Date:
10-Sep-03
FAX COMMUNICATION
B-Field Exposure From Induction Cooking
No. of Pages (including this one):
1
Appliances
Re:
To:
Clementine Viellard, Albert Romann, Urs Lott, and Niels Kuster
Zurich, July 2006 (revised July 2007)
B-field Exposure From Induction Cooking Appliances
1
2
Executive Summary
Induction cooking uses the fact that alternating magnetic fields generate heat in the ferromagnetic cooking vessel due to magnetic hysteresis and induced eddy currents. Safety concerns
about exposure to magnetic stray-fields have arisen. Studies by Suzuki [1] and Yamazaki [3]
have assessed these magnetic fields with contradicting findings.
The objective of this study was to assess the maximum exposure that arises during use of
induction cooking devices. Three devices currently available on the Swiss market were selected:
the built-in appliances 1 and 2 (Electrolux GK58TCi and Gaggenau CI 261 110) and the portable
appliance 3 (Inducs SH/BA 5000). The appliances were mounted on wooden supports allowing
measurement in close proximity to the hobs without disturbing the B-field.
Fifteen pots and pans of different sizes and shapes, as well as of various materials were
evaluated in single and multi-hob use in order to select a worst-case set of pots corresponding
to the worst-case B-field exposure. A standard set of pots was also defined according to the
European Norm EN50366. The Narda probe ELT-400 specifications were validated, and the
probe was used for the characterization of the induction cookers in the time- and frequencydomains (cooking signal frequency, dependency on the heat setting, etc.).
In the first step, the spatial B-field exposure was evaluated according to EN50366 [4] (i.e.,
at a measuring distance of 30 cm using the standard set of pots). All three appliances met the
compliance criteria of ICNIRP [2] for incident B-fields by a margin larger than 14 dB (see Table
11).
In the second step, the worst-case exposure was evaluated as a function of pot and heating
configurations. It was demonstrated that different pot and and heating configurations can
result in exposures that well exceed +10 dB of the standard EN50366 configurations at the same
distance (see Figure 55). In addition, the field distribution has a strong negative gradient in the
direction of larger distances. Therefore, DASY4 was enhanced to enable 3D field scanning using
the NARDA probe. At the very short distance of 1 cm, the fields can be more than 30 dB larger
than at 30 cm (see Table 14). Combining the results from worst-case configurations and short
distance measurements, the standard EN50366 values can be exceeded by 37 dB (see Figure 55).
The uncertainty of the evaluation was determined to be 1.5dB (k=2).
The third task was to evaluate the findings with respect to compliance testing. Assuming
the distribution of Appliance 3, exposure close to the appliance could be as much as 37 dB or
a factor of 70 above the ICNIRP safety limits. A very simple approximation suggest that the
induced currents for such a worst-case compliant appliance would exceed the basic restrictions
by nearly a factor of 10. In other words, the current standard EN50366 for compliance testing
does not prevent exposures far above the basic restrictions and therefore needs revisions.
To obtain the scientific basis for a sound and reliable compliance test procedure, systematic
evaluations of induced currents as a function of human anatomy and field distributions are
necessary and recommended.
B-field Exposure From Induction Cooking Appliances
3
Figure 1: a) Comparison between the compliance value (B-field measured according to EN50366)
and the maximum B-field measured in the worst-case scenarios at 1 cm and 30 cm of Appliance 3.
b) B-field measured according to EN50366 and c) B-field measured in the worst-case appliance
(horizontal plane at height of appliance).
B-field Exposure From Induction Cooking Appliances
4
Contents
1 Executive Summary
2
2 Introduction
2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
6
6
3 Experimental Setup
3.1 Setup Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Induction Cookers . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Induction Cookers Available on the Swiss Market . . . . .
3.2.2 Characteristics of the three selected Induction Cookers . .
3.3 Pots and Pans . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Norm, Manufacturer Instructions and the Standard Set of
3.3.2 Worst-Case Set of Pots . . . . . . . . . . . . . . . . . . .
3.4 B-field Narda Sensor ELT-400 . . . . . . . . . . . . . . . . . . .
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Pots
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4 Evaluation of Measurement System
4.1 Sensor Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Time- and Frequency-Domain Characterization of the Induction Cookers .
4.2.1 Time-Domain Characterization . . . . . . . . . . . . . . . . . . . .
4.2.2 Frequency-Domain Characterization . . . . . . . . . . . . . . . . .
4.2.3 B-Field Exposure Dependency on the Heat Settings . . . . . . . .
4.3 Uncertainty Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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5 Assessments According to EN50366 conditions
5.0.1 Measuring Conditions in EN50366 . . . .
5.0.2 Appliance 1 . . . . . . . . . . . . . . . . .
5.0.3 Appliance 2 . . . . . . . . . . . . . . . . .
5.0.4 Appliance 3 . . . . . . . . . . . . . . . . .
5.0.5 Discussion . . . . . . . . . . . . . . . . . .
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6 Worst-Case Exposure Conditions for Varied Loading of the Induction Cookers
6.0.6 Predominant Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.0.7 Negligible Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1 Induction Cookers B-Field Assessment . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 Definition of Several Scenarios Corresponding to Standard and Worst-Case
Exposures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 Appliance 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.3 Appliance 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.4 Appliance 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.5 B-field Measurement Above the Appliances . . . . . . . . . . . . . . . . .
6.1.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
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59
66
68
7 Approximation of Induced Current Density
70
8 Conclusion
72
B-field Exposure From Induction Cooking Appliances
9 Acknowledgments
5
73
B-field Exposure From Induction Cooking Appliances
2
6
Introduction
2.1
Background
Induction cooking has gained popularity since its introduction in the 1980s. It combines the
advantages of traditional gas and electric cooking, i.e., fast heat adjustment, precise cooking
temperature control and a glass-ceramic cooking top, allowing easy and fast cleaning. Additionally, the surroundings of the hob barely heat up, thus minimizing the risk of burning. However,
this also allows a person to stay in the vicinity of the cooking area, where the magnetic flux
density can have large values because of the proximity to the current coils.
Induction cookers generate strong magnetic fields by passing an alternating current (frequencies typically a few tens kHz) through the coils in the hob. When magnetic-material pans are
placed on the hob, the magnetic field generates heat in the ferromagnetic cooking vessel due to
magnetic hysteresis and induced eddy currents.
Devices emitting electromagnetic fields (EMF) are present in our living environment in ever
increasing numbers. This stimulates public concern about possible health effects due to exposure
to EMF. Safety concerns about exposure to magnetic stray-fields have arisen, since the fields
from the induction cooker cannot be fully shielded and are induced in the close vicinity of the
user (e.g., pregnant cook).
Studies by Suzuki and Yamazaki have assessed the magnetic fields with opposite findings.
Suzuki [1] reported that the maximum induced currents (numerical dosimetry) in the vicinity
of an induction cooker did not exceed the basic restrictions provided by ICNIRP [2]. However,
in a recent publication, Yamazaki [3] assessed the magnetic field from an induction cooker hob
(1.3 kW) and reported B-field values exceeding the ICNIRP guidelines: B-fields (rms-value)
greater than 14µT were measured for the fundamental frequency of 26.1 kHz, whereas the ICNIRP derived limit (general public exposure, 3 to 150 kHz) is only 6.25µT [2].
2.2
Objective
The objective of this study was to assess the exposure that arises during use of induction cooking
devices. This was achieved by the detailed evaluations of three devices currently available on
the Swiss market (one portable and two built-in appliances):
• First, the B-field exposure was assessed according to the European Norm EN50366 [4].
• Secondly, the worst-case field distributions were determined through measurements with
combinations of pots and pans of different sizes, shapes, and materials in single and multihob use.
• Third, the adherence of the compliance procedure of the European Norm EN50366 to the
basic restrictions of ICNIRP were discussed.
B-field Exposure From Induction Cooking Appliances
3
7
Experimental Setup
3.1
Setup Design
In order to find and evaluate the worst-case B-field exposure that arises during the realistic
use of induction cookers, the experimental setup shown in Figure 2 has been implemented. It
consists of:
• Three induction cooking appliances currently available on the Swiss market (winter 2005/06):
one portable and two built-in devices. All three appliances are mounted on a wooden support, allowing measurements in close proximity (see 3.2).
– Appliance 1: Electrolux GK58TCi (built-in)
– Appliance 2: Gaggenau CI 261 110 (built-in)
– Appliance 3: Inducs SH/BA 5000 (portable)
• 16 pots and pans of different sizes, shapes, and materials, including a standard set of pots
and a worst-case set of pots (definition in 3.3).
• The Narda exposure level tester ELT-400 (Narda GmbH, Pfullingen, Germany), used in
all measurements of the magnetic flux density (see 3.4).
• The DASY4 dosimetric assessment system from SPEAG (Zurich, Switzerland), used to
map the spatial B-field for each device. The software and hardware of the DASY4 system
has been extended to also allow 3D-scanning with probes from other manufacturers.
3.2
3.2.1
Induction Cookers
Induction Cookers Available on the Swiss Market
Market review A review of induction cooking equipment available on the Swiss market is
presented in Table 1 (as of 1. Dec. 2005). The distinction was made between built-in and
portable appliances. The portable appliances typically provide higher power for the hobs, which
may correspond to a higher B-field exposure, and a larger variety of designs, control options and
accessories. The built-in equipment can be divided into four groups with similar technical data:
Miele/Fors, AEG/Electrolux, Bauknecht/IKEA and Siemens/Gaggenau. Only the layout of the
hobs and control panel is different. It can be assumed that different brands source modules from
the same few OEM (original equipment manufacturers).
B-field Exposure From Induction Cooking Appliances
8
Figure 2: Experimental setup to assess the B-field exposure. The Narda probe ELT-400 is
attached on the DASY4 robot system, and the induction cooking appliance is mounted on a
wooden support.
Model
68001K-in
GK58TCi
ETPI5640
ELDIG HOB V00
E775501
CI 261 110
GK46TI
VI 411-110
AEG
Electrolux
Bauknecht
IKEA
Siemens
Gaggenau
V-Zug
Gaggenau
Inducs
Inducs
Inducs
SH/BA 3500
SH/BA 5000
SH/DU/BA 5000
portable induction cookers
KM492
SIR60FB
Miele
Fors
built-in induction cookers
Manufacturer
Brand
3.5
5.0
5.0
(3.5)
(2.5)
1.7 (2.5)
1.7 (2.5)
1.4 (1.9)
1.4
2.2 (3)
2.2 (3)
1.8 (2.3)
1.4
Front-left
5.0
-
(1.8)
2.2 (3.3)
2.2 (3.3)
1.4 (1.9)
1.8
1.8 (2.3)
1.8 (2.3)
1.4
1.8 (2.3)
Front-right
-
-
(2.5)
1.7 (2.5)
1.7 (2.5)
2.2 (3)
2.2
1.8 (2.3)
1.8 (2.3)
1.8
2.2 (3)
Rear-left
-
-
(3.2)
1.2 (1.8)
1.2 (1.8)
1.8 (2.3)
1.4
1.2 (1.5)
1.2 (1.5)
2.2 (3)
1.8
Rear-right
Rated hob power [kW] (with booster function)
3.5
5.0
10.0
3.5
7.4
6.8
6.8
6.8
6.8
7
7
7.2
7.2
Rated total power
(no booster)
0
0
0
1
4
4
4
4
0
4
4
2
2
Hobs with
booster function
Front
Front
Front
Front
Rear
Front
Front
Rear
Rear
Front
Front
Rear
Rear
Strongest
hob location
Table 1: Selection of induction cooking equipments available on the Swiss market (as of 1. Dec. 2005). Other manufacturers such as
Brandt-Fagor, Haier and Maytag are not present on the Swiss induction cooker market. Whirlpool is represented by Bauknecht. This list
contains the most widely available equipment but is not complete.
B-field Exposure From Induction Cooking Appliances
9
B-field Exposure From Induction Cooking Appliances
10
In Table 1, the maximum rated power of each hob is given without and with the booster
function for each model. The total power of the appliance is also provided (sum of the specified
maximum power of each hob without booster). The booster function permits the application
of additonal power to a specific hob, with the limitation that not all hobs are available for use;
otherwise the maximum rated power would be exceeded. Table 1 lists the number of hobs with
the booster function available.
built-in equipment characteristics All built-in appliances in Table 1 have four hobs, except
the Gaggenau VI 411-110 model, which has a single-hob. The hobs have different sizes, with
diameters varying from 10 cm to 23 cm. The appliances have a compact layout (approx. 60 cm
width, 55 cm length) without extra warming zones. The surface is a glass-ceramic plate, with an
integrated control panel in the front. All built-in appliances in Table 1 require two-phase mains
power (230 V and 16 A), and suitable pots are automatically detected. The booster function is
available on two or all four hobs or is not available. The booster function is usually labeled P
(for power) and corresponds to the highest heat setting.
