Effects on the Human Body and Assessment Methods of Exposure to Electro-Magnetic-Fields Caused by Spot Welding

Effects on the Human Body and Assessment Methods of Exposure to

Electro-Magnetic-Fields Caused by Spot Welding

Peter Mair

Fronius International GmbH, Wels, Austria [email protected]

Abstract

Ongoing public discussions on adverse effects of electromagnetic fields (EMF) on the human body and international as well as European legislation require the evaluation of workplaces with respect to EMF and therefore the development of assessment methods for these fields and their biological effects. This is of particular importance in special areas like resistance welding where high field-strengths occur and simple procedures to show compliance with the exposure limits are generally not applicable. This paper gives a summary of the EMF relevant parameters in welding applications, permissible exposure values and possible assessment procedures with a focus on methods using numerical simulation.

Keywords: Resistance welding, EMF, biological effects, assessment methods

1. INTRODUCTION

Possible adverse effects of electromagnetic fields originating from high-voltage power lines and mobile phones have given rise to discussions in both scientific and public communities for years. In many countries recommendations and mandatory legal regulations to limit exposure to non-ionising radiation exist, some of them covering the whole electromagnetic spectrum from d.c. to microwaves, some dealing only with power-frequency fields or the frequency bands used for radio-communication. Further information on national requirements can be found at the WHO1 homepage. In the European Community the responsibility for employers to assess EMF on workplaces exists, as well as mandatory requirements for placing resistance welding equipment on the European Market, addressed to manufacturers and importers.

Resistance welding uses high electric currents up to several hundreds of kilo-Amperes. These currents are provided by the welding power source, flow through the welding cables and gun, through the work-piece and back to the power source. At any location magnetic fields are generated by the current-flow, however the parts of the electric circuit closest to the welder are the predominating field sources.

The field-properties around welding equipment are primarily defined by the properties of the welding current, which is dependent on the welding process and the capabilities of the power-source used. Moreover, the topology of the current path (e.g. gun and cable geometry /dimensions and their position in relation to the position of the welders body is relevant for the level of exposure.

2. LEGAL AND NORMATIVE REQUIREMENTS 2.1 Workers protection in Europe

Within the framework of “Council Directive 89/391/EEC on the introduction of measures to encourage improvements in the safety and health of workers at work” the “Directive 2004/40/EC of the European Parliament and of the Council on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields)” [1] [2] has been published in the Official Journal of the European Communities (OJ) in April 2004. It has to be implemented by member states until 2008 and addresses employers. However, as in many cases the employer will not be qualified to carry out the workplace evaluation required in the directive without external expertise, manufacturers of welding equipment will likely be required to assist, at least by providing technical data. The protection levels (called “Action Values” and “Exposure Limit Values”) provided are based on well established, scientifically based guidelines, the exposure limits do not cover static fields.

2.2 EMF and CE Marking in Europe

Compliance with the Low Voltage Directive (LVD) 73/23/EEC [3] [4] and the Machinery Directive 98/37/EC [5], a mandatory requirement for CE Marking and therefore for placing resistance welding equipment on the European Market, is based on respecting the “Principal Elements of the Safety Objectives” listed in these directives. These safety objectives, among other requirements, include the “exclusion of risks due to radiation”. These directives address manufacturers and importers of resistance welding equipment.

2.3 Exposure guidelines 2.3.1 General principles

There are several internationally accepted guidelines on permissible levels of exposure to electromagnetic fields, e.g. published by ICNIRP2 and IEEE3. Many international standards and the European legislation are based on the ICNIRP “Guidelines on Limits of exposure to Static Magnetic Fields” (1994) [6] and “Guidelines for Limiting Exposure to Time Varying Electric, Magnetic and Electromagnetic Fields (up to 300GHz)” (1998) [7].

Exposure Guidelines usually follow a two tier philosophy, i.e. they contain two different sets of limits, one for the general public and a second set for occupational exposure. This separation is based on the special protection needs of sensitive groups of the population like children or diseased people. Moreover the limits for the general public consider the possibility that the EMF might be present also during the vulnerable phase of sleep (e.g. exposure due to high voltage power lines, public power-distribution transformers or base stations for mobile communication networks). Generally the limits for occupational exposure have to be applied for evaluation of welding workplaces, general public limits may be relevant for nearby areas of public access (e.g. offices or work sites in public places).

