WO2010008279A1

WO2010008279A1 - Heating element and method for operating such a heating element

Info

Publication number: WO2010008279A1
Application number: PCT/NL2009/050427
Authority: WO
Inventors: Gradus Johannes Kloppers; Reinardus Hermanus Bernardus Deenen; Martijn Van Dongen
Original assignee: Otter Controls Limited
Priority date: 2008-07-15
Filing date: 2009-07-14
Publication date: 2010-01-21
Also published as: NL2001806C2; EP2314129A1; CN102160456A

Abstract

The temperature of a heating element is commonly regulated to prevent dangerous situations overheating of said heating element. The invention relates to a heating element. The invention also relates to an electrical appliance comprising such a heating element. The invention further relates to a method for operating a heating element, in particular a heating element according to the invention.

Description

Heating element and method for operating such a heating element

The invention relates to a heating element. The invention also relates to an electrical appliance comprising such a heating element. The invention further relates to a method for operating a heating element, in particular a heating element according to the invention.

The temperature of a heating element is commonly regulated to prevent dangerous situations overheating of said heating element. To this end, the known heating element comprises a heating track and connected thereto an assembly of an analogous sensor and an electronic regulator, wherein the analogous sensor may be formed by e.g. a thermocouple or a thermistor (NTC or PTC resistance). The known heating element has several drawbacks. A first drawback of the known heating is that the heating element is constructively relatively complex, wherein the amount of wiring is substantial, in particular in case the heating element comprises multiple heating zones. A further drawback of the known heating element is that the heating element is merely adapted to measure the temperature of the heating track itself, wherein the temperatures of thermally commonly more critical components in the direct surroundings of the heating element are disregarded. This not only forms an important restriction of the known safety provision, wherein the risk of overheating of peripheral components and/or media cannot be monitored accurately, but does moreover complicate an effective and optimal regulation of the heating process to be performed by the heating element.

It is an object of the invention to provide an improved temperature regulated heating element.

This object can be achieved by providing a heating element according to the preamble, comprising: at least one electrically conductive heating track being made of a material having a temperature dependent resistance, at least one sensing element connected to said heating track for measuring the resistance of said heating track, and a control unit configured to allow the sensing element to measure the resistance of said heating track at least in a switched-off state of the heating track. By monitoring the temperature dependent resistance of the heating track at least in a switched-off state of the heating track, a relatively reliable, realistic temperature indication of the direct environment of the heating element can be obtained. In a (temporarily) switched-off state of the heating track, the heating element will commonly cool down and will adapt the temperature of its direct environment substantially determined by the temperature of a solid, liquid or gaseous volume to be heated by the heating element according to the invention. By measuring the resistance of the temporarily non-operational heating track, a realistic temperature indication of the peripheral components and/or media surrounding the heating element can be obtained. Based on this environmental temperature the heating element can be controlled such that situations of overheating of the heating element and components surrounding the heating element can be prevented. Moreover, in this manner, the performance of the heating element can be controlled in a more accurate manner as a result of which the heating process of the heating element according to the invention can be optimised. In case, for example, the heating element would be incorporated in a grill for heating a cooking plate of said grill, switching off the heating track of the heating element would result in a temperature levelling between the heating track and the (already heated) cooking plate. By measuring the actual resistance of the heating track in this levelling state (switched-off state) the actual temperature of the cooking plate can be determined based on which the control of the heating element can be adapted. Overheating of the cooking plate can thus be prevented and an optimum control of the temperature of the cooking plate and hence of the cooking process can be secured. A further advantage of the heating element according to the invention is that the heating track itself is used as thermistor for determining the temperature of the heating track as a result of which a relatively simple and efficient construction is provided. Hence, there is no need to apply additional thermistors and consequently additional wiring to determine the temperature of the heating track. Due to the simple construction of the heating element according to the invention the cost price of the heating element will commonly be less than the cost price of known temperature regulated heating elements.

