Flat heating element

Abstract

The present invention relates to a heating element for heating at least one surface contacted at times by a user, with a plurality of heating conductors or heating conductor sections that are directly or indirectly connected electrically to one another, at least in part by mutual contact, with at least one heating conductor or heating conductor section which, upon exceeding a permissible maximum temperature, at least temporarily loses its electrical conductivity at least in part. It is provided that the electrical resistance of at least two heating conductors or heating conductor sections is dependent at least in part on its mechanical strain, that these heating conductors or heating conductor sections are electrically connected to one another in parallel, and that they are separated from one another at least in sections by a spacing zone in order to prevent electrical connection between them in spacing zone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Details of the invention will be explained below. These discussions are intended to make the invention comprehensible. They have only the character of examples, however. It goes without saying that individual or multiple described characteristics can also be omitted, modified or added. The characteristics of different embodiments can also be combined with one another. Reference will be made below to:

FIG. 1, a plan view of a flat heating element;

FIG. 2, an enlarged cross section of an electrical individual strand;

FIG. 3, an enlarged perspective view of a complete bundle of heating conductors; and

FIG. 4, a side view of a vehicle with heated seat, in a partial vertical section.

DETAILED DESCRIPTION OF THE INVENTION

Figure shows a heating element 1. It comprises a carrier layer 3. Carrier layer 3 is preferably produced from a textile material such as a knit or a felt material. In the present case it has rectangular dimensions. Two electrodes 5, 5′ are arranged along the long edges of carrier layer 3. The electrodes are formed from, for instance, conductive threads, copper stranded wires, electrically conductive adhesives, electrically conductive foils, combinations thereof or the like.

Electrodes 5, 5′ are arranged at least roughly parallel to one another, and are each connected at one of their ends via power supply line 7, 7′ to a current or voltage source 70 (shown in FIG. 4).

Bundles 9, 9′, 9″ of heat conductors 11, 11′, 11″ run at an angle (α) to the electrodes. In the present case, these bundles 9, 9′, 9″ are arranged perpendicular to the electrodes. They are arranged on carrier layer 3. They are preferably fixed there with adhesive. They can also be laid on, sewn on or knitted on.

At least one heating conductor 11, 11′, 11″ comprises at least one filament in the form of an individual strand 16 of a plastic according to FIG. 3, which is metallized, i.e., coated with a metal cladding layer 14, in this case with a silver coating. Inner strand 15 is made of an elastic, tear-resistant and temperature-resistant synthetic material, particularly a thermoplastic, in particular, polyamide, which is very rupture-resistant, tear-resistant and temperature-resistant. It is coated with a cladding layer 14 of nickel, gold, silver or gold-silver alloy that can be applied, in particular, by electroplating. Cladding layer 14 is very ductile and thus very resistant to reversed bending over a long service life. Inner strand 15 is very tear-resistant and resistant to reversed bending.

The diameter of inner strand 15 can lie between about 0.01 mm and about 1 mm, while a reasonable diameter for the entire strand 16 can be from 0.02 mm to 3 μm. It can further be provided that inner strand 15 and cladding layer 14 have cross-sectional areas in a ratio between 1:4 and 10:1, preferably that inner strand 15 and cladding layer 14 have roughly equal cross-sectional areas.

Individual strand 16 has an electrical resistance that is dependent on its past. If individual strand 16 is subjected to a mechanical strain, elongation for example, then the electrical resistance rises with increasing elongation. If the individual strand is then relaxed again, the resistance declines, but is higher than in the unstressed state prior to elongation. The resistance value measured after elongation in the relaxed state is higher the more intensively the individual strand was stretched.

The resistance thus records the past mechanical strain and its magnitude.

If individual strand 16 is heated to a temperature of roughly 120° C. for a thermoplastic, then the resistance value returns to the initial value before elongation. A heat treatment thus erases the memory.

If the maximum elongation does not exceed 10%, then this process can be repeated arbitrarily often.

For an elongated individual strand 16, the resistance is strongly, exponentially dependent on the elongation. If several individual strands 16 are bundled in the form of threads into a strand bundle 17 as in FIG. 3, this yields a considerably weaker, but still exponential, dependency of resistance on elongation. This can be explained by the fact that the individual strand has microscopic or macroscopic interruptions in the silver layer, which are then bridged to a large extent by the adjacent individual strands in the thread by contact among one another. The memory property is preserved, however, even for strand bundle 17 bundled together from several individual threads 16.