2.3.2 Basic Restrictions

The so called Basic Restrictions limit the occurring intra-corporal biological processes caused by EMF to levels below the established thresholds values for adverse effects. Consequently

2 _{International Commission on Non-Ionizing Radiation Protection, http://www.icnirp.de/}

the quantities used to describe the Basic Restrictions can not be measured directly, analytical calculations or numerical simulations, based on equivalent field-sources and models for the human body have to be performed to derive the magnitude of these metrics. The ICNIRP values for limitation of induced current densities in Central Nervous System (CNS) tissues, the most relevant biological effect of fields produced by resistance welding equipment, for occupational exposure are given in Figure 1.

The Basic Restrictions must not be exceeded in any practical exposure situation. If this can not be guaranteed for a specific situation, measures have to be taken to reduce exposure to levels below the limits.

In resistance welding applications this can primarily be achieved by respecting minimum distances from the welding gun and cables to the body of the welder, the application of advanced technologies and control algorithms or the use of specially designed welding cables are further options to reduce magnetic fields at the source.

1 10 100 1,000 10,000 100,000

1.E- 01 1.E+01 1.E+03 1.E+05 1.E+07

frequency [Hz] J [ m A /m 2]

induced current density occupational exposure 0.0001 0.001 0.01 0.1 1 10 100 1000

1E- 01 1E+01 1E+03 1E+05 1E+07

frequency [Hz]

B [m

T

]

magnetic flux density occupational exposure

Figure 1: ICNIRP Basic Restrictions and Reference Levels for occupational exposure up to 10 MHz

2.3.3 Reference Levels

In addition to the Basic Restrictions derived levels of directly measurable quantities are provided in exposure guidelines for simplified exposure assessment. These are called Reference Levels (ICNIRP) or MPE values (IEEE) and are given as the electric field-strength E [V/m], the magnetic field-strength H [A/m] or the magnetic flux-density B [T] in the frequency range relevant for resistance welding.

There is a frequency dependency of the relevant coupling mechanisms, such as magnetic induction in a conducting loop where the loop voltage is directly proportional to the frequency of the magnetic field, which can be seen in the course of the Reference Level curves compared to the Basic Restrictions. As discussed before, mainly the magnetic field is relevant for resistance welding. The ICNIRP Reference Levels for magnetic flux-density, occupational exposure, are given in Figure 1.

When deriving these Reference Levels, conservative models considering maximum coupling conditions (e.g. worst-case field orientation with respect to the affected biological “receiving”

structures and homogenous field-distribution) have been used. Therefore, if the Reference Level is not exceeded, the Basic Restriction will most probably be respected.

This worst-case principle implies that if the Reference level is exceeded, it will not automatically mean that also the Basic restriction is exceeded and there is over-exposure. So if the first (simple) evaluation step of measuring magnetic field-strength or flux-density leads to results above the Reference Levels, further steps like the assessment of intra corporal biological effects have to be taken.

2.4 Standards

The Directives mentioned above are “New Approach” directives, which means that no details regarding technical requirements for conformity assessment are included. One way to show compliance is testing against “Harmonized European Standards” i.e. standards published in the Official Journal of the European Communities4.

Standards for the protection from radiation emitted by machinery, EN 50198-1, -2 and -3 were published by CEN in 2000 and 2002 [8] [9] [10]. They contain Reference Level based principles of product classification and marking and basic measurement guidelines, however without giving detailed procedures useful for the assessment of resistance welding equipment. Product standards for EMF conformity assessment of resistance welding equipment under the LVD are currently developed by CENELEC TC26A/WG2 and TC26B/WG2. Drafts of these documents, prEN 50445 [11] and prEN 50505 [12], were recently circulated for enquiry and will be under final vote by June 2007. They contain references to limits, test set-ups and calculation / simulation procedures. Future editions may also include assessment procedures specifically taking care of the special requirements of Directive 2004/40/EC.

3. BIOLOGICAL EFFECTS

3.1 Forces and adverse effects on medical implants

The dominating effect on the human body due to static fields are resulting forces on ferromagnetic elements (e.g. metallic implants) or moving charges (e.g. blood ions). Even under worst case conditions the levels of static fields generated in resistance welding applications are below the established permissible levels.