Preferably, the control unit is further configured to allow the sensing element to measure the resistance of said heating track in a switched-on state of the heating track. This allows the sensing element to continuously measure the electrical resistance of the heating track, both in a switched-on and in a switched-off state of the heating track. The advantage of measuring the resistance of the heating track is to monitor the temperature of the heating track during operation. Due to this safety provision a situation of overheating over the heating element can be anticipated, and hence be prevented. It should be clear that measuring the resistance of the heating track, both in a switched-on state and in a switched-off state leads to bifunctionality of the heating element according to the invention: measuring the resistance of the heating track in the switched-off state of the heating track is favourably intended to detect the temperature of the direct environment of the heating element, while measuring the resistance of the heating track in the switched-on state of the heating track is favourably intended to detect the temperature of the heating track and hence element itself in order to prevent dangerous situations of overheating. It would be conceivable that the control unit is configured to switch on and off the sensing element during a switched-off stage of the heating track. In this manner multiple temperature samples are obtained as the element cools. Analysis of the temperature decay function of the heating element during the interruption enables an estimation of specific environmental characteristics, such as a solid or liquid volume to be heated, as well as the time required to heat the liquid or solid volume to a selected temperature.

The control unit is commonly not merely configured to allow the sensing element to measure the electrical resistance of the heating track, but is preferably also adapted to actively switch the heating track. The control unit is preferably configured to alternately switch on and off the heating track. To this end, the control unit preferably comprises at least one timer for switching the heating track. More preferably, the control unit is adapted to switch on the heating track a period of time after the heating track has been switched off, wherein said period of time is between 1 and 10 seconds. This latter period is commonly sufficient to allow the heating track to cool down to the environmental temperature and to come to a reliable and realistic determination of the environmental temperature of the heating element. On the other hand, this period is purposively kept short, preferably as short as possible, to keep the interruption or disturbance of the heating process performed by the heating element as small as possible. The optimum duration of the switched-off state is commonly dependent on the specific application, technical design and dimensioning of the heating element. In this context it is noted that the control of the heating track may be dependent on the last measured resistance. This provides a self-regulating heating element with which a heating process can be optimised, and wherein the risks of dangerous situations can be minimised. The electrical resistance of the heating track as detected by the sensing element will be converted to a temperature value. This conversion may be done remotely e.g. by a computer connected to the heating element according to the invention. However, it is commonly more preferably in case the control unit is adapted to convert the measured resistance of the heating track into a temperature value of the heating track. Assumed that the Temperature Coefficient of Resistance (TCR) of the heating track is known, based on the measured resistance the actual temperature of the heating track can be calculated by means of the following formula:

T_end = T_{begm +} ^R;^nd *£^* (formula 1)

wherein: - T_end is the actual elevated temperature of the heating track

Tbegin is the initial temperature (commonly room temperature) of the heating track,

Rend is the measured resistance of the heating track at elevated temperature,

Rbegin is the initial resistance of the heating track at the initial temperature of the heating track, and

TCR is the Temperature Coefficient of Resistance.

It is commonly favourable to calibrate the control unit before first use, and more preferably periodically, to secure an accurate conversion of the measured resistance into a temperature value. Periodically (re)calibrating the control unit is commonly favourable, since there may be a drift of resistance and hence a drift of the TCR of the heating track over life. Calibration can be performed in various manners, wherein one manner is formed by measuring the electrical resistance of the heating track at at least one predefined temperature of the heating track e.g. by placing the heating element in a temperature conditioned space, such a conditioning oven.

In a preferred embodiment the heating element comprises a heat-conducting substrate for heating. The heating-conducting substrate will commonly be in contact with a medium (a gas, liquid, and/or solid) to be heated. The heat-conducting substrate may be made of metal and ceramic. In case the substrate is electrically conducting, the heating track is preferably positioned at a distance from said substrate to prevent short- circuiting within the heating element. The space between the heating track and the substrate to be heated may be filled by an air gap. However, preferably, the heating element further comprises at least one first dielectric layer arranged on the heat- conducting substrate, wherein the at least one electrically conductive heating track is arranged on said first dielectric layer. This latter basic construction of a heating element is also known as a thick film element. Commonly, the substrate is formed by a stainless steel or ceramic plate, onto which successively the first dielectric layer and resistive paste are applied. The individual layers are commonly applied by screen printing, wherein each layer is dried and fired afterwards. A thick- film element is preferable over another conventional (sheathed) heating element, since a thick- film has a relatively low (thermal) mass, a small heat storage capacity, a quick response, and is able to spread and to emit heat equally to the medium to be heated. Different preferred embodiments of the thick-film heating element are presented below.

