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Low frequency magnetic fields and exposure of users

Written by Administrator
Tuesday, 01 March 2011 13:14

March 2011

By Stéphane GRAVELEAU, Research & Development Department, SREM TECHNOLOGIES (France)
Edited by Pierre CHEMIN and Patrick DUBOSC

Though this paper is a kind of answer to an article (1) published in the French technical journal MESURES, issued in January 2011, we think it will be valuable for all those involved in Magnetic Testing, users, managers and equipment manufacturers. The paper published in Mesures gives disputable pieces of information, either from the technical or from the medical point of view.

As for the medical point of view, we are not in a position to give any clue, this being out of our field of knowledge. Nevertheless, we think this paper is far from balanced, going toward one target. It is based on a controversial report (Bio Initiative Report), using irrelevant words full of tension. This may be understood following the link:

True, the 2004/40/CEE European Directive, issued in April 2004(2), shall be transposed as a French law by 2012. It will be then the regulation with which every equipment shall be compliant. Let us have a look to the chapters dealing with low frequency magnetic fields--the only area of interest for the MT world, when many other aspects are dealt with in this document. Exposure to magnetic fields is described in this Directive through the “Exposure Limit values” as the “density of the current” flowing through the human body:

Basic restrictions for time varying electric and magnetic fields for frequencies up to 10 MHz
on the basis of the various frequencies that are recognized as having harmful effects on the human cardiovascular system or the central nervous system (such as dizziness).
These acute effects are mainly short, and from a scientific point of view, there is no need to modify these figures for short exposures. However, as these exposure limit values are based on their effects on the central nervous system, they allow for the accepting of higher densities of current in other body tissues with the same exposure conditions.
Unfortunately, these physical quantities cannot be directly measured. To help in the defining of a quick and simple assessment of the exposure by measuring a magnetic field, “action values” have been defined.
These action values are obtained from the guidelines laid down by the International Commission on Non-Ionizing Radiation Protection (ICNIRP)(3). They are set out in Table 2 of the Annex to the Directive (13 frequency ranges that apply to all electromagnetic fields and are based on directly measurable parameters) and the Table 6 of the ICNIRP Guidelines.
Compliance with the exposure limit values is mandatory, while compliance with action values is not. But if action values are complied with, they assure of the compliance with the exposure limit values. At 50 Hz, the reference level is set at 0.5 mT, equivalent to a 400 A/m magnetic field in the air; nevertheless, many harmonics are produced, which shall be considered.

Furthermore, note that ICNIRP published new guidelines(4), after reviewing papers published since 1998. The "action value" is less constraining and is upped from 0.5 mT at 50 Hz to 1 mT. These new guidelines should be taken into account in the new directive currently prepared and due to replace the 2004’s.

Once the action values are exceeded, employers must devise and implement an action plan comprising technical and/or organizational measures intended to prevent exposure from exceeding the exposure limit values (modification of working methods, choice of appropriate work equipment, better design of work stations, etc.). However, employers are not obliged to do so if they prove that there are no risks to the health of workers.

Though this Directive is not yet mandatory to date, SREM TECHNOLOGIES is committed since long ago to its consequences, as it is easy to see after the technical documents and notes on our website. Since 2006, we use a measuring equipment dedicated to the measuring of the environment of magnetic fields sources, and we know very well what our equipment produces. All our qualified people are ready to answer any question from our customers. The magnetic environment of a magnetic field source is very dependent on the inspection parameters, and a thorough knowledge of the equipment in use is the first step to assess the situation and think of the simple means to lower exposure, if need arises.

After these explanations about the Directive, it is obvious that, technically, the article displays also some points which come against each other, or are even false, that we would like to point out:

• First it is clear this paper has one main purpose: to frighten its readers, for instance when comparing an exposure limit value of 10 mA/m², which cannot be directly measured, to a magnetic field on a part’s surface measured at 2,400 to 4,000 A/m, a figure said as high versus this exposure limit value. It is a crude comparison. In fact, if a 4,000 A/m DC magnetic field exists on a part, the figure in the human body will be zero mA/m², as a DC magnetic field does not give any induction. The figure is then well below the 10 mA/m² limit! If an AC magnetic field is used, it is completely impossible, even utopian, to assess a simple correlation between the field on the part and the current density on the human body, so many factors being involved: frequency, shapes of the magnetic circuit and of the part, distance from the magnetic circuit, heterogeneity of the human body, etc.