Passive and active medical implants like cardiac pacemakers might be susceptible to static and time varying electromagnetic fields produced by resistance welding equipment. This issue is important for welders wearing such implants and has to be considered carefully, however it is not further discussed in this paper. Procedures to deal with these risks are currently developed in CENELEC TC106X WG15 and will lead to a dedicated European EMF standard. One of the difficulties when dealing with active medical implants is the susceptibility of these devices, which considerably varies for different products, even for different settings of the same implant model.

3.2 Stimulation

Time varying magnetic fields induce eddy currents in structures of conductive material. The published average conductivity σ of living tissues is 0,2 S m-1, which is sufficiently high to create relevant currents in body tissues. These currents, flowing in cross-sections of the body

are characterised by the resulting current density J, usually expressed in mA m-2. Cells may be stimulated by these currents if their amplitude is high enough, the time duration of the induced current exceeds the “response time” of the cell and the rise time of the excitation is faster than an established threshold value. These conditions for effects lead to frequency dependent levels of permissible induced current density and corresponding effects, a summary of published estimated threshold ranges is given in Figure 2.

P established range of the perception threshold

L established range of the let-go threshold

F established range of the threshold for irreversible cardiac fibrillation

all ranges are given from 0,5% to 99,5% probability

The ICNIRP limit-for induced current density, occupational exposure is indicated in orange

Figure 2: Stimulation effects of induced current densities in the frequency range up to 100kHz

3.3 Heating

At frequencies above 100kHz the energy of an electromagnetic field will be partially absorbed in body tissues, which can lead to dielectric whole-body or local heating effects. Typically the amplitudes of relevant frequency components of fields generated during welding are low. The presently available information does not suggest excessive thermal effects due to the electromagnetic fields produced by the resistance welding equipment.

4. PROPERTIES OF THE ELECTROMAGNETIC FIELD 4.1 Electric field

The working voltages used in resistance welding are low compared to the voltage levels capable of causing biological effects inside the human body. Therefore the studies regarding EMF and electric welding are currently confined to the assessment of magnetic fields and their effects.

4.2 Magnetic field

The body of the welder is exposed to magnetic fields generated by the welding current in the cables to the welding gun, the gun itself and inside the welding equipment. As the current amplitudes in resistance welding and therefore the resulting field-strengths are high, the magnetic field is the predominating field metric to be considered when evaluating EMF exposure.

4.2.1 Field distribution

The spatial distribution of the field is depending on the type of and distance to the source. Although field-based assessment of EMF exposure generally assumes uniform field distribution, this is never the case in typical resistance welding situations. The field gradient

0.1 1 10 100 1000 1 10 100 1,000 10,000 100,000 Frequency [Hz] J [ A/ c m 2] P 99,5% L F 99,5% 99,5% 0,5% 0,5% 0,5% J [uA cm -2 ] frequency [Hz]

(i.e. the decrease of the field-amplitude with distance d) varies from 1/d to 1/d 3, with 1/d for

straight single conductors (e.g. single sided welding tool), 1/d 2 for parallel pairs of conductors

carrying opposite currents (welding gun cables) and 1/d 3 for coils and transformers (e.g.

welding gun window and internal parts of equipment).

Figure 3 shows the highly non-uniform, analytically calculated field distribution in a sector of the horizontal X-Z plane at the vertical centre of a resistance welding gun (approximated 100x50cm rectangular source model) at distances up to 1m to the conducting parts of the arms, based on a welding current of 20kA.

0 0.3 0.6 0.9 1.2 1.5 0.00 0.25 0.50 0.75 1.00 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 00 95-100 90-95 85-90 80-85 75-80 70-75 65-70 60-65 55-60 50-55 45-50 40-45 35-40 30-35 25-30 20-25 15-20 10-15 5-10 0-5 B [mT] Z X

Figure 3: Field distribution in plane X-Z at the vertical centre of a resistance welding gun

4.2.2 Waveform of the field

The waveform of the magnetic flux-density is primarily dependent on the welding current waveform. Effects of ferromagnetic and conductive material close to the weld (e.g. the work-piece itself , parts of the equipment cabinet or metallic structures at workplaces) may lead to distortions, of the waveform as well as the field distribution, and have to be considered, at least when evaluating workplaces with well defined, fixed configurations.

5. PROPERTIES OF THE WELDING CURRENT

In resistance welding a wide range of current waveform is used, from d.c. or sinusoidal a.c. current sequences to complex waveforms, e.g. consisting of successive phases for preheating and welding. A straight-forward pre-assessment method, eliminating the need for complex field measurements, is the analysis of the welding current.

