Electromagnetic fields on manual medium frequency/inverter spot welding guns

Status:

completed 10/2011

Aims:

Manual medium frequency or inverter based spot welding guns are increasingly being used in the automotive industry for the welding of car bodyshells. These guns use a high direct current with an alternating current component at frequencies of between 50 Hz and 100 kHz. As a result, the welder may be exposed to magnetic fields exceeding the permissible values stated in UVV accident prevention regulation BGV B11. Studies conducted in the past by the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) on manual spot welding guns without integral transformer showed that exceeding of the permissible values stated in the UVV is not synonymous with exceeding of the limit values, for example those for the current densities induced in the human central nervous system (CNS). The objective of this project was to determine the boundary conditions under which this result can be transferred to medium frequency spot welding guns. The project was also to examine whether the additional limit values which will apply in the future for the peripheral nervous system (PNS) are observed. Furthermore, the results were to support the development of a method for assessing exposure based upon the welding current.

Activities/Methods:

The study was based upon the magnetic flux density characteristics recorded during in-plant measurements on medium frequency spot welding guns. These characteristics observed in the field were used to determine the typical frequencies of medium frequency spot welding guns acting upon human tissue. Two different CAD models of medium frequency spot welding guns were also developed. The two models differed in their design and electrode interval. They were used to simulate a number of working positions during the use of spot welding guns. The exposure in a human body model with heterogeneous tissue structure was computed systematically for these working positions. The current density induced in the human body and the electrical field strength were calculated as a function of the frequency and the distance between the body model and the welding guns. The results were compared with the limit values stated in the BGV B11 (UVV) accident prevention regulation and the ICNIRP guidelines. A method was also developed by which, at a known welding current, the exposure can be assessed as a function of the welding current frequency and the electrode interval.

Results:

The following interrelationships were identified:
The electric field strength induced in the tissue of the body model is dependent upon the electrode interval of the medium frequency spot welding gun, the distance between the body model and the electrode gap, and the welding current and its frequency.
Large electrode intervals, small distances from the medium frequency spot welding gun, exposure of the physiology to high frequencies, and high welding currents give rise to high electric current densities and field strengths in the tissue of the human body model.
The magnetic field of the spot welding gun studied induces electric fields in the tissue of the body model which vary in strength as a function of the current and frequency. At the currents and frequencies commonly used, the limit values for the peripheral and central nervous systems (PNS and CNS) may be exceeded.
On medium frequency spot welding guns, the limit values for the PNS and the CNS are exceeded at different welding currents and field strengths. Above a frequency of 400 Hz on spot welding guns with small electrode intervals, the limit values for the CNS may be violated at a current approximately ten times that at which the limit values for the PNS are exceeded. Below this frequency, this ratio falls. At a frequency of approximately 50 Hz, the limit values for the CNS and PNS are exceeded at the same welding current. On a spot welding gun with large electrode interval, this is the case at a frequency of approximately 200 Hz. Below this frequency (200 Hz), at a given welding current, the limit values are exceeded first for the CNS, then for the PNS.
Hazards to persons in the form of magnetic fields cannot be ruled out on medium frequency spot welding guns with large electrode intervals at the welding currents commonly in use at the present time.
The welding current, particularly the AC component, should therefore be limited. On the spot welding gun with a large electrode interval, the current or the magnetic field must be lower than that on the spot welding gun with the small electrode interval by a factor of approximately 100.
The results also show that the method developed can be used to simplify the current standard method for estimation and evaluation of the magnetic fields on spot welding guns. At present however, the method is suitable for use only for two particular electrode intervals. Complex exposure analyses could be avoided if comparable analyses were to be performed for further sizes and a catalogue created, for example of permissible currents.

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