WO2008150171A1

WO2008150171A1 - Heating element and electrical appliance provided with such a heating element

Info

Publication number: WO2008150171A1
Application number: PCT/NL2008/050359
Authority: WO
Inventors: Reinardus Hermanus Bernardus Deenen; Jan Hendrik Willem Oonk; Johannes Gerardus Maria Gelinck
Original assignee: Otter Controls Limited
Priority date: 2007-06-06
Filing date: 2008-06-06
Publication date: 2008-12-11
Also published as: CN101772985A; CN101772985B; WO2008150172A1; EP2163130B1; NL2000685C2; EP2163130A1

Abstract

The use of enamel as dielectric intermediate layer in the manufacture of heating elements is known. The dielectric enamel layer is herein arranged on a generally metal substrate for heating, after which metal heating tracks are arranged on the dielectric enamel layer by means of silkscreen techniques. The invention relates to an improved heating element. The invention also relates to a liquid container provided with such a heating element.

Description

Heating element and electrical appliance provided with such a heating element

The invention relates to a heating element. The invention also relates to an electrical appliance provided with such a heating element.

The use of enamel as dielectric intermediate layer in the manufacture of heating elements is known. The dielectric enamel layer is herein arranged on a generally metal substrate for heating, after which metal heating tracks are arranged on the dielectric enamel layer by means of silkscreen techniques. Such a heating element is for instance described in Netherlands patent application NL 1014601. Described herein is a heating element, for instance for heating liquid in liquid containers or for heating of heating plates, wherein heat is generated by conducting electric current through the at least one heating track. The heating track is herein arranged via a dielectric layer on a substrate for heating. The intermediate layer with dielectric properties not only provides for a good transmission of the generated heat to the substrate for heating, but also for an electric barrier between the - usually metal - substrate for heating and the heating track, whereby short-circuiting in the heating element can be prevented under normal operating conditions. The dielectric can moreover function as protection against overheating. The heating element according to NL 1014601 is provided for this purpose with an ammeter which can detect the leakage current through the dielectric. The leakage current coming from the heating element depends partly on the electrical resistance of the dielectric. Because the electrical resistance of the dielectric, at least in a determined temperature range, in turn depends on the temperature, and this dependence can in principle be predetermined, the detection of the leakage current through the dielectric provides insight into the temperature thereof. The leakage current which can be detected in simple manner with an ammeter therefore forms a measurement value with which the temperature of the dielectric, and thus of the heating element, can be determined. A protection against overheating can be easily built in by coupling the ammeter to a control for the heating element, whereby the supply of current to the heating element can be reduced or even wholly interrupted when a pre-defined minimal leakage current is detected. Although the known heating element provides a simple detection of temperature changes and protection against overheating, separate provisions must generally be made to enable proper detection of the leakage current. It is thus usually necessary on occasions to for instance amplify or, conversely, attenuate the current strength of the leakage current. It has also been found that the leakage current is generally difficult to detect if the heating element is provided with earthing. In that case a galvanically separated transformer system will have to be incorporated in the earth wire, which is time-consuming.

The international patent application WO2006083162 in the name of applicant provides an improved heating element for detecting a temperature change in the heating element with a view to protection against overheating. The improved known heating element comprises a substrate on which are successively arranged a first dielectric layer, an electrically conductive sensor layer, a second dielectric layer and a heating track. The second dielectric layer will generally have a thickness here of about 100 μm. Owing to the particular assembly of the dielectric a leakage current flowing in the second dielectric layer will preferably be diverted to the sensor layer, since in such a case the first dielectric layer acts as electrically more insulating layer (relative to the second dielectric layer). A possible detection of this leakage current by an ammeter or voltmeter coupled electrically both to the electrically conductive layer and to a controller for selectively switching off the heating element, hereby also becomes possible for very low current strengths or voltages, without separate provisions having to be made for this purpose. However, in addition to the particular advantage of the improved known heating element, the improved known heating element also has a drawback. Consumer market organisations like KEMA and UL, presently namely require that the heating elements are still protected against overheating in case the controller on the heating element fails in order to further prevent dangerous situations.

The invention has for its object, while retaining the advantage of the prior art, to provide an improved heating element with which the above stated drawback can be obviated.