Portable equipment characteristics The characteristics of portable induction cooking appliances are similar to those of built-in equipment. The portable Inducs appliances have one
or two hobs, which have a higher rated power than built-in appliances. The Inducs model
SH/DU/BA 5000 operates on a three-phase mains power supply.
Comparison and selection of three induction cookers Table 1 facilitates the comparison
and selection of three representative appliances. It is expected that the appliances exhibiting
the worst-case B-field exposure will be those with the highest hob rated power. The position of
the strongest hob with respect to the user is also considered and given for each model in Table
1 (front or rear). To perform a worst-case assessment, appliances with the strongest hob in the
front were selected.
One portable and two built-in appliances have been selected for experimental evaluation.
From groups with similar technical data, only the cheapest was chosen. It has been decided
not to follow the initial proposal, that one device shall be as similar as possible to the original
cooker used by Yamazaki et al. (domestic induction cooker, 1.3 kW, with a casting pot filled
80 % with water), since it is a much low power device and such devices are not readily available
on the Swiss market. The selection is depicted in Figure 3 and includes:
• Appliance 1: Electrolux GK58TCi (built-in)
• Appliance 2: Gaggenau CI 261 110 (built-in)
• Appliance 3: Inducs SH/BA 5000 (portable)
3.2.2
Characteristics of the three selected Induction Cookers
Dimensions and coordinate system The dimensions of the three appliances and the coordinate systems used in the measurements are shown in Figures 4 and 5.
Numbering of the hobs The hobs are numbered as shown in Figure 6. The rated hob power
for each appliance is given in Table 1.
B-field Exposure From Induction Cooking Appliances
11
Figure 3: Selected induction cookers. a) appliance 1: Electrolux GK58TCi, b) appliance 2:
Gaggenau CI 261 110, and c) appliance 3: Inducs SH/BA 5000.
Figure 4: Top view, dimensions (in mm) and coordinate system for a) appliance 1 (Electrolux)
and b) appliance 2 (Gaggenau).
Control panel and heat setting
• Appliances 1 and 2 (Electrolux and Gaggenau, respectively) The control panel
allows heat setting selection varying from 0 to 9 (minimum to maximum power). The
booster function (heat setting P ) is available for all 4 hobs, but not simultaneously. For
appliance 1, the heat setting P is available simultaneously with hobs 1 and 2, or with hobs
3 and 4. For appliance 2, the heat setting P is available for hob 1 if hob 3 is switched off
and reciprocally (same for hobs 2 and 4).
• Appliance 3 (Inducs) The heat setting varies from 0 to 12 (minimum to maximum
power). There is no booster function with appliance 3.
Mounting All three appliances are mounted on 20 mm thick plywood supports allowing close
proximity measurements (1 cm on each side). The mounting has been performed according
to the manufacturer’s specifications. The entire support system base is wooden without any
metallic parts in the vicinity of the appliances, thus eliminating the possibility of field distortions.
B-field Exposure From Induction Cooking Appliances
12
Figure 5: Dimensions (in mm) and coordinate system of appliance 3 (Inducs), top view and side
view.
Figure 6: The hobs 1 to 4 correspond, respectively, to the front-left, front-right, rear-left and
rear-right hobs. The front, rear, left and right sides of the appliance are indicated, as well as
the measuring distance and probe-center/hob-center distance.
The dimensions of the supports and the corresponding pictures are given in Figures 7 and 8,
respectively.
Measuring distance and probe-center/hob-center distance The measuring distance,
defined as the distance between the surface of the appliance and the closest point of the sensor
surface, according to EN50366, will be used. However, the corresponding distance between the
probe-center and hob-center is also relevant, since it is directly related to the B-field measured
(see Figure 6). Both distances are given in Table 3 for all appliances and hobs.
B-field Exposure From Induction Cooking Appliances
13
Figure 7: Dimensions of the wooden supports (in mm): a) appliances 1 (Electrolux) and 2
(Gaggenau), and b) appliance 3 (Inducs).
Figure 8: Induction cookers with wooden supports: a) appliance 1 (Electrolux), b) appliance 2
(Gaggenau) and c) appliance 3 (Inducs).
3.3
Pots and Pans
The B-field exposure has been assessed for several new and used pots and pans of different sizes
and shapes, as well as various materials. These are depicted in Figure 10.
3.3.1
Norm, Manufacturer Instructions and the Standard Set of Pots
EN50366 test conditions The test conditions for induction hobs given in norm EN50366
specify that standard measurements should be performed using an enameled steel cooking vessel,
filled to 50 % of its capacity with tap water. The smallest vessel recommended in the instructions
is used. If no recommendations are provided, the smallest standard vessel that covers the marked
cooking zone is used.
Manufacturers instructions The hobs diameter of the three appliances are given in Figures
4 and 5. The Inducs manual (appliance 3) states that the minimum diameter for pots and pans
B-field Exposure From Induction Cooking Appliances
14
is 12 cm without specification of pot thickness. The Electrolux manual (appliance 1) specifies a
bottom thickness of 2 to 3 mm for enameled steel pots, and 4 to 6 mm for stainless steel pots,
without specification of minimal pot diameter. Instructions concerning suitable pots and pans
are given in the manuals of all appliances:
• Only ferromagnetic pans are suitable for induction cooking. They can be made of steel,
enameled steel, cast-iron or special stainless steel. Aluminum, copper, brass, glass, ceramics or porcelain pots must not be used. To check whether a pot is suitable, the use of a
magnet is suggested: If it sticks the pot is suitable for induction cooking.
• Cooking with an empty pan should be avoided. The hob immediately switches off in the
case of overheating. This function is available on all three appliances.
• Pots and pans must be centered on the hobs to avoid nonuniform heating.
• The pot must have a minimum diameter which is related to the hob dimension. If the pot
is too small or of unsuitable material, the hob will not switch on. This function is available
on all three appliances. It is recommended to always choose a properly sized pot for the
quantities of food to save energy.
• Metallic objects such as cans, utensils, jewelry, cutlery, watches, aluminum foil paper, and
lids should not be put on the hobs to avoid the danger of burning.
Standard set of pots In order to comply with the test conditions specified in EN50366, a
standard set of pots has been defined (see Figure 9). It corresponds to the pots 3, 14a, 14b
and 13, with bottom diameters of 22 cm, 18 cm, 18 cm and 14.5 cm, respectively. These pots
correspond to the smallest standard vessel that covers the marked cooking zone for all three
appliances. The standard pots are not of enameled steel but of stainless steel, since stainless
steel cooking vessels are sold in the largest volume on the Swiss market.
Figure 9: Standard set of pots.
3.3.2
Worst-Case Set of Pots
However, despite the manufacturer instructions, users do not always follow the included recommendations, and not everyone operating an induction cooker has read the manual. When
consumers buy a new induction cooking appliance, they are likely to continue using old pans
if they appear to work, despite not being perfectly adapted for induction cooking. In order to
perform a worst-case B-field exposure and to select a worst-case set of pots, 16 pots and pans
were assessed (Figure 10). Their characteristics are given in Table 2.
B-field Exposure From Induction Cooking Appliances
15
Figure 10: Pans, frying-pans and pots used during the experiments to define the worst-case set
of pots.
Description
frying-pan
small saucepan
large pot
saucepan, bottom not flat
non-paramagnetic Saucepan
large enameled pot
small enameled pot
small enameled pot
wok
cast-iron pot
small pot
small pot
large pot
small pot
medium pot
medium pot
Pot
1
2
3
4
5
6
7
8
9
10
11a
11b
12
13
14a
14b
IKEA
IKEA
IKEA
2nd hand
2nd hand
2nd hand
2nd hand
2nd hand
2nd hand
Migros
Migros
Migros
Migros
IKEA
IKEA
IKEA
shop
shop
shop
shop
shop
shop
Manufacturer
/ Distributor
stainless steel, aluminum
stainless steel, aluminum
stainless steel, aluminum
stainless steel,
non-paramagnetic stainless
enameled steel
enameled steel
enameled steel
steel
cast-iron
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
stainless steel
Material
Dimensions
Bottom Diameter Height
[cm]
[cm]
22
5
15
10
22
19
18
10
18
10
18
11
14
8
14
12
12
9
17
11
16
9
16
9
22
11
14.5
13
18
14.5
18
14.5
Table 2: Characteristics of pans, frying-pans and pots used in the experiments.
Capacity
[l]
2
2
10
2
2
3
1.5
1.5
4
3.5
2
2
5
2.5
4.5
4.5
no
yes
yes
no
no
no
yes
no
no
yes
yes
yes
yes
yes
yes
yes
Lid
yes
yes
yes
no
no
no
no
no
no
yes
yes
yes
yes
yes
yes
yes
New
B-field Exposure From Induction Cooking Appliances
16
appliance 3 (Inducs)
hob
measuring distance [mm]
dist. probe/hob center [mm]
measuring distance [mm]
dist. probe/hob center [mm]
appliance 2 (Gaggenau)
hob
measuring distance [mm]
dist. probe/hob center [mm]
measuring distance [mm]
dist. probe/hob center [mm]
appliance 1 (Electrolux)
hob
measuring distance [mm]
dist. probe/hob center [mm]
measuring distance [mm]
dist. probe/hob center [mm]
542.5
252.5
1
512.5
222.5
1
front side
1
10
322.5
300
612.5
front side
2
3
10
262.5 462.5
300
552.5 752.5
front side
2
3
10
262.5 442.5
300
552.5 732.5
772.5
482.5
4
752.5
462.5
4
717.5
427.5
1
722.5
432.5
1
rear side
1
10
262.5
300
552.5
rear side
2
3
10
417.5 217.5
300
707.5 507.5
rear side
2
3
10
392.5 212.5
300
682.5 502.5
487.5
197.5
4
482.5
192.5
4
512.5
222.5
1
512.5
222.5
1
left side
1
10
262.5
300
552.5
left side
2
3
10
522.5 222.5
300
812.5 512.5
left side
2
3
10
512.5 202.5
300
802.5 492.5
812.5
522.5
4
812.5
522.5
4
822.5
532.5
1
792.5
502.5
1
right side
1
10
262.5
300
552.5
right side
2
3
10
232.5 532.5
300
522.5 822.5
right side
2
3
10
212.5 522.5
300
502.5 812.5
Table 3: Measuring distances and corresponding probe-center/hob-center distances. According to EN50366, the measuring distance is
the distance between the surface of the appliance and the closest point of the sensor surface. The front, rear, left and right sides of the
appliances and the hob numbering are shown in Figure 6.
522.5
232.5
4
492.5
202.5
4
B-field Exposure From Induction Cooking Appliances
17
B-field Exposure From Induction Cooking Appliances
18
Worst-case set of pots A worst-case set of pots has been selected (see Figure 11), composed
of pots 8, 7, 13 and 2 with bottom diameters of 14 cm, 14 cm, 14.5 cm and 15 cm, respectively.
Figure 11: Worst-case set of pots.
3.4
B-field Narda Sensor ELT-400
The magnetic flux density (B-field) was measured using the Narda exposure level tester ELT-400
(see Figure 12).
Figure 12: Narda exposure level tester ELT-400.
The probe is especially designed for investigating the magnetic field produced by household
equipment and other electrical devices. The DASY4 system was extended to enable use with
the Narda probe ELT-400. This involved mechanical, electrical and software modifications.
The reference test method described in EN50366 is implemented in the Narda probe, and the
measured B-field is given in [µT], or as a percentage of the ICNIRP guideline [2]. The 3Dsensor system covers the frequency range from 1-30 Hz to 400 kHz. The B-field is measured
using three orthogonal coils with a common center point (allowing isotropic measurement).
The cross sectional area is 100 cm2 (standard-compliant), and the probe’s external diameter is
125 mm. The signal voltages in the coils are digitized and evaluated by a digital signal processor
(DSP), which calculates in real-time the root-mean-square (rms) value of the B-field, according
to EN50366 [4]. The rms-value of the magnetic flux density B is calculated from the rms-values
of each of the three measurement axes (Bx , By and Bz ) according to Equation (1).