When using spectral analysis, the decrease of amplitudes within the frequency spectrum of the welding current, consisting of a fundamental frequency (a.c. or pulse frequency) and its harmonics (multiples of the fundamental) depends, on the analogy of time domain methods, on the current rise- and fall-times. Fast changes in the current will produce harmonics with considerable amplitudes up to higher frequencies compared to smooth waveforms. The simplest case for an alternating current, disregarding the pulsed nature of a spot weld, will be a purely sinusoidal signal shape with only one single frequency component of the spectrum at the fundamental frequency, e.g. 50Hz.

Also other a.c. components of the current-spectrum exist, which are based on the technology used in the welding power source, both for a.c. and d.c. welding. The amplitudes of these

Z X

X [m] Z [m]

ripple currents due to “imperfect” smoothing of the secondary circuit of the power-source are, compared to the peak current value, usually quite low, however they will considerably contribute to the total exposure due to the resulting higher frequencies, linked to lower permissible field-strength values and increased coupling to biological structures.

Figures 4a and 4b show the welding current waveform and r.m.s. amplitudes of the spectral components of a typical thyristor-controlled, a.c. resistance welding equipment. Varying deviations from a purely sinusoidal waveform, due to the influence of the phase angle control, can be clearly identified during the start-, weld- and end-phases.

time [ms] -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 0 50 100 150 200 250

synthesis welding current

Figure 4a: Waveform of typical a.c. welding current, the amplitude is given in [A]

0.01 0.1 1 10 100 1000 10000 4 43 82 121 160 199 239 278 317 356 395 434 473 512 551 590 629 669 708 747 frequency [Hz]

Figure 4b: Spectral components of typical a.c. welding current the r.m.s amplitudes are given in [A] on a logarithmic scale

Figures 5a and 5b show the welding current waveform and r.m.s. amplitudes of the spectral components of a typical thyristor-controlled, d.c. resistance welding equipment. The superimposed 300Hz ripple current, due to six pulse-rectification and phase angle control, can be clearly identified during the quasi-static current phase and in the spectrum chart.

time [ms] 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 20 40 60 80 100 120 140 160

synthesis welding current

0.01 0.1 1 10 100 1000 10000 7 75 143 211 279 348 416 484 552 620 688 757 825 893 961 1029 1097 1165 1234 1302 frequency [Hz]

Figure 5b: Spectral components of typical a.c. welding current the r.m.s amplitudes are given in [A] on a logarithmic scale

6. ASSESSMENT OF NON SINUSOIDAL SIGNALS 6.1 General

As the Reference Levels (Action Values) and Basic Restrictions (Exposure Limits) are defined as frequency dependent values, exposure assessment of signals consisting of multiple frequency components is not trivial. Generally all components with “additive” effects (i.e. within a frequency range causing basic effects like stimulation or heating) have to be considered, with the provision that compliance of each component with the relevant limit is not sufficient. In contrast to technical emission restrictions, e.g. to achieve electromagnetic compatibility, spectral components have to be weighted, based on their corresponding limit, and summed. The total “exposure quotient”, i.e. the degree of exploitation of the permissible values, shall be below 1. Several methods with different complexity are applicable, they are discussed in the following paragraphs.

6.2 Frequency domain - conservative summation

For simple, but worst case summation of induced current densities Equation 1, for magnetic flux densities Equation 2 may be applied.

∑

∑

∑

Jt is the total relative induced current-density, expressed as a fraction of the limits

Ji is the induced current-density component at frequency fi

JL,i is the corresponding current-density limit at frequency fi

Bt is the total relative magnetic flux-density, expressed as a fraction of the limits

Bi is the magnetic flux-density component at frequency fi

fsco is the summation cut off frequency, where thermal effects get dominant

b is a fixed magnetic flux-density value, valid above fsco

As no phase information is used in these summation formulae, this method can lead to significant overestimation of exposure as not all (sinusoidal or co-sinusoidal) frequency components reach their peak values at the same time, on the contrary, even partial cancellation effects typically occur. However, this procedure is suitable for evaluation of

exposure due to multiple sources (equipment) at a single workplace, where a defined phase relationship does not exist.

