Technical field

The invention relates to a device for overvoltage protection comprising a terminals for connection to protected circuit, while between the terminals there is arranged a current path in which a protective element is connected, while in the current path is created a place for purposeful cutting-off of the current path due to change of properties of the protective element and to this point a device for status signalling of protective element is assigned.

Background art

From WO 2007/017736 A1 and documents mentioned in the background art of WO 2007/017736 A1 there are known the devices for overvoltage protection comprising a protective element formed of a non-linear resistance element - varistor. Varistor due to its loading by electric current and by an impulse loading of a protected network gradually decreases the value of its electric resistance. Due to this, the electric current running through varistor is increasing and temperature of varistor increases as well. Therefore the device for overvoltage protection is provided with thermal cut-off device (hereinafter referred to as TCD) which once a certain temperature of varistor is achieved cuts-off the current path of the device for overvoltage protection, through which varistor is disconnected from the protected circuit or electric network (hereinafter referred to as electric network only), which secures that due to passing electric current may not occur further warming of varistor above the permissible limit.

, Cutting off the varistor from the electric network is signalled, namely optically directly on the device for overvoltage protection and possibly through remote control by means of transmitting a suitable signal. Once the varistor is disconnected from the electric network, this network is further not protected and so it is necessary to restore the protected status by replacement of the device for overvoltage protection, or of its part comprising the disconnected protective element. The whole effect of gradual warming of varistor through its "ageing" is expressively a long-term effect, markedly longer than the undesirable effects in electrical network, against whose effects varistor protects the connected devices. The period of "ageing" of varistor nevertheless depends also on a quantity of impulse loads incurred by overvoltage in the electric network, when during a very short period relatively great current is flowing through the current path and which also reduce the service life of varistor.

In electric network also other effects occur when TCD is subject to action of considerably different conditions, which it must manage in a manner securing a safe operation of an instrument (device for overvoltage protection) even in situations when the protective elements (varistors) are overloaded and their voltage failure occurs. In such situation it is required, that even at a high short- circuit current, declared for the given instrument, the TCD is not disconnected, but the circuit is cut-off in the overcurrent protection positioned in the circuit before the device for overvoltage protection. Disconnection of TCD could cause occurance of electric arc between the disconnected elements with danger of fire.

Requirements for a safe functioning of device for overvoltage protection are shown in respective national or international regulations (standards), e.g. CSN EN 61643-11 , which also specifies conditions for correct functioning of the device for overvoltage protection in situation of electric breakdown of varistor, for maximum short-circuit current in the point of connection of the device for overvoltage protection. OSN EN 61643-11 specifies these requirements including evaluation of results of tests in paragraph 7.7.3. of the above mentioned regulation.

Another situation arises from the point of view of TCD disconnection, if after varistor electric breakdown the short-circuit current is considerably lower than the maximum short-circuit fault tolerance of the device for overvoltage protection. Reaction of overcurrent securing elements which are positioned before the device for overvoltage protection is significantly prolonged and manner of loading the TCD is considerably changed. This fact has been reflected in A11 change of the OSN EN 61643-11 regulation, which comprises extended testing of short-circuit fault tolerance of the device for overvoltage protection. This extension is specified in paragraph 7.7.3. b) of the mentioned norm and its changes are designated as ..testing by low short-circuit current".

This new testing in an expressive manner enlarges requirements as to parameters of TCD, especially at the devices for overvoltage protection on basis of varistor for higher energies (category I). This testing requires a long- term (seconds in order) acting of low short-circuit current, which substantially changes thermal loading of the whole current path of the device for overvoltage protection. The current path, including TCD, is considerably warmed-up. This warming results in disconnection of the TCD even during passing the low short- circuit current, which may cause occurrence of electric arc with possible fire. As for the TCD there is by a regulation prescribed maximum temperature, at which its disconnection must occur, the low-fusing solders with respect to this condition are used for connection of elements of current path, while the melting temperature of the solders lies considerably lower, than the melting temperature of the remaining elements of the current path.

Taking into account that the low-fusing solder connects elements of the current path, it is required that the solder has the best possible electrical conductivity, which is of course lower than the electrical conductivity of other elements of current path, which are usually made of copper (of course with exception of varistor, that fulfils by its strongly non-linear characteristic the protective function of overvoltage protection).