In a strand bundle 17, 30 to 50 individual strands 16, for example, can be twisted together into a component thread 18, several of which can in turn be twisted into an electrical complete bundle 19. Thus a heating conductor 11, 11′, 11″ that can be sewn without problems can be formed with a plurality of individual strands 16. If heating conductor 11, 11′, 11″ is penetrated by a needle then only a few individual strands 16 are damaged, without significantly influencing the overall functioning or the electrical or mechanical properties of the complete bundle of conductor strands.

If a network structure is produced from a strand bundle and operated between two electrodes, then two opposed, self-reinforcing mechanisms act on the construction. Strand bundles with a memory property in the network become high-resistance due to mechanical strain. Less high-resistance strand bundles allow more current to flow and therefore become hotter, whereby their memory is better erased and they are returned to the low-resistance initial state. The higher-resistance strand bundles do not become so hot, due to the lower current flow, whereby they more or less retain their memory, and can become even more resistant due to further mechanical strain. Thus low-resistance current paths are formed over time in the network. If the network is very large, then very high current loads can develop in individual low-resistance strand bundles, thus causing this heating strand bundle to melt. This is remedied here by subdividing the entire network into sufficiently small independent bundles 9, 9′, 9″ of heating conductors 11, 11′, 11″ and thus limiting it. Hence the maximum possible current for a low-resistance current path is limited to a noncritical value.

Bundles 9, 9′, 9″ of heating conductors 11, 11′, 11″ preferably do not touch one another. A sufficient number of bundles 9, 9′, 9″ are preferably provided to uniformly heat a base surface 4 to be heated that is provided between electrodes 5, 5′.

Heat conductors 11, 11′, 11″ are preferably arranged in meandering form on or inside a ribbon-like strip 20. Strip 20 can be formed by a fabric underlayer, an adhesive strip or the like. It can, however, also be a purely conceptual delimitation of a space in which heating conductors 11, 11′, 11″ of a heating bundle 9, 9′, 9″ are arranged.

Heating conductors 11, 11′, 11″ can expediently also be laid down in a zigzag shape, in a straight line or in some other manner. At least one of heating conductors 1, 11′, 11″ is expediently arranged such that it crosses a plurality of other heating conductors 11, 11′, 11″ and thus creates connection points 12 to other heating conductors 11′, 11″.

In the present case, heating conductors 11, 11′, 11″ of a bundle 9, 9′, 9″ are laid down in a wave or sinusoidal shape with equally large “amplitude” and an identical center line. They are also offset from one another by the fraction (divided by the number in the bundle (3 in this case)) of a “wave” along the centerline of the bundle.

The individual bundles 9, 9′, 9″ of heating conductor 11, 11′, 11″ in the present example run in a straight line from one electrode 5 to the other electrode 5′. However, they can also be laid in a meandering shape, curved shape, or in some other way.

For the fixation of the entire arrangement it can be expedient, as in the example of an embodiment, to arrange a cover layer 2 on carrier layer 3 such that electrodes 5, 5′ and heating conductors 11, 11′, 11″ are embedded between cover and carrier layer.

It can be expedient for the heating element to be built into a vehicle seat, a steering wheel, an arm rest, a heating blanket or the like. FIG. 4 shows a heating element that is built into a seat 150 of a vehicle 1000. The heating element can be situated in a seat insert or, as here, between the covering surface and the seat cushion. It can also be expedient for the heating element to be built into a larger subsystem to supply the seat user with heating, cooling, ventilation, etc.

It is expedient if:

heating conductor 11, 11′, 11″ and/or heating conductor section 13 extends at least in part between two electrodes 5, 5′,

heating conductor 11, 11′, 11″ and/or heating conductor section 13 is elongated—in particular, strand-like—at least in part,

in operation, current flows through heating conductor 11, 11′, 11″ and/or heating conductor section 13 substantially in the direction of its longest extension direction.

It is expedient if at least two heating conductors 11, 11′, 11″ and/or heating conductor section 13 and/or bundles 9, 9′ of heating conductors are separated from one another over substantially the entire distance between two electrodes 5, 5′.

