FI68746C - ELECTRIC EQUIPMENT - Google Patents

Abstract

1. Methof of manufacturing a flat heating element (5) with a fuse (7'; 7") against over-heating, whereby a foil (1) is rolled from electrical resistance material, the said foil being cut into a - for instance - serpentine pattern of interconnected resistance strips which is embedded in or applied on an insulating body, characterised in this, that paths, wires or similar formations of use material (2) of tin or a lead-tin or lead-tin-antimony or lead-bismuth alloy are, in a prescribed pattern, applied on or inserted in the foil of resistance material (1) made of a lead-antimony alloy, that the foil is rolled out in a uniform thickness in a further operation, that the foil composed of resistance material (1) and rolled-in fuse material (2) is cut so as to produce a predetermined pattern of interconnected resistance strips each of which incorporates at least one thermal fuse (7'; 7") of fuse material (2) at predetermined places.

Description

The present invention relates to a method of manufacturing a flat heating element having a fuse against overheating, in which a film of electrical resistance material is rolled, which is cut into, for example, a meander-shaped pattern of interlocking resistance strips, this pattern being either embedded or placed therein.

Such thermocouples are well known from U.S. Patent Nos. 3,263,307 and 3,336,557 (both to 0.G. Lund et al.) And U.S. Patent Nos. 4,025,893 and 4,092,626 to both HA Bergersen. and one of the main features of these inventions is that the metal strips forming the resistance element are made of a material with a melting point below 200 ° C. When such elements are used as electric heating elements for residential use and the installation requires close contact with combustible materials such as wood and wallpaper, it is important that the temperature 20 in and around the heating elements does not exceed 150 ° C at any point. This requirement can be met by using resistance strips consisting of an alloy containing 61.5% tin, 37.7% lead and 0.8% antimony, since this alloy has a melting point of 183 ° C.

U.S. Patent No. 3,423,574 discloses a heating plate comprising thermal fuses and thermostats. These elements consist of special parts that are connected to the heat plate after the resistance-strip formation has been completed. Such a structure, in which the thickness and width of the fuses and thermostats exceed the resistance strip, cannot easily be used for ceiling or wall heating. U.S. Patent No. 3,108,175 discloses a heating blanket with split thermostatic elements. However, the size of the thermostat elements is considerably larger than the heat conductor and the overall structure cannot be used for ceiling or wall heating.

A thermocouple in which the metal strips forming the resistance elements are welded in between two layers of insulation 2 68746 acts as a large thermal fuse if it is exposed to abnormal situations, e.g. if it is covered in a harmful manner by a heat insulating material. As the temperature in these cases approaches its melting point (170 ° C), the mechanical strength of the strips is quite low, so that in this situation the film can easily break at any time, even before reaching the melting point. The moment a fracture begins to form in the strip, the cross-sectional area of the strip decreases and therefore 10 it melts immediately, resulting in the current path being interrupted. After this, the heating element must be replaced with a new one.

However, there are also known resistance elements which are provided with only one additional fuse. Such elements are described, for example, in [JS Patent No. 15,417,229 (G.J. Shomphe et al.), And the apparatus shown herein has a thermal fuse installed for each different heating element. Such devices are protected against overheating in cases where the fuse element itself overheats. Such overheating occurs either because the fuse element itself is particularly well insulated or because the thermocouple draws too much current for some reason. In this case, the fuse melts at a certain temperature when it overheats and turns off the power, so that the heating element itself does not necessarily need to be replaced, but if part of the heating element slightly away from the fuse overheats, the wallpaper and surrounding areas may catch fire before the fuse cuts off the power path. The protection against fire is therefore lower in this solution.

Therefore, it is assumed that the safest type of resistor element is that described in the first part of the prior art description. Ts. elements which as such form a large fuse element. However, experiments have shown that it is not necessary that every square centimeter of the resistance strips be able to melt at the desired, low temperature.