B=
q
Bx2 + By2 + Bz2
(1)
B-field Exposure From Induction Cooking Appliances
19
The measurement time interval is 250 ms. The rms-value, given in [µT], is continuously
integrated over four intervals, and the integration time of the rms calculation is fixed at 1 s. The
Narda ELT-400 probe can also operate as an active probe, using the three separate, analog signals
corresponding to the three spatial axes x, y, and z. Connecting it to an external oscilloscope or
spectrum analyzer allows temporal and spectral measurements.
In this project, the Narda probe ELT-400 is used in field strength mode, which is a measurement
of the magnetic flux density based on broadband evaluation of the signal, for the bandwidth
30 Hz to 400 kHz. The measurement uncertainty of the instrument is ±4 % (50 Hz to 120 kHz).
The frequency response for the selected bandwidth limits 30 Hz and 400 kHz is −3 dB ±1 dB.
The settings selected are given in Table 4. The intrinsic noise is 70 nT and 320 nT for the low
and high range settings, respectively.
Table 4: Settings of the Narda probe ELT-400.
Mode
Range
Overload Limita
Nominal Measurement Rangeb
Intrinsic noise (rms)
a
Low
32 µT
2 µT
70 nT
320 µT
High
320 µT
20 µT
320 nT
Maximum measurable rms-value of a sine wave.
Maximum measurable rms-value of a signal of any shape
with a crest factor (peak value/rms-value) of less than 22.
b
B-field Exposure From Induction Cooking Appliances
4
4.1
20
Evaluation of Measurement System
Sensor Characterization
Helmholtz coil The uncertainty of the Narda instrument was evaluated using the Helmholtz
coil, designed to test the T-coil compatibility of mobile phones (ANSI63.19). It is comprised
of a pair of identical circular magnetic coils (diameter 284.8 mm) that are coaxially aligned
and separated by a distance equal to the radius of the coil. Each coil carries an equal electrical
current. The cylindrical region extending between the centers of the two coils and approximately
1/5 of their diameters has a magnetic field with high spatial uniformity. The Helmholtz coil is
connected to the waveform generator (Agilent 33120A). The corresponding electronic circuit is
given in Figure 13.
Figure 13: Electronic circuit of the Helmholtz coil connected to the waveform generator (Agilent
33120A).
The current I in the Helmholtz coil and the voltage Vout at the output of the waveform
generator are given in Equations (2) and (3), respectively.
I=
Vout = Vs
V R2
Vout
=
R2
Rcoil + R2
(2)
Rcoil + R2
1
≈ Vs
Rs + Rcoil + R2
2
(3)
In the frequency domain, the current I(ω) in the Helmholtz coil is given in Equation (4),
where I(ω) = Iejωt and Vout (ω) = Vout ejωt+α are the current and voltage in the complex
domain, ω = 2πf is the angular velocity and f is the frequency. The cut-off frequency is given
by fc = R2 /2π(L1 + L2 ) and is here fc ≈ 20 kHz for the Helmholtz coil. The frequency response
of the coil is shown in Figure 14 b).
I(ω) =
Vout (ω)
R2 + jω(L1 + L2 )
(4)
An approximation of the B-field at the center point of the Helmholtz coil system is given in
Equation (5), where R is the radius (Raverage = 142.4 mm), N is the number of turns in each
coil (N = 20) and I is the current in the coils [6].
B-field Exposure From Induction Cooking Appliances
21
4
µ0 N I
B = ( )3/2
(5)
5
R
The generated B-fields in the Helmholtz coil are 8.6 µT and 0.86 µT when applying Vout =
10 Vpp and 1 Vpp (peak-to-peak), in the frequency range corresponding to a flat response.
Frequency response The specifications of the Narda probe ELT-400 indicate a flat frequency
response for the frequency range from 30 Hz to 400 kHz. To validate this, the experimental setup
shown in Figure 15 was used. The probe is at a fixed position in the center of the Helmholtz coil,
and the waveform generator applies a sinusoidal voltage, with the frequency stepwise increased
from 1 Hz to 800 kHz (amplitude 10 Vpp ). The frequency response of the Narda probe ELT-400
is shown in Figure 14.
Figure 14: a) Narda probe ELT-400 frequency response, calculated from the frequency responses
of the Narda probe (ωc1 = 30 Hz and ωc2 = 400 kHz) and the Helmholtz coil (ωc3 = 20 kHz),
as well as the frequency responses of the Helmholtz coil system. b) Deviation from the flat
frequency response between 30 Hz and 400 kHz in [dB].
The combined frequency response of the Narda probe (ωc1 = 30 Hz and ωc2 = 400 kHz) and
the Helmholtz coil (ωc3 = 20 kHz) was measured using the Narda probe ELT-400. The frequency
response of the Helmholtz coil system was then assessed by measuring VR2 as a function of the
frequency (see Figure 13). The frequency response of the Narda probe ELT-400 was calculated
B-field Exposure From Induction Cooking Appliances
22
from those two measurements. As given in the specifications, the cut-off frequencies for the
Narda probe ELT-400 are around 30 Hz and 400 kHz, and the frequency response is flat between
those frequencies; the error is smaller than ±0.8 dB. The error in [dB] was calculated using the
Equation (6):
Error[dB] = 20 log(B/Baverage )
(6)
Figure 15: Spherical isotropy of the Narda probe ELT-400. The probe was attached to the
DASY4 robot arm and centered in the Helmholtz coil. A waveform generator (Agilent 33120A)
applied a sinusoidal voltage (frequency 20 kHz) to the Helmholtz coil. The B-field values were
recorded while rotating the probe around its z-axis. Error in [dB] of the B-field measured
when applying in the coil a) 1 Vpp , top measurement, b) 1 Vpp , side measurement, d) 10 Vpp ,
top measurement, and b) 10 Vpp , side measurement; e) diagram of top measurement and side
measurement; f) measurement setup of the spherical isotropy of the Narda probe.
B-field Exposure From Induction Cooking Appliances
23
Spherical isotropy The experimental setup shown in Figure 15 f) was used to evaluate the
spherical isotropy of the Narda probe ELT-400. The probe rotates around its vertical z-axis,
and the B-field is measured as a function of the probe angle of rotation (from -180◦ to 180◦ ).
The measurements are performed on the top and at the side of the Helmholtz coil (see Figure
15 e) ). The uncertainty due to deviations from spherical uncertainty has
√ been determined by
the worst-case values assuming rectangular distribution, i.e., ± 0.2 dB/ 3.
Linearity In order to check the linearity of the probe, a sinusoidal voltage was applied to the
Helmholtz coil, with the amplitude varying between 0.1 Vpp and 10 Vpp (frequency of 20 kHz).
The experimental setup shown in Figure 15 f) was used. The probe was centered in the Helmholtz
coil. The B-field was measured as a function of the voltage applied to the coil as shown in Figure
16. The linearity is very good for B-fields greater than 0.3 µT, which corresponds to a deviation
from linear response < ±0.05 dB. The error increases for lower fields and equals 2.8 dB for
B-fields of ≈ 0.06 µT (0.1 Vpp ).
Figure 16: Linearity of the Narda probe ELT-400. a) The B-field is measured for amplitudes
between 0.1 Vpp and 10 Vpp (frequency 20 kHz). b) Deviation from linear response in [dB].
B-field Exposure From Induction Cooking Appliances
4.2
4.2.1
24
Time- and Frequency-Domain Characterization of the Induction Cookers
Time-Domain Characterization
Measurements The alternating magnetic fields generated by the induction cookers were measured with the Narda probe ELT-400. The analog output signals of the probe were recorded with
a digital oscilloscope (Agilent 54622D mixed signal oscilloscope). The B-fields were measured
in front (measuring distance 1 cm) of the largest hobs: hobs 1, 2 and 1 for appliances 1, 2 and
3, respectively. For each experiment the standard pot 3 with lid was centered on the hob and
filled with 3 liters of tap water. The time-domain signals corresponding to appliances 1, 2 and
3 are given in Figures 17, 18 and 19, respectively.
Figure 17: Time-domain signals of the alternating magnetic fields generated by appliance 1. Graphs a) and b) correspond to the heat settings 4 and P. The fundamental signal
frequencies are, respectively, 41.7 kHz and 17.5 kHz. The signal rms-value increases approximately by a factor of 5 for heat settings from 4 to P. Graphs c) and d) correspond to the heat
settings 4 and P and show a signal envelope with a frequency of 100 Hz that remains constant
for all heat settings. Graphs e) and f) correspond to the heat settings 1 and 3. For heat settings
1 to 4, the signal is pulse-width modulated, with a frequency of 0.5 Hz.
B-field Exposure From Induction Cooking Appliances
25
Figure 18: Time-domain signals of the alternating magnetic field generated by appliance 2. Graphs a) and b) correspond to the heat settings 4 and P. The fundamental signal
frequencies are, respectively, 52.1 kHz and 21.2 kHz. The signal rms-value increases approximately by a factor of 2.5 for heat settings from 4 to P. Graphs c) and d) correspond to the heat
settings 4 and P and show a signal envelope with a frequency of 100 Hz that remains constant
for all heat settings; d) also shows a 50 Hz component. Graphs e) and f) correspond to the heat
settings 1 and 4. For heat settings 1 to 4, the signal is pulse-width modulated, with a frequency
of 0.5 Hz.
Discussion The two built-in appliances and the portable appliance present similar characteristics. The signal rms-value increases with the heat setting for all three appliances. As shown
in Figures 17 a) and b), 18 a) and b) and 19 a) and b), the rms-value increases approximately
by a factor of 5, 2.5 and 4 when the heat setting varies from the lowest to the highest values,
for appliances 1, 2 and 3, respectively. At the opposite, the frequency of the signal decreases
when the heat setting increases (see Table 5). Figures 17 a) and b), 18 a) and b), and 19 a)
and b) also show that the shape of the signal changes for different heat settings. For all three
appliances, the signal shape is a triangle at the lowest heat setting and changes into a sinusoidal
at the highest heat setting. Figures 17 c and d), 18 c) and d), and 19 c and d) display the
signals envelopes. The envelope frequency (100 Hz, 100 Hz and 300 Hz for appliances 1, 2 and 3,
B-field Exposure From Induction Cooking Appliances
26
Figure 19: Time-domain signals of the alternating magnetic field generated by appliance 3. Graphs a) and b) correspond to the heat settings 4 and 12. The fundamental signal
frequencies are 62.7 kHz and 21.7 kHz, respectively. The signal rms-value increases approximately by a factor of 4 for heat settings from 4 to 12. Graphs c) and d) correspond to the heat
settings 6 and 10 and show a signal envelope with a frequency of 300 Hz that remains constant
for all heat settings. Graphs e) and f) correspond to the heat settings 1 and 3. For heat settings
1 to 4, the signal is pulse-width modulated, with a frequency of approximately 0.5 Hz.
respectively), does not depend on the heat setting (see Table 5). It is assumed that the 100 Hz
envelope component is caused by the power supply operating from a single-phase 50 Hz mains,
and the 300 Hz component by the three-phase power supply. As shown in Figures 17 e) and f),
18 e) and f), and 19 e) and f), the signal is pulse-width modulated for all three appliances and
at the heat settings 1 to 4. The duty cycles (ratio of time on to total cycle time) are given in
Table 6.
4.2.2
Frequency-Domain Characterization
The determination of the spectral content of the B-field in the frequency range from 10 Hz to
400 kHz was performed with the Narda probe ELT-400 (analog signal output) connected to a
B-field Exposure From Induction Cooking Appliances
27
Table 5: Signal and envelope modulation frequency of appliances 1, 2 and 3 for the heat settings
4 and P or 12.
heat setting
appliance 1
appliance 2
appliance 3
signal frequency
[kHz]
4
P or 12
42
18
52
21
63
22
envelope frequency
[Hz]
4 to max. (P or 12)
100
100
300
pulse-width modulation
frequency [Hz]
1 to 4
0.5
0.5
0.5
Table 6: Duty cycle of appliances 1, 2 and 3 for heat settings 1 to 4, where the signal is pulsewidth modulated (no pulse-width modulation for higher settings).
heat setting
appliance 1
appliance 2
appliance 3
1
20
15
40
duty cycle [%]
2
3
4
5
35 65 95 100
30 55 75 100
60 60 100 100
spectrum analyzer (Rohde & Schwarz FSP30). The signals were measured at the front side of
the appliances for each hob with a measuring distance of 1 cm. For each experiment the standard
pot 3 with lid was centered on the hob and filled with 3 liters of tap water.