When heating element 1 is connected to power, then current flows from the supply device via supply line 7 into electrode 5. Since electrode 5 is markedly more conductive electrically than heating conductors 11, 11′, 11″, the heating current distributes itself uniformly onto bundles 9, 9′, 9″ of heating conductors 11, 11′, 11″ that are connected to the electrode. The current then flows from electrode 5 through heating conductors 11, 11′, 11″ of base surface 4 to be heated to electrode 5′ and from there via supply line 7′ back to the current/voltage source. The current amperage of the heating current here is, for instance, between 4 and 5 A at an operating voltage of 12 V.

It can be expedient to sew heating element 1 into a cover or the like.

From the above, it can be seen that the invention can also be described with reference to the following paragraphs.

A. Heating element (1) for heating at least one surface contacted at times by a user, with a plurality of heating conductors (11, 11′, 11″) or heating conductor sections (13) that are directly or indirectly connected electrically to one another, at least in part by mutual contact, with at least one heating conductor (11, 11′, 11″) or heating conductor section (13) which, upon exceeding a permissible maximum temperature, at least temporarily loses its electrical conductivity at least in part, characterized in that the electrical resistance of at least two heating conductors (11, 11′, 11″) or heating conductor sections (13) is dependent at least in part on its mechanical strain, in that these heating conductors (11, 11′, 11″) or heating conductor sections (13) are electrically connected to one another in parallel, and in that they are separated from one another at least in sections by a spacing zone (22) in order to prevent an electrical connection between them in spacing zone (22).

B. Heating element (1) according to Paragraph A, characterized in that heating element (1) is furnished with at least two electrodes (5, 5′) for supplying heating element (1) with power and with a plurality of heating conductors (11, 11′, 11″) and/or heating conductor sections (13), which are arranged, passing over at least a part of base surface (4) to be heated, between electrodes (5, 5′) and electrically connected thereto.

C. Heating element (1) according to any of the preceding paragraphs A or B, characterized in that heating element (1) comprises at least two bundles (9, 9′, 9″) of heating conductors (11, 11′, 11″) and/or heating conductor sections (13), and in that the two bundles (9, 9′, 9″) and/or heating conductor sections (13) are arranged spaced apart from one another in order to prevent electrical contact between the two bundles (9, 9′, 9″).

D. Heating element (1) according to Paragraph A, characterized in that a bundle (9, 9c, 9²) comprises at least two, preferably at least three heating conductors (11, 11′, 11″), which are preferably fixed on base surface (4).

E. Heating element (1) according to any of the preceding paragraphs A through D, characterized in that at least one heating conductor (9, 9′, 9″) is formed at least in a certain proportion from copper, carbon particles, carbon fibers, carbonized plastic filaments, silver, gold, polyamide, and/or combinations thereof, and/or that a heating conductor (11, 11′, 11″) comprises one or more monofilament heating conductor strands, one or more multifilament heating conductor strands, and/or one or more mechanical reinforcement devices, wherein these constituents are preferably arranged parallel, concentrically, or in a zigzag, meandering and/or spiral shape relative to the running direction of heating conductor (11, 11′, 11″)

F. Heating element (1) according to any of the preceding paragraphs A through E, characterized in that at least one bundle (9, 9′, 9″) of heating conductors (11, 11′, 11″) comprises a ribbon-like strip (20), inside of which heating conductors (11, 11′, 11″) are arranged, preferably one alongside the other.

G. Heating element (1) according to any of the preceding paragraphs A through F, characterized in that at least two, preferably all, heating conductors (11, 11′, 11″) of a bundle (9, 9′, 9″) are arranged at least in a considerably large part of their extent spaced apart from one another—preferably also spaced apart from all other heating conductors possibly present—in bundle (9, 9′, 9″), in that at least one heating conductor (11, 11′, 11″)—preferably all of them—comprises a plurality of bends and/or inflection points in order to form a plurality of connection points 12 to at least one—preferably all—other heating conductors (11, 11′, 11″), at which the heating conductors (11, 11′, 11″) involved are electrically connected to one another, and which are preferably arranged distributed over substantially the entire length of bundle (9, 9′, 9″) and/or heating conductor (11, 11′, 11″).

H. Heating element (1) according to any of the preceding paragraphs A through G, characterized in that at least two heating conductors (11, 11′, 11″) are different from one another in their structure or material composition, and/or at least two are of the same type.

