ELECTRICAL FUSE

Abstract

1. An electrical fuse comprising a housing (1) having a first end and a second end, wherein the electrical fuse further comprises two terminal closures (2) individually arranged at the first and second ends of the housing (1), two contacts (3), and at least one fusible element (4) of electrically conductive material arranged within the housing (1), wherein the contacts (3) are electrically conductively connected by the fusible element (4) and pass axially through the terminal closures (2) from the housing (1), characterized in that the terminal closures (2) are made of electrically non-conductive material. 2. Electrical fuse according to claim 1, characterized in that the electrically non-conductive material of the terminal closure (2) is a plastic material. 3. Electrical fuse according to claim 2, characterized in that the electrically non-conductive material of the terminal closure (2) is glass fiber modified polyamide. 4. Electrical fuse according to any of the preceding claims, characterized in that the terminal closures (2) are pressed to the outer surface of the housing (1). 5. Electrical fuse according to any of the preceding claims, characterized in that each contact (3) comprises a first contact part (3) electrically connected in a conductive manner to the fusible element (4) and a second contact part (3) extending perpendicularly from the first contact part (3), wherein the second contact part (3) passes axially through the terminal closure (2).

Description

ELECTRICAL FUSE TECHNICAL FIELD The invention relates to an electrical fuse designed to protect devices, including appliances and parts thereof, that use the electrical network for their operation, against overcurrents. PRIOR ART Due to their compact size and small dimensions, electrical fuses are frequently used in a wide variety of applications to protect devices, including appliances and their components, that operate on mains electricity from overcurrents. In addition to their small size, modern electrical fuses offer users highly useful features thanks to their high breaking capacity, low electrical losses, and wide range of rated currents and ampere-second characteristics. They are typically constructed with a tubular housing containing a fusible element inside a cavity, which can be filled with an extinguishing agent, usually silica sand. At both ends of the tubular housing are metal seals and corresponding contacts (also called terminals).However, metal closures often represent exposed metal surfaces that pose a risk to electrical safety due to accidental contact by people or can allow the unintentional formation of a short circuit if a metal part comes into contact with adjacent surfaces. Electrical fuses are also described in patent literature, for example, in document WO99 / 40599 A1. A fuse cartridge according to this document comprises a fuse housing filled with an extinguishing medium, cover plates fixed laterally to the fuse housing, and a main fuse conductor electrically connected to contact pins protruding through said cover plates. The contact pins are adapted for installation in a fuse holder, through which they are connected by conductors to the protected device and to the low-voltage electrical distribution network. The improved fuse according to WO2013 / 063037 A1 has a fuse body made of an electrically insulating material with a cavity extending from a first end of the fuse body to a second end. The fusible element is located within the cavity. Insulated plugs are located in the cavity at the first and second ends of the fuse body, adhering to the inner surface of the fuse body and forming seals that close the inner cavity. Furthermore, the fuse may include terminations applied to the ends of the fuse body in electrical contact with the fusible element. These terminations are made of an electrically conductive material. US6653925 B1 describes a fuse with insulation of the thermal fuse conductors by means of an insulating tube and, at the same time, describes a method for its manufacture. A fuse according to US5905426 A comprises an electrically insulating elongated tube, a pair of blade-type contacts projecting axially outward from opposite ends of the tube, at least one fusible element disposed in the tube and electrically connected between the terminals, and a pair of metallic end closures disposed at opposite ends of the tube. Electrically insulating elements are disposed between the end closures and the blade-type contacts. Document WO2018 / 089156 A1 describes a fuse comprising a fuse body, a fusible element comprising a first termination protruding from the first end of the fuse body and a second termination protruding from the second end of the fuse body, and end closures. The material of the end closures is not specified in this document. The technical solution according to Czech utility model no. 19694 refers to an electric fuse, consisting of an insulating body, provided with cylindrical caps at opposite ends and equipped with a continuous cavity with at least one usable strip conductor, wherein the continuous cavity is filled with an extinguishing agent and is provided with plugs at the terminal parts. US2011298577 A1 describes an electrical fuse with one or more counter-holes in the inner wall of a hollow tube that serves as the fuse housing. The fuse has a first and a second end cap connected to the corresponding ends of the tube, where these caps are made of an electrically conductive material. The documents mentioned typically use metal caps, or caps made of electrically conductive material in general. Therefore, there is a risk of electric shock if a person accidentally touches these metal caps. This problem is either not addressed in the documents mentioned or is resolved by a disadvantageous modification of the fuse. For example, according to US5905426 A, electrically insulating elements must be provided between the terminal closures and the corresponding