The solder connecting elements of current path, with respect to solder ^' s lower electric conductivity when compared with other elements of current path, also gets quicker warmer due to passing electric current, which may be compensated to a certain level by a size of soldered area connecting the elements of current path. It is generally known that electrical conductivity of metals strongly correlates with temperature conductivity. This results into a fact, that if the layer of solder is sufficiently thin, the heat spreading from the solder into elements of current path occurs simultaneously during warming of the solder. This thermal mechanism ensures relatively low temperature of the solder only at the situation, when temperature of elements of the current path is relatively low. If, nevertheless this temperature raises to the solder melting temperature, there occurs an undesirable disconnection of TCD instead of cutting-off the circuit in overcurrent protection, which is positioned before the device for overvoltage protection.

The goal of this invention is to propose suitable means which prolong the period for cutting off the TCD behind the by regulation defined conditions for acting of the low short-circuit current so that there occurs disconnection of device for overvoltage protection from the electric network by means of protection positioned before the said device for overvoltage protection.

Principle of the invention

The goal of the invention has been achieved by a device for overvoltage protection whose principle consists in that in the place of purposeful cutting-off of the current path there is formed an additional fusible thermal stop with pre-set parameters of melting-down, while the elements of current path being purposefully disconnected are connected by means of first solder and the additional fusible thermal stop is formed by a second solder, while the second solder has the same or approximately the same temperature of melting as the first solder and the second solder has a lower value of heat conductivity than the first solder.

Hence, the principle of the invention is in the prevention of disconnection of TCD immediately after melting the solder connecting the elements of the current path, so that temperature of this solder, which is already in liquid status, may further be increased till boiling-point of this solder, through which the time for passing of the electric current through TCD without undesirable disconnection of TCD is considerably prolonged, which enables to increase dimensioning of overcurrent protection which is positioned before the device for overvoltage protection in compliance with requirements of provisions of the regulation (standard) mentioned in the background art. Due to time span of reacting of low short-circuit current and setting of conditions of its reacting, thus also an excessive thermal reacting towards the surroundings of TCD does not occur. The preferred embodiments of the invention are the subject of the dependent claims.

Description of the drawing

The invention is schematically represented in the drawing where Fig. 1 shows an exemplary embodiment of the device for overvoltage protection with slide-in protective element, Fig. 2 an exemplary embodiment of the place X of purposeful cutting-off of the current path with two-stage signalling of protection status, Fig. 2a ground plan of one embodiment of the place X of purposeful cutting-off of the current path according to Fig. 2, Fig. 2b shows ground plan of another embodiment of the place X of purposeful cutting-off of the current path according to Fig. 2, Fig. 2c ground plan of another embodiment of the place X of purposeful cutting-off of the current path, Fig. 2d modified embodiment of the place X of purposeful cutting-off of the current path from Fig. 2b, Fig. 3 other exemplary embodiment of the place X of purposeful cutting-off of the current path with three-stage signalling of protection status, Fig. 3a a cross-section through one embodiment of the place X of purposeful cutting-off of the current path, Fig. 3b, a cross-section through another embodiment of the place X of purposeful cutting-off of the current path, Fig. 4 scheme of another embodiment of device for overvoltage protection with the place X of purposeful cutting-off of the current path according to the invention, Fig. 5 another exemplary embodiment of the place X of purposeful cutting-off of the current path.

Examples of embodiment

The invention shall be described on particular examples of the device for overvoltage protection in the form of slide-in protective elements 1, which are replaceably mounted in a holder 0, which is created as a profile approximately of ,,U" letter. In one holder 0 there may be arranged side by side several slide-in protective elements 1, e.g. for each phase of the three-phase power line, etc. There may be also more single-pole holders 0 connected into one unit, e.g. by means of rivets. The holder 0 comprises not represented terminals for connection of electric conductors of the protected circuit. The holder 0 is provided with means for mechanical connection of the slide-in protective element Λ_ and further comprises not represented contacts for electrical connection of the current path of the slide-in protective element 1 to holder 0. The device for overvoltage protection is provided with local and possibly also with remote signalling of protection status, which is either two-stage or three- stage.

