The invention refers to arrangement of protection in an electric circuit, namely in such a circuit, which includes at least a direct or alternative voltage source together with at least one load, and moreover also a suitable protecting assembly with integrated varistor serving for interruption of the electric circuit as soon as excessive current or overvoltage would occur. In the view of the International Patent Classification, such invention belongs to electricity, namely to basic electric components, which are suitable for overvoltage protection of a direct current circuit, namely for automatic interruption due to undesired changes within the circuit during its regular operation, namely to the class H 01 H3/087.

The purpose of the invention is to protect efficiently either a direct or an alternative current circuit both against excessive current and also against overvoltage, which may occur e.g. by lightning, and moreover, on the one hand mainly in a direct circuit, also to avoid generation of electric arc and its negative impacts within the circuit, and on the other hand to assure a sufficient selectivity of such protection, so that after each interruption it would be clear, if the interruption has happened due to excessive current or due to overvoltage.

Such circuits usually comprise a direct voltage source and at least one electric load, which are electrically connected with each other by means conductors. In the one of said conductors, a fuse with a melting member is in series connected with said source and said load, and is able to protect the circuit against excessive current. In addition to that, an overvoltage protecting assembly is available in said circuit, which is however in parallel connected with the source, load and fuse, and includes a varistor and a further fuse with a melting member, which are in series connected with each other. A suitable overvoltage protecting assembly is disclosed e.g. in WO 2012/026888 Al or EP 2 609 600 Bl.

Said overvoltage protecting assembly, which is known to those skilled in the art as a varistor fuse, is in fact a combination of a varistor and a classic fuse with a melting member, which are each per se tubular shaped and are inserted within each other. Said fuse with a melting member consists of an electric insulating tubular housing, which is on its both sides closed by electric conductive covers, which therefore serve as conductive terminals for transferring the electric current throughout the fuse. The varistor comprises a tubular casing, consisting of suitable material, the electric conductivity of which depends on voltage and is essentially increased as soon as the voltage exceeds a pre-determined value. Two separate coatings, consisting of a conducting material, are each per se arranged on the inner and the outer surface of said varistor casing, and serve as electrodes of said varistor. The melting member within said fuse is on the one hand connected with one of both electric conductive covers and is on the other hand soldered to one of both electrodes of the varistor, while the residual varistor electrode is connected with the residual cover on the fuse, so that an electric circuit is established through the varistor and the fuse, in which the fuse and the varistor are connected in series. As soon as the overvoltage occurs, conductivity of the varistor increases, upon which the excessive electricity is lead to the earth. In situation of excessive current, even if the varistor would be damaged or disabled, the conductivity thereof may increase even if the absence of overvoltage. Various situations may however occur in view of possible electric current values. In the case of low current values within the range of several mA up to 1 A, the varistor casing starts heating gradually, and the solder between the varistor and the melting member of the fuse starts melting, and when it is molten, the contact between the varistor electrode and the melting member of the fuse is interrupted. In the case of medium current value through the varistor, e.g. within the range between 1 A and several 10 A, a so-called M-effect is exploited within a weak area of the melting member due to the presence of solder, which absorbs the heat, which is generated in said weak area of the melting member and also in the varistor, so that the process is then performed essentially quicker as it would be performed without heating of the varistor. In the third situation, when the electric current value is high and is e.g. within the range between 100 A up to several kA, the electric resistance of the varistor as such is not very high, but the melting member in the fuse is functioning in short circuit, so that said melting member is then molten within a relatively short time period, e.g. within several ms. Such interruption of the melting member, in particular when it is surrounded by granulated material like e.g. silica, should in an alternative electric circuit lead to a quite regular interruption thereof. However, in a high- voltage direct circuit, the tendency of generating an electric arc between both interrupted terminals is extremely high, and in addition to that, various gasses may result from such burning of the arc, which may then lead to damages of casing or even to explosion. The arc of course continues burning even after that, which means that it can still bum in a completely unprotected environment.

It is also known in the prior art, that in a direct electric circuit, comprising an electric source, at least one load as well as a classic fuse with a melting member suitable for interruption of said circuit when excessive current or short circuit would occur, which is in serial connected with said load, a required overvoltage protection of said circuit can be achieved by integration of an overvoltage protecting assembly, which is within said circuit connected in parallel to said load and said melting fuse. An apparatus for interruption of an electric circuit is disclosed in EP 2 208 208 Bl and comprises a varistor, in which by overloading or overheating thereof the current is redirected towards a melting fuse, which is in parallel connected therewith. Such measure should prevent the device against damages, when the electric arc would start burning. However, the electric current must always flowing through the varistor, and moreover, a desired selectivity is neither enabled nor even mentioned within said document.