The invention provides for this purpose a heating element, comprising: a substrate for heating, at least one first dielectric layer arranged on the conductive substrate, at least one electrically conductive heating track arranged on the first dielectric layer, and 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. In case of overheating of a heating element according to the invention, the thermal protective means, or at least a part thereof, will become electrically conductive and a short circuit between separate track sections of the heating track and/or between the covered track section and another part of the heating element is created. 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. The heating element may be provided with a control unit to ensure that the element does not reach excessive temperatures by preventatively switching off the heating element at a predetermined critical temperature. However, in abnormal conditions, for example if a primary protection facility fails, then the destruction of the heating track as such will result into an permanent interruption of the heating track and hence render the heating element open circuit in a safe and controlled manner, as a result of which dangerous situations for consumers can be prevented. Preferably, at least a part of said thermal protective means is electrically insulating below said predetermined temperature and electrically conductive above said predetermined temperature. The predetermined temperature at which the thermal protective means is arranged to generate a short-circuit is dependent on the specific construction and appliance of the heating element. However, commonly the thermal protective means will become electrically conductive above a temperature of substantially 400 ⁰C.

In a preferred embodiment said thermal protective means further covers at least one reference part of the heating element positioned at a distance of said covered heating track section, wherein during operation a potential difference is present between said covered heating track section on one side and the said covered reference part of the heating element on the other side. Since the thermal protective means covers and hence is connected either directly or indirectly to both the heating track section and the reference part, a short circuit between these elements will be realised at a predetermined critical temperature, wherein the thermal protective means in fact forms a bridge for facilitating the formation of an arc between said heating track section and said reference part. A higher potential difference between the heating track section and the reference part will promote the functioning of the overheating protection. In order to control the magnitude and location of the arc to be formed between the covered heating track section and the reference part, it is commonly advantageous that the mutual distance between said heating track section covered by said thermal protective means and said reference part is smaller than the mutual distance between an adjacent heating track section and said reference part. In this manner the bridge or arc to be formed can be kept locally at a predetermined position securing a relatively quick, safe, and efficient destruction of the heating track. The reference part can be of various nature. In a preferred embodiment said reference part is formed by another heating track section. However, it is also conceivable that the reference part is formed by an earthed wire.

In an alternative preferred embodiment 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. Optionally said reference is formed by the sensor track. By positioning both the at least one heating track and the at least one sensor track between the first dielectric layer and the second dielectric layer the heating track and the sensor track can be arranged on the first dielectric layer in a single pressing run, which considerably simplifies the production process for manufacturing the heating element according to the invention. In this way the distance between the heating track and the sensor track can moreover be kept relatively large (generally about 500 μm) in simple manner, whereby the chance of accelerated breakdown between the heating track and the sensor track as a result of gas bubbles possibly situated between the two tracks can be reduced significantly. An additional advantage is that in this way the heating track is substantially fully protected by the dielectric layers, which enhances the lifespan of the heating element according to the invention. Using the heating element according to the invention leakage currents can thus be measured in relatively efficient manner and at very low current strengths and/or voltages, whereby the (exceeding of a critical) temperature of the heating element according to the invention can be measured relatively quickly and accurately. Although a single first dielectric layer and a single second dielectric layer are in general usually applied in the heating element according to the invention, it is likewise possible to envisage a plurality of first dielectric layers, preferably arranged on each other, and/or a plurality of second dielectric layers, preferably arranged on each other, being applied in the heating element. The different first dielectric layers can herein be of differing composition and thickness. The same applies for the second dielectric layers in the case they are applied. Additional sensor tracks and/or additional heating tracks can optionally be arranged between the different first dielectric layers (and/or second dielectric layers) in order to enable optimizing of the safety and/or the power of the heating element. The heating track and the sensor track are preferably designed such that there is sufficient potential difference between the two tracks in operative mode to enable the forcing of a leakage current at sufficiently high temperature which flows from the heating track with a high potential to an adjacent part of the sensor track with a low potential.

In a preferred embodiment of the heating element according to the invention the electrical resistance of the first dielectric layer is higher than the electrical resistance of the second dielectric layer at substantially the same temperature. Owing to the further increased electrically insulating action of the first dielectric layer relative to the second dielectric layer, an even more sensitive leakage current measurement is found to be possible. It is advantageous here when the first electric layer is situated closer to the surface for heating than the second dielectric layer. During overheating a leakage current will occur which will flow from the heating track to the adjacent sensor track via the second dielectric layer. The leakage current will here not flow via the first dielectric layer, or at least hardly so, which could result in a dangerous situation for a user of the heating element. Owing to measurement or at least detection of the leakage current, combined if desired with a control of the heating element as already described above, in this preferred embodiment a very sensitive and rapidly responding protection against overheating is obtained. This embodiment has the additional advantage here that the protection against overheating gains in reliability and can for instance withstand improper use. The operation of the protection is thus to a large extent insensitive to whether or not the heating element, and in particular the substrate for heating, is earthed.