In an example of embodiment represented in Fig. 1 there is the slide-in protective element 1 positioned in the holder 0. In current path of slide-in protective element 1 there is connected at least one non-linear resistance element, for example varistor 2 or a group of parallel connected varistors 2, as a protective element is. In lower section of the slide-in protective element 1 is situated a cut-off device 3 of varistor 2 from the protected circuit whose task is, once the defined conditions (especially upon reaching the specified temperature of varistor 2 or of current path) are fulfilled to purposely cut-off the current path of the slide-in protective element 1_in an exactly defined point . The point, in which the purposeful and defined cutting-off the current path of the slide-in protective element Λ_ occurs in the meaning of the previous sentence, will be for purposes of this description called as place X of purposeful cutting-off of the current path. The cut-off device 3 is coupled with means for signalling the protection status. For local signalling of protection status in the protective element 1 there is in swivelling manner mounted a lever 4 of optical signalling, which is with its first end 40 coupled with the cut-off device 3 and is with its second end 4J. assigned to a window ^O of optical signalling in the protective element 1.

The Fig. 2 shows place X of purposeful cutting-off of the current path by means of the cut-off device with two-stage signalling of protection status from the Fig. 1. In this embodiment is to the lower side of electrode 20 of varistor 2 by means of the first solder 2Λ_ connected one of "L" shaped bent end 50 (hereinafter referred to as "L" end 50 only) of a flat electrically conductive stranded wire 5, whose second end is connected with the non-represented contact of the slide-in protective member Λ_. The ,,L" end 50 of the stranded wire 5 is strengthened to improve stiffness, e.g. by means of welding the individual wires forming the stranded wire or by means of connecting the elemental wires of stranded wire 5 by means of the first solder 21 etc. Against the ,,L" end 50 of the stranded wire 5 is leaning an end 300 of spring-loaded shifting member 30 of the cut-off device 3_. In the not represented example of embodiment the ,,L" end 50 of the stranded wire 5 is by means of the first solder 2J. connected to the upper side of electrode 20 of varistor 2. The stranded wire 5 and electrode 20 are further provided with a through hole, in which there is positioned the fusible pin 220 or fusible rivet made of the second solder 22. Not represented second electrode of varistor 2 is connected with the not represented contact of the slide-in protective element 1 on opposite polarity, e.g. with a second contact of the slide-in protective element 1 or contact of further non-linear protective element upon serial arrangement of non-linear resistance elements etc.

In example of embodiment represented in the Fig. 3, on which the place X of purposeful cutting-off of the current path by means of the cut-off device with three-stage signalling of protection status is illustrated, there is to the lower side of electrode 20 of varistor 2 by means of a first solder 21 connected the ,,L" end 50 of the stranded wire 5, whose second end is connected with the not represented contact of the slide-in protective member. The ,,L" end 50 of the stranded wire 5 is modified to increase tightness, e.g. by welding individual wires forming the stranded wire 5 or connecting the wires of the stranded wire 5 by means of the first solder 21., etc. The stranded wire 5 and electrode 20 are further provided with a through hole, in which is positioned the fusible pin 220 or fusible rivet made of the second solder 22. On upper side of electrode 20 of varistor 2 there is by means of the third solder 23 mounted the angle stop 7, against which is leaning the end 300 of spring-loaded shifting member 30 of the cut-off device 3. In the not represented example of embodiment the ,,L" end 50 of the stranded wire 5 is by means of the first solder 21 connected to upper side of electrode 20 of varistor 2, while the angle stop 7 is by means of the third solder 23 mounted on the upper side of the ,,L" end 50 of stranded wire 5. The not represented second electrode of varistor 2 is connected with the not represented contact of the slide-in protective element 1 on opposite polarity, e.g. with second contact of the slide-in protective element 1 or contact of further non-linear protective element upon serial arrangement of non-linear resistance elements, etc.