I. Heating element (1) according to any of the preceding paragraphs A through H, characterized in that at least one heating conductor (11, 11′, 11″) comprises at least one conductor strand (10), in particular, an individual strand (16), which comprises at least one inner strand (15) and at least one, preferably electrically conductive, cladding layer (14) sheathing this inner strand (15), and/or which comprises nickel-plated carbon fibers or consists substantially of a nickel alloy or pure nickel, in particular, as a multifilament strand.

J. Heating element (1) according to any of the preceding paragraphs A through I, characterized in that at least one inner strand (15) comprises a material that is heat-resistant at least to 75° C., preferably to 150° C., preferably to 300° C., preferably to 500° C., preferably to 1000° C.

K. Heating element (1) according to any of the preceding paragraphs A through J, characterized in that inner strand (15) is metallizable, in that cladding layer (14) is applied electrolytically to inner strand (15), and/or that cladding layer (14) is integrally connected to inner strand (15).

L. Heating element (1) according to any of the preceding paragraphs A through K, characterized in that inner strand (15) has a diameter of less than 100 μm, preferably less than 10 μm, preferably less than 1 μm, preferably less than 0.1 μm, preferably less than 0.01 μm, and/or in that the thickness of cladding layer (14) is between 0.2 and 2 μm, preferably between 0.5 and 1.5 μm, preferably between 0.8 and 1.2 μm, and/or in that individual strand (16) and/or heating conductor (11, 11′, 11″) has a thickness of less than 1 mm, preferably less than 0.1 mm, preferably less than 10 mm.

M. Heating element (1) according to any of the preceding paragraphs A through L, characterized in that inner strand (15) is produced at least in part from a synthetic material that is preferably elastic and tear-resistant, preferably at least in part, better yet, substantially entirely, from a thermoplastic, polyamide, carbon fiber, polypropylene, polyester or polyimide, and/or at least in part from glass silk and/or steel, and/or in that the material of inner strand (15) has a higher resistance to reversed bending and/or a markedly higher material price and/or a lower tensile or compressive strength than the material of cladding layer (14).

N. Heating element (1) according to any of the preceding paragraphs A through M, characterized in that cladding layer (14) has a surface that is chemically inactive under ordinary environmental conditions at least on its surface facing outward (relative to the inner strand).

O. Heating element (1) according to any of the preceding paragraphs A through N, characterized in that cladding layer (14) contains metal, preferably produced at least in part from an alloy of nickel with phosphorus components, of silver, copper and/or of gold, and preferably of an alloy formed substantially entirely of silver, copper, gold and/or nickel, in that cladding layer (14) comprises a metal the surface of which can be passivated, and/or in that the surface of cladding layer (14) is oxidized and/or chromated.

P. Heating element (1) according to any of the preceding paragraphs A through O, characterized in that the surface of cladding layer (14) is coated, in particular, with a plastic and/or a lacquer and/or at least in part with polyurethane, PVC, PTFE, PFA and/or polyester.

Q. Heating element (1) according to any of the preceding paragraphs A through P, characterized in that at least one conductor strand (1, 2, 3a, 10) comprises a plurality of individual strands (16), preferably more than five, preferably more than 50, preferably more than 100, and preferably more than 300.

R. Heating element (1) according to any of the preceding paragraphs A through Q, characterized in that a plurality of individual strands (16) one or more conductor strands (1, 2, 3a, 10) is provided, which are electrically insulated from one another at least in sections, preferably in that at least one individual strand (16) is at least partially insulated by means of an insulation layer (18) on its cladding layer (14), and/or in that conductor strand (10) and/or an individual conductor (16) is sheathed at least in sections with a plastic (72), in particular, with a heat-activatable plastic.

S. Heating element (1) according to any of the preceding paragraphs A through R, characterized in that several individual strands (16) collected into a strand bundle (17) are provided and in that preferably several strand bundles (17) and/or bundles of strand bundles (17) are collected into an overall bundle (19), wherein the conductor strand (10) and/or at least one individual strand (16) preferably has a spiral-shaped arrangement, preferably by being twisted, stranded or plaited.

T. Heating element (1) according to any of the preceding paragraphs A through S, characterized in that conductor strand (2, 3a, 10) and/or an individual strand (16) has a electrical resistance between 0 and 3 W/m, preferably between 0 and 2 W/m, preferably between 0.1 and 0.3 W/m, and/or in that at least one conductor strand (1) for heating element (20) has an electrical resistance between 0.1 and 3 W/m, preferably between 0.2 and 0.5 W/m.