In the case that a single heating track and a single adjacent sensor track are applied, such a track configuration is usually also referred to as a bifilar track. At least a part of the heating track and at least a part of the sensor track are preferably given a spiral form. In this way the substrate can be heated in relatively complete and efficient manner by the heating track, wherein a possible leakage current can be measured relatively effectively and reliably. The shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track is here more preferably substantially constant, whereby a substantially parallel orientation of the heating track and the sensor track can be realized. In an alternative preferred embodiment it is also possible to envisage reducing the shortest mutual distance between the heating track and the sensor track in position-selective manner in order to be able to predefine, and thereby optimize, the location of the occurrence of a leakage current. In a particular preferred embodiment the shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track lies between 100 μm and 800 μm, preferably between 400 μm and 600 μm, and more preferably amounts to substantially 500 μm. In this way the substrate can on the one hand be heated sufficiently in that the power density per substrate area can in this manner be kept sufficiently high, and reliable detection of a leakage current can on the other hand be guaranteed.

The at least one heating track and/or the at least one sensor track are preferably coupled to a control unit. Using the control unit a leakage current can on the one hand be detected and the heating element can on the other hand be (de)activated, and more preferably regulated. For the purpose of detecting the leakage current it is also advantageous if the sensor track is coupled electrically to an ammeter and/or a voltmeter. A leakage current can be detected in relatively simple and inexpensive manner by applying the ammeter and/or the voltmeter. The ammeter and/or the voltmeter will here generally also take an earthed form in order to be able to detect a potential difference between the fixed world and the sensor track.

In a particular preferred embodiment, the second dielectric layer and the thermal protective means are mutually integrated. This implies that the second dielectric layer becomes conductive to a certain extent at a first critical temperature to allow a leakage current through said second dielectric layer. This leakage current can be detected, for example by using a sensor track as described above. In case the detected leakage current exceeds a predetermined value the heating element can be switched off electronically . In the unfavourable case that this first overheating prevention facility fails during overheating of the heating element, then the second dielectric layer will become more and more conductive in order to allow a short circuit of the heating element at a second critical temperature, at which a part of the heating track covered by the second dielectric layer will be melted and/or evaporated, resulting in a destruction of the heating track. In this particular preferred embodiment the actual functionality thus depends on the actual temperature of the second dielectric layer.

If desired, the dielectric can be assembled from a dielectric layer of a polymer and a dielectric layer of enamel. Most preferably however, both dielectric layers are manufactured from enamel. Enamel compositions particularly suitable for this application are marketed under the name Kerdi. The use of an enamel layer as dielectric in the manufacture of, among other products, electrical heating elements is per se known, for instance from NL 1014601. The dielectric herein provides for electrical insulation of the electrical resistance, which generally consists of a metallic track. The manufacture of the dielectric from enamel results here in a mechanically relatively strong dielectric which conducts heat relatively well.

The composition of the enamel for both dielectric layers can be selected within wide limits, this subject to the desired electrical properties, particularly at temperatures occurring during use. The specific electrical resistance of a common enamel composition is generally high at room temperature, usually higher than 1.5*10¹¹ Ω-cm, but can fall drastically as temperatures increase to for instance a typical value of 1.5.10⁷ Ω-cm at 180-400° Celsius. A (relatively small) leakage current through the dielectric becomes possible at such a resistance. The conductivity of an enamel composition can be readily adjusted by for instance making variations in the alkali metal content and/or by adding conducting or, conversely, electrically insulating additives.

In a particular preferred embodiment the dielectric comprises a first and/or a second dielectric layer of an enamel composition and an electrically conductive layer which is assembled from metals and/or semiconductors and/or other conductive materials such as for instance graphite and so forth. A heating element according to the invention which operates particularly well has the feature that the alkali metal content of the enamel composition of the first dielectric layer is lower than that of the second dielectric layer. The manufacture of each layer of the dielectric from an enamel composition which differs only in the alkali metal content has the additional advantage that an optimal adhesion is achieved between the layers. The difference in coefficient of expansion of the layers is moreover relatively small, so that the mechanical stresses in the material are minimized, which results in an improved durability of the dielectric, and therefore also of the heating element.

