Investigation of ELF Magnetic Fields Caused by Resistance Welding Machines

Due to the high output currents of resistance welding machines the magnetic fields caused by these machines attract special attention. In resistance welding and in other high-current applications the magnetic fields occurring in the surrounding of the installations show a pulsating character. Resulting from the connection of the character of the magnetic field with the currents induced in human beings a specific assessment of pulsed magnetic fields is necessary. According to this requirement in the new German guideline concerning occupational field exposure BGV B11 additionally to exposure limits for continuous fields also rules for pulsed fields are given. In contrast to the situation in case of continuous fields where fixed exposure limits exist, the permissible exposure limits for pulsed fields have to be calculated on the basic of the occurring field parameter waveform applying a calculation procedure given in the mentioned guideline. Due to their principles of operation and welding current adjustment the concrete waveforms of output current and related magnetic field of resistance welding machines differ from the fundamental waveforms which are given in BGV B11 as the basis for the exposure limit determination. In the paper the application of the BGV B11 regulations to resistance welding machines is illustrated. By the example of an a.c. resistance welding machine the procedure to ascertain the exposure limits is presented and several influencing factors concerning the resulting limit values are discussed. The exposure limits are compared with the actual magnetic flux density values occurring in the surrounding of the machine and a practical method to determine minimum distances ensuring compliance with the given regulations is shown.