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Therefore, it is an object of the present invention to provide a new and improved method of manufacturing a thermocouple, wherein the finished thermocouple has the excellent thermal properties and installation properties of existing thermocouples, but at the same time provides adequate protection against fire.

This is achieved by manufacturing a heating element according to the appended claim. By using the heating element according to the invention, the advantage is achieved that overheating causes the current path to be interrupted in certain places. This is expected to improve safety and reduce the risk of overheating.

The thermal fuses must be distributed in such a way that although the resistance element, in addition to the covering caused by normal installation, is partially covered by various objects such as tablecloths, furniture, blankets, etc., the surface temperature in the covered area does not exceed a certain critical value. The degree of unauthorized masking that occurs depends on the material and size of the masking article, and in the worst case, the transfer of heat from a given area will be effectively prevented. The experiments make it possible to determine the maximum surface area that can be covered or insulated in such a heating element without this leading to a surface temperature exceeding the critical value 25 at any point within the covered area. This applies to temporary investment or unfavorable investment in the covered sector. The size of this area is hereinafter referred to as the critical area. Therefore, thermal fuses must be distributed in such a way that at least one such fuse starts to operate if the area larger than the critical area is covered to such an extent that heat transfer away from this area is almost completely prevented.

However, it is quite unlikely that any particular area of the thermocouple will in practice actually be effectively thermally insulated. In experiments performed to determine the critical surface area, we used a 100 mm 4,68746-thick mineral wool mat as an additional cover outside the normal surface area. The following arrangement was used to mimic the heating element of the ceiling installation: a heating element measuring 500 x 1200 mm with winding lead / antimony strips laminated between plastic sheets, a 200 mm thick mineral wool in a horizontal position and a 12 mm thick table which the latter was directed downwards. In stable situations, a current of 210 W / in was applied to this heating element, and the maximum temperature of the table top was 78 ° C.

To mimic the harmful covering of the roof area, we placed pieces of mineral wool, 100 mm thick, pressed in against the underside of the table top. The critical temperature was chosen to be 175 ° C, at which point the area of the critical 2 area turned out to be 400 cm. However, the size of the critical surface area decreases with decreasing thickness of the table top and also decreases if the table top is replaced with materials that conduct less heat per minute. The most detrimental form of the critical area is considered to be the circle, while the square gives a rather good result. If the element is not covered by more than 2,400 cm, i.e. the heat transfer out of this area is so large (mainly laterally) that no critical temperature occurs. The best known process for manufacturing such thermocouples is by starting with a block which is a volatile metal alloy and rolling it into a metal film having a thickness of 5 to 25. This film or sheet is then cut to obtain the desired resistance strips, e.g. in the form of a meander, while the resistance strips are laminated together with the insulating material on one or both sides. When a suitable resistance element according to the present invention is to be manufactured, it is important that the main metal component is inexpensive, and lead is still considered to be a more suitable material. However, other metals must be mixed with lead to reduce its brittleness, mainly about 1% antimony.

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Divided, separate thermal fuses are provided by the topical application of a metal layer that mixes with lead to give a metal composition with a melting point below 200 ° C. Tin or a lead / tin / antimony mixture is considered a suitable alloying element because the eutectic mixture between tin and lead melts at a temperature of about 183 ° C. One possibility is also to use a lead / bismuth mixture with approximately the same melting point.

Po. To facilitate understanding of the invention, several embodiments thereof will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 shows three different principles for arranging a thermal fuse layer; Fig. 2 schematically shows how the precautionary agent 15 can be placed on the underlying metal film; Figure 3 shows alternative fuses placed on a heating element; and Figure 4 shows examples of separate thermal fuse arrangements for resistor elements.