In addition to the specific resistance of a dielectric layer already described above, the breakdown voltage of such a layer, preferably an enamel layer, is also important. The breakdown voltage is the level of the electrical potential difference over the dielectric layer at which an electric current (with a much greater current strength than a leakage current) begins to flow through the layer. Breakdown can result in undesirable adverse effect on, and even irreparable disintegration of the dielectric layer and also the whole heating element. In order to ensure maximum safety in an electrical heating element, the breakdown voltage of the dielectric must be sufficiently high in accordance with regulations of certifying organizations such as KEMA and ISO, preferably at least 1250 V (alternating voltage) relative to the earth.

Preferably, at least a part of the thermal protection means are made of a glass or a ceramic, possibly mixed with a NTC material or a polymer. Since the demands on the thermal protection are a low melting point and high leakage current at elevated temperatures, it is possible that due to the chemical composition of the thermal protection, the expansion coefficients of the material of the thermal protection and the heating track and sensor layer or cover layer do not match. If the expansion coefficients do not match, cracks will appear in the applied thermal protection. This will be disadvantageous for the working of the thermal protection. In that case, it is beneficial that the material of the thermal protection is in a sintered state, possibly mixed with a polymer material to make the thermal protection watertight. However, mixing in a polymer material will increase the temperature at which the thermal protection will start to function due to the increased resistance of the thermal protection. Mixing in an NTC material in the thermal protection will lower the temperature at which the thermal protection will start to function. It is commonly also advantageous to mix in a certain amount of so-called Black Cover Coat, an organic substance provided with Mica, to achieve a solid adhesion at lower firing temperatures (400 ⁰C) of the thermal protective means to the heating track and/or the first dielectric layer. The application of Black Cover Coat is moreover favourable due to its watertight properties which commonly improves the lifespan of the heating element as such. By selecting the electrical resistance at a given temperature of the first dielectric layer significantly higher than that of the second dielectric layer, the second dielectric layer will at least partly transmit current at a given moment when the electrical resistance overheats. In such a case the first layer will transmit substantially no current, or in any case less. The heating element according to the invention is therefore resistant to high voltage, even if the element continues heating at too high a temperature due to a failure of the electronic regulation or the switching member/relay connected thereto. During this process the electrical resistance track will then burn through (like a melting fuse) as a result of the above described conductive layer or sintered glass layer, and after this process the first dielectric layer ensures that a sufficient dielectric strength always remains relative to the earth or the consumer. The heating element according to the invention is therefore intrinsically safe.

It is noted that the breakdown voltage of a dielectric is determined by a plurality of factors, including among others the layer thickness of the dielectric, the enamel composition and structural defects such as gas inclusions and the like present in the dielectric. A good adhesion of the dielectric layer, in this case the enamel composition on the surface for heating (generally of steel, aluminium and/or a ceramic material), is also important.

A particularly suitable enamel composition for application in a dielectric layer of the heating element, preferably the first dielectric layer, comprises between 0 and 10% by mass OfV₂O₅, between 0 and 10% by mass of PbO, between 5 and 13% by mass of B2O3, between 33 and 53% by mass of SiO₂, between 5 and 15% by mass OfAl₂O₃, between 0-10% by mass of ZrO₂ and between 20 and 30% by mass of CaO. If desired, the preferred composition also comprises between 0 and 10% by mass Of Bi₂O₃. Such a composition results in an enamel layer with an improved durability when used in heating elements. The enamel composition can be melted relatively easily and herein has a favourable viscosity, whereby it can be applied easily to different types of surface. The enamel composition adheres particularly well to metals, in particular to steel, more particularly to ferritic chromium steel, and still more particularly to ferritic chromium steel with numbers 444 and/or 436 according to the American AISI norm. The maximum compressive stress of the enamel layer which can be obtained from the enamel composition lies in the range between 200 - 250 MPa for the new composition. For known enamel compositions the maximum compressive stress generally lies in the range of 70 - 170 MPa. The preferred enamel composition furthermore has a high temperature resistance so that prolonged exposure to temperatures up to about 53O⁰C, with peak loads up to 700⁰C, does not cause problems. A first dielectric layer on the basis of the preferred enamel composition therefore has little risk of breakdown, in other words is less susceptible to degeneration owing to prolonged load at a high voltage than known enamel compositions. The properties of the enamel composition are furthermore such that the chance of crack formation in a dielectric layer manufactured therefrom is reduced in the case of temperature changes. The preferred enamel composition has the additional advantage that dielectric layers with the desired properties can be applied to the surface for heating in small layer thicknesses. This enhances the heat conduction.