6.3 Frequency domain - phase summation

A way to avoid overestimation when assessing coherent multi-frequency signals, which are always the nature of non-sinusoidal or pulsed welding currents and the magnetic field generated by them, is to consider the phase relationship between the spectral components. Equation 3 is an example for phase corrected summation of current densities.

1 ) 2 cos( , ≤ + +

∑

Ji is the induced current-density component at frequency fi

JL,i is the corresponding current-density limit at frequency fi

fi is the frequency of the spectral component i

θi is the phase angle of the spectral component at frequency fi

ϕi is the phase angle of the biological weighting mechanism inside a nervous cell

6.4 Time domain assessment

This approach, in contrast to the methods above, is based directly on the course of the magnetic field, without the need to perform spectral analysis. The derivatives of the magnetic flux density are used to assess the magnitudes of induced current densities. Some international and national documents (e.g. CENELEC prEN 50505 or the German EMF regulation BGV B11 [13]) contain detailed procedures, which are not further discussed here. A disadvantage of these methods may be that some of them include subjective approximation or separation steps for different parts of the signal, e.g. the basic wave-shape and superimposed ripple components of the welding current / magnetic field, which are difficult to include in computerized assessment routines.

7. FIELD BASED ASSESSMENT 7.1 Measurement

Measurements of the magnetic field-strength or flux-density are a fast and simple way to evaluate the EMF exposure at welding workplaces against Reference Levels, where this worst case approach is acceptable. Because of the multi-frequency field generally produced by typical welding currents and the vector-characteristic of the field (i.e. the amplitude of the measuring result is depending of the orientation of a single-axis measuring instrument) the field probe used for the assessment of time-varying EMF in resistance welding applications should ideally fulfil the following requirements:

• Broad frequency and magnitude range to properly cover all relevant field components, e.g. a bandwidth from d.c. to some 100 kHz and B ranges up to some 100 mT.

• Isotropic characteristics of the used measuring transducer to avoid scanning for worst case probe orientation with single-axis field probes.

• Internal 3D computation and weighting of the measured values. Probes showing the result as a percentage of the applicable Reference Level are commercially available.

Figure 6 shows two different measurement situations with manual welding guns, using field sensors containing Hall-elements and pick-up coils with isotropic properties. The different size of the sensors leads to diverging spatial averaging effects, which is an important detail to be considered, in particular when measuring highly non-uniform fields.

Figure 6: Typical measurement scenarios with different field sensors

7.2 Analytical calculation

Simple analytical models may be used to evaluate the field generated from the welding circuit. For equipment with single-sided welding tools, where two separate cables are used to connect the welding tool and the return cable, a single infinitely long straight wire model may be used. For the hose pack of double-sided welding tools, where both cables are connected to the welding gun, a parallel, infinitely long straight wire pair model may be used. For welding arms a rectangular model may be applied. The calculation results may be used for comparison to reference levels or as input data for further procedures such as current density modelling. The basic equations for a single infinitely long straight wire (4), a simplified pair of wires approach (5) and a rectangular circuit (6), based on the welding current I flowing in these

structures are given below.

π

μ

d

I

B

2

=

y

Ia

B

π

μ

≈

d is the distance to the wire

(

)

∫

×

₋

_′

−

′

=

4

r

r

r

r

l

Id

B

_r

_r

r

r

r

π

μ

The example given in Figure 3 was calculated using a software tool based on the model for a rectangular circuit (6). However, the application of these simple models is confined to approximated considerations. For the assessment of real exposure situations very often numerical simulation methods have to be used in order to obtain realistic results.

7.3 Numerical simulation 7.3.1 Input data and methods

These method may be used to derive magnetic field strength values, based on the welding current and sophisticated models, e.g. for the evaluation of complex spatial field distributions due to system geometries and specific workplace properties. The calculation results may be used for comparison to limits or as input data for further procedures such as current-density modelling.

Numerical field simulations may be performed by using the finite element method (FEM), the scaled frequency finite-difference time-domain (FDTD) method or other available software codes. Some of the commercially available packages provide integrated CAD-tools, which can easily be used to model the current path and the most important elements of the welding equipment, together with relevant metallic parts in the working environment.

Figure 7: Examples for numerical simulations of magnetic flux density distribution, influences of metallic objects [16](left) and field produced by a complex source[15](right)

Figure 7 shows examples for field distribution scenarios established by using numerical simulation tools, illustrating the influence of ferromagnetic objects (e.g. the work-piece or other metallic structures) and complex field sources close to the welders body.