Dependency of the signal frequency on the heat setting for each hob The signal
frequencies of all hobs were measured for different heat settings (see Figure 20). As shown in
Figures 17, 18 and 19, the frequency decreases when the heat setting increases. The frequency
ranges are 18−45 kHz, 21−52 kHz and 22−63 kHz for appliances 1, 2 and 3, respectively. The
electronic schemes of the appliances were not provided, and the appliances were not opened.
However, in order to explain why the signal frequency decreases for higher heat settings, a
simple circuit comprised of a coil L, a series resistor R and a AC voltage source U is considered.
The B-field increases with the AC current in the coil. The current in the coil is given by
I(ω) = V (ω)/(R + jωL), where I(ω) = Iejωt and V (ω) = V ejωt+α are the current and voltage
in the complex domain, and ω = 2πf is the angular velocity. If R, L and V are fixed, the current
in the coil I is proportional to 1/f (for | ωL | R).
Harmonics contribution Figure 21 shows the power spectral distribution of the B-field
emitted by appliances 1, 2 and 3 for frequencies up to 400 kHz. This frequency range corresponds
to the Narda probe bandwidth (30 Hz−400 kHz). The ratio between the fundamental and the
harmonics peaks is calculated using Equation (7):
P1[dB] −P2[dB]
P1[mW ]
10
= 10
P2[mW ]
(7)
where P1[dB] − P2[dB] is the difference in [dB] between the fundamental and harmonic peaks
(power spectrum), and P1[mW ] /P2[mW ] is the corresponding ratio. The contribution of the
B-field Exposure From Induction Cooking Appliances
28
Figure 20: Dependency of the signal frequency on the heat setting. Graphs a), b) and c)
correspond to appliances 1, 2 and 3, respectively.
harmonics is calculated for appliances 1, 2 and 3 in Tables 7, 8 and 9, respectively. These Tables
show that the contribution in terms of power of all harmonics does not exceed 1 %, 2.5 % and
1.8 % of the fundamental signal for appliances 1, 2 and 3, respectively.
The total harmonics contribution is also given in terms of B-field weighted with the frequency
f , which is more meaningful for evaluating the contribution to the induced current J in the
body (see Chapter 7). If Pf i (in [%]) is the contributions in term of power of the harmonics at
the frequencies fi = 3f, 5f , etc., the induced current is proportional to:
q
q
q
q
J ∝ f1 Pf 1 + f2 Pf 2 + ... ∝ f (3 P3f + 5 P5f + ...) ∝ f
X q
i Pif
(8)
i
4.2.3
B-Field Exposure Dependency on the Heat Settings
As shown in Figure 22 a), for appliance 1, the B-field exposure increases linearly for higher heat
settings. Figures 22 b) and 22 c) show that this dependency follows a second and fourth order
polynomial for appliances 2 and 3, respectively.
B-field Exposure From Induction Cooking Appliances
29
Table 7: Appliance 1, Calculated harmonics contribution in [%], in terms of power.
Heat setting
Fundamental frequency f [kHz]
3f
5f
7f
Contribution of the
9f
harmonics [%]
11f
13f
15f
17f
19f
Total in terms
of
power
[%]
P p
J ∝ i i Pif [%]
4
41.7
0.8
0.08
0.03
0.006
0.916
5.91
5
33.0
0.5
0.05
0.02
0.008
0.003
0.581
4.93
6
28.8
0.5
0.06
0.01
0.006
0.003
0.0016
0.5806
4.88
7
24.0
0.4
0.05
0.01
0.004
0.003
0.0016
0.0008
0.4694
4.37
8
22.5
0.3
0.04
0.008
0.003
0.002
0.001
0.0008
0.0005
0.3553
3.76
9
19.8
0.3
0.03
0.008
0.003
0.001
0.0008
0.0005
0.0003
0.0003
0.3439
3.52
P
18.0
0.3
0.03
0.005
0.003
0.001
0.0005
0.0004
0.0003
0.0002
0.3404
3.52
Table 8: Appliance 2, Calculated harmonics contribution in [%], in terms of power.
Heat setting
Fundamental frequency f [kHz]
2f
3f
4f
Contribution of the
5f
harmonics [%]
6f
7f
8f
Total in terms
of
power
[%]
P p
J ∝ i i Pif [%]
4
50.1
1.6
1.6
3.78
5
46.5
0.4
0.4
1.19
6
39.4
0.7
0.7
2.55
7
31.2
0.05
0.8
0.85
3.12
8
26.1
0.03
0.6
0.1
0.73
4.51
9
23.4
0.07
1.4
0.03
0.2
1.7
7.13
P
21.30
0.05
2.0
0.02
0.3
0.05
2.42
9.32
Table 9: Appliance 3, Calculated harmonics contribution in [%], in terms of power.
Heat setting
Fundamental frequency f [kHz]
3f
5f
7f
Contribution of the
9f
harmonics [%]
11f
13f
15f
17f
Total in terms
of
power
[%]
P p
J ∝ i i Pif [%]
4
62.9
1.3
0.2
1.5
5.60
5
50.8
1.3
0.2
0.03
1.53
6.60
6
46.0
1.3
0.2
0.05
1.55
6.92
7
41.3
1.6
0.1
0.05
0.02
1.77
8.39
8
30.9
1.3
0.2
0.03
0.02
0.01
0.003
1.563
9.57
9
24.7
0.6
0.1
0.03
0.004
0.002
0.002
0.001
0.739
7.38
10
23.5
0.5
0.1
0.02
0.008
0.002
0.0008
0.0006
0.0005
0.6319
7.11
11
22.6
0.4
0.08
0.03
0.01
0.004
0.001
0.0004
0.0003
0.5257
7.15
12
22
0.3
0.08
0.03
0.02
0.005
0.002
0.0008
0.0004
0.4382
7.64
B-field Exposure From Induction Cooking Appliances
30
Figure 21: Harmonics contribution, in [dB] (power spectrum, 10 dB per division). Graphs a) and
b), c) and d), e) and f) correspond to appliances 1, 2 and 3, with hobs 1, 2 and 1, respectively
(largest hobs).
Figure 22: Measured B-field exposure for different heat settings, for a) appliance 1,
b) appliance 2 and c) appliance 3. Measurements performed at the largest hob: hobs 1, 2
and 1 for appliances 1, 2 and 3, respectively, with pot 3 and lid, measuring distance of 5 cm and
max. heating setting. Other measurement conditions according to EN50366.
B-field Exposure From Induction Cooking Appliances
4.3
31
Uncertainty Budget
Table 10 gives a list of probable sources of error and an evaluation of the uncertainty budget,
as given in the IEEE 1528-2003 standard [9]. The uncertainty was assessed in the region of
the test specification limit (6.25 µT) and for the full range of measurements. Moreover, during
all experiments, the random components of uncertainty were reduced by performing
repeated
√
measurements (averaging N = 4, random components of uncertainty divided by N = 2). The
maximum uncertainty limits have been evaluated as follows [7, 8, 9]:
• Narda probe ELT-400 measurement uncertainty. The specification given by the
manufacturer is ±4 % or ±0.34 dB for a signal in the frequency range 50 Hz to 120 kHz.
• Error due to non-perfect frequency response of the probe. The Narda probe
ELT-400 frequency response deviation is less than ±0.80 dB or ±9.6 %.
• Error due to non-perfect spherical isotropy of the probe. As described in Chapter
4.1, the Narda probe ELT-400 spherical isotropy is very good, and the deviation is less
than ±0.2 dB or ±2.3 %.
• Error due to non-perfect linearity of the probe. The deviation from linear response
is < ±0.05 dB for B-fields exceeding 0.3 µT (see Chapter 4.1). The error increases for lower
fields and equals 2.80 dB for B-fields of 0.06 µT. However, most of the B-fields measured
exceeded 0.3 µT, and the uncertainty ±0.05 dB or ±0.6 % is considered.
• Centering of the pot on the hob. As discussed in Chapter 6.0.6, for pot displacements
of ±4 mm (in every direction relative to the hob center) a maximum error of 11 % or
±0.90 dB is introduced.
• Water volume and evaporation. As shown in Chapter 6.0.7, the B-field variation due
to changes in the volume of water in the pot is smaller than ±2.45 % or ±0.21 dB.
• Probe alignment with respect to the appliance edge. The probe alignment with
respect to the appliance edge was verified before for each measurement with a worst-case
accuracy of ±4 mm. It was calculated (Figures 24 to 29) that the corresponding B-field
variations are smaller than ±2.2 % or ±0.19 dB when measuring in front of each hob.
• Probe positioning on the DASY4 robot arm. The mount made especially to fix the
Narda probe ELT-400 on the robot arm has a notch which very precisely defines the probe
position (±0.5 mm). This error was considered to be negligible.
• Robot arm positioning. The DASY4 robot arm positioning and repeatability is better
than ±1 mm. The corresponding measurement uncertainty was considered to be negligible.
• Variation in the mains power supply. The transfer function between the mains power
supply and B-field variations was not assessed and assumed to be negligible.
B-field Exposure From Induction Cooking Appliances
32
Table 10: Measurement uncertainty evaluation. As described in IEEE 1528-2003, the tolerance,
the probability distribution (R = Rectangular distribution), the divisor used to get standard uncertainty, the sensitivity coefficient ci , the uncertainty components ui and the combined standard
uncertainty uc are given.
Error Description
Narda probe ELT-400
- Narda probe ELT-400 specification
- Error due to non-perfect frequency response
- Error due to non-perfect spherical isotropy
- Error due to non-perfect linearity
System repeatability
- Centering of the pot on the hob
- Water volume and evaporation
- Alignment of probe to appliance edge
- Probe positioning on the robot arm
- Robot arm positioning
- Variation in the mains power supply
Combined standard uncertainty uc
Coverage factor for 95 %
Expanded uncertainty
Tol.
(±dB)
Prob.
Dist.
Div.
ci
ui
(±dB)
vi
±0.34
±0.80
±0.20
±0.05
R
R
R
R
√
√3
√3
√3
3
1
1
1
1
±0.20
±0.46
±0.12
±0.03
∞
∞
∞
∞
±0.90
±0.21
±0.19
-
R
R
R
√
√3
√3
3
1
1
1
±0.52
±0.12
±0.11
∞
∞
∞
±0.75
∞
kp= 2
±1.50
All error contributions considered are given in Table 10 and are assumed to be random and
uncorrelated. In a conservative evaluation of all uncertainties, the error distribution is assumed
to be rectangular. The combined uncertainty uc is obtained by taking the square root of the
sum of squares of the individual uncertainties ui according to [7, 8, 9]:
v
u n
uX
uc = t u2i
(9)
i=1
Throughout this report all B-fields measured are given with the uncertainty uc = ±1.50 dB.
B-field Exposure From Induction Cooking Appliances
5
33
Assessments According to EN50366 conditions
One objective of this study was to measure whether the B-field exposure from the three selected
induction cookers complies with the derived and basic restrictions of the ICNIRP guidelines,
when assessed according to the European Norm EN50366.
5.0.1
Measuring Conditions in EN50366
The European Norm EN50366 describes a method for the evaluation and measurement of the
electromagnetic field in the vicinity of appliances such as induction hobs and hotplates.
Figure 23: Measuring distances, according to EN50366. Lines (A, B, C, D) indicate the measuring positions for an induction cooker with 4 hobs in operation. The probe is here positioned on
line A. [4]
Measuring distance and operator distance, according to EN50366 The measuring
distance is the distance between the surface of the appliance and the closest point on the sensor
surface. The operator distance is the distance between the surface of the appliance and the closest
point of the head, limbs or torso of the operator. For each cooking zone, the measurements are
made along four vertical lines (A, B, C, D) at a measuring distance of 30 cm from the edges of
the appliance (see Figure 23). The measurements are made up to 1 m above the cooking zone
and 0,5 m below it. The measurement is not made at the rear of the appliance if the appliance
is intended to be used when placed against a wall.
