Foetal exposure to magnetic field produced by induction hobs Bor Kos1,2, Blaž Valič2, Tadej Kotnik1, Damijan Miklavčič1 and Peter Gajšek2 1Faculty of Electrical Engineering, University of Ljubljana, Slovenia 2Institute of Non-ionizing Radiation, Ljubljana, Slovenia Results 7 a b 6 Measurement at x=1 cm Computation at x=1 cm Measurement at x=10 cm Computation at x=10 cm Measurement at x=30 cm Computation at x=30 cm EN 62233 reference level 5 B [µT] Introduction Induction hobs are electrical cooking appliances that use intermediate-frequency magnetic fields to heat the cooking vessel directly (1). The induction hobs typically operate in the frequency region of between 20 and 100 kHz, and can deliver of up to 3.6 kW (2). There are few reports on human exposure to the magnetic fields generated by induction cookers. Measurements of domestic appliances have been reported (3,4), and the fields were below the ICNIRP (5) reference levels. Close to the devices, the magnetic field approaches, and in some operating conditions even exceeds the reference levels for magnetic flux density. 4 3 2 1 0 0 10 20 30 40 50 z [cm] Figure 1: Measured and computed BRMS at Figure 2: Model setup in front of various distances and heights from the appliance. the appliance: a) 30 weeks pregnant b) 26 weeks pregnant. Model M1 M2 Max E (whole body) [V/m] Max E (UFU) [V/m] Max J (whole body) [mA/m2] Max J (UFU) [mA/m2] 0.111 0.029 47.1 47.1 0.660 0.079 42.3 23.3 E field 999th per-1000 (whole body) [V/m] E field 99th percentile (UFU) [V/m] J field 999th per-1000 (whole body) [V/m] J field 99th percentile (UFU) [V/m] 0.038 0.024 27.5 28.8 0.080 0.033 17.9 18.8 % of basic restriction Internal E field (ICNIRP 2010) Internal E field UFU Induced J field (ICNIRP 1998) Induced J field UFU 2.3% 0.62% 67% 67% 14% 1.66% 60% 33% E field (whole body 999th per-1000) E field (UFU 99th percentile) J field (whole body 999th per-1000) J field (UFU 99th percentile) 0.8% 0.52% 39% 41% 2% 0.69% 26% 27% a b 0 dB -10 dB Methods The standard EN 62233 (6) details a measurement procedure in which the magnetic flux density is measured along a vertical line at 30 cm from the front of the device, averaged over an area of 100 cm2. The measurement of magnetic flux density was performed using a calibrated Narda ELT-400 field meter at 30, 10 and 1 cm from the front of the appliance. The induction cooker was modeled as three vertically displaced, concentric current loops. The middle current loop represents the main (source) coil, while the upper and lower current loops represent the eddycurrents in the base of the pot and in the ferrite flux guides below the main coil, respectively (1). We used the magneto quasi-static low-frequency solver implemented in SEMCAD X v 14.4 (SPEAG, Zurich, Switzerland). The data show a good agreement, particularly at 10 cm horizontal distance from the appliance. Using SEMCAD X, we computed induced currents and electric fields in two pregnant women models, one built in-house from CT images (7), and the other based on MRI images (8). Each human model was positioned standing upright with the nearest point of the body being 180 mm from the center of the cooker, while in the lateral direction, the centers of the cooker and the body were aligned. Vertically, the models were positioned with their lowest point 850 mm below than the surface of the induction cooker. J basic restriction @ 35 kHz: 70 [mA/ m2] E basic restriction @ 35 kHz: 4.725 [V/m] Table 1: Computed values of induced E fields. -20 dB -30 dB -40 dB -50 dB Figure 3: Computed values in: a) 26 week pregnant, b) 30 weeks pregnant female models. 0 dB equals 0.1 V/m. The induced electric fields and current densities were computed in two pregnant models and are shown in Table 1. The results show that while the induced electric field is far below the basic restrictions, the induced currents could exceed the 1998 basic restrictions under normal operation. Figure 3 shows a cross section of E through the centre of the body in both models. Conclusion The results show that the magnetic fields produced by induction cookers do not cause the basic restriction for internal electric field (ICNRIP 2010) to be exceeded in cfoetuses. Maximum exposure of a foetus is less than 2 % of the new ICNIRP guidelines for limiting exposure to low-frequency electromagnetic fields. The induced currents according to the 1998 Guidelines are closer and could concievably exceed their respective basic restriction. References 1. 2. 3. 4. 5. 6. 7. 8. Acero J et al. Domestic Induction Appliances. IEEE Ind. Appl. Mag. 2010 3;16(2):39-47. Millan I et al. Resonant inverter topologies for three concentric planar windings applied to domestic induction heating. Electronics Leers. 2010 Aug 19;46(17):1225-U84. Stuchly M and Lecuyer D. Electromagnetic-Fields Around Induction-Heating Stoves. Journal of Microwave Power and Electromagnetic Energy. 1987;22(2):63-69. Viellard C et al. B-Field Exposure From Induction Cooking Appliances. Zurich, Swierland: 2007. 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