In a preferred embodiment the heating element further comprises: at least one electrically conductive sensor track arranged on the first dielectric layer at a distance from the heating track, and at least one second dielectric layer arranged on the first dielectric layer, which second dielectric layer connects to at least a part of the heating track and to at least a part of the sensor track. Preferably, the sensor track is enclosed between both dielectric layers. More preferably, at almost the same temperature the electrical resistance of the first dielectric layer is higher than the electrical resistance of the second dielectric layer. In case the temperature of the heating element will exceed a predetermined critical value, a leakage current will flow from the heating track via the second dielectric layer to the sensor track. By sensing this leakage current, for example by connecting the sensor track to an ammeter or voltmeter, overheating of the heating element and therefore a situation of running dry can be detected in a relatively sensitive and reliable manner. The heating track may be positioned between the first dielectric layer and the second dielectric layer, resulting in a more or less parallel layout of the heating track with respect to the sensor track. Alternatively, the at least one heating track is positioned on a side of the second dielectric layer remote from the first dielectric layer, which could be favourable in case the sensor track would for example formed by a sensor grid. Preferred embodiments are disclosed in more in detail in the international application WO 2006/083162 and in the non-published international application PCT/NL2008/050360 which are both incorporated herein by reference in its entirety and for all purposes.

In another preferred embodiment the heating element further comprises thermal protective means covering at least one heating track section, said thermal protective means being adapted to generate a short circuit of said at least one covered heating track section at a predetermined increased temperature of the heating track thereby increasing the temperature of said at least one covered heating track section such that said at least one covered heating track section will be destroyed at least partially rendering said heating track irreversibly interrupted. Due to this short circuit the thermal protective means and hence the covered heating track section will reach a high temperature being such that the covered heating track section will melt and/or evaporate, and will hence be destroyed. Preferably, the control unit is also connected to the heating element to ensure that the element does not reach excessive temperatures by preventatively switching off the heating element at a predetermined critical temperature.

The sensing element can be of various nature, but preferably comprises an ammeter to (indirectly) measure the resistance of the heating track. By means of an ammeter the electrical current I through the heating track can be detected. The electrical resistance R of the heating track can be determined by means of the formula:

R = - (formula 2)

wherein:

R = the electrical resistance of the heating track

V = the electrical voltage applied to the heating track

I = the electrical current measured by the ammeter.

Preferably the heating element comprises both a primary electrical circuit to be connected to be to electric mains to perform the heating process and a secondary electrical circuit to apply a low voltage to the heating track in case the heating track is disconnected from the electric mains. The heating element comprises at least one switch to switch between both circuits. It may be clear that the primary electric circuit and the secondary electric circuit may be mutually integrated at least partially.

In case an ammeter will be used to determine the actual temperature of the heating track, formula 1 and formula 2 will be combined leading to:

L

T ^L en,,d, - L b end = T begin + V - egin (formula 3)

I begin ^■ TCR

The invention also relates to an electrical appliance comprising at least one heating element according to the invention. Preferably, the electrical appliance is formed by an appliance chosen from the group consisting of: a grill plate, a teppanyaki, a sandwich maker, a contact grill, a deep-frying pan, a radiator, a water kettle, a (dish-)washing machine, and a hot beverage maker. Other appliances that may incorporate a heating element according to the invention include wallpaper strippers, steam irons, water purifiers, food steamers, dishwashers, floor cleaner, carpet, curtain or furniture cleaners and sterilization equipment for medical, dentistry or food sterilisation applications. The appliance may be portable, or form part of a domestic, industrial, commercial or laboratory processing unit. In a preferred embodiment the electrical appliance comprises multiple heating elements to be able to provide a larger heating surface and eventually multiple heating zones. The control unit could alternately switch the heating elements in order to monitor the direct environment of each heating element in turn. In another preferred embodiment the electrical appliance comprises a single heating element, wherein said single heating element comprises multiple heating tracks, wherein the control unit could alternately switch the heating tracks in order to monitor the direct environment of each heating track in turn. It is expected that one of the best electrical appliances incorporating one or multiple heating elements according to the invention is formed by a grill plate, where the type and number of food articles being cooked, as well as the volume of each food article and the movement of the food with respect to the grill plate will have an almost immediate effect on the temperature of the heating element. Hence, during a switched-off state of the heating track of the heating element, the temperature of the food articles can be measured relatively quickly and accurately. By analysing multiple resistance samples - and hence indirectly multiple temperature samples - taken during this interrupted switched-off state an estimation can be made of the readiness of the food articles. This information can be provided to a user and/or can be used to further control the grill plate. The invention also relates to a method for operating a heating element, in particular a heating element according to the invention, comprising the steps of: A) switching on the heating track, B) switching off the heating track a period of time after the heating track had been switched on according to step A), and C) measuring the resistance of the heating track in a switched-off state of the heating track. Preferably, the method further comprises step D) comprising determining the temperature of the heating track based upon the measured resistance of the heating track. The control unit will preferably alternately switch on and off the heating element to allow measure the resistance of the heating track during a switched-off state of the heating track. Hence, steps A) and B) are preferably repeated at least once. More preferably, all steps A)-D) are repeated at least once. The time duration between switching off the heating element according to step B) and switching on again the heating element according to step A) is dependent on the specific application of the heating element, but is commonly between 1 and 10 seconds. The method may further comprise a calibration step to calibrate the heating element prior to step D) in order to secure an accurate conversion of the measured resistance into a realistic determination of the actual temperature of the heating track. This calibration step may be performed before first use and more preferably periodically, e.g. monthly or annually. It is commonly not required to calibrate the heating element every time step D) has to be performed.