As shown in Figure 1, there are many different ways of placing thermal fuses in resistor elements. The main substance 1 or the base material in Fig. 1a is provided with an inner 2 which is a fuse, while in Fig. 1b the fuse is placed between two thin films of the base material 1 and in Fig. 1a the base material film is cut through its entire cross section The cross-section according to Ib can be obtained by rolling, but the embodiment of Fig. 1a further comprises soldering before rolling. Figure 2 schematically shows 30 different ways of placing fuse strips on the film of the base material 1, which has not yet been cut. If the substances 1 and 2 in Figs. 68746 6 (migration) so that a small zone in the finished mixture of substances obtains a melting point with the desired value, i. about 180 ° C. However, it may be more desirable that already in the production process, i.e. before the heating element 5 is put into normal use, care is taken to ensure that the contact surface between the base material and the fuse is pre-mixed, so as to ensure the desired melting point immediately after the first hour of use.

When the heating element is manufactured by the above-mentioned 10 or other methods, an element is obtained which has the property that all the separate, divided fuses form alloys, so that fuse zones with a ratio of about 60% tin to 40% lead are obtained. At least one of these zones melts and cuts off 15 power paths if the area, which is the so-called greater than the critical area, is covered in an unauthorized manner by a thermal insulator. This is the case no matter where the covered area is located in the element.

Although po. the invention relates generally to the substances lead and tin and lead and bismuth, can be po. other substances are used within the scope of the invention. For example, steel can be used as the base material and silver / copper as the base material, or chromium / nickel as the base material and brass as the base material, etc.

Figure 3 shows three alternative ways of placing the fuse strip on the heating element 5. For simplicity, the electric resistance strips 6 are shown in a tortuous shape, but it is clear that the resistance strips can be arranged in any suitable way.

30 Although the fuse strips are shown in the figure as intact lines, it is clear that the fuse strips do not form a current path along these shown lines. This is particularly true of Figures 3a and 3c, where the final product appears approximately as a section of the heating element 35 shown in Figure 4a. However, in Fig. 3b, in which the fuse element strips are arranged so as to be parallel to the longitudinal parts of the bend of the heating element 7 68746, the fuse strips are conductive for a considerable part of their length.

In all parts of Figure 3, a separate thermal fuse is provided for the cat-5 of the fuse strip 7 and the resistance strip 6 at the cross or overlap points of the cat. Po. according to the invention, the separate thermal fuses must be divided so that the at least one fuse is always completely or partially covered by the critical area 8, which is temporarily placed on the heating element 5. As explained, the fuse action, i.e. it melts because it is covered, causing its temperature to exceed a critical value and cut off the current path. If, on the other hand, the covered area is smaller than the critical area, heat will be dissipated from the site so that no critical temperature is generated even during continuous loading.

Figures 4a and 4b show diagrammatically an enlarged section of a part of the heating element of Figures 3a and 3b comprising a resistance strip 6 provided with a plurality of separate thermal fuses, 7 'and 7 ", respectively.

Fuse strips should preferably be placed on the base film of the resistor before the desired shape is cut into the base film to obtain the desired resistance strips (Figure 2). This ensures that the fuse strips are also cut and that the desired number of separate thermal fuses on the resistance strips is obtained.

Another possible way is to place separate fuse element or fuse zones on the resistance strips after the base sheet has been cut into the desired shape, but this method is relatively complicated and also difficult to achieve suitable contact between the two substances.

Po. in all embodiments of the invention, sufficient, desired contact, or even a mixed belt zone, can be achieved with the two materials, as long as the surface of the materials is clean and free of corrosion and oxide films to provide contact.

Claims (1)

A method of manufacturing a flat heating element (5) with a fuse (7 '; 7 ") against overheating, wherein a film (1) is rolled from an electrical resistive material, which is cut into a meander-shaped pattern of interlocking resistor strips, this pattern being embedded either in the insulating piece or placed on it, characterized in that the strips, metal wires or similar fuse material assemblies (2) of tin or lead-tin or lead-tin-antimony or lead-bismuth alloy are mounted inside or on a film of resistance material consisting of lead-antimony alloy that the film is rolled to a uniform thickness in the next step, that the film combined with the resist material (1) and the precautionary material (2) rolled therein is cut to form a predetermined pattern of interdependent resistor strips each containing at predetermined points a thermal fuse (7 '; 7 ") consisting of a fuse material.