text: fig1 shows a first exemplary embodiment of the device 1 according to the invention , which has two connection terminals 2 and 3 , with which the device 1 is connected in series into a pole 4 , in other words a conductor , of a dc voltage network , which is not illustrated in any further detail in the figures . the pole 4 has the line inductance l n illustrated schematically in fig1 . the device 1 has an operating current path 5 , which is bypassed by a disconnection branch 6 . a mechanical switch 7 which opens quickly and an inductive component part 8 having the inductance l 3 are arranged in the operating current path 5 . the inductive component part 8 is a coil or an inductor , for example . a power switching unit 9 is arranged in the disconnection branch 6 , said power switching unit having a series circuit comprising two - pole sub modules 10 , of which only one sub module 10 is illustrated in fig1 . each sub module 10 has an energy store 11 , with which two series circuits 12 and 13 , each comprising two power semiconductor switches 14 which can be switched on and off , such as igbts , igcts or gtos , for example , are connected in parallel . a freewheeling diode 15 is connected in parallel in opposition with each power semiconductor switch 14 . a first sub module connection terminal 16 is connected to a potential point between the power semiconductor switches 14 of the first series circuit 12 , and the second sub module connection terminal 17 is connected to the potential point between the power semiconductor switches 14 of the second series circuit 13 . thus , the capacitor voltage drop uc across the energy store , in this case a capacitor 11 , a zero voltage or else the inverse capacitor voltage — uc can be generated at said sub module connection terminals 16 and 17 . in addition , the power switching unit 9 comprises diverters ( not illustrated in the figures ) for dissipating the energy released during switching . if the power semiconductor switches 14 of the sub modules 10 are driven in such a way that a zero voltage is generated at the sub module connection terminals 16 and 17 , a direct current can flow via the entire power switching unit 9 . owing to the ohmic resistance of the numerous series - connected power semiconductors 13 , 14 , such a high voltage drop is set across the power switching unit 9 , however , that a direct current flows exclusively via the operating current path 5 when the mechanical switch 7 is closed . in the exemplary embodiment shown in fig1 , the current flows from the left to right , i . e . from the connection terminal 2 to the connection terminal 3 . if there is a short circuit in the direction of said current flow downstream of the device 1 , a steep rise in the current flowing via the operating current path 5 takes place . the thus induced change in the magnetic flux in the inductive component part 8 induces a voltage in the inductive component part 8 , which ensures that the short - circuit current commutates from the operating current path 5 into the disconnection current branch 6 . the inductive component part 8 has an inductance l 3 which is low in relation to the line inductance l n , but high in relation to the inductance of the power switching unit 9 . the constant load current continues to flow in the operating current path 5 even in the event of a short circuit , however . this load current can likewise be commutated into the disconnection branch by corresponding driving of the power switching unit 9 by virtue of such a voltage being set at the power switching unit 9 that a circulating current flows in the mesh formed from the operating current path 5 and the disconnection branch , which circulating current is in opposition to the operating current in the mechanical switch 7 and to the short - circuit current components flowing there and ensures a zero crossing of the total current . the mechanical switch 7 can then open virtually in the deenergized state . the power which is required by the power electronics 9 to generate a current zero crossing in the mechanical switch 7 is therefore considerably reduced by the inductive component part 8 . fig2 shows a further exemplary embodiment of the device 1 according to the invention , which differs from the exemplary embodiment illustrated in fig1 in that the commutation means have two inductive component parts 18 and 19 , which are in turn both in the form of a coil or inductor . the inductive component part 18 is arranged between one of the connection terminals 2 and a first junction 20 between the operating current path 5 and the disconnection current branch 6 . the further inductive component part 19 is only in the disconnection branch 6 . it can be seen that the inductive component parts 18 and 19 are coupled inductively to one another by a core 21 consisting of a ferromagnetic material , in this case iron . the component part 19 has a winding sense which is opposite the winding sense of the component part 18 . in other words , the inductive component parts 18 and 19 have opposite