Operating mode, according to EN50366 An enameled steel cooking vessel, filled to 50 %
of its capacity with tap water, is placed centrally on the cooking zone to be measured. The
smallest vessel recommended in the instructions for use is used. If no recommendations are
provided, the smallest standard vessel that covers the marked cooking zone is used. The bottom
B-field Exposure From Induction Cooking Appliances
34
diameters of standard cooking vessels are: 110 mm, 145 mm, 180 mm, 210 mm and 300 mm. The
hobs are operated in turn at the highest power setting, with all other cooking zones uncovered.
The measurements are made after stable operating conditions have been reached. If no stable
conditions can be reached, an appropriate observation time (e.g., 30 s) shall be defined to be
sure to get the maximum value with fluctuating field sources.
Implementation of EN50366 in this study The B-field exposure from the three appliances
was assessed according to the operating mode given in EN50366, but with minimal adaptations.
The measurements were made along vertical lines at a distance of 30 cm from the edges of the
appliance (see Figure 23), but also at measuring distances of 1, 5 and 10 cm. The front, rear,
left and right sides of the appliances (see Figure 6) were assessed: additional measurements
were also made at the rear of the appliance, since modern kitchens often feature cooking islands
which allow the cook to access the hobs from all sides. For these experiments, the standard
set of pots was used (pots 3, 14a, 14b and 13, with bottom diameters of 22 cm, 18 cm, 18 cm
and 14.5 cm, respectively, see Figure 9). The pots used were not made of enameled steel but
of stainless steel, which is the most common vessel type available for induction cooking on the
market. For each single-hob measurement the pot of corresponding size was filled to 50 % of
its capacity with tap water and centered on the hob. Lids were placed on the pots to achieve
more stable conditions during the measurements, and to prevent excessive evaporation in the
laboratory. The measuring range along the z-axis was [−0.4...0.4 m] for appliances 1 and 2, and
[−0.7...0.3 m] for appliance 3, due to the finite travel of the DASY4 robot arm. For Appliance
3, the measurements according to EN50366 were performed using pots 3 and 13 (largest and
smallest pots of the standard set of pots, see Figure 9), since no vessel was recommended in the
instructions and no cooking zone was marked on the appliance. The only relevant information
given in the Inducs manual is to use a pot with a minimum diameter of 12 cm. The B-field
measured for all three appliances was compared to the ICNIRP limit, which is 6.25 µT for
general public exposure. The results are presented below.
5.0.2
Appliance 1
The B-field measured in the vicinity of appliance 1 is shown in Figures 24 (front and rear sides)
and 25 (left and right sides). The B-field does not exceed the ICNIRP limit (6.25 µT) for a
measuring distance of 30 cm according to EN50366.
Figures 24 a) (hob 1, front side) and 24 g)(hob 3, rear side) show that the ICNIRP limit is
exceeded for a measuring distance of 1 to 5 cm.
5.0.3
Appliance 2
Figures 26 (front and rear sides) and 27 (left and right sides) depict the B-field measured in the
vicinity of appliance 2. The B-field does not exceed the ICNIRP limit (6.25 µT) for a measuring
distance of 30 cm according to EN50366.
Figures 27 c) (hob 3, left side), 27 e) (hob 1, right side) and 27 h)(hob 4, right side) show
that the ICNIRP limit is exceeded for a measuring distance of 1 to 5 cm. In Figure 27 f)(hob 2,
right side), the ICNIRP limit is exceeded for a measuring distance of 5 to 10 cm.
B-field Exposure From Induction Cooking Appliances
35
Figure 24: Appliance 1, single-hob measurement using the standard set of pots (front
and rear sides of the appliance). Graphs a) to d) correspond to front side B-field measurements along the z-axis, in front of a) hob 1, b) hob 2, c) hob 3 and d) hob 4, at measuring
distances of 1, 5, 10 and 30 cm. Graphs e) to h) correspond to rear side B-field measurements
along the z-axis, in front of e) hob 1, f) hob 2, g) hob 3 and h) hob 4, at measuring distances
of 1, 5, 10 and 30 cm. The other measuring conditions were according to EN50366. Hobs 1 to 4
were evaluated with single-hob measurements using the standard set of pots (pots 14a, 3, 14b
and 13, centered on hobs 1, 2, 3 and 4, respectively).
B-field Exposure From Induction Cooking Appliances
36
Figure 25: Appliance 1, single-hob measurement using the standard set of pots (left
and right sides of the appliance). Graphs a) to d) correspond to left side B-field measurements along the z-axis, in front of a) hob 1, b) hob 2, c) hob 3 and d) hob 4, at measuring
distances of 1, 5, 10 and 30 cm. Graphs e) to h) correspond to right side B-field measurements
along the z-axis, in front of e) hob 1, f) hob 2, g) hob 3 and h) hob 4, at measuring distances
of 1, 5, 10 and 30 cm. The other measuring conditions were according to EN50366. Hobs 1 to 4
were evaluated with single-hob measurements using the standard set of pots (pots 14a, 3, 14b
and 13, centered on hobs 1, 2, 3 and 4, respectively).
B-field Exposure From Induction Cooking Appliances
37
Figure 26: Appliance 2, single-hob measurement using the standard set of pots (front
and rear sides of the appliance). Graphs a) to d) correspond to front side B-field measurements along the z-axis, in front of a) hob 1, b) hob 2, c) hob 3 and d) hob 4, at measuring
distances of 1, 5, 10 and 30 cm. Graphs e) to h) correspond to rear side B-field measurements
along the z-axis, in front of e) hob 1, f) hob 2, g) hob 3 and h) hob 4, at measuring distances
of 1, 5, 10 and 30 cm. The other measuring conditions were according to EN50366. Hobs 1 to 4
were evaluated with single-hob measurements using the standard set of pots (pots 14a, 3, 14b
and 13, centered on hobs 1, 2, 3 and 4, respectively).
B-field Exposure From Induction Cooking Appliances
38
Figure 27: Appliance 2, single-hob measurement using the standard set of pots (left
and right sides of the appliance). Graphs a) to d) correspond to left side B-field measurements along the z-axis, in front of a) hob 1, b) hob 2, c) hob 3 and d) hob 4, at measuring
distances of 1, 5, 10 and 30 cm. Graphs e) to h) correspond to right side B-field measurements
along the z-axis, in front of e) hob 1, f) hob 2, g) hob 3 and h) hob 4, at measuring distances
of 1, 5, 10 and 30 cm. The other measuring conditions were according to EN50366. Hobs 1 to 4
were evaluated with single-hob measurements using the standard set of pots (pots 14a, 3, 14b
and 13, centered on hobs 1, 2, 3 and 4, respectively).
B-field Exposure From Induction Cooking Appliances
39
Figure 28: Appliance 3, with pot 3 (large pot, standard set of pots). The B-field is
measured along the z-axis, on the a) front side, b) rear side, c) left side and d) right side of
the appliance using pot 3 at measuring distances of 1, 5, 10 and 30 cm. The other measuring
conditions were according to EN50366 with the pot centered on the hob.
Figure 29: Appliance 3, with pot 13 (small pot, standard set of pots). The B-field is
measured along the z-axis, on the a) front side, b) rear side, c) left side and d) right side of
the appliance using pot 13 at measuring distances of 1, 5, 10 and 30 cm. The other measuring
conditions were according to EN50366 with the pot centered on the hob.
B-field Exposure From Induction Cooking Appliances
6
41
Worst-Case Exposure Conditions for Varied Loading of the
Induction Cookers
The influence of several parameters was evaluated to assess the worst-case exposure.
6.0.6
Predominant Parameters
Pot-hob centering The predominant parameter is the centering of the pot on the hob. The
B-field measured at a fixed position of the probe is minimal for perfect centering. However,
the B-field increases by factors of approximately 3, 15 and 10 when the pan is displaced by
8 cm for appliances 1, 2 and 3, respectively (see Figure 30). For non-centering by ±4 mm in
each direction, the B-field measured increases by approximately 2.5-4 %, 3-11 % and 5-10 % for
appliances 1, 2 and 3, respectively.
Figure 30: Pot-hob centering, a) appliance 1, b) appliance 2 and c) appliance 3. For
each appliance, pot 3 (large pot, standard set of pots) was centered on the largest hob (hobs
1, 2 and 1 for appliances 1, 2 and 3, respectively). The Narda probe measured the B-field at
a fixed position (front side of the appliance in front of the largest hob; measuring distance of
1 cm and heat setting 7 for appliances 1, 2 and 3). The pot was then displaced in the x and
y directions, and the corresponding B-field was measured (the probe remained fixed). Other
measuring conditions were according to EN50366.
Pot characteristics Other parameters leading to significant variation of the B-field include
the bottom diameter, shape, bottom flatness and material grade (see Figure 31). Generally, the
B-field measured increases with smaller bottom diameter, and it is larger when the pots are old
or not perfectly flat.
Single-hob versus multi-hob use Simultaneous operation of several hobs is possible with
appliances 1 and 2. This multi-hob use increases the total B-field exposure since the area of
exposition is enlarged (see 6.1). However, in single-hob as well as in multi-hob use the B-field
emitted by each hob is highly localized in the vicinity of the particular hob. Thus for multi-hob
use the maximum B-field is still localized in front of the largest hob. Moreover, for multi-hob use
it was found experimentally that the maximum B-field exposure does not necessarily correspond
to all four hobs switched on simultaneously. The maximum B-field exposure corresponding to
B-field Exposure From Induction Cooking Appliances
42
Figure 31: Influence of pot characteristics (bottom diameter, shape, bottom flatness
and material grade) on the B-field measured. a) Normalized B-field measured for each
pot (pots ranked from smallest to largest bottom diameter). b) Standard set of pots; the
measured B-field decreases when the bottom diameter increases. c) For four pots with the same
bottom diameter, the B-field measured depends on the pot characteristics (material, flatness).
Measurements using appliance 3; the pots are centered and filled to 50 % of their capacity with
tap water; lids used when available.
the worst-case scenario depends strongly on the pot, hob and appliance combination (see Chapter
6.1.1).
6.0.7
Negligible Parameters
The following parameters have also been evaluated and can be neglected since their contribution
amounts to less than 5 % of the B-field variations. These include the volume of water in the
pot, content of the pot (e.g., tap water, salt-water, vegetable oil), lid on pot, handle position,
proximity to a magnet or paramagnetic object (cooking utensil).
Volume of water in the pot Stable operation conditions are reached when the water boils,
i.e., after approximately 4 to 6 minutes (H2 O phase transition). While the water is boiling,
the B-field is almost constant (see Figure 32) and the field variations remain less than ±2.45 %
for appliances 1, 2 and 3. Figure 32 shows that for all three appliances, the B-field measured
does not significantly depend on the duration of the measurement (the power supplied by the
appliance is stable over time). Moreover, the B-field measured does not depend on the volume
of water in the vessel, except when there is almost no water remaining in the pot (in that
case the pot quickly becomes very hot and the B-field increases rapidly before the appliance
automatically stops).
B-field Exposure From Induction Cooking Appliances
43
Figure 32: Influence of the volume of water and the duration of measurements on
the B-field measured, a) appliance 1, b) appliance 2, and c) appliance 3. For each
appliance, pot 3 (large pot, standard set of pots) is centered on the largest hob (hobs 1, 2 and 1
for appliances 1, 2 and 3, respectively). The pot is initially filled with 3 l of tap water. Measuring
distance of 1 cm, heat setting 8. The B-field is measured over a period of 30 minutes, and the
water evaporation is reduced using a lid.
Pot content Figure 33 compares the B-field measured using salt-water (see Figure 33 a) and
vegetable oil (see Figure 33 b) compared with tap water. In both cases, the corresponding
B-field variations are less than ±1 % (∆B/Baverage = ±0.55 % and ∆B/Baverage = ±0.85 %,
respectively).
Figure 33: Influence of pot content (salt-water, vegetable oil and tap water) on the
B-field measured. a) Salt-water. Measurement of the B-field with salt added to tap water
(1 l of tap water at 25◦ C reaches saturation with approximately 400 g of salt). Appliance 1, pot
3 without lid initially filled with 1 l of tap water, hob 1, measuring distance 1 cm, heat setting 8.
b) Comparison between tap water and vegetable oil. Appliance 3, pots 11a and 11b filled with
1 l of tap water and 1 l of vegetable oil, respectively, without lid, measuring distance 1 cm, heat
setting 8.
Lid on pot The B-field measured slightly increases when using a lid on the pot; however this
variation is less than 3 %. Nevertheless, the rate of evaporation is reduced by approximately
B-field Exposure From Induction Cooking Appliances
44
a factor of 4 when using a lid, allowing longer measurement times, while maintaining constant
measuring conditions. A lid was used for all of the DASY4 robot measurements.