contacts. This complicates manufacturing and increases production costs. Therefore, it would be desirable to find a solution that eliminates the risk of electric shock to people or the risk of short circuits due to contact with conductive surfaces. At the same time, the proposed solution should be characterized by a simple construction, undemanding in terms of both cost and technology. DESCRIPTION OF THE INVENTION The aforementioned deficiencies are mitigated to some extent by an electrical fuse comprising a housing with a first end and a second end, wherein the electrical fuse further comprises two terminal closures individually arranged at the first and second ends of the housing, two contacts, and at least one fusible element of electrically conductive material disposed within the housing. The contacts are electrically conductively connected by means of a fusible element and pass axially through the terminal closures to the outside of the housing. The essential feature of the electrical fuse according to the present invention is that the terminal closures are made of electrically non-conductive material. The fact that the terminal closures are made of electrically non-conductive material eliminates the risk of electric shock to people if they touch these closures. It also minimizes the risk of an unintentional short circuit if the terminal closure comes into contact with any of the adjacent metal surfaces. Furthermore, producing terminal closures from an electrically non-conductive material is simpler and less expensive than producing them from metal. There is also no need to provide the fuse with additional insulation between the terminal closures and the contacts, which preserves its simple construction and reduces manufacturing costs. Thus, end seals close the housing at both ends and include openings for the passage of contacts. The contacts pass through these openings axially, meaning they exit the end seals axially from the housing, i.e., in the direction of the housing axis. Therefore, the contact can pass through the end seal in a direction coinciding with this axis, i.e., normally through the center of the end seal; however, it can also pass through the end seal in a direction offset from the housing axis, i.e., it can pass through the end seal at a different location, for example, closer to the edge of the end seal. If we say that the contacts are electrically conductively connected to the fusible element, this conductive connection can be made, for example, by the contact and the fusible element forming two separate parts that are electrically conductively connected to each other during production. However, the contacts can also be connected by a fusible element in such a way that they are an extension of the fusible element at both ends. In other words, this means that the contacts can be formed from the same piece as the fusible element and, together with the fusible element, form a single integral part. The transition between the fusible element and the contacts is then formed by a thinning of the portion of the fusible element located inside the housing relative to the contacts, or by a thickening of the contacts relative to the fusible element. Both terminal closures can be made of the same electrically non-conductive material. Alternatively, the individual contacts can be made of different electrically non-conductive materials. The electrically non-conductive material of the terminal closure is preferably a plastic material. The plastic material can be any electrically non-conductive plastic material, or an electrically non-conductive composite material of which at least one component is plastic. The plastic material can also be a blend, that is, a mixture of two or more compatible polymers. More specifically, the plastic material can be, for example, one of the thermoplastics (polyamide, polycarbonate) or thermosets, which generally exhibit greater resistance to thermal stress than thermoplastics. The electrically non-conductive material of the terminal closure is preferably glass fiber-modified polyamide. This material is hard, flexible, and withstands thermal loads well. The proportion of glass fiber additive can be from 15 to 30%, or another suitable proportion of this additive can be selected. The terminal closures are preferably manufactured by injection molding of plastic material, although alternatively, they can be manufactured using additive manufacturing technology (3D printing), i.e., the terminal closure can be a 3D-printed part. Additive manufacturing technology can be advantageously used for small production runs. Preferably, the terminal seals are pressed onto the outer surface of the housing. This allows the terminal seals to withstand the overpressure that occurs inside the housing when an electric arc is broken. The adhesion strength of the terminal seals, i.e., the force required to press them in, is preferably greater than 100 N. Alternatively, the terminal seals may be bonded to the outer surface of the housing. Each contact preferably comprises a first contact portion electrically and conductively connected to the fusible element and a second contact portion extending perpendicularly from the first contact portion, wherein the second contact portion passes axially through the terminal closure. In this way, the contact may have an L-shaped form. This contact arrangement is preferred because it is also suitable for miniature fuses designed for surface-mount (SMD) installation on printed circuit boards. This first contact portion is adapted to the cross-section of the housing; that is, it has a circular cross-section if the housing has a circular cross-section, or a square or other type of cross-section if the housing has such a square or other type of cross-section.The corresponding end of the fusible element is electrically connected to this first part of the contact in a conductive manner. DESCRIPTION OF THE DRAWINGS The essence of the invention is