winding directions . in this case , the number of windings of the two inductive component parts 18 , 19 can be the same . given a constant load current , which flows from left to right in fig2 , as indicated , the inductance of the inductive component parts 18 and 19 is irrelevant . owing to the voltage drop across the power switching unit 9 , the constant load current flows via the operating current path 5 , however . in this normal state , the mechanical switch 7 is of course closed . in the event of a short circuit downstream of the device 1 in the direction of current flow , a current rise di / dt of greater than zero arises . if the changing current flows both through the inductive component part 18 and through the inductive component part 19 , said current generates a change in the magnetic flux , which is compensated for in the core 21 , via which the inductive component parts 18 and 19 are coupled . for this current , therefore , virtually no inductance is effective . a current flowing via the operating current path , on the other hand , flows exclusively through the inductive component part 18 with the inductance l 1 . this inductance l 1 is effective for said current and ensures commutation of said current into the disconnection branch 6 . the power switching unit 9 can now generate a circulating current , which only reduces the remaining substantially constant component of the load current to zero by generation of a countercurrent in the switch 7 . if the number of windings of the inductive component part 19 in the disconnection branch 6 is less than the number of windings of the inductive component part 18 , the inductance l 2 of the inductive component part 19 in the disconnection branch is less than the inductance l 1 of the inductive component part 18 . a change in current , such as in the case of a current rise owing to a short circuit in l 1 , for example , results in a change in the magnetic flux in the core 21 . a magnetic flux is also induced in the inductive component part 19 , and this counteracts the cause thereof . owing to the lower turns number of the inductive component part 19 , more current will flow there than via the inductive component part 18 , with the result that the magnetic fluxes in the core 21 can compensate for one another . the current therefore increases to a greater extent in the component part 19 than the fault current . the additional current rise causes a circulating current in the mesh formed by the disconnection branch 6 and the operating current path 5 . the circulating current is effected by the lower inductance l 2 of the inductive component part 19 in the disconnection branch . it flows in the same direction as the short - circuit current in the disconnection branch 6 . however , in the operating current path 5 , it flows in the opposite direction to the load current flowing there and the short - circuit current component which is lower there . this circulating current therefore results in a reduction in the current in the mechanical switch 7 . given suitable dimensioning of the component parts , the current via the switch 7 can be virtually suppressed . the additional intervention of power electronics of the power switching unit 9 for generating a current zero crossing in the switch 7 is therefore unnecessary . the commutation is effected purely by passive components , namely by the inductive component parts 18 and 19 and the inductive coupling thereof via the core 21 . fig3 shows a further exemplary embodiment of the device 1 according to the invention , which differs from the exemplary embodiment shown in fig2 in that the second inductance 19 is no longer arranged in the disconnection branch 6 , but in the operating current path 5 . in this variant , therefore , the constant load current flows in the direction indicated by the arrows in fig5 in the low - resistance operating current path 5 and , as in the other exemplary embodiments , causes a certain bias of the core 21 . a current gradient brought about owing to a short circuit , for example , in the inductive component part 18 with the inductance l 1 effects a change in magnetic flux in the core 21 , as a result of which a current is set in the inductive component part 19 with the inductance l 2 , which current is in opposition to its cause and suppresses a change in flux in the core 21 . if the inductive component part 18 and the inductive component part 19 each have the same number of windings , a change in current di / dt in the inductive component part 18 with the inductance l 1 generates a change in magnetic flux . in the inductive component part 19 with the inductance l 2 , the same current rise is set owing to the same turns number , in order that the two magnetic fluxes in the core 21 compensate for one another . the current rise in the inductive component part 19 and therefore also in the switch 7 is negative with respect to the constant load current and therefore sooner or later results in a current equal to zero in the switch 7 . the lower the ratio between the inductances