Handle position In the case of frying-pan and saucepans, the handle position has a noticeable
influence on the B-field exposure. However, no general rule could be established, and the
variation of the B-field depends strongly on the pot, hob and appliance combination. For that
reason, the most appropriate handle position was chosen: For single-hob measurement, the
handle was aligned on the right side (y-direction), and for multi-hob measurement, the handle
was aligned on the right for hobs 1 and 3, and on the left sides for hobs 2 and 4 (y-direction).
Proximity to a magnet and paramagnetic object (cooking utensil) The manufacturer
recommends not to place cooking utensils, aluminum foil or magnets on the hob when it is
switched on (risk of burning). Measurements of the B-field variation were performed, and it
appears that depending on the object (e.g., magnet and paramagnetic utensil) and depending on
the location (between the appliance and the probe, on the side of the hob), the B-field variation
is −5 % < ∆B/Baverage < 1.5 %.
6.1
Induction Cookers B-Field Assessment
The second objective of this study was to evaluate the worst-case exposure for realistic use of
the three selected induction cooking devices, and to assess whether compliance is maintained
with the ICNIRP guideline (6.25 µT for general public exposure).
6.1.1
Definition of Several Scenarios Corresponding to Standard and Worst-Case
Exposures
In Chapter 6, the influence of several parameters was evaluated to assess the worst-case exposure.
The predominant parameters (see Chapter 6.0.6) were used to define the exposure scenarios,
combining single- and multi-hob use, pans centered and not, and the standard or worst-case set
of pans (see 3.3).
Standard exposure scenarios The standard set of pots (see Chapter 3.3.1) was used in the
scenarios summarized in Table 12. 14 standard scenarios were selected, combining single- and
multi-hob use, and pan centered and not. For Appliance 3, the single-hob standard measurements were performed using pots 3 and 13 (largest and smallest pots of the standard set of pots,
see explanation in Chapter 5.0.1).
Figure 34 shows the pot locations for non-centered measurements. In order to measure the
worst-case exposure on each side for multi-hob measurements with appliance 1, the heat settings
(P, P ) were selected for hob pairs (1, 2), (3, 4), (1, 3) and (2, 4) for front, rear, left and right side
measurements, respectively. Similarly, for non-centered pots on appliance 3, the pot location
was chosen to differ for the front, rear, left and right side measurements (see Figures 34 c) and
34 f) ). Appliance 3 is portable. The cook can choose to place it according to his needs and
available space, and thus it may be accessed from the rear, left and right sides. For this reason,
the B-field exposure of appliance 3 was assessed from all sides.
Worst-case exposure scenarios The worst-case set of pots (see 3.3.2) was used in the
scenarios given in Table 13. A total of 12 scenarios were selected, combining single- and multi-
B-field Exposure From Induction Cooking Appliances
45
Table 12: Standard scenarios (using the standard set of pots) defined to measure the B-field
emitted by the 3 induction cookers using the DASY4 robot system.
Single-hob measurement
Multi-hob measurement
Appliance
1
2
3
3
1
1
2
Hob(s)
1 to 4
1 to 4
1
1
1
2
3
4
1
3
1
2
3
4
Pot(s)
3, 14a, 14b, 13
3, 14a, 14b, 13
3
13
3
14a
14b
13
3
14a
14a
3
14b
13
Heat setting
P
P
12
12
P
P
8
7
P
P
9
9
9
9
App. sides
4
4
4
4
4
Centered
yes/no
yes/no
yes/no
yes/no
yes/no
4
yes/no
4
yes/no
Table 13: Worst-case scenarios (using the worst-case set of pots), defined to measure the B-field
emitted by the 3 induction cookers using the DASY4 robot system.
Single-hob measurement
Multi-hob measurement
Appliance
1
2
3
1
1
2
Hob(s)
1
2
1
1
2
3
1
2
1
2
3
Pot(s)
8
8
8
8
13
2
8
13
7
8
2
Heat setting
P
P
12
P
P
8
P
P
9
P
9
App. sides
4
4
4
4
Centered
yes/no
yes/no
yes/no
yes/no
4
yes/no
4
yes/no
B-field Exposure From Induction Cooking Appliances
46
hob use, and pan centered and not. The worst-case exposure depends on the combinations of
pot/hob/appliance. The pots for the worst-case scenarios where chosen experimentally.
Figure 34: Localization of the pots during standard and worst-case, single- and multihob measurements: a) and d) appliance 1, b) and e) appliance 2, c) and f) appliance 3.
6.1.2
Appliance 1
Single-hob measurement using the standard set of pots Figure 35 shows the B-field
measured in the vicinity of appliance 1, with only hob 1 switched on and using standard pot
3 centered and non-centered. The B-field does not exceed the ICNIRP limit (6.25 µT) for a
measuring distance of 30 cm, according to EN50366.
Figures 35 a) to e), f), g) and h) show that the ICNIRP limit is exceeded for measuring
distances of 2, 3, 1 and 3 cm for the front, rear, left and right sides of the appliance, respectively.
Figures 35 i) and j) show the B-field for pot 3 centered and not (front side of the appliance, see
Figure 34 a) ). The ICNIRP limit is exceeded for measuring distances of 2.2 cm and 5.5 cm when
the pot is centered and non-centered, respectively. The maximum B-field values at a measuring
distance of 1 cm are 7.2 µT (pot centered) and 13 µT (pot non-centered).
B-field Exposure From Induction Cooking Appliances
47
Multi-hob measurement using the standard set of pots Figure 36 shows the B-field
measured in the vicinity of appliance 1, with hobs 1, 2, 3 and 4 switched on simultaneously
(multi-hob measurement) and using the standard set of pots centered and non-centered. The
B-field does not exceed the ICNIRP limit (6.25 µT) for a measuring distance of 30 cm, according
to EN50366.
Figures 36 a), e) and f) show that the ICNIRP limit is exceeded for measuring distances
of 1 to 5 cm and 5 to 10 cm when the pots are centered and non-centered, respectively. The
corresponding maximum rms-values of the measured B-field are 8.6 µT and 13 µT (measuring
distance of 1 cm).
Figure 37 depicts the B-field measured in the vicinity of appliance 1, with hobs 1 and 2
switched on simultaneously (multi-hob measurement) and using the standard set of pots centered. The maximum rms-value of the B-field when only two hobs are switched on is 8.5 µT
(measuring distance of 1 cm), which is almost equal to the B-field measured when all four hobs
are switched on.
Single-hob measurement using pot 8, worst-case set of pots Figure 38 shows the Bfield measured in the vicinity of appliance 1, with only hob 1 switched on and using pot 8
(worst-case set of pots) centered and non-centered. The B-field does not exceed the ICNIRP
limit (6.25 µT) for a measuring distance of 30 cm, according to EN50366.
Figures 38 a), b) and e) (pot centered) and 38 f), g), h) and j) (pot non-centered) show that
the ICNIRP limit is exceeded for measuring distances of 9 cm and 15 cm when the pots are
centered and non-centered, respectively. The corresponding maximum rms-values of the B-field
measured are 21 µT and 38 µT (measuring distance of 1 cm).
Multi-hob measurement using the worst-case set of pots Figure 39 illustrates the Bfield measured in the vicinity of appliance 1, with hobs 1, 2 and 3 switched on simultaneously
(multi-hob measurement employing three hobs) and using the worst-case set of pots (pots 8, 13
and 2 on hobs 1, 2 and 3, respectively) centered and non-centered. The B-field does not exceed
the ICNIRP limit (6.25 µT) for a measuring distance of 30 cm, according to EN50366.
Figures 39 a), b) and e) (pot centered) and 39 f), g), h) and j) (pot non-centered) show that
the ICNIRP limit is exceeded for measuring distances of 9 cm and 15 cm when the pots are
centered and non-centered, respectively. The corresponding maximum rms-values of the B-field
measured are 21 µT and 36 µT (measuring distance of 1 cm).
The same results are found in Figure 40, when two hobs instead of three are switched on
simultaneously. The values are the same for one, two or three hobs switched on simultaneously
when using the worst-case set of pots. However, the B-field exposure area is larger with multihob than with single-hob measurement.
B-field Exposure From Induction Cooking Appliances
48
Figure 35: Appliance 1 single-hob measurement using the standard set of pots: In
Graphs a) to d), the B-field is measured in the vertical plane (y, z) at the front side of the
appliance, at measuring distances of a) 1 cm, b) 5 cm, c) 10 cm and d) 30 cm. In Graphs e) to
h), the B-field is measured in the horizontal plane. Graphs e), f), g) and h) correspond to the
front, rear, left and right sides of the appliance, respectively. Graphs i) and j) show a comparison
between i) pot centered, and j) pot non-centered; the B-field is measured in the horizontal plane,
at the front side of the appliance. Hob 1 is switched on (single-hob measurement), using the
standard pot 3 centered; other measuring conditions according to EN50366.
B-field Exposure From Induction Cooking Appliances
49
Figure 36: Appliance 1 multi-hob measurement (four hobs) using the standard set of
pots. Comparison of the B-field when the pots are a) to d) centered, and e) to h)
non-centered (front side of the appliance): The B-field is measured in the vertical plane
(y, z), at measuring distances of a) 1 cm, b) 5 cm, c) 10 cm and d) 30 cm. Hobs 1, 2, 3 and 4
are switched on simultaneously (multi-hob measurement), using the standard set of pots (pots
3, 14a, 14b and 13 on hobs 1, 2, 3 and 4, respectively); other measuring conditions according to
EN50366.
B-field Exposure From Induction Cooking Appliances
50
Figure 37: Appliance 1 multi-hob measurement (two hobs) using the standard set
of pots (front side of the appliance): The B-field is measured in the vertical plane (y, z),
at measuring distances of a) 1 cm, b) 5 cm, c) 10 cm and d) 30 cm. Hobs 1 and 3 are switched
on simultaneously (multi-hob measurement), using the standard set of pots (pots 3 and 14a on
hobs 1 and 3, respectively), other measuring conditions according to EN50366. The pots are
centered.
B-field Exposure From Induction Cooking Appliances
51
Figure 38: Appliance 1 single-hob measurement using the worst-case set of pots
(front side of the appliance). Comparison of the B-field when the pot is a) to e)
centered, and f ) to j) non-centered: In Graphs a), b), c), d), f), g), h) and i), the B-field is
measured in the vertical plane (y, z) at measuring distances of 1, 5, 10, 30, 1, 5, 10 and 30 cm,
respectively. In Graphs e) and j), the B-field is measured in the horizontal plane. Hob 1 is
switched on (single-hob measurement), and pot 8 (worst-case set of pots) is used.
B-field Exposure From Induction Cooking Appliances
52
Figure 39: Appliance 1 multi-hob measurement (three hobs) using the worst-case set
of pots (front side of the appliance). Comparison of the B-field when the pot is a)
to e) centered, and f ) to j) non-centered: In Graphs a), b), c), d), f), g), h) and i), the
B-field is measured in the vertical plane (y, z), at measuring distances of 1, 5, 10, 30, 1, 5, 10
and 30 cm, respectively. In Graphs e) and j), the B-field is measured in the horizontal plane.
Hobs 1, 2 and 3 are switched on simultaneously (multi-hob measurement), using the worst-case
set of pots (pots 8, 13 and 2 for hobs 1, 2 and 3, respectively).
B-field Exposure From Induction Cooking Appliances
53
Figure 40: Appliance 1 multi-hob measurement (two hobs) using the worst-case set
of pots (front side of the appliance). Comparison of the B-field when the pot is a)
to e) centered, and f ) to j) non-centered: In Graphs a), b), c), d), f), g), h) and i), the
B-field is measured in the vertical plane (y, z), at measuring distances of 1, 5, 10, 30, 1, 5, 10
and 30 cm, respectively. In Graphs e) and j), the B-field is measured in the horizontal plane.
Hobs 1 and 2 are switched on simultaneously (multi-hob measurement), using the worst-case set
of pots (pots 8, and 13 for hobs 1 and 2, respectively).
B-field Exposure From Induction Cooking Appliances
6.1.3
54
Appliance 2
Single-hob measurement using the standard set of pots Figure 41 shows the B-field
measured in the vicinity of appliance 2, with hob 2 switched on using pot 3 (standard set of pots)
centered (front side measurement). The B-field does not exceed the ICNIRP limit (6.25 µT) for
a measuring distance of 30 cm, according to EN50366. The limit is not even exceeded at 1 cm
off the edges of the appliance.