In embodiment of the place X of purposeful cutting-off of the current path according to the Fig. 2 and 3 the first solder 21_ features a good electric as well as thermal conductivity and serves to conduct electric current between the electrode 20 of varistor 2 and the stranded wire 5. The second solder 22 when compared with the first solder 2J. features a lower electric as well as thermal conductivity. The value of thermal conductivity of the second solder 22 is at least by one degree lower than the value of thermal conductivity of the first solder 21_. The not represented second electrode of varistor 2 is connected with the not represented other contact on opposite polarity, e.g. the second contact of the slide-in protective element or contact of further non-linear protective element upon serial arrangement of non-linear resistance elements, etc.

The Fig. 2a and 2b schematically represent ground plan of arrangement of the place X of purposeful cutting-off of the current path from the Fig. 2. Electric current pass in direction of Y arrows, it is known it tends to pass through the shortest possible path or through the place of the lowest electric resistance. This is also related to a course of thermal field in the ground plan of connection of electrode 20 and the stranded wire 5, from which it is obvious, that the highest temperature is in the area A, through which the electric current pass preferably. The heat generated by passing of electric current through the current path thanks to thermal conductivity of materials gradually expands from the area A to the area B, which is the farthest from the area A. Due to limited distance of the areas A and B, after a certain period of passing of electric current there is a sufficient warming of the area B, in which or in whose surroundings is arranged the fusible pin 220 or fusible rivet, while the heat is transferred into the fusible pin 220 or fusible rivet only from its immediate surroundings, i.e. from walls of the hole, in which the fusible pin 220 or fusible rivet is inserted. For the purpose that the fusible pin 220 or fusible rivet stops to prevent a controlled displacement of the ,,L" end 50 of the stranded wire 5 from electrode 20 and thus also the purposeful cutting-off of the current path in the place X it is nevertheless necessary, that the fusible pin 220 or fusible rivet is molten in its whole cross-section, which process, due to a worse thermal conductivity of the second solder 22, of which the fusible pin 220 or fusible rivet is made, takes a longer time than it takes to melt-down a thin layer of the first solder 21..

Fig. 2c schematically represents ground plan of another arrangement of the place X of purposeful cutting-off of the current path, when elements of the current path are formed of the strip 9 mounted in elongated hole in electrode 20 of varistor 2. The strip 9 is by means of the first solder 21. connected with electrode 20 and one of its ends is coupled with tension spring 90, by which the strip 9 is pulled out the hole. On its opposite end the strip 9 is provided with a hole into which is inserted the fusible pin 220 or fusible rivet formed of the second solder 22. For the purpose that the fusible pin 220 stops to prevent a controlled pulling out the strip 9 from electrode 20 and thus also the purposeful cutting-off of the current path in the place X it is nevertheless necessary, that the fusible pin 220 is molten in its whole cross-section, which, due to a worse thermal conductivity of the second solder 22, of which the fusible pin 220 or fusible rivet is made, takes a longer time than it takes to melt-down a thin layer of the first solder 21..

Fig. 2d schematically represents ground plan of arrangement of the place X of purposeful cutting-off of the current path modified towards the Fig. 2b by the nose C_arranged in the area B of the "L" end 50 of the stranded wire 5 and electrode 20 of varistor 2. Electric current pass in direction of Y arrows, it is known it tends to pass through the shortest possible path or through the place of the lowest electric resistance. This is also related to a course of thermal field in the ground plan of connection of electrode 20 and the stranded wire 5, from which it is obvious, that the highest temperature is in the area A. The heat generated by passing of electric current through the current path expands thanks to thermal conductivity of materials gradually from the area A to the area B and into the nose C. With respect to a greater distance of the area A and the fusible pin 220 or fusible rivet in the nose C, than the distance in embodiment in the Fig. 2b, after a certain period, which is longer than at embodiment in the Fig. 2b, through passing of electric current there is sufficient warming of the fusible pin 220 or fusible rivet, into which the heat is expanding only from its immediate surroundings, i.e. from walls of the hole, in which the fusible pin 220 or fusible rivet in the nose C is inserted. For the purpose that the fusible pin 220 or fusible rivet stops to prevent a controlled displacement of the ,,L" end 50 of the stranded wire 5 from electrode 20 of varistor 2 and thus also the purposeful cutting-off of the current path in the place X it is nevertheless necessary, that the fusible pin 220 or fusible rivet is molten in its whole cross-section, which, with respect to mounting of the fusible pin 220 or the fusible rivet in the nose C due to a worse thermal conductivity of the second solder 22, of which the fusible pin 220 or fusible rivet is made, takes a longer time than it takes in a case of embodiment according to the Fig. 2b.

