Human exposure to EMF is a subject strongly felt by public opinion in several countries. For this reason, many studies have been carried out in order to accurately assess human exposure and to design field mitigation systems for power sources. The definition of reference levels and basic restrictions for practical hazard assessment related to EMF are presented in the International Commission on Non-ionizing Radiation Protection (ICNIRP) guidelines, which cover occupational and general public exposure. Recently, the new Physical Agents European Directive (2004/40/EC modified by 2008/46/EC), which will probably be implemented in 2012, is expected to reconsider reference levels; however, the improvement in the mitigation systems for workers will remain an essential goal in the near future, especially in medical environment. The expertise required to handle this subject involves measurement capabilities, computational skills and knowledge of magnetic materials. The first are needed to assess human exposure, the last is fundamental to define efficient mitigation systems. Over the last ten years INRIM has developed a three dimensional experimentally validated model of a power substation (in 2000), supported many industrial activities by designing and realizing new shielding systems, and participated in numerous research projects dedicated to mitigation and human exposure assessment.
The Electromagnetism Division is currently involved in the development of the aforementioned topics and contributes to defining the state of the art by improving the reference standards and by developing measurement techniques and new numerical constitutive models of magnetic materials. Traceable calibration in reference fields and proper estimation of measurement uncertainty are essential requirements to ensure accuracy and comparability of the measurements carried out in the process of assessing human exposure. Moreover measurement accuracy is essential for the validation of the numerical models employed in the simulated analysis of mitigation systems. Time-varying reference electric, magnetic and electromagnetic fields are generated at INRIM by parallel plates systems, Helmholtz coil based systems, waveguides (TEM and GTEM) and in an anechoic chamber (fig. 1) from a few hertz up to 4 GHz [1-2].

Fig. 1 Characterisation of a biconical antenna in the INRIM anechoic chamber.

Topics of interest include the analysis of meter behaviour in the presence of distorted and/or pulsed magnetic field waveforms and of digitally modulated electromagnetic fields in relation to the evaluation of the exposure indexes. With an integrated numerical-experimental approach, studies are carried out to identify and quantify significant uncertainty components, such as those introduced by the dielectric support of the electric field probe and by the spatial non-uniformity of the field distributions.
During 2008 the Electromagnetism Division developed computational tools, based on the Boundary Element Method, able to estimate the current density and the SAR (Specific Absorption Rate) induced inside a phantom exposed to electromagnetic fields from power-frequency to radiofrequency (fig. 2). Thus, a simple human model (based on the Reference Man, IEC 62226-3-1) is now available to evaluate the exposure of both general public and workers in typical industrial environments [3].

Fig. 2 Horizontal axis pipe induction heater. Chromatic map of the current density induced in the human body in the x-z plane. Indications are in Ampere per square meter. The pipe has an internal radius of 0.3 m, axial length of 1.5 m, magnetomotive force equal to 150 kA; the body axis is located at 1.2 m from the pipe axis.

The next target is the definition of a model able to relate SAR to the surface field measured on human phantoms (for instance, the specific anthropomorphic mannequin "SAM" phantom). This tool will be one of the elements of a hybrid experimental/numerical non-invasive procedure, providing traceable metrology for SAR. This work is developed in the framework of the EU supported project iMERA Plus "Traceable measurement of field strength and SAR for the Physical Agents Directive" with the aim of defining measurement references for the SAR in the whole ICNIRP guidelines frequency range. With reference to mitigation, starting from the late 80's INRIM has developed dedicated formulations to open boundary shielding problems. Recently the multiple scale expansion theory (MSET) has been applied to the analysis of heterogeneous thin structures employed for magnetic shielding. Attention has been focused on grid shields, since these structures are extremely useful when thermal dissipation is a restriction. The problem has been successfully faced on the one hand, by applying thin-shell approximation to Maxwell equations and, on the other hand, by developing a homogenisation technique, based on the MSET, in order to replace the heterogeneous structure with an equivalent homogenous one [4].
A study called "Powerfield" has been carried out in recent years with the aim of creating a simple 2D tool able to perform the mitigation design of cable systems. To this end, a suitable user interface has been developed including a solver based on the thin-shell formulation. As a result, with this numerical code a significant reduction of computational and training time is obtained with respect to the use of classical finite element tools. In recent months INRIM has been involved in the research project "Mitigation of magnetic fields produced by MRI: shielding solutions by superconductive and traditional elements" promoted by the Italian Ministry of Research and University (MIUR). Finally, it is worth mentioning that a significant contribution to the definition of general criteria concerning low frequency magnetic shielding has been given by INRIM participation in technical committees [5].

[1] M. Borsero, G. Vizio, D. Parena, V. Teppati: "Synthetic TDR measurements for TEM and GTEM cell characterization", IEEE Trans. on Instr. and Meas., Vol. 56, pp. 271-274, 2007.
[2] M. Chiampi, G. Crotti, D. Giordano: "Set up and characterization of a system for the generation of reference magnetic fields from 1 to 100 kHz", IEEE Trans. on Instr. and Meas., Vol. 564, pp. 300-304, 2007.
[3] O. Bottauscio, M. Chiampi, L. Zilberti: "Boundary Element Approaches for the evaluation of Human Exposure to LF Electromagnetic Fields", IEEE Trans. on Magn., Vol. 45, pp. 1674-1677, 2009.
[4] O. Bottauscio, M. Chiampi, A. Manzin, P. E. Roccato, M. Zucca: "A multiscale approach to the analysis of magnetic grid shields and its validation", J. of Comput. Physics, Vol. 22, pp. 1470-1482, 2007.
[5] Technical brochure "Mitigation techniques of power-frequency magnetic fields originated from electric power systems", edited by Cigré (February 2009).