further clarified by examples of its embodiment, which are described using the accompanying drawings, where in: Fig. 1 shows an electrical fuse in a first exemplary embodiment according to the present invention, Fig. 2 shows an electrical fuse in a first exemplary embodiment according to the present invention in longitudinal section, and Fig. 3 shows an electrical fuse in a second exemplary embodiment according to the present invention in longitudinal section. EXAMPLES OF IMPLEMENTATION OF THE INVENTION The invention will be further clarified with embodiment examples with reference to the corresponding drawings. In a first exemplary embodiment, as shown in Fig. 1 and Fig. 2, the electrical fuse comprises a housing 1, two terminal closures 2, two contacts 3, a fusible element 4 made of electrically conductive material, and an extinguishing agent 5. This first exemplary embodiment will now be described in detail. In the first exemplary embodiment indicated and illustrated, the housing 1 is tubular in shape, i.e., it comprises a first and a second end of the housing 1 and further a continuous cavity opening at these two ends of the housing 1. The housing 1 has, in one exemplary embodiment, a circular cross-section, alternatively a square cross-section or a cross-section of another shape, provided that such cross-section allows the arrangement of the fusible element 4 within the housing 1. The housing 1 is made of an electrically non-conductive material, exemplarily ceramic (for example, steatite), non-conductive composite materials, or glass. To close the housing 1 at its first and second ends, the electrical fuse comprises two end seals 2. These end seals 2 are arranged individually at the first and second ends of the housing 1, respectively, where they are specifically pressed against the outer surface of the housing 1 to withstand the overpressure generated inside the housing 1 during the interruption of an electric arc. The adhesion strength of the end seals 2, i.e., the force required to press them together, is exceptionally greater than 100 N. To eliminate the risk of electric shock injury, terminal closures 2 are made of electrically non-conductive material, specifically plastic. This electrically non-conductive material can be either a single material or a composite material (e.g., blends, i.e., mixtures of two or more compatible polymers). In the most preferred embodiment of terminal closures 2, they are manufactured by injection molding from a glass fiber-modified polyamide material, typically with an addition of 15 to 30% glass fibers. However, the proportion of glass fibers can vary. In other embodiments, terminal closures 2 are made, for example, from thermoplastics (polyamide, polycarbonate, etc.).or thermosets, which generally exhibit greater resistance to thermal stress than thermoplastics. The above specific examples of materials are intended to be illustrative and not an exhaustive list of applicable materials. A specialist would have no problem using any other known electrically non-conductive material as well. In the first illustrated exemplary embodiment of the fuse according to the present invention, a fusible element 4 made of an electrically conductive material, for example, copper, silver, or various alloys, is arranged in the housing 1. This fusible element 4 is generally known in the prior art and is also called a fusible conductor. The fusible element 4 is exemplified by a wire or strip shape. As shown in longitudinal section in Fig. 2, the fusible element 4 is arranged inside the housing 1 and extends axially between both ends of the housing 1. It is the fusible element 4 that provides protection against overcurrent, as it melts if an excessively high current flows through it. This principle and the specific materials of the fusible conductor are well known in the prior art. As can also be seen in Fig.2, the casing 1 is filled with an extinguishing agent 5, for example, silica sand, to improve the extinguishing of the electric arc and the interruption of the subsequent current. The fusible element 4 electrically connects the contacts 3 in a conductive manner, wherein in the first illustrated exemplary embodiment both contacts 3 are implemented in an L-shaped form and these contacts 3 are electrically connected conductively to both ends of the fusible element 4. As can be seen in Fig. 2, the contacts 3 comprise a first part that is disposed at the corresponding end of the housing 1, i.e., at the point where the continuous cavity of the housing opens. This first part of the contact 3 is adapted to the cross-section of the housing 1, i.e., it has a circular cross-section if the housing 1 has a circular cross-section, or a square or other type of cross-section if the housing has such a square or other type of cross-section. The corresponding end of the fusible element 4 is electrically connected conductively to this first part of the contact 3.In this context, it should be mentioned that the end cap 2 is adapted to cover the corresponding end of the housing 1, including the first part of the contact 3. Specifically, this means that the end cap 2 comprises a cavity whose inner diameter corresponds to the outer diameter of the housing 1 and the first part of the contact 3, or is slightly larger than the outer diameter of the housing 1 and the first part of the contact 3. Thus, the end cap 2 can be fitted over the first part of the contact 3 and the outer surface of the housing 1, as shown in Fig. 2. Each contact 3 further comprises, in the first exemplary embodiment according to Fig. 1 and Fig. 2, a second part of the contact 3 that is perpendicular to the first part of the contact 3 and passes axially through the corresponding terminal closure 2 to the outside of the housing 1. For this reason, an opening is made in each terminal closure 2 for the passage of the second part of the contact 3. The contacts 3 are made of electrically conductive material, typically metal, and