In the not represented example of embodiment the fusible pin 220 or fusible pin is positioned in otherwise shaped or positioned nose C, which may have in the place of its transition into the ,,L" end 50 of the stranded wire 5 and electrode 20 of varistor 2 a reduced cross-section, i.e. to have a neck, by which the heat transfer into the nose C is slowed down as well as melting-down of the fusible pin 220 or fusible rivet.

Thickness of the layer of the first solder 21_ is in the tenths of millimeter and moreover the first solder 2J. has a good thermal conductivity. Diameter of the fusible pin 220 made of the second solder 22 is considerably greater, e.g. equals to one (1) millimeter or more, and moreover the second solder 22 features a worse thermal conductivity, preferably at least by one degree. The first and the second solder 21., 22 have the same or approximately the same melting temperature. The possible third solder 23 has a lower melting temperature than is the melting temperature of the first and second solder 21., 22, while electric conductivity of the third solder 23 in solution according to the invention does not play any important role. In the represented examples of embodiment the first solder 2J. and the second solder 22,due to requirement as to different thermal conductivity, are also of a different chemical composition, thus these are different materials. Generally it could be said, that the lower the electric conductivity of the used solder is, the smaller is also its thermal conductivity and thus it takes longer to melt-down the used solder in the whole cross-section or volume.

At a normal status or occurrence of overvoltage, when the accompanying effect is a slow warming of the current path, the embodiment according to the Fig. 2, 2a, 2b, 2d works so, that by a sufficient and slow warming of electrode 20 of varistor 2 there occurs melting of the first as well as the second solder 21., 22 and the spring-loaded shifting element 30 shifts and by its end 300 pushes away the ,,L" end 50 of the stranded wire 5 from electrode 20 of varistor 2, by which it cuts-off of the current path in the place X and disconnects the protective element (varistor 2) from the electric network. Through displacement of the shifting member 30 there occurs signalling in change of status of overvoltage protection from status ..functioning protection" to status ..non-functioning protection".

At a normal status or occurrence of overvoltage, when the accompanying effect is a slow warming of the current path, the embodiment according to the Fig. 2c works so, that by a sufficient and slow warming of electrode 20 of varistor 2 there occurs melting of the first as well as the second solder 21., 22 and the spring-loaded strip 9 pushes out from the hole in electrode 20 of varistor 2, by which it cuts-off of the current path in the place X and disconnects the protective element (varistor 2) from the electric network and there occurs signalling in change of status of overvoltage protection from status ..functioning protection" to status ..non-functioning protection".

At a normal status or occurrence of overvoltage, when the accompanying effect is a slow warming of the current path, the embodiment according to the Fig. 3 works so, that by a sufficient and slow warming of electrode 20 of varistor 2 there occurs first melting of the third solder 23 and the angle stop 7 by acting of the spring-loaded shifting member 30 advances up to the ,,L" end 50 of the stranded wire 5. Upon this first movement of the shifting member 30 there occurs signalling in change of status of overvoltage protection from status ..functioning protection" to the status ..approaching failure of protection". Through further slow warming of electrode 20 of varistor 2 the first as well as the second solder 21., 22 get molten and the spring-loaded shifting member 30 moves further and by its end 300 pushes away the angle stop 7 as well as the ,,L" end 50 of the stranded wire 5 from electrode 20 of varistor 2, by which it cuts-off of the current path in the place X and disconnects the protective element (of varistor 2) from the electric network. By this further movement of the shifting member 30 there occurs signalling in change of status of overvoltage protection from status ..approaching failure of protection" to the status ..non-functioning protection".