7.3.2 Pros and cons of numerical field simulation

• PRO The input metric is the welding current, which can easily be measured with commonly available instruments

• PRO Simulation of non uniform fields, due to realistic source geometries (welding guns and cabling) and magnetically effective parts of the environment, is possible

• PRO Simulation is time saving compared to measurements at fine, three dimensional grids

• PRO Field averaging over the volume of the exposed body can be done automatically, however this may lead to problems in highly non-uniform fields [14]

• PRO The result may be used as input data for further simulations, e.g. of induced current densities in various body models

• CON Realistic simulation of complex source structures (e.g. internal components of the equipment) is still difficult or impossible

• CON The results have to be compared to the conservatively derived Reference Levels, which may lead to overestimation of exposure and therefore to unnecessarily stringent application restrictions

8. DIRECT ASSESSMENT OF BIOLOGICAL EFFECTS 8.1 Body models

The human body represents a structural receiving antenna when exposed to electromagnetic fields, therefore its geometrical and electrical properties play a major role in EMF assessment. Many analytical and numerical models for the evaluation of intra-corporal effects, such as induced current densities, replace the body by simple homogeneous structures like disks, cubes or spheroids with spatially uniform conductivity. For these homogeneous models a fixed conductivity, representing an average value over the entire body, is used which is usually not frequency dependent.

This makes the computations easier, however the results of such procedures tend to overestimate the actual exposure considerably. Simple models are a practical way for assessment if the field-strength is expected to be well below the permissible levels.

Examples for currently available body models of different complexity are given in Figure 8.

Figure 8: Body models for evaluation of exposure to EMF : 2D homogeneous disc model 3D homogeneous cubic model, 3D homogeneous spheroid model

and 3D complex anatomical model (from left to right)

The high-resolution three-dimensional anatomical model is based on the Visible Human Project® 5, where slices of a human body were analysed, using CT and MRI technology. The tissue types and their position in the numerous slices were mapped and an extensive set of digitised data collected. This results in a heterogeneous (up to 115 different body tissues) numerical model, which allows to reproduce a realistic exposure scenario with a spatial resolution of down to 1mm.

The resulting detailed anatomical structure is combined with data from Brooks Air Force Base 6 _{, where the dielectric properties of all identified tissues (relative permittivity ε´ and}

conductivity σ) have been studied and mathematical models for the frequency behaviour, valid over a wide frequency range, were developed. Examples for the frequency dependent

5 _{http://www.nlm.nih.gov/research/visible/visible_human.html} 6_{http://www.brooks.af.mil/AFRL/HED/hedr/reports/dielectric/Report/Report.html#Mes_Acc_Freq_Range} R d A I x y

values of conductivity σ [S m-1] of Central Nervous System tissues, which are of primary interest for the evaluation of effects of induced current densities, are given in Figure 9.

0.01 0.1 1 0.01 0.1 1 10 100 1000 10000 frequency [kHz] c o n d u c ti v ity [S /m ] GRAY MATTER WHITE MATTER CEREBELLUM EYE - SCLERA

NERVE / SPINAL CORD

Figure 9: σ values of Central Nervous System tissues in the frequency range 10Hz – 10MHz

8.2 Analytical calculation

The simplest model is based upon the hypothesis of coupling between an external magnetic field, uniform and time-varying, and a homogeneous disk of given conductivity, used to model the part of the human body under consideration. Such models are used for example in ICNIRP guidelines.

The objective of such a model is to propose a simple method to assess induced currents and internal fields. This very first approach is simple and gives conservative values of electrical quantities to be calculated. For time-varying magnetic fields, the calculation assumes that the body or the part of the body exposed is a circular section of radius r , with conductivity σ.

Induced currents are distributed inside the disk, following a rotation symmetry around the central axis of the disk. The value of induced currents is minimum at the centre (null formally) and maximum at the edge of the disk. The calculation is made under maximum coupling conditions i.e. with a uniform magnetic field perpendicular to this disk. In this case, the induced current density at radius r is given by:

dt dB r J r 2 ) ( =

σ

dB/dt is the rate of change of the magnetic flux density (measured or calculated)

r is the radius of the conductive disk

σ is the electrical conductivity, usually specified as 0,2 S m-1

Considering a sinusoidal single frequency magnetic field of frequency f, it gives:

rfB

These basic formulas show that the amplitude of the induced currents is indirectly proportional to the rise and fall times, respectively directly proportional to frequency and amplitude of the welding current.