U. Heating element (1) according to any of the preceding paragraphs A through T, characterized in that conductor strand (1, 2, 3a, 10) is worked into a knit and/or a braid, in that it is laid on a textile and fixed on the textile by means of a sewing or knitting thread, in that it is woven into a textile as a sewing thread, and/or that it is adhered to at least one textile and/or adhered between two textile layers.

V. Seat (150), in particular, a vehicle seat, characterized in that it is equipped with at least one heating element (1) according to one of the preceding paragraphs A through U.

W. Vehicle (1000), in particular, a motor vehicle, characterized in that it is equipped with at least one heating element (1) and/or seat (150) according to one of the preceding paragraphs A through V.

LIST OF REFERENCE SYMBOLS

• b Width of the bundle
• 1 Heating element
• 2 Cover layer
• 3 Carrier layer
• 4 Base surface to be heated
• 5, 5′ Electrode
• 7, 7′ Connecting lines
• 9, 9′ Bundles of heating conductors
• 10 Conductor strand
• 11, 11′, 11″ Heating conductors
• 12 Connecting points of the heating conductors
• 13 Heat conductor section
• 14 Cladding layer
• 15 Inner strand
• 16 Individual strand
• 17 Strand bundle
• 18 Component thread
• 19 Overall bundle
• 20 Ribbon-like strip
• 22 Spacing zone
• 70 Power source
• 15 Seat
• 1000 Vehicle

Claims

1. A heating element for heating at least one surface contacted at times by a user comprising: a plurality of heating conductors or heating conductor sections that are at least in part directly or indirectly connected electrically to one another, at least in part by mutual contact, with at least one heating conductor or heating conductor section which, upon exceeding a permissible maximum temperature, at least temporarily loses its electrical conductivity at least in part, characterized in that the electrical resistance of at least two heating conductors or heating conductor sections is dependent at least in part on its mechanical strain, in that these heating conductors or heating conductor sections are electrically connected to one another in parallel, and in that they are separated from one another at least in sections by a spacing zone in order to prevent an electrical connection between them in the spacing zone.

2. The heating element according to claim 1 wherein the heating element comprises at least two electrodes for supplying heating element with power and with a plurality of heating conductors and/or heating conductor sections, which are disposed over at least a part of a base surface to be heated and further disposed between at least one electrode and electrically connected thereto.

3. The heating element according to claim 1 wherein the heating element comprises at least two bundles of heating conductors and/or heating conductor sections, and further wherein the two bundles and/or heating conductor sections are disposed apart from one another in order to prevent electrical contact between the two bundles.

4. The heating element according to claim 3 wherein the bundle comprises at least two heating conductors, which are fixed on the base surface.

5. The heating element according to claim 3 wherein in that at least one bundle is formed at least of a certain proportion or combinations thereof selected from the group consisting of copper, carbon particles, carbon fibers, carbonized plastic filaments, silver, gold, or polyamide, and that the heating conductor further comprises one or more monofilament heating conductor strands, one or more multifilament heating conductor strands, and/or one or more mechanical reinforcement devices, further wherein these strands are disposed parallel, concentrically, or in a zigzag, meandering and/or spiral shape relative to the running direction of heating conductor.

6. The heating element according to claim 3 wherein in that at least one bundle of heating conductors comprises a ribbon-like strip, inside of which heating conductors are optionally arranged one alongside the other.

7. The heating element according to claim 3 wherein in at least two heating conductors within the bundle are significantly disposed apart from one another in that at least one heating conductor comprises a plurality of bends and/or inflection points in order to form a plurality of connection points together with at least the one other heating conductor, at which the heating conductors involved are electrically connected to one another, and which are optionally distributed over substantially the entire length of the bundle and/or heating conductor.

8. The heating element according to claim 1 wherein at least two heating conductors are different from one another in their structure or material composition, and/or at least two are of the same type.

9. The heating element according to claim 1 wherein at least one heating conductor further comprises at least one conductor strand, in particular, an individual strand, which comprises at least one inner strand and at least one cladding layer that is electrically conductive sheathing this inner strand, and/or which comprises nickel-plated carbon fibers or consists substantially of a nickel alloy or pure nickel, in particular, as a multifilament strand.

