Occupational Exposure to ELF Electromagnetic Fields

根据在电力、焊接、电磁感应加热、铁路、钢铁等职业场所测量电磁暴露的经验，对7名学生在实验室时间进行实验时的电磁场暴露量进行了调查。实验期间，5人携带EMDEX xii仪表和2台EMDEX MATE仪表。他们还进行了与电力系统有关的实验。结果表明，在继电和焊接实验中，暴露时间加权平均值均超过100 μT。研制了一种新型的测量直流电源焊接时产生的交流磁场叠加在直流磁场上的测量仪表。一些霍尔元件成功地串联在一起，以检测和观察这种磁场。为了减少焊工对磁场的暴露，提出了几种缓解措施：(1)焊工应远离携带焊接电流的回路线导体；(2)线导体应足够近，使磁场相互抵消。(3)长线导体不应在地面上作环形敷设。介绍了用阻抗法评价焊机内部感应电流密度的方法。计算机模拟结果表明，该方法适用于将最大感应电流与ICNIRR规定的基本限制进行比较。

On the basis of the experience of measuring electromagnetic exposures in the occupation sites like electric power, welding, electromagnetic induction heating, railroad and iron steel works, a survey was made of measuring the exposure of 7 students performing experiments in the lab time to magnetic fields. The five of them carried EMDEXII meters and the two EMDEX MATE meters during the lab. They also performed experiments associated with electric power systems. As a result, it has been found that time-weighted average of the exposure exceeds 100 μT for relay and welding experiments.A novel meter was developed for measuring the AC magnetic field superimposed on DC one that appears at the time of welding using a DC source. Some Hall elements were successfully connected in series to detect and observe such a magnetic field.In order to reduce the exposure of welders to magnetic fields, several mitigation measures were proposed : (1)A welder should be out of the loop line conductor carrying a welding current, (2)The line conductor should be close enough so that the magnetic fields are cancelled each other. (3)A long line conductor should not be placed in a loop manner on the ground. The impedance method was introduced to evaluate the induced current density in the welder. It is found from the computer simulation that this method is applicable to compare the maximum induced current to the basic restriction specified by ICNIRR.