8.3 Numerical current density simulation 8.3.1 General

Models which take the non-uniform field characteristics and/or the properties of a heterogeneous body into consideration are more realistic than those which are based on uniform fields and/or homogenous conductive discs. Induced current densities may be derived by simulation of currents in 2 or 3 dimensional structures (2D or 3D computations).

8.3.2 Input data and methods

The input data may be a given (measured or simulated) time-varying magnetic field distribution, alternatively the simulation procedure may include the calculation of the two or three dimensional field distribution in the body model, based on the field source geometry an the actual welding current.

Numerical current density simulations may be performed by using the finite element method (FEM), the impedance method, the scaled frequency finite-difference time-domain (FDTD) or other available tools. In addition to the procedures discussed in 7.3.1, the human body model is implemented by using the CAD tool integrated in the software package used.

Figure 10 shows examples for the current density distribution in a cut of the welders body for a given source topology and a sinusoidal welding current of 1000A at frequencies of 50Hz (left) and 5kHz (right) [15], with considerably higher results for 5kHz.

Figure 10: Simulation results in cuts of a welders body for 50Hz and 5kHz

The results of simulations at a set of representative frequencies can be used to assess the effects of any welding current waveform by summing the resulting current densities of all its spectral components, as discussed in 5 and 6. When these basic simulations are additionally performed at different distances to the welders body, they may be used to derive minimum distances and a “profile” showing the distance-exposure relation for the assessed welding situation.

Results for various exposure scenarios for arc welding are already available at FRONIUS, studies regarding representative resistance welding exposure situations, based on the same

principles but considering specific source topologies, especially for manually used welding guns at typical positions, are currently in progress.

Figures 11 and 12 show evaluation results for typical resistance welding current waveforms, based on arc welding topologies which are, however, similar to the exposure situation using a single sided welding tool (e.g. for car body repair).

time [ms] -1 -0.5 0 0.5 1 0 20 40 60 80 100 120 140 160 Σ R rel ( limit = +/- 1 ) -500 0 500 1000 1500 2000 2500 3000 3500 4000 4500

welding current [A]

summation R relevant synthesis welding current

Figure 11: Assessment results for 4000A d.c. welding current with 300Hz ripple

time [ms] -1 -0.5 0 0.5 1 0 50 100 150 200 250 Σ R rel ( limit = +/- 1 ) -4000 -3000 -2000 -1000 0 1000 2000 3000 4000

welding current [A]

summation R relevant synthesis welding current

8.3.3 Pros and cons of numerical current density simulation

• PRO The input metric may the welding current, which can easily be measured with commonly available instruments

• PRO Simulation of non uniform fields, due to realistic source geometries (welding guns and cabling) and magnetically effective parts of the environment, is possible

• PRO Use of heterogeneous body models and therefore elimination of overestimation due to simplified, conservative models is possible

• PRO Isolation of CNS current densities and the possibility of averaging over areas, as defined in the exposure guidelines, leads to a realistic relationship to exposure limits

• PRO Determination of realistic minimum distance and exposure distribution is possible

• CON Some details of simulation parameters (e.g. an-isotropic properties of a few body tissues or the optimum spatial resolution of the body model to be used) have to be finally defined in regulatory documents

• CON Realistic simulation of complex source structures (e.g. internal components of the equipment) is still difficult or impossible, this might be overcome by the use of (very time consuming) fine grid measurements as input data for the current density simulation

9. COMPARISON OF RESULTS

As the methods discussed in the previous paragraphs contain safety factors, conservative approximations and overestimations on different scales, they lead to diverging results, representing more or less conservative figures. The following table compares different assessment results for two resistance welding scenarios.