are adapted for connection to the protected device; for example, their second part is adapted for mounting in a fuse holder, through which and by means of conductors the contact 3 is subsequently connected to the protected device.In the first illustrated exemplary embodiment, the second part of contact 3 is longer than the first part of contact 3; however, in some of the alternative embodiments, the second part of contact 3 may be as long as the first part of contact 3 or even shorter. The first exemplary embodiment described above can be modified in various ways to create other alternative embodiments of the fuse. Alternatively, for example, the housing 1 is not filled with extinguishing agent 5. In another alternative embodiment, the fuse comprises a plurality of fusible elements 4, i.e., a plurality of fusible conductors. In yet another alternative embodiment, the terminal closures 2 are not pressed onto the outer surface of the housing 1, but are bonded to it. In another alternative embodiment of the fuse, which we will call the second exemplary embodiment and which is shown in Fig. 3, the contacts 3 are connected by a fusible element 4 such that they are an extension of the fusible element 4 at both ends. In other words, the contacts 3 are formed from the same piece as the fusible element 4 and together with the fusible element 4 form a single integral part. The transition between the fusible element 4 and the contacts 3 is then formed by either a thinning of the portion of the fusible element 4 that is located within the housing 1 relative to the contacts 3, or a thickening of the contacts 3 relative to the fusible element 4. This thinning of the fusible element 4, or the thickening of the contacts 3, can be seen in Fig. 3.In practice, only this thinned portion (corresponding to fusible element 4) will melt when an overcurrent is applied, while the thicker portions (corresponding to contacts 3) will remain intact. Thus, when fusible element 4 melts, the conductive connection between contacts 3 is broken. The other parts of the fuse, namely the housing 1 and the end closures 2, in the second exemplary embodiment are made in the same way as in the first exemplary embodiment described above, but the fuse in the second exemplary embodiment further comprises two fronts 6. As can be seen in Fig. 3, these fronts 6 are arranged at the corresponding ends of the housing 1, i.e., at the point where the continuous cavity of the housing opens. The fronts 6 are thus arranged analogously in the same space as the first parts of the contacts 3 according to the first exemplary embodiment shown in Fig. 1 and Fig. 2. Like these first parts of the contacts 3 in the first exemplary embodiment of the fuse, the fronts 6 in the second exemplary embodiment of the fuse are adapted to the cross-section of the housing 1 and the shape of the cavity of the end closure 2.Specifically, this means that the end cap 2 comprises a cavity whose inner diameter corresponds to the outer diameter of the housing 1 and the front 6, or is slightly larger than the outer diameter of the housing 1 and the front 6. In this way, the end cap 2 can be fitted onto the front 6 and the outer surface of the housing 1. Unlike the first part of the contacts 3 of the first exemplary embodiment, the fronts 6 of the second exemplary embodiment do not serve for the electrically conductive connection of the fusible element 4. Instead, each front 6 comprises an opening for the passage of a fusible element 4 terminated by contacts 3. In addition, further openings are provided in both end closures 2, these openings being arranged coaxially with the openings in both fronts 6. This allows the contacts 3, which terminate the fusible element 4, to pass through the fronts 6 and then through the end closures 2 axially outside the housing 1. In the second exemplary embodiment, the front 6 is in contact with the corresponding contact 3 but is not electrically conductively connected to this contact 3.Alternatively, the second exemplary embodiment can be modified so that the front 6 is conductively connected to the corresponding contact 3. INDUSTRIAL APPLICABILITY The electrical fuse described above can be used to protect various devices against overcurrents, including appliances and parts thereof, which use the electrical network for their operation. List of reference numbers 1 - casing 2 - terminal closure 3 - contact 4 - fusible element 5 - extinguishing agent 6 - front

Claims

1. An electrical fuse comprising a housing (1) having a first end and a second end, wherein the electrical fuse further comprises two terminal closures (2) individually disposed at the first and second ends of the housing (1), two contacts (3), and at least one fusible element (4) of electrically conductive material disposed within the housing (1), wherein the contacts (3) are electrically conductively connected by the fusible element (4) and pass axially through the terminal closures (2) from the housing (1), characterized in that the terminal closures (2) are made of electrically non-conductive material.

2. An electrical fuse according to claim 1, characterized in that the electrically non-conductive material of the terminal closure (2) is a plastic material. 3.An electrical fuse according to claim 2, characterized in that the electrically non-conductive material of the terminal closure (2) is glass fiber modified polyamide.

4. An electrical fuse according to any of the preceding claims, characterized in that the terminal closures (2) are pressed against the outer surface of the housing (1).

5. An electrical fuse according to any of the preceding claims, characterized in that each contact (3) comprises a first contact portion (3) electrically conductively connected to the fusible element (4) and a second contact portion (3) extending perpendicularly from the first contact portion (3), wherein the second contact portion (3) passes axially through the terminal closure (2).