Electromagnetic fields on hand-held spot-welding guns

Status:

completed 04/2008

Aims:

During the use of spot-welding guns, strong pulsed magnetic fields occur in the direct vicinity of the welding gun. These fields are generated by pulsed electric currents in the magnitude of a few kiloamps. Since welders generally use their hands to position and hold the spot-welding gun in front of the body during welding, a high level of exposure to magnetic fields must be anticipated. In the process, depending upon the welding current level and the orientation of the welder with respect to the welding gun, the permissible values for the magnetic flux density and flux density changes stated in Section 3 of the BGV B11 accident prevention regulation may be exceeded. A hazard to welders' health presented by strong magnetic fields cannot be ruled out, since a high level of exposure of the human body to magnetic fields may cause excitation of nerves and muscle cells. In a worst-case scenario, this may lead to ventricular fibrillation.

In order to permit assessment of whether a hazard to health exists, the exposure must be evaluated. BGV B11 sets out the permissible values. These values distinguish between basic restrictions, such as the electrical current density, which describes the effects of the magnetic field upon the human body, and the external field values, termed reference values, such as the electrical or magnetic current strengths. Since in the past, measurement of the electrical current density in the human body has been either extremely complex or impossible, evaluation of the exposure to magnetic fields is currently based upon the result of the comparison between the measured magnetic flux density and the permissible reference values stated in the BGV B11. Observance of these values ensures that the permissible values stated in the BGV B11 for the electrical current density in the human body, i.e. the basic restriction, are observed. This does not mean, however, that should the permissible value for the magnetic flux density be exceeded, the permissible value for the electrical current density, i.e. the basic restriction, is also exceeded. Consequently, should the permissible values for the magnetic flux density be exceeded, compliance with the permissible values for the basic restrictions must be ascertained by supplementary examination/analysis/simulation in order for the exposure to be evaluated. Owing to the technical difficulties associated with measurement of the basic restriction values, only very simple mathematical models have been employed to date for the use of these values for evaluation of the exposure. This has always led to the exposure being estimated conservatively. Improvements in simulation techniques now enable the values of the basic restriction in the human body to be measured without the use of complex instrumentation. Since the reference values are frequently exceeded in the vicinity of spot-welding guns owing to the high welding currents, the objective of the project was to determine, by measurement and simulation, whether the values of the basic restrictions stated in the BGV B11 are observed. In addition, the advantages presented by the use of basic restrictions for exposure evaluation purposes are to be identified, compared to the use of reference values for work in the proximity of spot-welding guns.

Activities/Methods:

The project was conducted in five stages. In stage I, the distribution of the magnetic flux density in the vicinity of the spot-welding guns was measured at several levels: in the laboratory, on two spot-welding guns (without integral transformer) with differing electrode intervals, as a function of the welding current, at different intervals from the welding gun.

In stage II, preliminary measurements of the field distribution were conducted on a user in the automotive industry on eight spot-welding guns (without integral transformer). In addition, the different working positions of the welder during welding in terms of the interval between body and welding gun were analysed during these measurements. The spot-welding guns were comparable to those studied in stage I.

In stage III, the field distribution was calculated by means of the EMPIRE simulation software based upon the geometry of the various spot-welding guns and the measured welding currents. In the process, consideration was given to comparison with and adjustment to the measured values as per stages I and II.

In stage IV, the body model of the EMPIRE simulation software was used to determine the basic restriction values (body current densities) for the external magnetic field distributions in the proximity of the spot-welding guns which were calculated in step III.

Finally, the results of measurement and simulation were evaluated in stage V.

Results:

The results of measurement at the selected workplaces and on the spot-welding guns in the laboratory show that violation of the limit values for the magnetic flux density (reference values) cannot be excluded in practice during welding with hand-held spot-welding guns.

In addition, the exposure of a welder on hand-held spot-welding guns was evaluated in this project for the first time by the analysis of body current densities. For frequently recurring work situations, body current densities were analysed and visualized in a three-dimensional field simulation in multiple layers of the body (genitals, torso, neck, head). The results were compared with the basic restriction values currently applicable for the central nervous system (spinal cord and brain), and evaluated. It was found here - as a function of the interval, position and orientation of the spot-welding gun with respect to the body model - that in various fat and muscle tissues and in the spinal fluid (liquor), violation of the limit values is possible, but that the limit values are observed within the central nervous system (brain and spinal cord). In all work situations studied, a maximum of 10% (1 mA/m²) of the basic restriction was exploited in the central nervous system, even though the magnetic flux densities lay above the reference values.

The low degree of exploitation of the basic restrictions permits the conclusion that in real-case work situations on comparable hand-held spot-welding guns, the exposure of the welder lies below the limit values (basic restrictions) stated in BGV B11. The new result of the project is consequently that no special measures are required for the protection of welders during work on the spot-welding guns studied or on similar equipment.

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