A further electric circuit interrupting device, which is disclosed in EP 2 537 163 Bl, in addition to a plurality of components like PTC, GDT, L comprises a rotating disc for interrupting of the circuit, which essentially increases complexity of the appliance and also makes manufacturing thereof much more difficult.

Still further, a multiple phase energetic system is disclosed in US 2019/0252142, in which each phase is furnished with an overvoltage protecting assembly, consisting of a varistor, an interrupting assembly as well as of a fuse, which are in series connected with each other. Also in in this case, the expected selectivity is not even mentioned.

The most relevant arrangement of such known arrangement of an electric circuit is schematically presented in Fig. 1. Therein, an overvoltage protecting assembly comprises a varistor and a secondary fuse, which are in serial connected with each other. The varistor is suitable for protecting the load against overvoltage, and the fuse serves for short circuit protection of the varistor when it becomes damaged, namely in a so called thermal runaway. Such approach is however connected with lack of selectiveness between the primary and secondary fuse. Each required technical performances of the primary fuse are determined by cross-section of the electric conductor towards the load, while the technical performances of the secondary fuse depend on varistor and corresponding level of overvoltage pulses, which is known to those skilled in the art as “surge rating”. Requirements, which are dictated by determining of primary and secondary fuse are therefore in contradiction with each other. In the primary fuse, a required resisting against overvoltage pulses dictate as large as possible cross-section of the melting member, while the interrupting efficiency is much better when a melting member with a smallest possible cross-section is used. Consequently, increasing of crosssection on the primary fuse leads to slowing-down its reactivity to potential irregularities within the electric circuit. As already mentioned, a thermal protection of the varistor can be achieved by means of a resilient contact member, which is soldered to the electrode on the varistor casing. When the current is flowing through such spring-like contact, the speed of its reacting is downward limited by value of the current, which has crucially impacts the reaction speed. As soon as the solder is heated to a pre-determined value and starts melting, the contact terminals are separated apart from each other. In a high voltage direct circuit, due to relatively slow deflecting of the contact and also due to a relatively short distance between both separated contact terminals, a huge problem may occur in connection with the electric arc, which starts burning there-between. In addition to that, varistors are pretty sensitive components in view of temperature, since the highest allowable and still acceptable temperature thereof in regular operating conditions should not exceed approx. 100°C.

When bearing in mind a required selectivity, it would be therefore desired and expected in such a circuit, that by each overloading, even when less current would flow through the secondary fuse than the through the primary fuse, the secondary fuse would have to become interrupted, but not also the primary one. By overvoltage, the excessive electric energy is via the varistor lead to the earth, and by damaged or destroyed varistor only the secondary fuse would become interrupted, but not also the primary one.

The present invention refers to arrangement of protection in an electric circuit, which comprises an electric voltage source, which is via primary electric conductors electrically connected with a load, wherein an electric fuse with a melting member is integrated in one of said primary conductors, and wherein said primary conductors are bridged by a secondary conductor, which includes an overvoltage protecting assembly with at least one varistor and a thermal sensor arranged in thermal conductive contact therewith, and moreover also at least one further electric resistant fuse with a melting member, which is electrically connected with said varistor.

The invention however proposes that said bridging electric conductor of the overvoltage protecting assembly, by which said primary electric conductors are bridged in the area between said voltage source and said load, in addition to said varistor with a belonging thermal sensor also includes a three-point switch with an interrupting member, which is in pre-determined conditions by means of a suitable triggering means displaceable from its initial first position, in which said bridging electric conductor of the overvoltage protecting assembly is conductive, into its second terminal position, in which said bridging electric conductor of the overvoltage protecting assembly is interrupted, while the electric resistance fuse is integrated in a parallel branch of said bridging conductor, which extends from the one of said primary conductors of the electric circuit and is ended at a distance apart from each residual primary conductor and also at a suitable pre-determined distance apart from the interrupting member of said switch, wherein said distance is determined in such a manner that the electric circuit througf said parallel branch and the belonging fuse is established only upon interruption of the switch within said bridging conductor, namely upon displacement of the interrupting member of the switch into its second terminal position in contact with said parallel branch.

