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Induction hobs

Induktionsherd
Induction hobs cook rapidly and save energy. They have long been used in commercial kitchens because of their advantages, and they are becoming increasingly common in domestic kitchens.
In an induction hob, the heat energy needed to cook the food is created by medium-frequency magnetic fields. These magnetic fields penetrate the base of the pan, where they create electric currents which heat the pan and its contents. Some of the magnetic fields are not absorbed by the pan, so magnetic fields may occur in the immediate vicinity of the hob.

It is not currently known whether magnetic magnetic fields originating from induction hobs represent a health risk. These magnetic fields can be reduced by correct use of the induction hob. The following tips will help you to get the best results:

  • Use the right size of cookware for the size of the cooking zone; don't put a small pan on a large zone, but use a pan that covers the cooking zone completely. Always place the pan in the middle of the cooking zone.
  • Don’t use damaged pans with buckled or rounded bases, even if they can still be heated without difficulty.
  • Persons standing close to the hob or who touch the worktop with their body during cooking are advised to use the rear cooking fields, or the front cooking fields at reduced power.
  • It is vital to use specially manufactured pans to ensure that energy is transmitted efficiently from the hob to the pan. They are labelled by the manufacturer as suitable for induction cooking. The best pans to use are the ones supplied with the hob.
  • Exposure to magnetic fields can be reduced greatly by keeping a distance of 5-10 cm between your body and the hob.
  • Don’t use metal cooking spoons to prevent leakage current from flowing through your body.
  • People with a cardiac pacemaker or an implanted defibrillator should talk to their doctor before using an induction hob.

Detailed information

Electrical induction has been used for years in industry to heat electrically conducting components in a wide variety of applications. The primary use of this heating principle in the domestic setting is in induction hobs. Heat is generated directly in the pan and not, as with conventional hobs, conducted through the cooking zone to the pan. Induction hobs have a number of advantages: a rapid response time, rapid onset of cooking, shorter cooking times, energy-saving heat generation, no hot cooking zones and a correspondingly lower risk of burns and fire.

1. Technical information

Frequency: 20 – 100 kHz
Output: up to 7500 W

The principle of induction cooking 
Beneath each cooking zone of the induction hob there is a coil through which a medium-frequency alternating current (20 - 100 kHz) flows. This creates a magnetic field of the same frequency which passes unobstructed through the ceramic cover of the hob and penetrates the pan sitting on the cooking zone (Figure 1). The magnetic field creates a circular current in the electrically conductive base of the pan (eddy current). This principle is called induction. The base of the pan is made of a material in which the heat-loss of the eddy current is as high as possible at the frequency being used. This happens in ferromagnetic materials. In these materials the alternating field is forced into the outer layer of the pan base (skin effect), which increases the resistance of the material to the current and produces intense heat. The alternating magnetic field within the base of the pan also repeatedly magnetises and demagnetises the material, and this creates additional heat (hysteresis loss) [1].
induktionskochherd
induktionskochherd
 1 Induction coil  5 Indication of operation
 2 Temperature sensor  6 Base of pan made of ferromagnetic material
 3 Thermal insulation  7 Alternating electromagnetic field
 4 Glass ceramic plate  
Stray fields
The magnetic field which is not captured by the induction in the pan is called a stray field. It is most likely to occur when the cooking zone is not completely covered by the pan [3]. Since the eddy current in the base of the pan creates a magnetic field which opposes the magnetic field created by the hob, the field created by the hob and consequently the stray field are both weakened.

Leakage current
The induction coil and the pan standing on the cooking zone form a capacitor. When the induction coil is switched on, the pan is charged electrically. If the pan is touched by a person, a small current (leakage current) may flow through that person’s body [7].

Typical output
Appliances designed for use in the home usually have four cooking zones with different outputs ranging from 1200 to 3600 Watts. The total output of built-in units is approximately 7500 Watts. Cooking zones can be operated for a brief period at increased output (booster or power function) in order to start the cooking process rapidly or to heat water quickly.

Regulating heating power
Heating power can be regulated using various methods which affect the properties of the magnetic fields. Common methods include:
  • Regulation using the frequency of the alternating current: The induction hob constitutes an electrical oscillating circuit which carries the maximum current at resonant frequency.  If the frequency deviates from the resonant frequency, both current and output are reduced. (Example: full output at the resonant frequency of 17.5 kHz, output is four times lower at 41.7 kHz.)
  • Regulation using pulse-amplitude modulation: Output is regulated by switching the magnetic field on and off periodically at lower cooking settings. One pulse every two seconds is typically used, with the duration of the pulse varying according to the selected output. The resulting magnetic fields are pulsed at a frequency of 0.5 Hz with varying pulse length.