l 2 / l 1 , the earlier a current zero crossing in the switch 7 arises . in this case too , the commutation of the current from the operating current path 5 into the disconnection branch 6 can be assisted by the power electronics of the power switching unit 9 . given suitable dimensioning of the inductive component parts 18 and 19 , however , commutation exclusively by means of the passive inductive component parts 18 and 19 is possible . fig4 shows a further variant of the invention . it can be seen that , in addition to the component parts already described in connection with fig2 , which are configured correspondingly here or can be configured correspondingly here , further inductive component parts are provided . thus , a second inductive component part 30 in the form of a coil or inductor with the inductance l 3 is arranged between the second connection 3 and the second junction 22 . the inductive component part 30 is inductively coupled to an inductive component part 31 arranged in the disconnection current branch 6 , which component part 31 has the inductance l 4 . again a core 21 consisting of a ferromagnetic material is used for the inductive coupling . the mode of operation of the two additional inductive component parts 30 and 31 corresponds to the mode of operation already described of the inductive components 18 and 19 . the inductive component part 31 can of course also be arranged in the operating current path 5 within the scope of the invention , on the basis of the exemplary embodiment shown in fig3 . fig5 shows , with respect to a device 1 shown in fig2 , schematically with the aid of the dashed line , the profile of the current i s in the operating current path 5 from left to right in the event of a short circuit at time t 0 . in fig5 , t 0 is at 2 ms . the current profile i s experiences a zero crossing very quickly , i . e . after 500 μs , at t 1 , where the following applies : t 1 = t 0 + 500 μs . the 500 μs are in this case to be understood purely by way of example . however , it is essential that the duration after the occurrence of the fault up to the first current zero crossing is much less than 1 ms . since the customary switching delay time is approximately 2 ms , the first current zero crossing cannot be used to open the mechanical switch 7 virtually in the deenergized state . however , it is apparent from fig5 that a second current zero crossing arises at time t 3 . since a very large amount of time can pass between t 0 and t 3 , it is not possible to wait for the second current zero crossing in some applications . as has already been mentioned several times , however , active commutation means can be part of the disconnection unit 9 , drive a circulating current in the mesh formed from the operating current path 5 and the disconnection current branch 6 , which circulating current is in opposition to the current in the switch 7 . this circulating current accelerates the occurrence of the second current zero crossing . the current profile i s in the switch 7 with active commutation is illustrated schematically in fig5 with the aid of the continuous line . it can be seen that a second current zero crossing can thus be brought about even after 2 ms , wherein the current then remains virtually at zero . furthermore , fig5 shows a third current profile by means of a dotted line . such a current profile can be obtained using a device as shown in fig6 , which largely corresponds to the exemplary embodiment shown in fig2 , but a thyristor 32 is arranged in the operating current path 5 , with a surge arrestor 33 connected in parallel with said thyristor . the thyristor 32 is a controllable power semiconductor switch which cannot be actively switched off , however . during rated operation , the current via the operating current path 5 and therefore via the thyristor 32 and the mechanical switch 7 flows from connection 2 to connection 3 , for example . during rated operation , the thyristor 32 is permanently refired in order to ensure its conductivity . in the event of a short circuit downstream of the device 1 according to the invention in the direction of current flow , a current rise d i arises after d t , with the result that the inductive component parts 18 and 19 which are coupled inductively to one another via the ferromagnetic core 21 ensure a quick current zero crossing at time t 1 ( see fig5 ), i . e . already after 500 μs . since the thyristor 32 is not conductive in the reverse direction , the current flowing via the operating current path 5 remains at zero after time t 1 . if the firing commands to the thyristor 32 are suppressed , the current flow remains equal to zero even when a positive current wishes to be set again , i . e . from connection terminal 2 to connection terminal 3 . a corresponding hold - off time for the thyristor 32 is provided . the surge arrestor 33 , which is connected in parallel with the thyristor 32 , is a varistor , for example . this varistor is dimensioned such that , in the case of the occurrence of trv , practically no current