In a preferred embodiment the method comprises step E) comprising comparing the resistance measured during step C) with a predefined critical resistance. In case the actual resistance measured exceeds (or comes close(r)) to the critical resistance the control of the heating element may be adapted. This adaptation of the control of the heating element may imply e.g. extension of the switched-off state, shortening the switched-on state of the heating element, and/or actively lowering the heating capacity of the heating element. By (re)actively adapting the control of heating element overheating of the heating element and of peripheral equipment surrounding the heating element can be prevented. In this context it is noted that the critical resistance can be either an upper threshold value or a lower threshold value, dependent on the nature of the heating track. In case the heating track is substantially made of a PTC material, the critical resistance will form an upper threshold value; in case the heating track is substantially made of an NTC material, the critical resistance will form a lower threshold value. Preferably, the method comprises step F) comprising measuring the resistance of the heating track in a switched-on state of the heating track. Due to this safety provision a situation of overheating over the heating element can be anticipated, and hence be prevented. In case an approaching situation of overheating is detected, the timer of the control unit can be regulated e.g. to elongate the switched-off state of the heating element to allow the heating element to cool down more intensively to prevent overheating of the heating element. It would also be conceivable that the timer of the control unit is overridden in this case, wherein the heating element may be switched off in a forced manner.

The invention will be elucidated on the basis of non- limitative exemplary embodiments shown in the following figures. Herein:

Figure 1 shows a schematic cross section of a heating element according to the invention,

Figure 2 shows a schematic view of a method for operating the heating element according to figure 1 , and

Figure 3 shows a water kettle incorporating a heating element according to the invention.

