WO1999031692A1

WO1999031692A1 - A device for switching

Info

Publication number: WO1999031692A1
Application number: PCT/SE1998/001190
Authority: WO
Inventors: Per Skytt; Hans Bernhoff; Jan Isberg; Mats Ekberg
Original assignee: Abb Ab
Priority date: 1997-12-17
Filing date: 1998-06-17
Publication date: 1999-06-24
Also published as: WO1999034496A1; AU8051798A; WO1999031692A8; AU8051898A

Abstract

A device for switching electric power comprising at least one electric switching arrangement. The latter comprises at least one first switch element (10) which represents an electrode gap. The latter is convertible between an electrically substantially insulating state and an electrically conducting state. Furthermore, the switch element comprises means (25) for causing or at least initiating the electrode gap or at least a part thereof to assume electrical conductivity. The means (25) for causing or at least initiating the electrode gap to assume conductivity are adapted to supply energy to the electrode gap in the form of radiation energy to bring the gap or at least a part thereof to the form of a plasma by means of this radiation energy. A second switch element (40) of a mechanical type is arranged to close a shunt line parallelly over the first switch element, such that the voltage loss over the latter is minimised.

Description

A switching device

FIELD OF THE INVENTION AND PRIOR ART

This invention is related to a device according to the pre- characterising part of enclosed claim 1. The device according to the invention may be used in any connection for switching purposes. Particularly preferred are applications where high power is to be switched. In reality, high voltage connections and electric power transmission applications are involved. A preferred, but not restricting, application of the device according to the invention is to protect, in an electrical power plant, an electrical object from the consequences of faults, primarily as far as current is concerned but also voltage. Besides, the invention comprises a method for protection of the object.

The electric object in question may be of arbitrary nature as long as it is contained in an electric power network and requires protection against fault -related over-cur- rents, i.e. in practice short-circuit currents. As an example, it may be mentioned that the object may be formed by an electric apparatus having a magnetic circuit, e.g. a generator, transformer or motor. Also other objects may be in question, e.g. power lines and cables, switch gear equipment etc. The present invention is intended to be applied in connection with medium and high voltage. According to IEC norm, medium voltage refers to 1-72,5 kV whereas high voltage is >72,5 kV. Thus, transmission, sub- transmission and distribution levels are included. In prior power plants of this nature one has resorted to, for protection of the object in question, a conventional circuit -breaker (switching device) of - such a design that it provides galvanic separation on breaking. Since this circuit breaker must be designed to be able to break very high currents and voltages, it will obtain a comparatively bulky design with large inertia, which reflects itself in a comparatively long break-time. It is pointed out that the over-current primarily intended is the short-circuit current occurring in connection with the protected object, for instance as a consequence of faults in the electric insulation system of the protected object. Such faults means that the fault current (short-circuit current) of the external network/equipment will tend to flow through the arc. The result may be a very large breakdown. It may be mentioned that for the Swedish power network, the dimensioning short-circuit current/fault-current is 63 kA. In reality, the short-circuit current may amount to 40-50 kA.

A problem with said circuit-breaker is the long-break time thereof. The dimensioning break-time (IEC-norm) for completely accomplished breaking is 150 milliseconds (ms) . It is associated to difficulties to reduce this break-time to less than 50-90 ms depending upon the actual case. The consequence thereof is that when there is a fault in the protected object, a very high current will flow through the same during the entire time required for actuating the circuit-breaker to break. During this time the full fault current of the external power network involves a considerable load on the protected object. In order to avoid damage and complete breakdown with respect to the protected object, one has, according to the prior art, constructed the object so that it manages, without appreciable damage, to be subjected to the short-circuit current/fault current during the break-time of the circuit breaker. It is pointed out that a short-circuit current (fault current) in the protected object may be composed of the own contribution of the object to the fault current and the current addition emanating from the network/equipment. The own contribution of the object to the fault current is not influenced by the functioning of the circuit -breaker but the contribution to the fault current from the network/equipment depends upon the operation of the circuit breaker. The requirement for constructing the protected object so that it may withstand a high short-circuit current/fault current during a considerable time period means substantial disadvantages in the form of more expensive design and reduced performance.