Figure 41: Appliance 2, single-hob measurement using the standard set of pots:
Hob 2 (largest) is switched on, with pot 3 centered (front side measurement). The B-field
was measured in the vertical plane (y, z) at measuring distances a) 1 cm and b) 30 cm; other
measuring conditions according to EN50366.
Multi-hob measurement using the standard set of pots Figures 42 and 43 depict the
B-field measured in the vicinity of appliance 2 (multi-hob measurement using the standard set
of pots centered and non-centered, respectively) with hobs 1, 2, 3 and 4 simultaneously switched
on (multi-hob measurement) using the standard set of pots (pots 14a, 3, 14b and 13 on hobs 1, 2,
3 and 4, respectively). The B-field does not exceed the ICNIRP limit (6.25 µT) for a measuring
distance of 30 cm, according to EN50366.
However, the maximum rms-values of the B-field measured at the front side of the appliance
are 2.5 µT and 8.5 µT for a centered and non centered pots, respectively (measuring distance
of 1 cm, see Figures 42 a) and 43 a). Figures 42 c), e) and g) (pots centered) show that the
ICNIRP limit is exceeded for a measuring distance of 1 to 5 cm. The corresponding maximum
rms-values of the B-field measured are 13.5 µT, 9 µT and 7 µT for the rear, left and right sides
of the appliance, respectively. Figure 43 also shows that when the pots are non-centered, the
ICNIRP limit is exceeded for a measuring distance ranging from 1 to 5 cm.
Single-hob measurement using pot 8, worst-case set of pots Figure 44 shows the Bfield measured in the vicinity of appliance 2 with hob 2 switched on (single-hob measurement)
using the worst-case pot 8. The B-field does not exceed the ICNIRP limit (6.25 µT) for a
measuring distance of 30 cm, according to EN50366.
Figures 44 a), (pot centered), e) and f) (pot non-centered) show that the ICNIRP limit is
exceeded for measuring distances of 1 to 5 cm and 5 to 10 cm when the pot is centered and
non-centered, respectively. The corresponding maximum rms-values of the B-field measured are
7.5 µT and 16 µT (measuring distance of 1 cm).
B-field Exposure From Induction Cooking Appliances
55
Figure 42: Appliance 2, multi-hob measurement using the standard set of pots, with
pots centered: The B-field is measured in the vertical plane (y, z), at measuring distances of
a), c), e) and g) 1 cm and b), d), f) and h) 30 cm; a) and b) front side, c) and d) rear side, e)
and f) left side and g) and h) right side of the appliance. Hobs 1, 2, 3 and 4 are switched on
simultaneously (multi-hob measurement) using the standard set of pots (pots 14a, 3, 14b and
13 on hobs 1, 2, 3 and 4, respectively); other measuring conditions according to EN50366.
Multi-hob measurement using the worst-case set of pots Figure 45 shows the B-field
measured in the vicinity of appliance 2, with hobs 1, 2, and 3 switched on simultaneously
(multi-hob measurement) using the worst-case set of pots (pots 7, 8 and 2 on hobs 1, 2 and 3,
B-field Exposure From Induction Cooking Appliances
56
Figure 43: Appliance 2, multi-hob measurement using the standard set of pots with
pots non-centered (front side of the appliance): The B-field is measured in the vertical
plane (y, z), at measuring distances of a) 1 cm, b) 5 cm, c) 10 cm and d) 30cm. Hobs 1, 2, 3 and
4 are switched on simultaneously (multi-hob measurement), using the standard set of pots (pots
14a, 3, 14b and 13 on hobs 1, 2, 3 and 4, respectively); other measuring conditions according to
EN50366.
respectively). The B-field does not exceed the ICNIRP limit (6.25 µT) for a measuring distance
of 30 cm, according to EN50366.
Figure 45 a) (pots centered), as well as Figures e), f) and g) (pots non-centered) show that
the ICNIRP limit is exceeded for a measuring distance from 1 to 5 cm and 10 cm for a centered
and non-centered pots, respectively. The corresponding maximum rms-values of the B-field
measured are 8.5 µT and 13.2 µT (measuring distance of 1 cm).
B-field Exposure From Induction Cooking Appliances
57
Figure 44: Appliance 2, single-hob measurement using the worst-case set of pots.
Comparison of the B-field when the pot are a) to d) centered, and e) to h) noncentered (front side of the appliance): In Graphs a), b), c), d), e), f), g) and h), the B-field
is measured in the vertical plane (y, z) at measuring distances of 1, 5, 10, 30, 1, 5, 10 and 30 cm,
respectively. Hob 2 is switched on (single-hob measurement) using the worst-case pot 8.
B-field Exposure From Induction Cooking Appliances
58
Figure 45: Appliance 2, multi-hob measurement using the worst-case set of pots.
Comparison of the B-field with the pot a) to d) centered, and e) to h) non-centered
(front side of the appliance): In Graphs a), b), c), d), e), f), g) and h), the B-field is
measured in the vertical plane (y, z) at measuring distances of 1, 5, 10, 30, 1, 5, 10 and 30 cm,
respectively. Hobs 1, 2, and 3 are switched on simultaneously (multi-hob measurement), using
worst-case set of pots (pots 7, 8 and 2 on hobs 1, 2 and 3, respectively).
B-field Exposure From Induction Cooking Appliances
6.1.4
59
Appliance 3
Single-hob measurement using pot 3, large pot, standard set of pots Figures 46 and
47 depict the B-field measured in the vicinity of appliance 3 with pot 3 (large pot, standard set
of pots) centered and non-centered. Figure 46 depicts the front side measurements, and Figure
47 the measurements at the rear, left and right sides of the appliance. The B-field does not
exceed the ICNIRP limit (6.25 µT) for a measuring distance of 30 cm, according to EN50366.
However, at the front side of the appliance, Figures 46 f) and h) (pot non-centered) show
that the ICNIRP limit is exceeded for a measuring distance of 12 cm. The maximum rms-values
of the measured B-field are 3.7 µT and 18 µT (measuring distance of 1 cm) for a centered and
non-centered pot, respectively. Moreover, Figure 47 shows that the ICNIRP limit is exceeded
for a measuring distance of 5 cm, 4 cm and 3 cm for the rear, left and right sides of the appliance,
respectively, when the pot is centered, and 14 cm, 14 cm and 13 cm for the rear, left and right
sides of the appliance, respectively, when the pot is non-centered. The corresponding maximum
rms-values of the B-field measured (measuring distance of 1 cm) are 12.5 µT, 11 µT, 9 µT (rear,
left and right sides, pot centered) and 22.5 µT, 22.5 µT and 22 µT (rear, left and right sides, pot
non-centered).
Single-hob measurement using pot 13, standard set of pots, small size Figures 48
and 49 show the B-field measured in the vicinity of appliance 3 with pot 13 (small pot, standard
set of pots) centered and non-centered. Figure 48 depicts the front side measurements, and
Figure 49 the measurements at the rear, left and right sides of the appliance. The B-field does
not exceed the ICNIRP limit (6.25 µT) for a measuring distance of 30 cm, according to EN50366.
However, at the front side of the appliance, Figures 48 a) and e) (pot centered), as well as a
f) and h) (pot non-centered) show that the ICNIRP limit is exceeded for a measuring distance
of 4 cm and 19 cm for a centered and non-centered pot, respectively. The maximum rms-values
of the B-field measured are 9 µT and 34 µT (measuring distance of 1 cm) for a centered and
non-centered pot. Moreover, Figure 49 shows that the ICNIRP limit is exceeded for measuring
distances of 10 cm, 9 cm and 10 cm for the rear, left and right sides of the appliance, respectively
when the pot is centered, and 24 cm, 24 cm and 25 cm for the rear, left and right sides of the
appliance, respectively, when the pot is non-centered. The corresponding maximum rms-values
of the B-field measured (measuring distance of 1 cm) are 24 µT, 24 µT, 25 µT (rear, left and right
sides, pot centered) and 51 µT, 55 µT and 57 µT (rear, left and right sides, pot non-centered).
Single-hob measurement using pot 8, worst-case set of pots Figures 50 and 51 depict
the B-field measured in the vicinity of appliance 3 with pot 8 (worst-case set of pots) centered and
non-centered. Figure 50 depicts the front side measurements, and Figure 51 the measurements
at the rear, left and right sides of the appliance. The B-field does not exceed the ICNIRP limit
(6.25 µT) for a measuring distance of 30 cm, according to EN50366.
However, at the front side of the appliance, Figures 50 a), b) and e) (pot centered), as well as
f) and h) (pot non-centered) show that the ICNIRP limit is exceeded for a measuring distance
of 6 cm and 20 cm for a centered and non-centered pot, respectively. The maximum rms-values
of the B-field measured are 11.5 µT and 35 µT (measuring distance of 1 cm) for a centered and
non-centered pot. Moreover, Figure 51 shows that the ICNIRP limit is exceeded for measuring
distances of 13 cm, 13 cm and 14 cm for the rear, left and right sides of the appliance, respectively,
when the pot is centered, and 22 cm, 24 cm and 24 cm for the rear, left and right sides of the
appliance, respectively, when the pot is non-centered. The corresponding maximum rms-values
of the measured B-field (measuring distance of 1 cm) are 30 µT, 30 µT, 34 µT (rear, left and right
B-field Exposure From Induction Cooking Appliances
60
Figure 46: Appliance 3 and pot 3 (large pot, standard set of pots), front side of
the appliance. Comparison of the B-field with pot a) to e) centered, and f ) to h)
non-centered: In Graphs a), b), c), d), f) and g), the B-field is measured in the vertical plane
(y, z), at measuring distances of 1, 5, 10, 30, 1 and 30 cm, respectively. In Graphs e) and h), the
B-field is measured in the horizontal plane. Other measuring conditions according to EN50366.
sides, pot centered) and 50 µT, 60 µT and 60 µT (rear, left and right sides, pot non-centered).
B-field Exposure From Induction Cooking Appliances
61
Figure 47: Appliance 3 and pot 3 (large pot, standard set of pots). Graphs a) to f ),
g) to l) and m) to r) correspond to the rear, left and right sides of the appliance,
respectively. Comparison of the B-field with pot a) to c), g) to i), m) to o) centered,
and d) to f ), j) to l) and p) to r) non-centered: In Graphs a), d), g), j), m) and p), the
B-field is measured along the z-axis at measuring distances of 30, 10, 5 and 1 cm. In Graphs
b), e), h), k), n) and q), the B-field is measured in the vertical plane, at a measuring distance
of 1 cm. In Graphs c), f), i), l) o) and r), the B-field is measured in the horizontal plane. Other
measuring conditions according to EN50366.
B-field Exposure From Induction Cooking Appliances
62
Figure 48: Appliance 3 and pot 13 (small pot, standard set of pots), front side of
the appliance. Comparison of the B-field with pot a) to e) centered, and f ) to h)
non-centered: In Graphs a), b), c), d), f) and g), the B-field is measured in the vertical plane
(y, z), at measuring distances of 1, 5, 10, 30, 1 and 30 cm, respectively. In Graphs e) and h), the
B-field is measured in the horizontal plane. Other measuring conditions according to EN50366.
B-field Exposure From Induction Cooking Appliances
63
Figure 49: Appliance 3 and pot 13 (small pot, standard set of pots). Graphs a) to f ),
g) to l) and m) to r) correspond to the rear, left and right sides of the appliance,
respectively. Comparison of the B-field with pot a) to c), g) to i), m) to o) centered,
and d) to f ), j) to l) and p) to r) non-centered: In Graphs a), d), g), j), m) and p), the
B-field is measured along the z-axis at measuring distances of 30, 10, 5 and 1 cm. In Graphs
b), e), h), k), n) and q), the B-field is measured in the vertical plane, at a measuring distance
of 1 cm. In Graphs c), f), i), l) o) and r), the B-field is measured in the horizontal plane. Other
measuring conditions according to EN50366.
B-field Exposure From Induction Cooking Appliances
64
Figure 50: Appliance 3 and pot 8 (worst-case set of pots), front side of the appliance.