Upon short circuit, during which through the current path of the slide-in protective element Λ_ there passes still lower current than in case of occurrence of overvoltage, but this lower current is passing through the current path for a longer period and the accompanying effect is a quick warming of the current path, at embodiment according to the Fig. 2 as well as Fig. 3, happens that by a quick warming of the current path and due to small thickness of the first solder 2J. and possible third solder 23, the first solder 21 and the possible third solder 23 get quickly molten. Both these solders 21., 23 lose their strength, while the first solder 21 continues to lead electric current between electrode 20 and the stranded wire 5. Due to considerably greater thickness of material of the fusible pin 220 or fusible rivet made of the second solder 22, i.e. owing to a diameter of the fusible pin 220 or fusible rivet, the period of melting the fusible pin 22 or fusible rivet in its entire diameter is significantly longer than the melting period of the first solder 2Λ_ and third solder 23, because the fusible pin 220 or fusible rivet made of second solder 22 with lower thermal conductivity than the first solder 2J. becomes gradually warm by conducting the heat from its external perimeter towards its centre. Through a suitable selection of material differentness of the first solder 2J. and the second solder 22 (value of electric and thermal conductivity), in combination with selection of diameter of the fusible pin 220 or the fusible rivet, through selection of positioning of the fusible pin 220 or the fusible rivet in ground plan of the place X of purposeful cutting-off of the current path, through selection of size and shape of contact surfaces, selection of material and size and shape in cross-section of elements of the current path, etc., see Fig. 2a, 2b, 2c, 2d, it is then possible to influence and in its essence directly in advance set the parameters of melting, especially the melting period of the fusible pin 220 or fusible rivet in its entire cross-section and thus to influence or directly in advance set the period between occurrence of the short-circuit and a moment of purposeful cutting-off of the current path in the place X Thus, it could be relatively easily achieved that the period between occurrence of a short-circuit and the moment of purposeful cutting-off of the current path in the place X is longer than the period defined for cutting-off the circuit in overcurrent protection which is positioned before the device for overvoltage protection. The defined maximum period for cutting-off the circuit in overcurrent protection, which is positioned before the device for overvoltage protection usually is 5 seconds, which is also a period prescribed for safe disconnection of all devices connected to the protected circuit. Through this it is secured, that the prescribed cutting-off of the whole protected circuit happens in the defined place, i.e. in the overcurrent protection which is positioned before the overvoltage protection, outside the device for overvoltage protection itself, which enables to apply such a types of devices for overvoltage protection into these distribution lines and it also enables to increase dimensioning of the current protection positioned before the device for overvoltage protection in compliance with required provisions of the regulation mentioned in the background art.

Example of embodiment in the Fig. 4 represents device for overvoltage protection, whose cut-off mechanism with three-stage signalling of protection status comprises ,,T" lever 80, upon which the spring 8 is acting, while the "T" lever 80 is acting with its one arm 801 against the conducting connecting member 81.. The connecting member 8J. is on its end 810. i.e. in the place X of the purposeful cutting-off of the current path, by means of the third solder 23 connected with the angle stop 84, which is by means of the first solder 21 connected with electrode 82 of the non-linear resistance element (varistor). The angle stop 84 and electrode 82 of non-linear resistance element (varistor) are provided with a through hole in which there is positioned the fusible pin 220 or fusible rivet made of the second solder 22.

Embodiment according to the Fig. 4 may be without exerting any inventiveness modified into other concrete structural form falling into the range of this invention, e.g. into a form of embodiment with two-stage status signalling of overvoltage protection, in which there is not present any angle stop 84, and in which the end 810 of connecting member 8J. and electrode 82 of non-linear resistance element are connected by means of the first solder 21 and simultaneously are provided with through hole, in which there is positioned the fusible pin 220 or fusible rivet made of the second solder 22.

Example of embodiment in the Fig. 5 schematically represents a side view of another arrangement of the place X of purposeful cutting-off of the current path, where the current path is formed of electrode 20 of varistor 2, on whose upper side by means of the first solder 2J. there is connected the "L" end 50 of the stranded wire 5. In the zone outside the electrode 20 towards the ¹¹ L" end 50 of the stranded wire 5 in direction of arrow there is acting the disconnecting force F, e.g. developed by means of the not represented spring- loaded lever. Electrode 20 and the "L" end 50 of the stranded wire 5 are provided with a through hole, in which there is positioned the fusible rivet with heads 221 at its ends. The fusible rivet is made of the second solder 22. The heads 221 of the fusible rivet are abutting to outer surfaces of both connected parts 20, 5 of the current path, while the heads 221 of fusible rivet are forming an additional fusible thermal stop with defined parameters of melting. For the purpose that the heads 221. of fusible rivet stops to prevent a controlled and purposeful pushing away the "L" end 50 of the stranded wire 5 from the electrode 20 and thus also the purposeful cutting-off of the current path in the place X it is therefore necessary, that the heads 221 of the fusible rivet are totally molten in their entire height, which owing to a lower thermal conductivity of the second solder 22, out of which the fusible rivet is made, takes a longer period, than it takes to melt the thin layer of the first solder 21.. Once the heads 221 of the fusible rivet are molten in their entire height, the "L" end 50 of the stranded wire 5 is by action of the force F is pushed away from electrode 20, through which the purposeful cutting-off of the current path occurs in the place