Figure 1 shows a schematic cross section of a heating element 1 according to the invention. In this embodiment the heating element 1 makes part of a water kettle (not shown). The heating element 1 comprises a heating plate 2 for heating, manufactured from ferritic chromium steel with a chrome content of 18% by weight. It is also possible to apply another suitable metal or ceramic carrier, such as for instance decarbonized steel, copper, aluminium, titanium, SiN, AI2O3 and so on. The substrate 2 is arranged to have direct contact with water to be heated. A first dielectric enamel layer 3 is arranged on heating plate 2. The first enamel layer 3 has an enamel composition substantially as according to column HT of Table 1 below. An electrically conductive sensor layer in the form of a grid 4 is arranged on the first dielectric layer 3. Grid 4 is manufactured from for instance a thick film layer on the basis of ruthenium oxide (RuO₂) or other suitable conductive (thick film) layers with a suitable conductive material, such as for instance silver, palladium, nickel and so on, and/or combinations thereof. A second dielectric enamel layer 5 is arranged on the relatively conductive layer 4. The second enamel layer 5 is provided with a quantity of NTC material, in particular a quantity of the spinel structure of an oxide of nickel, manganese, cobalt and/or iron, in order to enable improvement of the conductivity of the second enamel layer 5, particularly at relatively low temperatures (< 300⁰C), whereby a leakage current can flow more easily through second enamel layer 5. The enamel composition of the second enamel layer 5 is chosen within the limits indicated in column LTl of Table 1 below, in which the content of the NTC material mixed with the enamel is not taken into account. On the second dielectric layer 5, which has a better electrical conduction than first dielectric layer 3, a heating track 6 is arranged which is used to generate heat to heat the substrate 2. The heating track 6 is made of a material having a temperature dependent resistance. The heating element 1 according to the invention has multiple thermal safety provision. In order to monitor the temperature of heating element 1 during use, the sensor layer 4, which has better conduction than both first layer 3 and second layer 5, provides the option of determining the leakage current through the second, relatively conductive layer 5. For direct measurement of the leakage current through first layer 3, a first ammeter 7 is connected between electrical resistance layer 6 and conductive layer 4. The magnitude of the measured leakage current is indicative of the magnitude of the highest temperature at a position on heating element 1. When a determined temperature is exceeded, the leakage current will increase sharply due to the reduced resistance of the second dielectric layer 5, so that this can be readily detected by the first ammeter 7. Because practically no leakage current flows through the first dielectric layer 3, it has been found that the measurement of the leakage current by ammeter 7 becomes much more accurate. Beside the leakage current detection provision, the heating element 1 additionally incorporates a second safety provision which will be elucidated hereinafter. The heating element 1 further comprises a second ammeter 8 (shown simplified) for measuring the magnitude of the electrical current running through the heating track 6. Based on the measured value, the resistance of the heating track 6 and hence the temperature of the heating track 6 can be determined. The second ammeter 8 is incorporated in both a powering circuit and a measuring circuit. The powering circuit is configured to power the heating element 1 to perform the heating process, wherein the heating track 6 is connect to the electric mains 9. The actual power to be applied to the heating track 6 is regulated by means of a regulating switch 10 to be controlled by a control unit 11. The regulating switch 10 also acts as main switch in this embodiment. In this exemplifying embodiment the measuring circuit is provided with a separate low- voltage source 12 to apply a low voltage to the heating track 6 in case the heating track 6 is disconnected from the electric mains. A circuit switch 13 is provided to switch between the powering circuit and the measuring circuit, wherein the circuit switch is controlled by the control unit 11. The voltage over the heating track 6 is measured by means of a voltmeter 14. In case the heating track 6 is switched off, which means in this embodiment that the heating track 6 is disconnected from the electric mains 9 and is connected to the low-voltage source 12, the temperature of the heating element 1 will lower and will level with the components and/or media, and in particular the water to be heated, directly surrounding the heating element 1. In this switched-off state the temperature of the heating track 6, and hence of the water to be heated, can be determined by means of the second ammeter 8, the voltmeter 14, and the control unit 11. Besides, since the second ammeter 8 is also incorporated in the powering circuit the temperature of the heating track 6 will or may also be measured in the switched-on stage, although this temperature will be merely independent of the temperature of surrounding components and/or media. In order to monitor the temperature of the direct surroundings of the heating element 1, among which the water to be heated, which gives more relevant information about potential situations of overheating and/or the progress of the boiling process, the control unit 11 is adapted to alternately switch on and off the heating track 6 by means of the circuit switch 13 thereby allowing the second ammeter 8 to (indirectly) measure the temperature of the heating track 6 in a switched-off state. This switched-off state preferably lasts between 1 and 10 seconds. To this end, the control unit 11 comprises a timer 15 to be able to periodically switch on and off the heating track 6. Based on the temperature determined by the control unit 9 the regulating switch 10 may be controlled by the control unit 11 to modify the power to be delivered to the heating track 6. In this manner situations of overheating of both the heating element 1 and the peripheral components and/or media can be prevented, and an optimal boiling process can be realised.

Table 1 : preferred enamel compositions in the heating element according to the invention Enamel composition LTl HT

Constituent % by weight % by weight

Li₂O 0-5

K₂O 0-15 0-10 Na₂O 0-10

CaO 20-40 20-40

Al₂O₃ 5-15 5-15

B₂O₃ 5-13 5-13

SiO₂ 33-53 33-53 ZrO₂ 0-10 0-10

PbO 0-10 0-10

V₂O₅ 0-10 0-10

Bi₂O₃ 0-10 0-10

Total 100 100

Figure 2 shows a schematic view of a method for operating the heating element 1 according to figure 1. The control unit 11 alternately switches on and off the heating track 6 of the heating element 1. In a switched-off state of the heating element 1 (indicated by the dotted box) the electrical resistance of the heating track 6 is determined by the second ammeter 8. The measured value of the resistance is compared by a comparator 16 with a predefined critical resistance. Both the measured value of the resistance and the comparison related information are provided to the control unit 11 upon which the control of the heating track 6 may be adapted to prevent overheating situations and/or to be able to optimise the boiling process.