2. Exposure of the user to stray magnetic fields

In a study commissioned by the FOPH in 2006, the stray magnetic fields of two built-in models with four cooking zones (hob 1 and hob 2) and a professional high-performance mobile unit with one cooking zone (hob 3) were measured [3].
The current standard for induction hobs [4] stipulates that the unit must comply with the reference value recommended by the ICNIRP (International Commission on Non-Ionising Radiation Protection) of 6.25 microtesla (µT) at a distance of 30 cm from the cooking field when one cooking zone is operated with a suitable pan which is large enough and centred on the cooking zone. All the units measured complied with this requirement.
However, in everyday use this condition may not be met. The effect on the stray field of several cooking zones being used at the same time or unsuitable pans being used or the pans not being centred on the cooking zone was therefore also investigated. The magnetic fields were measured between 1 cm and 30 cm away from the edge of the glass ceramic cooking field since it is not always possible to keep at least 30 cm away from the hob in practice. This applies particularly to pregnant women, children and people of small stature.

Using several cooking zones at the same time
The measurements showed that the stray fields produced in front of the hob by simultaneous use of several cooking zones are not much larger than those created by a single cooking zone.

Appropriate vs. inappropriate pan
The measurements were carried out using appropriate and inappropriate pans which were centred over the cooking zone.
  • Appropriate pans: Pans which are suitable for induction hobs AND / OR whose diameter is the same as that of the cooking zone.
  • Inappropriate pans: Pans which are not suitable for induction hobs OR whose diameter is not the same as that of the cooking zone. 

The stray fields measured with inappropriate pans were up to 3.5 times larger than those measured with appropriate pans (Figure 2).

magnetic field
Figure 2: Stray fields were measured at a distance between 1 and 30 cm using appropriate and inappropriate pans centred over the cooking zone.
Centred vs. not centred position on the cooking zone
An induction hob switches off automatically when the pan is removed from the cooking zone. The stray-field measurements compared suitable, exactly centred pans with suitable pans which were only so far off-centre that the hob did not switch off. Figure 3 shows that positioning the pan off-centre increases the stray field for the same pan by a factor of up to 5.
magnetic field
Figure 3: Stray fields were measured at a distance between 1 and 30 cm using centred and off-centre appropriate pans.

Appropriate pan, centred vs. inappropriate pan, off-centre
Figure 4 compares the stray fields from an appropriate, centred pan and an unsuitable, off-centre pan (worst case). The stray fields in the worst case are up to 9.5 times larger than the stray field generated by the use according to the standard.

magnetic field
FFigure 4: Stray fields were measured at a distance between 1 and 30 cm with appropriate, centred pans and inappropriate, off-centre pans.
The impact of distance on stray fields 

Stray fields are larger the closer to the cooking field they are measured. At a distance of 30 cm, all models comply with the reference value of 6.25 microtesla (µT) recommended by the ICNIRP. In most cases the stray field measured 1 cm in front of the edge of the cooking zone exceeds this reference value. With an off-centre placing the stray field reached the reference value at a distance of < 1 cm to 12 cm with appropriate pans and < 1 cm to 20 cm with inappropriate pans. All measurements were carried out with the hob at the highest setting. A distance of 1 cm is unlikely to occur in normal daily use and represents a worst-case scenario. None of the measurements exceeded the ICNIRP reference value at a distance of at least 5 - 10 cm, the distance most likely to occur in practice, when the pans were used correctly (suitable cookware, centred over the cooking zone).

3. Exposure of the user to induced body currents

The magnetic field originating from induction hobs leads to electrical currents running through the body of a person standing in front of the hob. In order to avoid acute reactions such as nerve or muscle stimuli, these currents may not exceed the respective reference values set by the ICNIRP for the general public.[TMX1]   

Body currents cannot be measured directly; they have to be calculated with computer simulations using virtual model persons. On behalf of the FOPH, the IT'IS research foundation in Zurich undertook such simulations for models standing directly by the worktop in front of the three tested induction hobs and who are cooking with properly positioned pans suitable for induction hobs. In addition to the magnetic currents, the simulations also took into account gender, age, build, anatomy, tissue characteristics and posture of the following virtual persons:

  • Woman, age: 26, height: 1.60 m, weight: 58 kg, not pregnant
  • Woman, age: 26, height: 1.60 m, three/seven/nine months pregnant
  • Foetuses in the third/seventh/ninth month
  • Girl, age: 5, height: 1.08 m, weight: 18 kg
  • Boy, age: 6, height: 1.17 m, weight: 20 kg
  • Boy, age: 14, height: 1.65 m, weight: 50 kg
  • Man, age: 34, height: 1.74 m, weight: 70 kg
  • Man, age: 37, height: 1.78 m, weight: 120 kg

The body currents were simulated for the entire body as well as specifically for the central nervous system (brain and spinal cord) of persons standing directly by the edge of the worktop. In this position, users tend to be a few centimetres away from the cooking field that is built into or on top of the worktop. The simulation assumed that the models were cooking with pans that were suitable for induction hobs and centred on the cooking zone, covering it up completely. Figures 5 and 6 depict the maximum exposures vis-à-vis the reference value.


Figure 5: Body currents measured throughout the entire body of models standing directly by the worktop of induction hobs, as a ratio of reference value. 100% corresponds to the reference value for the general public. Hob 1 and hob 2 are built-in units; hob 3 is a professional mobile unit.