The fusible pin 220 or the fusible rivet in the not represented examples of embodiment are of another shape, e.g. a shape of cuboid or of another suitable body. With respect to the fact that into the fusible pin 220 or fusible rivet the heat is transferring only by passing from its immediate surroundings, i.e. especially from the walls of elements of the current path, which are in contact with outer surface of the fusible pin 220 or fusible rivet, it is possible also by shaping of these contact surfaces, e.g. by bevelling edges of the holes or by recess V, to influence (to adjust) passing of heat into the fusible pin 220 or fusible rivet, as it is exemplary represented in the Fig. 3a and 3b.

The fusible pin 220 or fusible rivet is either without a head, or it is provided with round head or lens head or conical shaped or otherwise suitably shaped head, which is either arranged above the level of elements of current path being connected or it is countersunk into elements of current path being connected, similarly as it is at commonly known rivets or screws for machinery production.

From examples of embodiment it is also obvious that the invention may be without any further inventiveness applied in many existing as well as new structures of devices for overvoltage protection, namely because the represented fusible pin 220 or fusible rivet creates additional fusible thermal stop with pre-set parameters of melting. In the scope of skilled person then only by a simple provision there may be added addition thermal stop with pre-set parameters of melting in its whole cross-section into a suitable place X of purposeful cutting-off of the current path in variously designed devices for overvoltage protection, while it is possible that the additional thermal stop with pre-set parameters of melting in the whole cross-section is subject to shear, like in examples of embodiment in the Fig. 2 to 4, or it is in combination subject to tension and bending, like in example of embodiment in the Fig. 5, or it is subject to pressure or bending or it is subject to combination of individual types of tension. Nevertheless at all of these particular structural embodiments and to them corresponding types of embodiments of additional thermal stop with preset parameters of melting there will be preserved the basic principle of this invention, which is adding of additional thermal stop with pre-set parameters of melting in the whole cross-section into whatever place X of purposeful cutting- off of the current path of the device for overvoltage protection. At present, by means of this invention it is also possible to create ROHS compliant device for overvoltage protection without increased costs for special solders, i.e. totally without usage of lead solders, as the structure according to this invention enables to suitably combine the existing unleaded solders with necessary electric and thermal conductivity with unleaded solders with low electric and thermal conductivity. It is also apparent, that the invention without exertion of any inventive work, that means within the scope of abilities of a common specialist, may be used in other particular devices for overvoltage protection, e.g. in such, where the "U" holder and the slide-in protective elements 1 known from embodiment in the Fig. 1 are performed as a single-part body or as a body of totally different shape and size, or at totally new embodiments of the device for overvoltage protection or for purposeful cutting- off of the current path, etc.

In the whole text, under the term "pre-set parameters of melting" mentioned in connection with additional thermal stop there is understood a suitable combination of parameters of the used parts arranged in the current path, like the value of electric and thermal conductivity of the used solders and materials, melting temperatures of used solders and materials, dimensions of additional thermal stop (e.g. diameter of the fusible pin 220 or of fusible rivet), positioning of additional thermal stop (e.g. the fusible pin 220 or fusible rivet) in ground plan of the place X of purposeful cutting-off of the current path, size and shape of contact surfaces, material, size and shape of cross-section of elements in the current path, while combination of these parameters into an advantageous particular embodiment is for a common specialist upon knowledge of this invention obvious and without any further inventiveness simply feasible in principle on any structure of the device for overvoltage protection.

Industrial applicability

The invention is applicable in the sphere of protection of electric circuits against overvoltage.