Introduction to the Directive
The European Directive 2013/35/EU is concerned with the health and safety implications of exposing workers to electromagnetic fields and describes the requirements for working practices to reduce the impact of these fields. The Directive is only concerned with short-term exposures to electromagnetic fields and gives no consideration to long-term exposure or effects.
Two categories of effects are defined in the Directive. These are “direct biophysical effects” which are effects in the body caused by its presence in an electromagnetic field and “indirect effects” which are effects caused by the presence of an object in an electromagnetic field.
Direct biophysical effects can be in the form of thermal effects, non-thermal effects or limb currents. Thermal effects are tissue heating through the tissues of the body absorbing energy from the magnetic field. Non-thermal effects are where the electromagnetic field stimulates the nerves, muscles or sensory organs. This may have a negative effect on health but may also cause transient sensory symptoms such as vertigo, nausea or "seeing stars" which can affect the ability to work safely.
Indirect effects can include interference with medical devices, e.g. pacemakers, the projectile risk of ferromagnetic objects moved by the field, the initiation of electric detonators, spark discharges to flammable items or contact currents.
The Directive considers electromagnetic fields as separate electric and magnetic fields up to a frequency of 10 MHz and as a combined electromagnetic field from 100 kHz to 300 GHz. Welding is only concerned with frequencies up to approximately 100 kHz and does not create any significant electric field. As such, regarding welding, only magnetic fields will be considered and limited to 100 kHz consideration.
Two possible ways of quantifying the magnetic field strength limit are used. These are “exposure limit values” (ELVs) and “action levels” (ALs). These cover the internal magnetic field and the external magnetic field respectively. The internal field is caused by the coupling of the internal body currents to the external magnetic field.
Exposure limit values
Exposure limit values are based on experimentally established short-term effects such as thermal effects and stimulation of tissues. For low frequency (below 10 MHz) magnetic fields, the ELVs are specified in terms of an electric field induced in the body, with limits given in V/m. Two ELVs are of importance for the welding field and these are the “sensory effects ELVs” and “health effects ELVs”.
The sensory effects ELV define the limits above which workers may experience temporary sensory perceptions, such as retinal phosphenes (“seeing stars”) or vertigo or subtle changes in decision-making and reasoning. The health effects ELV define the limits above which workers might experience adverse health effects such as thermal effects or direct stimulation of tissues.
Health effects ELVs can be considered “hard limits” which are to not be exceeded in typical working practices. They are only allowed to be exceeded in “duly justified circumstances” as mentioned in the Directive, such as magnetic resonance imaging or military applications.
The sensory effects ELVs may be exceeded where justified by the practice or process, provided that they are only temporarily exceeded, the health effects ELVs are not exceeded, action is taken to ensure that any sensory effects are managed to avoid secondary safety hazards and workers are informed of the possible sensory effects. Temporary exceedance is not defined within the Directive.
As these ELVs are defined relative to an internal electric field, they are not measurable in situ. Currently, the most common way of assessing these limits is to perform significant computational simulations. Each simulation is only applicable for a single welding equipment geometry and operator position as changes to these factors alter how the magnetic field couples with the operator and changes the internal electric field. The EMFWELD software contains the data from many of these simulations.
Action levels
Due to the difficulty of assessing the internal electric field the Directive also defines action levels, which are limits for the external magnetic field. These are based on extrapolation of how an external magnetic field couples with the body to produce an internal electric field and as such are more conservative than the ELVs. However, the external magnetic field strength limits, given in Tesla, are much easier to assess using magnetic field probes in situ.
There are three specified action levels for low frequency magnetic fields. These are the “low”, “high” and “limb” action levels. The low action level relates to the sensory effects ELV and the high and limb action levels relate to the health effects ELV. The working principle of these action levels is that if they are not exceeded, the related ELVs are also not exceeded and so the working conditions are compliant with the Directive. One of the currently common ways of ensuring compliance with the Directive is to take the action levels as limits and not exceed them.