A particular preferred embodiment comprises a dielectric in which at least the lithium and/or sodium and/or potassium content of the first and the second dielectric layers differ from each other. It is advantageous herein if the enamel composition of the first dielectric layer is substantially free of lithium and/or sodium ions. In a preferred composition according to the invention the second dielectric layer comprises at least lithium and/or sodium ions.

In a preferred embodiment the enamel composition comprises between 0.1 and 6% by weight of potassium. Owing to the addition of potassium the load-bearing capacity of the adhesion of the enamel composition to the substrate surface is less critical. In an assembly of such an enamel composition with a substrate surface there occurs less deformation at increased temperatures, in particular in the case of overheating. This is particularly advantageous when the enamel composition is fired into a heating element. The compressive stress is reduced but is still high enough to prevent the undesired formation of hair cracks. At percentages of potassium higher than 6% by weight the chance of hair crack formation has however been found to increase. In combination with the absence of other alkali metal ions, in particular lithium and sodium, a low leakage current at increased temperatures also remains ensured.

In a preferred embodiment at least one electrically conductive element is arranged on the thermal protective means. Said electrically conductive element, such as for instance a layer of silver, may be arranged in position-selective manner on a side of the thermal protective means, commonly formed by a layer, remote from the heating track. The electrically conductive element facilitates the generation of a short circuit within the heating track and/or between the heating track section and reference part of the heating element as described above. The electrically conductive element is also adapted to function as additional safety provision in the case the electronic regulation fails at sufficiently high temperature. The electrically conductive element is adapted to be able to facilitate, and thereby guarantee, the flow of a current from the heating track section to a reference part with a lower potential when a critical temperature is exceeded. The electrically conductive element, for instance formed by a silver strip, more specifically functions here as a bridge for the purpose of being able to facilitate the generation of a destructive short circuit within the heating element. In a particular preferred embodiment the shortest distance between the heating track and the electrically conductive element lies between 5 μm and 50 μm, preferably between 12 μm and 20 μm.

The substrate for heating, on which the dielectric is arranged, can be manufactured from any heat-conducting material. The surface for heating is preferably manufactured substantially from metal, for instance steel and/or aluminium. Particularly advantageous is ferritic chromium steel, preferably with a chromium content of at least 10% by weight.

It is advantageous if the coefficient of expansion of the material from which the surface for heating is manufactured does not differ too much from the coefficient of expansion of the first dielectric layer, for instance no more than 20 to 45%, for instance relative to steel, more preferably no more than 20 to 35%. The coefficient of expansion of the second layer preferably does not differ any more than 0 to 25% relative to that of the first layer. A heating element is thus obtained which has been found to be very well able to withstand temperature changes. Particularly the formation of hair cracks in both the dielectric enamel layers according to the invention has been found to be hereby much less. It has been found that the chance of hair cracks increases again at a difference in coefficient of expansion lower than 20%. It will be apparent that the coefficient of expansion of an enamel composition can be readily adapted to the coefficient of expansion of the surface for heating by for instance adjusting the alkali metal content. Adjusting the potassium content in the enamel composition is recommended here, since the leakage current is hardly influenced hereby at increased temperature. Conversely, it is also possible to choose another material for the substrate for heating.

The invention also relates to an electrical apparatus provided with at least one heating element according to the invention. The heating element according to the invention can be applied in many fields. It is thus possible to use the element in a water boiler, wherein electrical safety is provided for the user. The heating element is also particularly suitable for application in 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; irons; 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.

