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.
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6.
7.
8.
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