• This paper states that several industries are affected by this question about the users’ exposure to magnetic field, but specifically points out the Non Destructive Testing method MT, which is a wrong assertion. In fact the action values when transformed to magnetic field figures are very dependent on the frequency, and lower and lower as the frequency increases; for example, H < 400 A/m for f = 50 Hz / H < 40 A/m for f = 500 Hz / H < 24.4 A/m for f = 5 kHz. When compared to other industries, MT produces very low frequency magnetic fields, with lower limitations attached. Even if there are harmonics, due to the inductive effect of parts, there are less current spikes, which limit the harmonics to some kilohertz. All the same, there is very often a confusion between magnetic fields and electric fields, both being low frequency fields, produced by high-voltage power lines. These are two different effects.

Current harmonics produced when using a thyristor-controlled magnetic bench

When dealing with thyristor-controlled power pack more and more often used on equipment, the paper states that these power packs give harmonics more dangerous than the fundamental frequency. That is true, as explained in a previous paragraph, with the maximum acceptable figure lowering as far as the frequency increases. Nevertheless we have performed tests whose result is that the exposure is higher at full power (the thyristor has then no action) when only the fundamental frequency is seen.

• As a final step, the author recommends using a swinging field 3D chamber, designed as a vertical well, supposed to lower the exposure of the user. We are very dubious. In fact, as for an encircling coil, the magnetic flux of such a piece of equipment closes the loop in the air, hence maybe in the area where the user is. It is likely that the magnetic field will be stronger than on any magnetic bench with a closed-loop circuit (those without any coil). In this latter case, the magnetic flow circuit is very well known as it goes through the bench. The underneath figure, taken from a standard bench, displays the area in which the user should not go after the 2004/40 Directive recommendations. It is then easy to see that the “No access” area is limited to the tray of the bench.

Just to be sure, as we were somewhat unconvinced of the objectivity of the author, who happens to be a manufacturer of 3D chambers, we made tests on one of its units. A meter-wide “restricted area” around the equipment was found. Be it vertical or not, the equipment would radiate the same way. To claim that a 3D chamber is less dangerous for the users than a magnetic bench is for us highly disputable.

As a conclusion, we question the objectivity of this author as well as his motives. Though we do not want to argument about the medical consequences for their health of an exposure of users to low-frequency magnetic fields, we think it important to emphasize that MT has been in use for almost 100 years; that SREM TECHNOLOGIES has been active for more than 60 years in this area; and that we NEVER got any information about an adverse effect of the magnetic field produced by either a SREM equipment or a competitor’s. A new, more stringent regulation may be enforced quite soon to further lower exposure and have users get more information about exposure. We approve such a move, and will be close to our customers for help, but let us go not too far! For sure, MT is still living, and will for a long time, as it is a most efficient and safe means to thoroughly check parts critical for safety.


(1) Marie-Line ZANI-DEMANGE with Éric CRESCENZO, Société IXTREM, Des techniques simples pour se protéger des champs électromagnétiques (Editor's note: Simple techniques to protect oneself from electromagnetic fields), N°831 of the MESURES journal, 15 Rue d’Oradour-Sur-Glane, F-75015 Paris (France), January 2011. Page 26.

(2) Directive 2004/40/EC of the European Parliament and of the Council of 29 April 2004 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields) (18th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC), Official Journal of the European Union, Volume 47, L 159, 30 April, 2004 and corrigenda L 184/1, 24 May.2004.

(3) Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz), International Commission on Non-Ionizing Radiation Protection (ICNIRP), Health Physics Journal, 74 (4): 494-522; 1998.

(4) Fact sheet on the guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz – 100 k Hz), International Commission on Non-Ionizing Radiation Protection (ICNIRP), Health Physics Journal, 99(6):818-836; 2010. Paper available on ICNIRP’s Website.

Last Updated ( Tuesday, 17 April 2012 17:48 )