10. The heating element according to claim 9 wherein at least one inner strand comprises a material that is heat-resistant at least to 1000° C.

11. The heating element according to claim 9 wherein the inner strand is metallizable, further wherein the cladding layer is applied electrolytically to inner strand, and/or that cladding layer is integrally connected to inner strand.

12. The heating element according to claim 9 wherein the inner strand has a diameter of less than 100 μm and further wherein the thickness of cladding layer is between 0.2 and 2 μm and further wherein the individual strand and/or heating conductor has a thickness of less than 1 mm.

13. The heating element according to claim 9 wherein the inner strand further comprises, at least in part, a synthetic material that is elastic and tear-resistant, further comprises, at least in part, a material selected from the group consisting of thermoplastic, polyamide, carbon fiber, polypropylene, polyester, polyimide, glass silk or steel, further the material of inner strand has a higher resistance to reverse bending and/or a markedly higher material price and/or a lower tensile or compressive strength than the material of cladding layer.

14. The heating element according to claim 9 wherein the cladding layer has a surface that is chemically inactive under ordinary environmental conditions at least on its surface facing outward relative to the inner strand.

15. The heating element according to claim 9 wherein the cladding layer comprises an alloy of: nickel with or without phosphorus components, silver, copper, gold and further wherein the cladding layer comprises a metal surface which can be passivated or is oxidized or is chromated.

16. The heating element according to claim 9 wherein the surface of cladding layer is coated with a plastic or a lacquer and at least in part with material selected from the group consisting of polyurethane, polyvinyl chloride, polytetrafluoroethylene, perfluoroalkoxy or polyester.

17. The heating element according to claim 1 wherein at least one conductor strand comprises a plurality of individual strands.

18. The heating element according to claim 17 wherein a plurality of individual strands or conductor strands at least in sections are electrically insulated from one another; and optionally, in that at least one individual strand is at least partially insulated by means of an insulation layer on its cladding layer, or in that the conductor strand and/or the individual strand is sheathed at least in sections with a plastic.

19. An automotive vehicle including at least one surface contacted at times by a user comprising: a planar heating element including a pair of spaced apart electrodes positioned generally along opposing edges of a textile carrier layer and being adapted to connect with a current or voltage source;a first heating conductor bundle, which includes a plurality of metal-clad strands having electrical resistance characteristics that are dependent upon mechanical strain, and which extends between the electrodes;a second heating conductor bundle, which includes a plurality of metal-clad strands having electrical resistance characteristics that are dependent upon mechanical strain, and which extends between the electrodes generally in parallel with the first heating conductor bundle,wherein the first and second heating conductor bundles are arranged in a spaced pattern relative to each other such that electrical connection points are created where the first and second heating conductor bundles make crossing contact;wherein upon exceeding a permissible maximum temperature, at least one of the bundles at least temporarily loses at least part of its electrical conductivity.

20. An automotive vehicle including at least one surface contacted at times by a user comprising: a planar heating element including a pair of spaced apart electrodes positioned generally along opposing edges of a textile carrier layer and being adapted to connect with a current or voltage source;a first heating conductor bundle, which includes a plurality of metal-clad strands having electrical resistance characteristics that are dependent upon mechanical strain, and which extends between the electrodes;a second heating conductor bundle, which includes a plurality of metal-clad strands having electrical resistance characteristics that are dependent upon mechanical strain, and which extends between the electrodes generally in parallel with the first heating conductor bundle; anda spacing zone that extends between the electrodes and generally parallel with the first and second heating conductor bundles and separates first and second heating conductor bundles from any other heating conductors on the heating element;the cladding of the metal-clad strands of the first and second heat conductor bundles comprises at least on metal selected from the group consisting of (a) an alloy of nickel with phosphorus components, silver, copper or gold, (b) a metal surface which can be passivated, oxidized or chromated; or both (a) and (b);wherein the surface of cladding is coated with a plastic or a lacquer and at least in part with material selected from the group consisting of polyurethane, polyvinyl chloride, polytetrafluoroethylene, perfluoroalkoxy or polyester;wherein the first and second heating conductor bundles are arranged in a spaced pattern relative to each other such that electrical connection points are created where the first and second heating conductor bundles make crossing contact; andwherein upon exceeding a permissible maximum temperature, at least one of the bundles at least temporarily loses at least part of its electrical conductivity.