Comparison of the B-field with pot a) to e) centered, and f ) to h) non-centered: In
Graphs a), b), c), d), f) and g), the B-field is measured in the vertical plane (y, z), at measuring
distances of 1, 5, 10, 30, 1 and 30 cm, respectively. In Graphs e) and h), the B-field is measured
in the horizontal plane.
B-field Exposure From Induction Cooking Appliances
65
Figure 51: Appliance 3 and pot 8 (worst-case set of pots). Graphs a) to f ), g) to l) and
m) to r) correspond to the rear, left and right sides of the appliance, respectively.
Comparison of the B-field with pot a) to c), g) to i), m) to o) centered, and d) to
f ), j) to l) and p) to r) non-centered: In Graphs a), d), g), j), m) and p), the B-field is
measured along the z-axis at measuring distances of 30, 10, 5 and 1 cm. In Graphs b), e), h),
k), n) and q), the B-field is measured in the vertical plane, at a measuring distance of 1 cm. In
Graphs c), f), i), l) o) and r), the B-field is measured in the horizontal plane.
B-field Exposure From Induction Cooking Appliances
6.1.5
66
B-field Measurement Above the Appliances
As indicated in Figure 52, the B-field was measured above the induction cookers to evaluate the
exposure in the vicinity of the pot.
With induction cookers, the surroundings of the hob do not heat significantly, allowing the
cook to stay in the vicinity of the cooking area, where the magnetic field exposure is at a
maximum.
Figure 52: Description of the B-field measurement above a) appliance 1, b) appliance
2 and c) appliance 3: The probe measures the B-field along the horizontal y-axis, at the front
sides of the appliances (single-hob measurements, with pots 3, 8 and 13 centered on the hobs).
The B-fields measured above the three appliances are given in Figure 53. The maximum
rms-values measured are 9.3 µT and 47 µT for pots 3 and 8, respectively (appliance 1), 9.5 µT
and 61 µT for pots 3 and 8, respectively (appliance 2), and 55 µT, 80 µT and 84 µT for pots 3,
13 and 8, respectively (appliance 3). These values exceed the ICNIRP guidelines by a factor of
1.5, 7.5, 1.5, 9.8, 8.8, 12.8 and 13.4, respectively.
B-field Exposure From Induction Cooking Appliances
67
Figure 53: B-field measurement above the appliances: a) and b) correspond to appliance
1, with a) pot 3 centered, b) pot 8 centered. c) and d) correspond to appliance 2, with c) pot
3 centered, d) pot 8 centered. e) to g) correspond to appliance 3, with e) pot 3 centered, f) pot
13 centered, and g) pot 8 centered. Measuring conditions are described in Chapter 6.1.1.
B-field Exposure From Induction Cooking Appliances
6.1.6
68
Discussion
The evaluation according to EN50366 (i.e., for a measuring distance of 30 cm) demonstrates
compliance according to the ICNIRP limit (6.25 µT) for all three devices with a margin larger
than 13 dB (see Chapter 5.0.1 and Table 11).
As demonstrated in this report, different pot and and heating configurations can results in
exposures that well exceeds +10 dB of the standard EN50366 configurations at same distance.
In addition, the field distribution has a strong gradient in the direction of larger distances (see
Table 14).
The maximum distance at which the ICNIRP limit of 6.25 µT is exceeded is 25 cm for Appliance 3 (pots non-centered). At 1 cm the limit is exceed by almost 20 dB. When measuring
above appliance 3 (single-hob measurement, pot 8 centered), the measured B-field exceeds this
guideline by over 22 dB. Since the induced currents are a function of the exposed cross-section
and not of local fields, these violations of the derived limits are not necessarily also a violation
of the governing basic restrictions.
Pot 3
Pot 13
Pot 8
Standard
Worst-case
Multi-hob
Worst-case
Standard
Single-hob
Worst-case
yes
no
yes
no
yes
no
yes
no
yes
no
yes
no
yes
no
yes
no
yes
no
yes
no
yes
no
Pot(s)
centered
3
5.5
1-5
5-10
9
15
9
15
1-5
1-5
1-5
5-10
1-5
10
12
4
19
6
20
front side
[cm]
3
1-5
3-5
13-14
9-10
24-25
13-14
22-24
other sides
[cm]
Meas. dist. at which
ICNIRP limit is exceeded
7.2
13
8.6
13
21
38
21
36
2.7
2.5
8.5
7.5
16
8.5
13.2
3.7
18
9.0
34
11.5
35
front side
[µT]
8.0
14
13.5
12.5
27.5
25
57
34
60
front side
[µT]
Max. B-field
at meas. dist.
of 1 cm
3.0
7.4
3.7
6.4
10
18
9.7
18
2.0
1.7
5.4
4.6
8.2
4.5
10
2.2
12
6.2
23
8.2
21
front side
[µT]
Max. B-field
at meas. dist.
of 5 cm
1.4
3.5
1.9
4.2
5.6
9.9
5.1
9.4
1.3
0.97
3.4
2.4
5.0
2.8
6.0
1.0
7.6
3.7
13
5.0
13
front sides
[µT]
Max. B-field
at meas. dist.
of 10 cm
0.21
1.0
0.39
1.1
1.0
1.8
1.0
1.9
0.24
0.26
0.94
0.61
1.2
0.68
1.6
0.15
2.0
0.91
2.9
1.2
3.2
front side
[µT]
Max. B-field
of meas. at
30 cm
Note:
†
For the built-in type appliances 1 and 2 only front-side exposure values are compared, as only the exposure in front of the appliance is most relevant.
‡
For the free-standing appliance 3 exposure values from all sides (front and other sides) are compared as the device is accessible from all sides.
Appliance 3
Multi-hob
Standard
Multi-hob
Worst-case
Single-hob
Single-hob
Worst-case
Standard
Multi-hob
Standard
Appliance 2
Single-hob
Standard
Appliance 1
Measurement
type
Set of pots
Appliance
-29
-15.9
-24
-15.1
-15.9
-10.8
-15.9
-10.3
-28
-28
-16.8
-20
-14.3
-19.3
-11.8
-32
-10.1
-16.8
-6.7
-14.3
-5.8
[dB]
Comparison
of meas. at
dist. 30 cm and
ICNIRP limit
31†
22†
27†
21†
26†
26†
26†
26†
21†
19.7†
19.5†
22†
22†
22†
18.3†
38‡
23‡
29‡
26‡
29‡
25‡
[dB]
Comparison
of meas. at
dist. 30 cm
and 1 cm
Table 14: Summary of the results discussed in Chapters 6.1.2, 6.1.3 and 6.1.4. The column ”other sides” corresponds to the rear, left and
right sides of the appliances.
B-field Exposure From Induction Cooking Appliances
69
B-field Exposure From Induction Cooking Appliances
7
70
Approximation of Induced Current Density
As shown in Chapters 6.1.2, 6.1.3, 6.1.4 and in Table 14, the B-fields measured around the
induction cookers are largely non-uniform.
The ICNIRP guidelines [2] state that the reference levels are intended to be spatially averaged
values over the entire body of the exposed individual, but with the important provision that
the basic restrictions on localized exposure are not exceeded. However, reliable estimations of
the current density J induced in the body at close proximity to the induction cookers can only
be obtained by appropriate simulation tools. Since these evaluations were not within the scope
of this project, we approximate J based on very simple models in order to identify possible
concerns of strong violations of the basic restrictions.
Figure 54: Magnetic field model used to roughly calculate the induced current density J in the
body at close proximity to the induction cookers. The B-field does not depend on the position.
The magnetic field model shown in Figure 54 was used to roughly evaluate the induced current
density J. The model assumes that the body has a homogeneous and isotropic conductivity,
using a plane wave approximation of the B-field. The induced current density J is derived from
Faraday’s law of induction (simple circular conductive loop model, Equation (10)) and Ohm’s
law (Equation (11)):
I
E dl = −
Z
L
S
∂B
· dA
∂t
J = σE
(10)
(11)
where E and B are the electric and magnetic fields, L and S are the contour and the surface,
and J and σ are the current density and the conductivity, respectively.
In this model, B does not depend on the position (see Figure 54). The B-field is given by
B(x, y, z, t) = B(z, t) = B0 sin(ωt), with ω = 2πf the angular velocity. The electric field E is
derived from Faraday’s law for a pure sinusoidal field at frequency f :
I
E dl = 8aE(t)
(12)
L
−
Z
S
∂B
· dA = 4a2 B0 ω cos (ωt)
∂t
(13)
B-field Exposure From Induction Cooking Appliances
| E(t) | = πaf B0
71
(14)
The induced current density J is derived from Ohm’s law:
| J(t) | = πaf σB0
(15)
The averaged B-field for the worst-case configuration of Appliance 3, averaged over the area
with a = 0.175m (dimensions of the numerical homogeneous human body, see [4]) at height z =
1 m, equals 18 µT (average B-field in the worst case plane). Assuming f = 20 kHz, σ = 0.2 S/m
(see [2]), the induced current density magnitude equals J = 39.6 mA/m2 (rms) compared to
the ICNIRP basic restriction for the general public exposure at 20 kHz, which equals 40 mA/m2
(rms). A device with similar field distributions but close to the limits at EN50366 configurations
(B=6.25 µT at a measuring distance of 30 cm, for standard measurement according to EN50366)
would induce a current density of 315 mA/m2 , which would be 18 dB above the ICNIRP limits.
In conclusion, the ICNIRP basic restriction may be exceeded by considerable margin while
the device would be considered compliant with EN50366. Further numerical simulations are
therefore necessary to evaluate the potential of violation, especially in pregnant women.
B-field Exposure From Induction Cooking Appliances
8
72
Conclusion
The objective of this study was to assess the maximum exposure that arises during the use of
induction cooking devices. Three devices currently available on the Swiss market were selected:
the built-in appliances 1 and 2 (Electrolux GK58TCi and Gaggenau CI 261 110) and the portable
appliance 3 (Inducs SH/BA 5000). The appliances were mounted on wooden supports allowing
measurement in close proximity to the hobs without disturbing the B-field.
Figure 55: a) Comparison between the compliance value (B-field measured according to
EN50366) and the maximum B-field measured in the worst-case scenarios at 1 cm and 30 cm
of Appliance 3. b) B-field measured according to EN50366 and c) B-field measured in the
worst-case configuration (horizontal plane at height of appliance).
B-field Exposure From Induction Cooking Appliances
73
Fifteen pots and pans of different sizes and shapes, as well as of various materials were
evaluated in single and multi-hob use in order to select a worst-case set of pots corresponding
to the worst-case B-field exposure. A standard set of pots was also defined according to the
European Norm EN50366. The Narda probe ELT-400 specifications were validated, and the
probe was used for the characterization of the induction cookers in the time- and frequencydomains (cooking signal frequency, dependency on the heat setting, contribution).
In the first step, the spatial B-field exposure was evaluated according to EN50366 [4] (i.e.,
at a measuring distance of 30 cm using the standard set of pots). All three appliances met the
compliance criteria of ICNIRP [2] for incident B-fields by a margin larger than 14 dB (see Table
11).
In the second step, the worst-case exposure was evaluated as a function of pot and heating
configurations. It was demonstrated that different pot and and heating configurations can
result in exposures that well exceed +10 dB of the standard EN50366 configurations at the same
distance (see Figure 55). In addition, the field distribution has a strong negative gradient in the
direction of larger distances. Therefore, DASY4 was enhanced to enable 3D field scanning using
the NARDA probe. At the very short distance of 1 cm, the fields can be more than 30 dB larger
than at 30 cm (see Table 14). Combining the results from worst-case configurations and short
distance measurements, the standard EN50366 values can be exceeded by 37 dB (see Figure 55).
The uncertainty of the evaluation was determined to be ±1.5dB (k=2).
The third task was to evaluate the findings with respect to compliance testing. Assuming
the distribution of Appliance 3, exposure close to the appliance could be as much as 37 dB or
a factor of 70 above the ICNIRP safety limits. A very simple approximation suggest that the
induced currents for such a worst-case compliant appliance would exceed the basic restrictions
by nearly a factor of 10. In other words, the current standard EN50366 for compliance testing
does not prevent exposures far above the basic restrictions and therefore needs revisions.
To obtain the scientific basis for a sound and reliable compliance test procedure, systematic
evaluations of induced currents as a function of human anatomy and field distributions are
necessary and recommended. The data obtained from this study indicates that an improved
procedure will not be considerable more costly than EN50366.
9
Acknowledgments
This study was supported by the Swiss Federal Office of Public Health (SFOPH/BAG).
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