As pointed out hereinabove, the invention is, however, not only restricted to protection applications. In other switching situations it is a disadvantage to have to resort to rather costly and bulky switching devices when high power is involved, for instance banks of semi- conductor components, in order to manage the switching function aimed at. Today's semi-conductor component, which preferably is produced in silicon even if other materials may be in question, has for practical reasons a restriction as to the maximum electric field strength which the component may withstand before an electrical breakthrough occurs in the semi-conductor material. This implicates immediately corresponding restrictions of the maximum electric voltage that the component may be subjected to. In particular in high voltage connections, one is therefore forced to couple in series (stack) a large number of semiconductor components in such a way that none of the components contained in the stack is subjected to a voltage which is above a safe level for the component.

Furthermore, complications may occur in the design of the semi-conductor component in that the semi-conductor mate- rial in itself endures to be subjected to, for instance compared with atmospheric air, very high electric field strength. The same is, however, not valid for the insulating material which necessarily must be present between those electrodes externally of the semi-conductor material between which the high voltage is placed. This also involves a restriction: In design of a semi-conductor component for high voltage use a careful balancing must be made between the electrical field strength in the semi- conductor material and the electric resistance in the surrounding insulating medium.

In several applications in electric power plants the components included therein are subjected to not only high electrical voltages but also to large electrical currents. When a current passes through a component having a certain resistance, considerable amounts of thermal energy (so- called Joule heat energy) which is proportional to the resistance in question and to the square of the current. Since each semi-conductor component has a small but negligible resistance, the maximum current that the component stack may endure is restricted. If very large currents are to be conveyed by the semi-conductor components one is forced to convey the current through several identical parallel current paths. The number of semi-conductor components increases, accordingly, multiplicatingly .

At high voltages and at large currents, a large number of semi-conductor components must be used. This results imme- diately in a lower reliability since all components must function in order to make the electric power plant, such as for instance a HVDC valve, to be in operation.

The fact that a large number of semi-conductor components are stacked means that they must be controlled with very high precision in time. Erroneous "timing" could for in- stance result in a far too high a voltage being applied over an individual component causing a certain failure and appendant removal from operation of the entire plant. The "timing" problem increases, of course, if a plurality of parallel current paths must be provided and synchronized.

OBJECT OF THE INVENTION

The primary object of the present invention is to provide a switching device better suited for switching high electric power in a rapid manner and to a comparatively low cost than switching devices used today, and where a very rapid operation of the switching arrangement should be associated with a good electric current closure, i.e. a low voltage loss across the switching arrangement.

A secondary object of the present invention is to devise ways to design the device and the method so as to achieve better protection for arbitrary objects and, accordingly, a reduced load on the same, a fact which means that the objects themselves do not have to be designed to withstand a maximum of short-circuit currents/fault currents during relatively long time periods.

SUMMARY OF THE INVENTION

According to the invention, the switching arrangement is designed in accordance with the characterizing part of claim 1. Since the electrode gap of the switch element is brought to an electrically conducting state by supplying energy directly to the electrode gap in order to establish ionisa- tion/plasma in the electrode gap, conditions are created for a very rapid operation of the switching arrangement according to the invention. The ionisation/plasma in the electrode gap causes/initiates an electrically conducting plasma channel having a very high conductivity so that very large cur- rents may be conveyed and this more specifically during relatively prolonged time periods without negative effects, which -is in direct contrast to conventional semi-conductor art. According to the invention, the switching arrangement also comprises a second switch element adapted to establish a galvanic contact between its contact members, the first and second switch elements being connected in parallel . Accordingly, this means that, when a closure of the switching arrangement is desired, the very rapid first switch element will be closed firstly as a conducting plasma channel is formed in the electrode gap thereof. Because the conductivity in the plasma channel is good in itself, the voltage loss across the first switch element becomes comparatively low, although this might be remarkable, and in some con- texts, undesired or even unacceptable. Here, advantage is taken of the second switch element, which, due to its somewhat slower operation closes for establishing a galvanic contact after that the first switch means has been activated to close and a relatively low voltage loss is present across the latter, something that, in its turn, means that the galvanic contact between the contact members of the second switch element results in a total closure in reality upon the closure of the second switch element, i.e. no or nearly no voltage loss at all across the switching arrangement.