The invention will be elucidated on the basis of non-limitative exemplary embodiments shown in the following figures. Herein: figure 1 shows a cross-section of a heating element according to the invention, figure 2 shows a bottom view of a part of the heating element according to figure 1 , figure 3 shows the progression of the specific resistance of the first dielectric layer and second dielectric layer forming part of the heating element according to figure 1 as a function of the temperature, figure 4 shows the progression of the measured current strength as the temperature increases through dielectric layers of different enamel composition, figure 5 shows a cross-section of a basic embodiment of the heating element according to the invention, figure 6 shows a cross-section of another preferred embodiment of a heating element according to the invention, figure 7 shows a cross-section of yet another preferred embodiment of a heating element according to the invention, and figure 8 shows a water kettle incorporating a heating element according to the invention. Figure 1 shows a cross-section of a heating element 1 according to the invention. Heating element 1 comprises a heating plate 2 for heating manufactured from ferritic chromium steel with a content of 18% by weight of chromium. It is also possible to apply another suitable metal or ceramic carrier, such as for instance decarbonized steel, copper, titanium, SiN, Al₂O₃ and so forth. 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. A heating track 4 and a sensor track 5 running parallel to heating track 4 are arranged on the first, relatively electrically insulating enamel layer 3 in a substantially spiral-shaped pattern, wherein the distance between the tracks amounts to about 500 μm. Heating track 4 and sensor track 5 are preferably manufactured from the same material, more preferably from silver, copper or an alloy of these or other metals, in order to be able to simplify and speed up the production process for the manufacture of heating element 1. A second enamel layer 6 is arranged on top of and between tracks 4, 5, wherein a sintered glass layer 7 with a relatively low melting point is arranged crosswise over two or more of the heating tracks 4. The enamel composition of second enamel layer 6 is selected within the limits indicated in column LTl of Table 1. Arranged on second enamel layer 6 is a metal strip 8, in particular a silver strip, which extends over both heating track 4 and sensor track 5 to a different part of heating track 4 via the second dielectric layer. Heating track 4 and sensor track 5 are electrically connected to a control unit 9. Control unit 9 is adapted here to regulate the current strength through heating track 4. Control unit 9 is coupled to a safety circuit 10 which can for instance be provided with a bimetal. Control unit 9 is also coupled to an ammeter 11 for measuring the leakage current through sensor track 5. Figure 2 shows a bottom view of a part of heating element 1 according to figure 1, in which the second enamel layer 6 is omitted from the figure for the sake of clarity. Figure 2 clearly shows that heating track 4 and sensor track 5 are arranged in substantially spiral shape and substantially parallel to each other on first enamel layer 3. Figure 2 also shows that silver strip 8 overlaps both sensor track 5 and heating track 4, in fact a plurality of heating track sections. The operation of heating element 1 can be described as follows. After activation of heating track 4 by control unit 9 heat will be generated in heating track 4, a substantial part of which heat is transferred to heating plate 2 via enamel layers 3, 6. Heating plate 2 will here generally be in contact with a liquid, to which the heat can then be relinquished. If however the heat developed by heating element 1 can no longer be transferred in adequate manner, the temperature of heating element 1 will rise. In order to be able to prevent overheating of heating element 1 and thereby the occurrence of hazardous situations, the composition of second enamel layer 6 is chosen such that the resistance decreases significantly when a critical temperature is exceeded, such that a leakage current will begin to flow from heating track 4 to sensor track 5 via second enamel layer 6 and possibly also via silver strip 8. A leakage current flowing through sensor track 5 can be detected by ammeter 11. If this leakage current measurement were to fail and further (over)heating of heating element 1 were to occur, the sintered glass layer 7 will then melt, which decreases the resistance of said glass layer 7 significantly. As a result, a short circuit will be created between the heating track 4 and the sensor track 5. Due to the (large) current running through this short circuit, the temperature of the glass layer 7 increases rapidly, resulting in an arc between the heating track 4 and the sensor track 5 destroying the heating track 4 at that position, whereby the operative mode of heating element 1 will be terminated. Further heating of heating element 1 will then also be no longer possible.

Figure 3 shows the progression of the specific resistance of the first dielectric layer and second dielectric layer forming part of the heating element according to figure 1 as a function of the temperature. As indicated in figure 3, the LTl and HT enamel compositions, among others, ensure that the specific electrical resistance R6 of second enamel layer 6 decreases at a lower temperature than the specific electrical resistance R3 of the first relatively insulating layer 3.

The leakage current characteristic measured with the ammeter for a number of dielectric layers is shown in figure 4 as a function of the temperature T. The leakage current I plotted on the vertical axis remains limited for relatively low temperatures T up to a point close to a determined initiating temperature, above which it suddenly increases rapidly. The initiating temperature greatly depends on the composition of the enamel layer. Figure 4 shows that the composition of the first layer, indicated with HT, has an initiating temperature which amounts to at least 500⁰C. The other four shown leakage current characteristics (designated with LTl) are representative of enamel compositions of the second layer. By adjusting the composition of the enamel compositions to the desired initiating temperature for the first and/or second dielectric layer, a temperature protection for heating element 1 can be realized using a relatively simple electrical circuit. Table 1: preferred enamel compositions in heating element 1 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