A preferred embodiment of the invention further proposes that said triggering means, by means of which in pre-determined conditions said interrupting member of the switch is displaced by shifting it from its initial first position, in which the bridging conductor is conductive, into its second end position, in which the bridging conductor is interrupted and the interrupting member of the switch is brought in contact with a parallel branch with integrated fuse, is conceived as a pyrotechnic switch with an actuator, which can be triggered on the basis of a signal received either from a magnetic Reed (NO) switch, which is integrated within a belonging electrically conductive branch between said bridging conductor and an actuator, or from a thermal sensor, which is arranged in a thermal conductive contact with the varistor and is integrated between said bridging conductor and the actuator within an electric conductive branch, which extends parallel with the previously mentioned branch with integrated magnetic Reed (NO) switch. During the regular operation of the electric circuit, said magnetic Reed (NO) switch is open, but as soon as an irregularity and related change of the magnetic field occurs, said magnetic switch is closed and the actuator is triggered, by which also the interrupting member in the switch is displaced, and wherein also said thermal sensor (50’) during the regular operation of the electric circuit (1) is interrupted, but becomes electrically conductive as soon as the temperature of the varistor reaches a pre-determined value.

Said Reed (NO) switch is preferably selected as a timer relay with a time delay of at least 1 ms, but limited to 2 ms at maximum, while said electric voltage source is either a direct voltage (DC) source or an alternative voltage (AC) source. The invention will also be described in more detail on the basis of an embodiment, which is schematically presented in the accompanied drawing, wherein

Fig. 1 presents a previously discussed state of the art arrangement of protection of an electric circuit according; and

Fig. 2 presents just one of possible embodiments of the invention in situation after interruption of the three-point switch, in which a pyrotechnic switch is used for the purposes of achieving said interruption.

As mentioned, the present invention refers to arrangement of protection in an electric circuit 1. Also the electric circuit according to Fig. 2 comprises an electric voltage source 2, which is via primary electric conductors 11, 12 electrically connected with a load 3, wherein an electric resistant fuse 4 with a melting member is integrated in one of said primary conductors 11, 12. Similar like in the prior art arrangement (Fig. 1) also in this case said primary conductors 11, 12 are bridged by a secondary conductor 13, which includes an overvoltage protecting assembly 5, which comprises at least one varistor 50 and a thermal sensor 50’ arranged in thermal conductive contact therewith, and moreover also at least one further electric resistant fuse 55 with a melting member, which is electrically connected with said varistor 50.

However, said bridging electric conductor 13 of the overvoltage protecting assembly 5, by which said primary electric conductors 11, 12 are bridged in the area between said voltage source 2 and said load 3, in addition to said varistor 50 with each belonging thermal sensor 50’ also includes a three-point switch 6 with an interrupting member 60, which is in pre-determined conditions by means of a suitable triggering means 7 displaceable from its initial first position, in which said bridging electric conductor 13 of the overvoltage protecting assembly 5 is conductive, into its second terminal position, in which said bridging electric conductor 13 of the overvoltage protecting assembly 5 is interrupted. Said electric resistance fuse 55 is integrated in a parallel branch 13’ of said bridging conductor 13, which extends from the one of said primary conductors 11, 12 of the electric circuit 1 and is ended at a distance apart from each residual primary conductor 11, 12 and also at a suitable pre-determined distance apart from the interrupting member 60 of said switch 6. Said distance is selected in such manner that the electric circuit throughout said parallel branch 13’ and the belonging fuse 55 is established only upon interruption of the switch 6 within said bridging conductor 13, namely upon displacement of the interrupting member 60 of the switch 6 into its second terminal position in contact with said parallel branch 13, wherein said distance is at least such, that in particular when a DC electric source is used, formation of an electric arc should be avoided upon interruption of the bridging conductor 13.

In a preferred embodiment it is due to a required sensibility in view of reacting to electric overloading further foreseen that said triggering means 7, by means of which in pre-determined conditions said interrupting member 60 of the switch 6 is displaced by shifting it from its initial first position, in which the bridging conductor 13 is conductive, into its second end position, in which the bridging conductor 13 is interrupted and the interrupting member 60 of the switch is brought in contact with a parallel branch 13’ with integrated fuse 55, is conceived as a pyrotechnic switch with an actuator 71. Said actuator 71 is triggered on the basis of a signal, which is received either from a magnetic Reed (NO) switch 75, which is integrated within a belonging electrically conductive branch 13” between said bridging conductor 13 and an actuator 71, or from a thermal sensor 50’, which is arranged in a thermal conductive contact with the varistor and is integrated between said bridging conductor 13 and the actuator 71 within an electric conductive branch 13”’, which extends parallel with the previously mentioned branch 13” with integrated magnetic Reed (NO) switch 75. Said Reed (NO) switch 75 is open i.e. interrupted during the regular operation of the electric circuit 1 , but as soon as an irregularity and related change of the magnetic field within the electric circuit 1 occurs, said magnetic switch is closed, which leads to triggering of the actuator 71. Said thermal sensor 50’ is during the regular operation of the electric circuit 1 also interrupted, but becomes electrically conductive as soon as the temperature of the varistor 50 reaches a pre-determined value, upon the current path is established towards the actuator 7 of the switch 6.

