.
Although sensitive people can already detect electrical fields at 1kV/m, be it from the vibrations of body hair or due to discharge from conducting objects near the human body, there is no known danger to health, even when exposed for long periods of time. Indirect effects on electronic implants, e.g. rarely used types of single-pole artificial pacemaker, can occur from a field strength of around 2.5 kV/m or 20 µT, but life-threatening results are very unlikely. However, uncomfortable stutter rhythms can occur, which is why those affected people should stay away from strong fields (BfS 1994).
The scientific literature yields numerous epidemiological studies which address possible links between exposure to fields and the risk of cancer among certain sections of the population. So far, despite sometimes considerable effort, the results have been contradictory. Direct comparisons are rendered more difficult by varying circumstances. There is a shared emphasis, however, on the need for more research into both the epidemiology and the mechanisms at play.
Limit and Recommended Values
The observed effects have been used by various national and international bodies to establish limit or recommended values for different frequencies and areas of exposure. In addition to limits on direct field impact (V/m, A/m) at the workplace and among the general population, there are also maximum limits for indirect field impacts, pacemakers, small transmitters, partial body exposure, exposure of short duration, pulsed radiation, etc.
Due to the different safety strategies which have been conceived for different sections of the population, it is difficult to compare the various limit and recommended values.
The International Committee on Non-Ionising Radiation Protection (ICNIRP, formerly INIRC) of the International Radiation Protection Association (IRPA) has defined a maximum admissible body current density of 10 mA/m2 (INIRC/IRPA 1990) which takes its lead from the body’s own physiological current densities. Acute danger to health from the disruption of nervous, muscular and cardiac functions only occurs at 10 – 100 times this amount (see Tab.1).
To protect the population at large, ICNIRP/IRPA recommends a further reduction by a factor of five, resulting in a body current density of 2 mA/m2.
To protect the general population and local neighbourhoods from harmful environmental impacts, this basic precautionary value has been used in German legislation to derive maximum limits for electric and magnetic field strengths at a frequency of 50 Hz. These values are legally binding under the provisions of the 26th Ordinance (26. BImSchV 1996), in force since 1 January 1997, regulating the Federal Pollution Control Law. The limits for low-frequency installations – defined for the purposes of the Ordinance as “stationary plant for the transformation and transmission of electricity at a voltage of 1000 V or more” – are:
To protect against harmful environmental impact, overhead and underground cables, overhead traction supply lines and electricity transformer stations must be constructed and operated in a manner to ensure that, within their zone of influence, at full capacity and taking account of exposure to other low-frequency installations, the limit values for electric field strength and magnetic flux density are not exceeded in buildings or on sites that are intended to be used by people on more than a purely intermittent basis.
Under certain circumstances, the limits for magnetic flux density may by exceeded by 100 % for a short duration, and electric field strength may be exceeded by 100 % within a small area. “For precautionary purposes, the construction of, or substantial modifications to, low-frequency installations close to dwellings, hospitals, schools, kindergartens, after-school care facilities, playgrounds or similar installations in these buildings or on these sites” must be carried out so that the maximum effective values reflect these limits. In addition, the State of Berlin recommends remaining well within these limits and, especially during planning, exploiting any potential there is for reducing the values in these specific areas to 10 %. These recommendations are based on the effects of electromagnetic fields on electric and electronic implants, which are not considered in the 26th BImSchV, and on publications by the Federal Agency for Radiation Protection (BfS 1994).
The limit values only apply to construction or substantially modification of installations. Installations which were built before the 26th BImSchV entered force must meet these requirements within three years of that date. It should also be noted that the limits need only be observed in areas where people are intended to be present on a more than intermittent basis. This does not cover, for example, agricultural land or railway station platforms. Although platforms may be in constant use, individual passengers do not essentially stay very long.
BG = sectoral employers’ liability associations (Berufsgenossenschaften)
UVV = Accident Prevention Regulations (Unfallverhütungsmaßnahmen)
As the limit values in the 26th BImSchV only concern certain installations – notably those with an operating voltage of 1000 V or more – it is often necessary in practice to consider the recommended values of the IRPA/ICNIRP (Tab. 3), which cover a far more comprehensive spectrum than those in the 26th BImSchV (see Fig. 6).
The IRPA/ICNIRP guidelines (ICNIRP/IRPA 1990, 1994, 1998) include both areas of public use and places of work. There are no limitations with regard to voltage levels or date of construction. The IRPA/ICNIRP also addresses d.c. fields, which are an important feature in medicine and industry. However, the IRPA/ICNIRP values are not legally binding, having only the status of a recommendation. Yet they are important enough for the legislator to state explicitly that the limits in the 26th BImSchV are oriented to IRPA/ICNIRP guidelines.
At workplaces not covered by the 26th BImSchV – workplaces where the occurrence of electromagnetic fields can be expected – Accident Prevention Regulations apply which have been formulated by the Berufsgenossenschaften (sector-based employers’ liability associations). These are currently being revised and will replace the previous recommendations of the Berufsgenossenschaften (the “Rules on Safety and Health Protection at Workplaces Exposed to Electric, Magnetic or Electromagnetic Fields”, drawn up by the association responsible for the sector of precision mechanics and electrical engineering) (BGF 1995).
Tab. 3 summarises once more the limit and recommended values for public areas and workplaces. The scope of application is explicitly limited to 50 Hz. For historical reasons, recommended limits for occupational exposure were published in the early years by both the VDE (VDE 0848 1995) and the IRPA, founded on the generally recognised effects of strong electric and magnetic fields. The different field strength limits proposed by the two organisations are simply due to the different models they used for translating the same primary base values into secondary values for external fields. The IRPA also defined limit values for the general population which are still in place today, whereas the VDE did not attempt to remedy this lack until it introduced Amendments 1-3 to its Standard 0848, Part 4. Given that the VDE, which represents the interests of the electrical industry, is surely not free of a certain vested interest, the limits proposed by the VDE were never an unequivocal match for those of the IRPA and never progressed beyond the stage of a proto-standard. As a result, the legislator in Germany did not choose to be guided by the VDE, accepting instead the internationally undisputed values of the IRPA as the basis for the maximum limits in the 26th BImSchV. As part of the EU harmonisation effort, the European Committee for Electrotechnical Standardisation (CENELEC) has also proposed limit values. However, these are not likely to be adopted by the EC, and will probably be replaced by a recommendation from the Council of Ministers within the framework of guidelines on physical standards for workplaces.