Principle Method EQ20* d.c. welding EQ20* a.c. welding Conservative summation of

magnetic flux densities Analytical calculation 18,20 30,40 Phase summation of

magnetic flux densities Analytical calculation 8,80 8,10 Conservative summation of

induced current densities

Numerical modelling

using an anatomical body model 1,08 1,71 Phase summation of

induced current densities

Numerical modelling

using an anatomical body model 0,60 0,53

*EQ20 Exposure quotient at 20cm distance, expressed as a fraction/multiple of the permissible value

Table 1: Assessment results for typical resistance welding current waveforms as given in 5, using different methods for the same scenario, including uncertainties

10. SUMMARY

Currently published exposure guidelines and regulatory requirements contain various methods of varying complexity for the assessment of human exposure to electromagnetic fields. Whereas most of them are suitable to show compliance with the mandatory requirements for

products or workplaces with low or medium EMF levels, only some of them are useful for resistance welding applications. Using advanced numerical simulation tools enables the parties involved in exposure evaluation to obtain realistic (and therefore less conservative) results and thus to avoid overly stringent application restrictions or other unnecessary measures, to the advantage of both users and manufacturers of resistance welding equipment. 11. REFERENCES

[1] Directive 2004/40/EC of the European Parliament and of the Council of 29 April 2004 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields) (18th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC), Official Journal L159, 30.4.2004, p.1-26

[2] Mandate M 351, standardisation mandate addressed to CEN, CENELEC and ETSI to develop harmonised standards for the assessment, measurement and calculation of workers' exposure to static magnetic and varying electric, magnetic and electromagnetic fields with frequencies from 0Hz to 300 GHz, European Commission, Employment and Social Affairs DG, Brussels, 17. Mai 2004

[3] Council Directive 73/23/EEC of 19 February 1973 on the harmonization of the laws of Member States relating to electrical equipment designed for use within certain voltage limits, Official Journal L077, 26.3.1973, p.29-33

[4] Mandate M 305, standardization mandate addressed to CEN, CENELEC and ETSI in the field of electro technology, information technology and telecommunications, European Commission, Enterprise Directorate-General, Brussels, 7. September 2000

[5] Directive 98/37/EC of the European Parliament and of the Council of 22 June 1998 on the approxima-tion of the laws of the Member States relating to machinery, Official Journal L207, 23.7.1998, p.1-46 [6] ICNIRP Guidelines 1994, International Commission on Non-Ionising Radiation Protection,

Guide-lines on limits of exposure to static magnetic fields, Health Physics, Volume 66, 1994, p. 100-106 [7] ICNIRP Guidelines 1998, International Commission on Non-Ionising Radiation Protection,

Guidelines for limiting exposure in time-varying electric, magnetic and electromagnetic fields (up to 300 GHz), Health Physics, Volume 74, Number 4, 1998, p. 494 - 522

[8] EN 50198-1:2000, Safety of machinery - Assessment and reduction of risks arising from radiation emitted by machinery - Part 1: General principles, CEN

[9] EN 50198-2:2002, Safety of machinery - Assessment and reduction of risks arising from radiation emitted by machinery - Part 2: Radiation emission measurement procedure, CEN

[10] EN 50198-3:2002, Safety of machinery - Assessment and reduction of risks arising from radiation emitted by machinery - Part 3: Reduction of radiation by attenuation or screening, CEN

[11] prEN 50445:2006, product family standard to demonstrate compliance of equipment for resistance welding, arc welding and allied processes with the basic restrictions related to human exposure to electromagnetic fields (0 Hz – 300 GHz), CENELEC TC26A WG2 draft for 2nd enquiry

[12] prEN 50505:2006, basic standard for the evaluation of human exposure to electromagnetic fields from equipment for resistance welding and allied processes, CENELEC TC26B WG2 draft for 1st enquiry [13] BGV B11, Electromagnetic fields, German accident prevention regulation (in German), June 2001 [14] Magnetic Field From Spot Welding Equipment, is the Basic Restriction Exceeded ?, Mohammad

Nadeem 1_{, Yngve Hamnerius} 1_{,Kjell Hansson Mild} 2/3 _{and Mikael Persson} 1_{, 1 Department of} Electromagnetics, Chalmers University of Technology, Goteborg, Sweden, 2 National Institute for Working Life, Umeo, Sweden, 3 Department of Natural Science, Orebro University, Orebro, Sweden, Bioelectromagnetics 25, p. 278-284, 2004

[15] Simulation of induced body current density during exposure to magnetic fields from welding, Yngve Hamnerius and Mikael Persson, Yngve Hamnerius AB, study for a group of sponsors, 2005

[16] IIW Doc. No. XII-1901-06, Numerical Simulation of Electromagnetic Field in Arc welding, Satoshi Yamane, Yasuko Yamamoto and Kenji Oshima, Saitama University, 2006