The action levels can be exceeded if it can be demonstrated that the related ELVs are not exceeded. Again, the sensory effects ELV can also be exceeded assuming the conditions described above are met.
Multi-frequency sources
The ALs and ELVs are frequency dependent, varying across the range of frequencies covered as shown in Figures 1 and 2. However, the majority of magnetic field sources (as are likely to be encountered in welding industries) are not single frequency, but instead have complex current and magnetic field waveforms which can contain many different fundamental and harmonic frequencies.
For example, while a single phase mains frequency pedestal resistance welder will have a fundamental frequency component of 50 Hz, it will also possess harmonic frequency components of 150 Hz etc. Also, if the waveform is “chopped”, the sudden drop in current from peak to zero can introduce even higher frequency components. An example magnetic field waveform is shown in Figure 3.
There are two approaches to assessing non-sinusoidal multiple frequency component waveforms. The first is to perform a Fast Fourier Transform on the waveform, to select out all of the frequency components. The magnetic field strength of each frequency component is then compared to the relevant limit and a summation is performed to determine whether the limit has been breached across the entire frequency spectrum (making use of certain rules and assumptions).
The second approach to assessing non-sinusoidal multiple frequency component waveforms is the “weighted peak method (filtering in time domain)” as mentioned in the Directive, which is the method EMFWELD uses. In this method, the collected magnetic field waveform is processed through a filter (either hardware analogue or simulated digital) which has an instantaneous amplifying response equivalent to the frequency dependent limits.
When a waveform passes through this filter, it is scaled to an output value, with an output value of 1 typically corresponding to 100% of the relevant limit. An example output of such a filter is shown in Figure 3. With the waveform as shown, the magnetic field strength would be assessed at 350% of the low AL.
This process is how handheld magnetic field measuring probes work, with real-time filtering of the magnetic field strength measured by a magnetic probe. The process can also be done post-measurement, by sampling the magnetic field at a sufficiently high frequency (~500 kHz) with a magnetic field probe and oscilloscope and then processing the magnetic field waveform later with a simulated digital filter. This is the EMFWELD method.
Requirements on employers
The Directive lays out certain requirements that employers must meet, beyond the simple magnetic field strength limits. These include performing a risk assessment of electromagnetic field exposure, health surveillance of employees, ensuring information is available to employees through both training and signage where applicable and managing risk from secondary dangers where applicable.
Risk assessments should take account of the magnetic field strength relative to the various applicable levels, the specific geometry of the magnetic field, alternative methods of working that can reduce the exposure to the electromagnetic field and several other factors described in the Directive.
If the magnetic field to which employees are exposed is demonstrated to be above the applicable action level or exposure limit value then action must be taken to reduce the magnetic field strength to below the relevant limits.
If it is demonstrated that the magnetic field to which employees is exposed is potentially going to lead to sensory effects, either by being above the low action level or sensory exposure limit value, then additional steps must be taken. It is necessary to ensure that any sensory effects will not cause any additional safety risks. It is also important to inform the workers who are exposed to the magnetic field of the possible sensory effects and place signage if needed. The possibility of sensory or health effects should also be considered as part of regular employee health surveillance.
Employees at particular risk
Certain employees are considered to be at particular risk from electromagnetic fields. These are employees who have either told their employer that they are pregnant or who have an active implanted medical device (AIMD) e.g. a pacemaker. It is recognised that these cases may experience interference problems at magnetic field strengths below the action level.
The typical approach to take is as that suggested in BS EN 50527-2-1, which states that “pacemakers are expected to work uninfluenced as long at the General Public Reference levels of Council Recommendation 1999/519/EC are not exceeded”. The general public levels are still defined in more recent ICNIRP guidelines, however EMFWELD does not currently assess against these levels. Work to develop this capability is ongoing.
Practical guide
Non-binding practical guides have been written to aid in implementation of the Directive and links to these are in the links section below.
Useful links:
Directive 2013/35/EU
International Commission on Non-Ionizing Radiation Protection (ICNIRP)
EU Non-binding practical guide
Volume 1 - Practical guide
Volume 2 - Case studies
Guide for SMEs