Figure 3 shows a water kettle 17 partially filled with water 18, said water kettle 17 incorporating a heating element 1 according to the invention. Details and embodiments about the heating element 1 applied are provided above in a comprehensive manner. It may be clear that the heating element 1 according to the invention may also be applied in numerous other devices, such as but not limited to flow through heaters; steam generators; (dish-)washing machines; humidifiers; milk and other liquid heaters; pipe heating devices for liquids; irons; cooking devices; such as cooker plates and grill plates; and appliances used in such household fields as cooking, heating and cleaning, plus process and machinery in commercial, industrial, catering, domestic, and office environments. The geometry of the heating element is non- limitative and can be of various nature, such as for example flat, domed or contoured.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. Heating element, comprising: at least one electrically conductive heating track being made of a material having a temperature dependent resistance, at least one sensing element connected to said heating track for measuring the resistance of said heating track, and a control unit configured to allow the sensing element to measure the resistance of said heating track at least in a switched-off state of the heating track.

2. Heating element according to claim 1, characterized in that the control unit is configured to allow the sensing element to measure the resistance of said heating track in a switched-on state of the heating track.

3. Heating element according to claim 1 or 2, characterized in that the control unit comprises at least one timer for switching the heating track.

4. Heating element according to claim 3, characterized in that the control unit is adapted to switch on the heating track a period of time after the heating track has been switched off, wherein said period of time is between 1 and 10 seconds.

5. Heating element according to one of the foregoing claims, characterized in that the control unit is adapted to convert the measured resistance of the heating track into a temperature value of the heating track.

6. Heating element according to one of the foregoing claims, characterized in that the heating element comprises a heat-conducting substrate for heating.

7. Heating element according to one of the foregoing claims, characterized in that the heating element further comprises at least one first dielectric layer arranged on the conductive substrate, wherein the at least one electrically conductive heating track is arranged on the first dielectric layer.

8. Heating element according to claim 7, characterized in that the heating element comprises at least one electrically conductive sensor track arranged on the first dielectric layer at a distance from the heating track, and at least one second dielectric layer arranged on the first dielectric layer, which second dielectric layer connects to at least a part of the heating track and to at least a part of the sensor track.

9. Heating element according to claim 7 or 8, characterized in that the heating element comprises thermal protective means covering at least one heating track section, said thermal protective means being adapted to generate a short circuit of said at least one covered heating track section at a predetermined increased temperature of the heating track thereby increasing the temperature of said at least one covered heating track section such that said at least one covered heating track section will be destroyed at least partially rendering said heating track irreversibly interrupted.

10. Heating element according to claim 9, characterized in that the thermal protective means is adapted to increase the temperature of the at least one covered heating track section such that said heating track section will be destroyed by melting and/or evaporating of said heating track section.

11. Heating element according to claim 9 or 10, characterized in that at least a part of said thermal protective means is electrically insulating below said predetermined temperature and electrically conductive above said predetermined temperature.

12. Heating element according to one of the foregoing claims, characterized in that the sensing element comprises an ammeter.

13. Heating element according to one of the foregoing claims, characterized in that the heating element comprises comparison means for comparing the measured resistance of the heating track with at least one predefined critical resistance value.

14. Electrical appliance comprising at least one heating element according to claims 1-13.

15. Electrical appliance according to claim 14, characterized in that the electrical appliance is formed by an appliance chosen from the group consisting of: a grill plate, a teppanyaki, a sandwich maker, a contact grill, a deep-frying pan, a radiator, a water kettle, a (dish-)washing machine, and a hot beverage maker.

16. Method for operating a heating element, in particular a heating element according to one of claims 1-13, comprising the steps of:

A) switching on the heating track,

B) switching off the heating track a period of time after the heating track had been switched on according to step A), and

C) measuring the resistance of the heating track in a switched-off state of the heating track.

17. Method according to claim 16, characterized in that the method further comprises step D) comprising determining the temperature of the heating track based upon the measured resistance of the heating track.

18. Method according to claim 16 or 17, characterized in that steps A) and B) are repeated at least once.

19. Method according to claim 18, characterized in that the heating element will be switched on again according to step A) after a period of time after switching off the heating element according to step B).

20. Method according to one of claims 16-19, characterized in that the method comprises step E) comprising comparing the resistance measured during step C) with at least one predefined critical resistance value.

21. Method according to one of claims 16-20, characterized in that the method comprises step F) comprising measuring the resistance of the heating track in a switched-on state of the heating track.