According to the invention, the secondary object discussed above is achieved by means of the content of claim 16. It is preferred that the switching arrangement in the form of an over-current reducing arrangement, activateable for reduc- tion of over-current by aid of an arrangement for detecting over-current conditions, is connected to the electric power plant in order to protect the object. Thereby, according to a preferred embodiment, the switching arrangement can form an over-current diverter for diverting over-currents to earth or to any other unit having a relatively low potential. Accordingly, the invention is based upon the principle of taking advantage of a rapidly operating switching arrangement which reduces the over-current without effectuating any real breaking thereof , to such an extent that the protected object will be subjected to substantially reduced loads and thereby a smaller amount of damages. The reduced over- current/fault current means, accordingly, that the total energy injection into the protected object will be substan- tially smaller than in absence of the switch means according to the invention.

The solution according to the invention based upon a switch means implies a particularly advantageous fulfilling of de- mands which may be set up in order to achieve a satisfactory protection function. Thus, a very rapid triggering may be achieved by the switch means so that occurring fault -related over-currents with a very small delay in time will be diverted via the switch means as soon as the electrode gap has adopted an electrically conductive condition. It is pointed out that the term "triggering" in this connection means bringing the switch means into an electrically conducting state. By means of the arrangement of the switch means, said switch means may easily be dimensioned to be able to conduct very large currents. In order to obtain a satisfactory protection function it is, namely, desirable that the current conducting channel, which is established through the switch means, has a very low resistance. This means the largest possible strain-relieving of the object, which is to be pro- tected from fault-currents . Besides, a switch means according to claim 1 may with a small effort be caused to function with a particularly high triggering safety. The triggering must not, in order to divert occurring fault -currents as soon as possible, therefore fail in a critical situation. The switch means according to the invention gives on the other hand rise to the possibility to dimensioning in order to achieve a very high electric strength in a non-triggered condition. The probability for a spontaneous breakthrough is thus to be at a minimum. It is especially preferred to thereby use at least one laser for triggering.

Preferable developments with respect to a.o. the means for supplying radiant energy to the electrode gap are defined in the enclosed claims. According to one embodiment, the radiant energy is supplied to the electrode gap in two or more spots or areas for the purpose of achieving the highest possible certainty with regard to bringing the electrode gap to assume an electrically conducting state. According to one alternative the energy supply means may be designed to supply the radiant energy along an elongated area in the con- duction path which is aimed at between the electrodes. According to an optimal embodiment this elongated area may, entirely or substantially entirely, bridge the gap between the electrodes. Although it is possible, in a case with two or more spots or areas for radiation supply, that these spots or areas are applied successively corresponding to the propagation with respect to the electrical conduction path between the electrodes in such a way that the spots or areas are successively applied with a time delay, it is, according to the invention, normally preferred to apply these spots or areas substantially simultaneously for making the electrode gap conducting momentarily.

Furthermore, the means for supply of triggering energy may according to the invention be adapted to apply the radiant energy in a volume having a tubular shape. This is particularly preferable when one of the electrodes comprises an opening, through which the radiant energy is supplied, and when the radiant energy supplied in a tubular volume is applied relatively close to the electrode provided with an opening. According to an alternative embodiment, the energy supply means may be designed to supply the radiant energy in a plurality of substantially parallel, elongated areas extending between the electrodes.

The radiant energy may also be supplied to the electrode gap transversely relative to an axis of the electrodes in one or more spots located between the electrodes.