Figure 6: Body currents measured in the central nervous system of models standing directly by the worktop of induction hobs, as a ratio of reference value. 100% corresponds to the reference value for the general public. Hob 1 and hob 2 are built-in units; hob 3 is a professional mobile unit. CNS = Central nervous system

The results show that the body currents emanating from the two built-in units fall below or right on the reference value for most models, with the exception of the woman who is nine months pregnant and the six-year-old child, both of whom show body currents above the reference value. The body currents generated by the professional high-performance mobile unit are mostly above the reference value (figure 5). The central nervous system currents are below the reference value for all models (figure 6).

4. Effects on health

To date no specific studies of the effect of induction hobs on health have been carried out. Medium-frequency magnetic fields of the kind generated by induction hobs can penetrate the human body, where they can induce electrical fields and currents. Very strong currents can possibly excite nerves of the central nervous system. The exposure limits of ICNIRP allow only currents, which are 50 times smaller than the threshold for stimulation of the central nervous system [5]. Acute effects can be prevented by compliance with the ICNIRP recommended threshold. You can ensure compliance with the ICNIRP recommended threshold by observing the tips listed under "Health risks and prevention". According to the World Health Organization (WHO), there is no compelling evidence of medium-frequency magnetic fields having long-term effects on health. [6]. However, it notes that few studies investigating this frequency range have been published. It is not possible to draw any conclusions from the small number of animal studies that have been carried out in the medium-frequency range. The human studies, most of which have looked at the risks posed by computer monitors, have not identified any impact on health. The extent to which these results can be extrapolated to induction hobs is not clear, since these appliances are different in terms of both the radiation which they emit and the size of the magnetic fields.
Effect on implanted electronic devices 

Some studies have looked at the way induction hobs affect implanted electronic devices [7-10]. The possibility cannot be excluded that stray magnetic fields generated by induction hobs may affect implanted electronic devices at short range; this has been demonstrated for unipolar cardiac pacemakers [10]. Also the effect of leakage current on unipolar cardiac pacemakers has to be borne in mind. People with unipolar pacemakers are advised not to touch pans for extended periods and not to use metal spoons for cooking [7]. It is vital for people with implanted electronic devices to read the safety advice provided by the manufacturer and talk to their doctor before using an induction hob. The likelihood of the implanted device being affected adversely is very low if the induction hob is used correctly.

5. Regulation in law

Induction hobs are low-voltage appliances which are regulated in Switzerland by the Regulation concerning electrical low-voltage appliances[11]. This regulation requires low-voltage appliances not to endanger either persons or objects when used correctly, where possible when used in a foreseeable incorrect manner, and when foreseeable faults occur. It also states that low-voltage appliances may only be marketed if they comply with the essential health and safety requirements of the European (EC) Low Voltage Directive.
Manufacturers of low-voltage appliances must obtain a Declaration of Conformity for a product before it can be brought onto the market; this declaration states that the product complies with the essential requirements. The essential requirements for individual products are specified in technical standards; the requirements that the electromagnetic fields created by domestic appliances have to meet are specified in EN 62233 [4]. The corresponding conformity criteria correspond to the limit recommended by ICNIRP[5].  Manufacturers are responsible for ensuring that their appliances comply with the conformity criteria; there is no comprehensive oversight of the market in Switzerland. The authorities check compliance with the regulations by inspecting random samples of products on the market.

6. Literature

1. Llorente S et al. A comparative study of resonant inverter topologies used in induction cookers. Seventeenth Annual IEEE Applied Power Electronics 2, 1168-1174. 2002.
2. Gaspard JY et al. Cuisson par induction: une nouvelle génération de systèmes inducteurs. Proceedings of Congrès Eurpéen L’induction et ses applications industrielles. 1991
3. Clementine Viellard et al. B-field exposure from induction cooking appliances. ITIS-Foundation, Zurich, July 2006. IT'IS report. See "Documents"
4. SN EN 62233 "
Electromagnetic fields around household and similar electrical appliances – Methods for evaluation and measurement”
5. ICNIRP. Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields up to 300 GHz. Health Phys. 75: 494-521. 1998. See "Further information"
6. WHO: Extremely Low Frequency Fields. Environmental Health Criteria Monograph No.238, 2007. See "Further information"
7. Irnich W, Bernstein AD. Do induction cook tops interfere with cardiac pacemakers? Europace. 2006;
8: 377-84. 8. Binggeli C et al. Induction ovens and electromagnetic interference: what is the risk for patients with implantable cardioverter defibrillators? J Cardio-vasc.Electrophysiol. 2005; 16: 399-401.
9. Rickli H et al. Induction ovens and electromagnetic interference: what is the risk for patients with implanted pacemakers? Pacing Clin Electrophysiol.2003, 26:1494-7.
10. Hirose M et al. Electromagnetic interference of implantable unipolar cardiac pacemakers by an induction oven Pacing Clin.Electrophysiol. 2005;28:540-8
11. SR 734.26: Verordnung vom 9. April 1997 über elektrische Niederspannungserzeugnisse (NEV). See "Further information"

Specialist staff: emf@bag.admin.ch
Last updated on: 08.11.2011

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