flow via the surge arrestor 33 is permitted . the mechanical switch in this exemplary embodiment can be opened in the fully deenergized state since a current flow from the thyristor is prevented . in this case , the thyristor only needs to be dimensioned for low voltages , voltages of markedly less than the dc rated voltage . fig7 shows the exemplary embodiment shown in fig6 , but the circuit can be used for both current directions . for this purpose , a further thyristor 34 is provided , which is connected in parallel in opposition to the first thyristor 32 . the mode of operation of the thyristors 32 , 34 corresponds to the mode of operation explained in connection with the device 1 shown in fig6 , however , wherein the firing of the thyristors is interrupted in any case in the event of a fault . fig8 shows a further configuration of the device 1 according to the invention , which corresponds to a so - called h circuit . in this exemplary embodiment of the device 1 according to the invention , two mechanical switches 7 and 23 are arranged in the operating current path 5 . furthermore , a shunt current branch 24 is provided , which extends connected in parallel with the operating current path 5 between the connection terminals 2 and 3 , wherein two junctions 20 and 22 are formed between the operating current path 5 and the shunt current branch 24 . two diodes 25 and 26 are arranged as power semiconductors with mutually opposite forward direction in the shunt current branch 24 . a central branch potential point 27 of the operating current path 5 is formed between the mechanical switches 7 and 23 of the operating current path 5 . the central branch potential point 28 of the shunt current branch is between the two diodes 25 and 26 . the disconnection branch 6 comprising the power switching unit 9 , which otherwise corresponds to the power switching unit 9 shown in fig1 to 6 , extends between said two central branch potential points 27 and 28 . inductive component parts 8 which have the inductance l 3 and l 3 ′, respectively , are again arranged between the central branch potential point 27 of the operating current path 5 and each mechanical switch 7 and 23 . during normal operation , owing to the high voltage drop of the constant load current across the power switching unit 9 , the current flows only via the operating current path 5 , therefore through both switches 7 and 23 and the inductive component parts 8 arranged there . in the event of a current rise in the event of a short circuit , both inductive component parts 8 wish to prevent a current rise . owing to the diodes 25 and 26 arranged in the shunt current branch 24 , however , this can only take place in the branch whose switch is intended to be opened . if the current flows from connection terminal 2 to connection terminal 3 , this is the mechanical switch 23 . the current then commutates into the disconnection branch 6 and flows via the diode 26 to the connection terminal 3 . fig9 shows a further exemplary embodiment of the device 1 according to the invention . this exemplary embodiment differs from the variant shown in fig7 in that , in addition to the two inductive component parts 8 in the operating current path 5 , a further inductive component part 29 is arranged in the disconnection branch 6 , wherein all three inductive component parts 8 and 29 are coupled inductively to one another via a core 21 consisting of a ferromagnetic material . during normal operation , the constant load current flows via the operating current branch 5 comprising the switches 7 and 23 . in the event of a current rise as a result of a short circuit , the inductive component parts 8 and 29 are effective , wherein , owing to the orientation of the diodes 25 and 26 , the current commutates into the disconnection branch 6 . owing to the inductive component part 29 in the disconnection branch 6 , which has an opposite winding direction and the same turns number with respect to the inductive component part 8 in the operating current path , an effective inductance of zero is set owing to the inductive coupling by the core 21 for the current commutated into the disconnection branch 6 . this favors commutation of the current into the disconnection branch 6 even more . in a further variant which is not illustrated in the figures , the third inductive component part 29 in the disconnection branch 6 is missing , wherein the inductive component parts 8 in the operating current path 5 are coupled to one another . in this variant , the current rise in the switch 23 to be opened is prevented . the constant load current also continues to flow via the switch 23 , however . in this configuration , therefore , the power electronics of the disconnection unit 9 needs to ensure active commutation of the remaining current flowing via the switch 23 . this can take place , for example , by inducing a current flowing via the junctions 27 , 28 and 22 , the direction of said current being in the opposite direction to the constant load current in the operating current path .