Al₂O₃ 5-15

B₂O₃ 5-13

SiO₂ 33-53

ZrO₂ 0-10

PbO 0-10

V₂O₅ 0-10

Bi₂O₃ 0-10

Total 100

Figure 5 shows a cross-section of a basic embodiment of the heating element 12 according to the invention. The heating element 12 comprises a heating plate 13 for heating manufactured from metal or ceramic. A dielectric enamel layer 14 is arranged on heating plate 13. The dielectric enamel layer 14 has an enamel composition substantially as according to column HT of Table 1 provided above. A heating track 15 is arranged on the enamel layer 14 in a substantially spiral-shaped pattern. Two heating track sections 15 a, 15b are directly covered by an initially electrically insulating thermal protective layer 16. In case of overheating of the heating track 15, the initially electrically insulating thermal protective layer 16 becomes electrically conductive resulting in a short-circuiting of the different heating track sections 15 a, 15b. During this short-circuiting state the temperature of the thermal protective layer 16, and hence of the track sections 15a, 15b will further rise, wherein an arc will be formed between said track sections 15a, 15b, resulting in melting and/or evaporation and hence in an irreversible destruction of at least one of the track sections 15 a, 15b rendering the heating track to become no longer operational. A silver strip 17 is applied on a side of the thermal protective layer 16 remote from the heating track 15 in order to facilitate the generation of a short-circuit. Figure 6 shows a cross-section of another preferred embodiment of a heating element 18 according to the invention. The heating element 18 comprises a heating plate 19 for heating manufactured from metal or ceramic. A dielectric enamel layer 20 is arranged on heating plate 19. The dielectric enamel layer 20 has an enamel composition substantially as according to column HT of Table 1 provided above. A heating track 21 is arranged on the enamel layer 20 in a substantially spiral-shaped pattern. A heating track sections 21a is connected to and covered by an initially electrically insulating thermal protective layer 22 made of glass and/or ceramic. The thermal protective layer 22 is further connected to an earth wire 23. The thermal protective layer 22 is further connected to another heating track section 21b by means of a conductive link 24, in particular a silver link 24. In case of overheating of the heating track 21, the initially electrically insulating thermal protective layer 22 becomes electrically conductive resulting in a short-circuit between the heating track 21 and the earth resulting in an instantaneous increase of the electrical current through heating track 21 and the thermal protective layer 22 causing the heating track section 21a covered by the thermal protective layer 22 to be destructed by an arc formed adjacent to said track section 21a. In case this primary short-circuit would fail, then a subsidiary short-circuit can be generated between the heating track sections 21, 21b. In this embodiment the thermal protective layer 22 and the conductive link 24 together form a thermal protective means in the context of this patent application.

Figure 7 shows a cross-section of yet another preferred embodiment of a heating element 25 according to the invention. The heating element 25 comprises a heating plate 26 for heating manufactured from metal or ceramic. A first dielectric enamel layer 27 is arranged on heating plate 26. The first enamel layer 27 has an enamel composition substantially as according to column HT of Table 1. A heating track 28 and a sensor track 29 running parallel to heating track 28 are arranged on the first, relatively electrically insulating enamel layer 27 in a substantially spiral-shaped pattern, wherein the distance between the tracks amounts to about 500 μm. Heating track 28 and sensor track 29 are preferably manufactured from the same material, more preferably from silver, copper or an alloy of these or other metals, in order to be able to simplify and speed up the production process for the manufacture of heating element 25. A second enamel layer 30 is arranged between a part of tracks 28, 29 in order to enable detecting a leakage current. A sintered glass layer 31, which could also be a molten glass layer or a ceramic layer (which may be provided NTC material), with a relatively low melting point is arranged crosswise over two heating track sections and a sensor track section thereby also covering said second enamel layer 30. The enamel composition of second enamel layer 31 is selected within the limits indicated in column LTl of Table 1. Heating track 28 and sensor track 29 are electrically connected to a control unit 32. Control unit 32 is adapted here to regulate the current strength through heating track 28. Control unit 32 is coupled to a safety circuit 33 which can for instance be provided with a bimetal. Control unit 32 is also coupled to an ammeter 34 for measuring the leakage current through sensor track 29. The sintered glass layer 31 acts as a fallback safety provision, in case the leakage current detection would fail. The sintered glass layer 31 is adapted to generate a short-circuit between the heating track 28 and the sensor track 29 resulting in an instantaneous heat development between (and in the direct region of) the covered parts of the heating 28 resulting in an irreversible devastation of the heating track as such. By means of these dual safety provisions dangerous situations for consumers can be prevented in an improved manner.

Figure 8 shows a water kettle 35 incorporating a heating element 36 according to the invention. Details and embodiments about the heating element 36 applied are provided above in a comprehensive manner. It may be clear that the heating element 36 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; irons; 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.