Said electric voltage source 2 is either a direct voltage (DC) source or an alternative (AC) voltage source, while said Reed (NO) switch 75 is a timer relay with a time delay of at least 1 ms, but preferably below 2 ms.

Thanks to previously disclosed features, reaction of such arrangement of protection to various irregularities is much more reliable than in previously available prior art solutions.

When overvoltage occurs within the circuit 1, the current path is established via the bridging conductor 13, namely via the interrupting member 60 of the conductive switch 6 and the varistor 50. The cross-section surface of the terminals in the area of said switch is much larger, namely at least for 10-times larger, than the cross-section of the melting member of the fuse 55, so that unintentional interruption of said current path is practically impossible. Duration of the current pulse in the case of lightning in an atmospheric discharge usually takes up to 1 ms, so that said Reed (NO) switch 75 is a time relay with time delay of at least 1 ms, and up to 2 ms at maximum, by which activation of overcurrent protection is avoided, whenever just a short-time voltage increasing occurs.

Whenever a short circuit is established through the varistor 50, a relatively strong magnetic field is generated around the conductor 13, which leads to closing of the Reed switch 75, upon which the electric current may flow towards the triggering means 7, which initiates displacement of the interrupting member 60 in the switch 6 from its initial position, in which the conductor 13 is conductive, into its second end position in contact with the parallel branch 13’ with integrated fuse 55, upon which the conductor 13 through the varistor 13 remains interrupted. The fuse 55 becomes overloaded and interruption of its melting member leads to interruption of the circuit throughout said parallel branch 13’. In such case, a short-circuit current needs not to flow through a current sensor, so that the reacting time is quite short, and also the Reed switch 75 is not quire reluctant. When compared with prior art solutions with a classic melting fuse according to Fig. 1, in this case the nominal current value does not dictate the capacity of current sensor.

If the varistor 50 becomes heating, the thermal sensor 50’ due to its thermally conductive relationship with the varistor 50 establishes a current path towards the actuator 71 of the triggering means 7, which then displaces the interrupting member 60 of the switch 6 from its first initial position, in which the conductor 13 through the varistor 50 is uninterrupted, into its second end position in contact with the parallel branch 13’ with integrated fuse 55, upon which the conductor 13 through the varistor remains interrupted. Also in such case the fuse 55 becomes overloaded, and interruption of its melting member leads to interruption of the circuit through said parallel branch 13’, wherein the current does not flow through the thermal sensor 50’, so that also the capacity of said thermal sensor 50’ is not dictated by current value, so that the thermal sensor 50’ can be therefore quite small and therefore also much more sensitive.

Thanks to the previously stated features, by irregular operation of the electric circuit 1 a reliable selectivity between the fuse 5 and protection of the varistor 50 against overcurrent is always enabled. In addition to that, a low thermal inertia of the thermal sensor 50’ and small dimensions of the Reed switch 75, when compared with prior art solutions, allow much quicker reaction, and the protecting assembly 5 can easily react to each irregularity within the electric circuit 1. Besides, the capacity i.e. nominal value of the fuse 55 in the protecting assembly 5, which is much lower than the nominal value of the fuse 4 in one of the primary conductors 11, 12, is practically independent on overvoltage pulse, so that such solution by means of said protecting assembly 5 is quite universal and is suitable for use in various solutions. Such arrangement of protection of an electric circuit 1 according to the invention further excels in a high interrupting capacity regardless to requirements in view of resistance to overvoltage pulses. Contrary to the fuse 4 and the varistor 50, which is in prior art solutions (Fig. 1) in series connected therewith, in the proposed arrangement (Fig. 2) the cross-section of terminals in the area of the interrupting member 60 in the switch 6 has no impact to interrupting capacity, and moreover, the reaction time in view of overcurrent protection is independent on intensity of the overvoltage pulse. Consequently, arrangement of protection according to the invention allows managing of extremely high overvoltage pulses without undesired switching-off due to overvoltage, which is achieved by sufficiently large cross-section of the interrupting member in the switch 6, by which simultaneously a suitable resistance against wearing or degradation during a long-term use is achieved.