# The EU Directive on Electromagnetic Fields--Practical Experience of Field Measurements
## 2 Extremely Low Frequency, ELF  
One of the most difficult operations to measure from a compliance point of view is non-sinusoidal field exposure. In the EU directive, the Weighted-Peak-Method in Time-Domain is specified as the reference method for the ELF part. The \"Non-binding guidefor practical implementation of the directive\" [3] gives an alternative method, the Time-Domain-Assessment-Method (TDAM). The problem with assessing these kinds of exposures has been discussed by several authors [3; 4; 5; 6; 7]. The problem with the different methods is also addressed at length in [8]. They point out that the TDAM method underestimates the exposure by up to a factor of 22 in certain situations. We will here look at some of the situations where the problem of assessment of non-sinusoidal signals occurs, and where we think the underlying biological knowledge is not sufficient and needs further studies. Spot welding in industrial settings uses very high currents, of the order of some to tens of kA, and the operator is standing rather close, with their body and their hands very close to the electrodes. Below are some examples with oscilloscope pictures from some of our recent measurements on welding machines in use in the car industry. The spot weld lasts from some to tens of Hz periods, up to 50 Hz. Often, operators are not using a zero-crossing relay for start of the weld, which can lead to quite high starting currents and transients. The waveform is often shaped by thyristors to obtain just the right amount of energy to produce the spot weld. As can be seen below in Figures 1 and 2, the same machine can have different settings depending on the material to be welded. This means that the measurements need to be taken for all applications of the machine. The harmonic content of the wave needs to be considered, since the use of the highest current setting is not the one giving the highest reading. This was the case below where the readings were lower when maximum current of 34 kA was used versus the other setting with 11 kA. In this case an amplitude-modulated current would have been a better choice than the thyristor-shaped reduction in current. This, since the latter-introduced harmonics which made the total exceed the action level, we see further in Figure 3.  
Figure 4 shows the shape of the curve when a newer welding machine is used. This gives a lower exposure of the operator. Figure 1: Example of the waveform from a spot-welding machine operating at 34 kA current. Measurements were taken with a Narda ELT 400 with a PICO oscilloscope attached. Time scale: 50 ms per division. Figure 4: Newer spot-welding machine which uses a technique with kHz in the pulse. This often means that the exposure to the operator is less than or within the action levels. Time scale: 5 ms/division. Figure 3: Frequency spectra of the weld in Figure 2. The waveshape with sharp edges leads to a substantial influence of harmonics with 150, 250 Hz, and so on. Figure 2: Same machine as in Figure 1 but now with another setting giving \(11\) kA current. Taking measurements for showing compliance with the directive for a spot-welding machine, it is necessary to have a peak hold instrument with a filter according to EU directives. It is quite likely that the readings will exceed the actions levels at some distance from the machine, and if the operator cannot move further away from the electrodes it will be necessary to evaluate if the exposure limit values (ELVs) are exceeded. To do this, numerical calculations are needed, also for the input to the waveform of the curves, plus a more complete measurement of the field distribution around the electrodes is needed. In most cases this is not an option, and the action levels become the de facto limits. [PERSON] et al. [9] calculated induced body currents in a full 3D human model of an operator of a spot-welding machine.