I 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: a substrate for heating, - at least one first dielectric layer arranged on the conductive substrate, at least one electrically conductive heating track arranged on the first dielectric layer, and 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.

2. Heating element according to claim 1, 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.

3. Heating element according to claim 1 or 2, 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.

4. Heating element according to one of claims, characterized in that said thermal protective means further covers at least one reference part of the heating element positioned at a distance of said covered heating track section, wherein during operation a potential difference is present between said covered heating track section on one side, and the said covered reference part of the heating element on the other side.

5. Heating element according to claim 4, characterized in that the mutual distance between said heating track section covered by said thermal protective means and said reference part is smaller than the mutual distance between an adjacent heating track section and said reference part.

6. Heating element according to claim 4 or 5, characterized in that said reference part is formed by another heating track section.

7. Heating element according to one of claims 4-6, characterized in that said reference part is earthed.

8. Heating element according to any of the foregoing claims, 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 one of claims 4-7, and claim 8, characterized in that said reference part makes part of the sensor track.

10. Heating element according to claim 8 or 9, characterized in that at least a part of the heating track and at least a part of the sensor track are given a spiral form.

11. Heating element according to one of claims 8-10, characterized in that the shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track is substantially constant.

12. Heating element according to one of claims 8-11, characterized in that the shortest mutual distance between at least a part of the at least one heating track and at least an adjacent part of the at least one sensor track lies between 100 μm and 800 μm, preferably between 400 μm and 600 μm.

13. Heating element according to one of claims 8-12, characterized in that the at least one heating track and the at least one sensor track are coupled to a control unit.

14. Heating element according to one of claims 8-13, characterized in that an ammeter is coupled electrically to the sensor track.

15. Heating element according to one of claims 8-14, characterized in that a voltmeter is coupled electrically to the sensor track.

16. Heating element according to claim 15, characterized in that at substantially the same temperature the electrical resistance of the first dielectric layer is higher than the electrical resistance of the second dielectric layer.

17. Heating element according to one of claims 8-16, characterized that the second dielectric layer and the thermal protective means are mutually integrated.

18. Heating element according to one of claims 8-17, characterized in that the first and/or the second dielectric layer are manufactured from an enamel composition.

19. Heating element according to claim 18, characterized in that the alkali metal content of the enamel composition of the first dielectric layer is lower than that of the second dielectric layer.

20. Heating element according to claim 18 or 19, characterized in that at least the lithium and/or sodium and/or potassium content of the first and the second dielectric layers differ from each other.

21. Heating element according to any of the claims 18-20, characterized in that the first dielectric layer is substantially free of lithium and/or sodium ions.

22. Heating element according to any of the claims 18-21, characterized in that the alkali metal content of the first and the second dielectric layer differ from each other.

23. Heating element according to any of the claims 18-22, characterized in that the enamel composition of the first layer is chosen such that as temperature increases it always has a higher electrical resistance than that of the second layer.

24. Heating element according to any of the foregoing claims, characterized in that the enamel composition of the first dielectric layer is chosen such that the breakdown voltage is higher than 1250 VAC.

25. Heating element according to any of the foregoing claims, characterized in that the thermal protective means are at least partially made of glass or ceramic.

26. Heating element according to claim 25, characterized in that the thermal protective means are at least partially applied in a sintered state.

27. Heating element according to any of the foregoing claims, characterized in that at least one electrically conductive element is arranged on the thermal protective means.

28. Heating element according to claim 27, characterized in that the shortest distance between the heating track and the electrically conductive element lies between 5 μm and 50 μm, preferably between 12 μm and 20 μm.

29. Heating element according to any of the foregoing claims, characterized in that the expansion coefficient of the material of which the substrate consists differs by no more than 20 to 45% from the expansion coefficient of the first and/or the second dielectric layer.

30. Enamel composition for application as first dielectric layer in a heating element according to any of the foregoing claims, comprising between 0 and 10% by mass of

V2O5, between 0 and 10% by mass of PbO, between 5 and 13% by mass OfB₂O₃, between 33 and 53% by mass of SiO₂, between 5 and 15% by mass OfAl₂O₃ and between 20 and 30% by mass of CaO.

31. Enamel composition for application as second dielectric layer in a heating element according to any of the claims 8-23, comprising between 0 and 5% by mass of Li₂O, between 0 and 15% by mass of K₂O and between 0 and 10% by mass OfNa₂O.

32. Electrical appliance, provided with a heating element according to any of the claims 1-29.