The switching arrangement according to the invention may be used with advantage for realizing various switching functionalities obtainable conventionally by means of semiconductor art. Expressed in other words, electrical components may be built by means of the switching arrangement ac- cording to the invention in suitable number, such electrical components having properties similar to those known per se within for instance semi-conductor art.

Further advantages and features of the invention, particu- larly with respect to the method according to the invention, appear from the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more specific description of an embodiment example of the invention follows hereinafter.

In the drawings :

Fig 1 is a purely diagrammatical view illustrating the basic aspect behind the solution according to the invention, Figs 2a-

2d are diagrams illustrating in a diagrammatical form and in a comparative way fault current developments and the energy development with and without the device according to the invention,

Fig 3 is a diagrammatical view illustrating a conceivable design of a device according to the invention,

Fig 4 is a diagrammatical, detailed view illustrating a possible design of the over-current reducing arrangement ,

Fig 5 is a view similar to Fig 4 of a different variant,

Fig 6 a side-view illustrating how a plurality of focal spots or regions are generated along an intended conducting path between the electrodes,

Fig 7 is a schematic view of an alternative, showing how, by means of a suitable optical system, an elongated focal region can be formed between the electrodes in and for closure of the current path between them,

Fig 8 is a diagrammatical view illustrating the device according to the invention applied in an electric power plant comprising a generator, a transformer and an electric power network coupled thereto,

Fig 9 is a diagrammatical view of the inventive switching arrangement in a series switching function,

Fig 10 a diagram illustrating a typical AC-current- /voltage characteristic for a discharge in gas, Fig 11 a diagrammatical view illustrating the switching arrangement according to the invention with first and second switch means to replace the switch means 10 according to fig. 3, and

Fig 12 a voltage-time diagram showing the operation of the switching arrangement according to fig. 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An electric power plant comprising a protected object 1 is shown in Fig 1. This object could for instance consist of a generator. This object is connected, via a line 2, to an external distribution network 3. Instead of such a network, the unit denoted 3 could be formed by some other equipment contained in the electric power plant. The electric power plant involved is conceived to be of such a nature that it is the object 1 itself which primarily is in- tended to be protected against fault currents from the network/equipment 3 when there occurs a fault in the object 1 giving rise to a fault current from the network/equipment 3 towards the object 1 so that the fault current will flow through the object. Said fault may con- sist in a short-circuit having been formed in the object 1. A short-circuit is a conduction path, which is not intended, between two or more points. The short-circuit may for instance consist of an arc. This short-circuit and the resulting violent current flow may involve considerable damages and even a total break-down of the object 1.

It is already pointed out that with at least some types of protected electrical objects 1, short-circuit currents/fault currents harmful to the object in question may flow from the protected object towards the network/equipment 3. Within the scope of the invention, it is intended to be used for protection purposes not only for protection of the object from externally emanating fault currents flowing towards the object but also from internal fault currents in the object flowing in the opposite di- rection. This will be discussed in more detail in the following .

In the following, the designation 3 will, to simplify the description, always be mentioned as consisting of an ex- ternal electric power network. However, it should be kept in mind that some other equipment may be involved instead of such a network, as long as said equipment causes violent current flows through the object 1 when there is a fault.

A conventional circuit breaker 4 is arranged in the line 2 between the object 1 and the network 3. This circuit breaker comprises at least one own sensor for sensing circumstances indicative of the fact that there is an over- current flowing in the line 2. Such circumstances may be currents/voltages but also other indicating that a fault is at hand. For instance, the sensor may be an arc sensor or a sensor recording short circuit sound etc. When the sensor indicates that the overcurrent is above a certain level, the circuit breaker 4 is activated for breaking of the connection between the object 1 and the network 3. The circuit breaker 4 must, however, break the total short circuit current/fault current. Thus, the circuit breaker must be designed to fulfil highly placed requirements, which in practice means that it will operate relatively slowly. In Fig 2a it is illustrated in a current/time-diagram that when a fault, for instance a short circuit in the object 1, occurs at the time t _au^t the fault current in the line denoted 2 in Fig 1 rapidly assumes the magni- tude i_. This fault current i]_ is broken by means of the circuit breaker 4 at t_, which is at least within 150 ms after tf_au_f ^pig ^2< illustrates the diagram i²-t and, accordingly, the energy developed in the protected object 1 as a consequence of the short circuit therein. The energy injection into the object occurring as a consequence of the short-circuit current is, accordingly, represented by the total area of the outer rectangle in Fig 2d.

It is in this connection pointed out that the fault current in Figs 2a-c and the currents in Fig 2d represent the envelope of the extreme value. Only one polarity has been drawn out in the diagram for the sake of simplicity.

The circuit breaker 4 is of such a design that it establishes galvanic separation by separation of metallic con- tacts. Accordingly, the circuit breaker 4 comprises, as a rule, required auxiliary equipment for arc extinguishing.

According to the invention the line 2 between the object 1 and the switching device 4 is connected to an arrangement generally denoted 5. This arrangement may in general regard be designated as a switching arrangement. In the application shown, the switching arrangement has the function of an arrangement reducing overcurrents towards the apparatus 1. The arrangement is actuatable for overcurrent reduction with the assistance of an overcurrent conditions detecting arrangement within a time period substantially less than the break time of the circuit breaker 4. This arrangement 5 is, accordingly, designed such that it does not have to establish any galvanic separation. Therefore, conditions are created to very rapidly establish a current reduction without having to accomplish any total elimination of the current flowing from the network 3 towards the protected object 1. Fig 2b illustrates in contrast to the case according to Fig 2a that the overcurrent reducing ar- rangement 5 according to the invention is activated upon occurrence of a short circuit current at the time tf_au]_t for overcurrent reduction to the level i₂ at the time t₂. The time interval tf_au__t-t₂ represents, accordingly, the reaction time of the overcurrent reducing arrangement 5. Since the task of the arrangement 5 is not to break but only reduce the fault current, the arrangement may be caused to react extremely rapidly, which will be discussed more closely later. As an example, it may be mentioned that current reduction from the level iτ_ to the level i₂ is intended to be accomplished within one or a few ms af- ter unacceptable overcurrent conditions having been detected. It is then aimed at to accomplish the current reduction in a shorter time than 1 ms , and preferably more rapidly than 1 microsecond.

As appears from Fig 1, the device comprises a further breaker generally denoted 6 and arranged in the line 2 between the circuit breaker 4 and the object 1. This further breaker is designed to break a lower voltage and current than the circuit breaker 4 and may, as a consequence thereof, be designed to operate with shorter break times than the circuit breaker. The further breaker 6 is arranged to break not until after the overcurrent from the network 3 towards the object 1 has been reduced by means of the overcurrent reducing arrangement 5 but substan- tially earlier than the circuit breaker 4. From that stated, it appears that the further breaker 6 should be coupled to the line 2 in such a way that it is the current reduced by means of the overcurrent reducing arrangement 5 which will flow through the further breaker and which, ac- cordingly, is to be broken by means thereof.

Fig 2b illustrates the action of the further breaker 6. This breaker is, more specifically, designed to break at the time t₃, which means that the duration of the current i reduced by means of the overcurrent reducing arrangement 5 is substantially delimited, namely to the time pe- lb

riod t₂-t₃. The consequence is that the energy injection into the protected object 1 caused by a fault current from the network 3 is represented by the surfaces marked with oblique lines in Fig 2d. It appears that a drastic reduc- tion of the energy injection is achieved. In this connection it is pointed out that since, according to a specific model, the energy increases with the square of the current, a reduction to one half of the current reduces the energy injection to a fourth. It is illustrated in Fig 2c how the fault current will flow through the arrangement 5.

The dimensioning of the arrangement 5 and the further breaker 6 is conceived to be carried out such that the arrangement 5 reduces the fault current and the voltage to be broken by means of the further breaker 6 to substantially lower levels. A realistic break time as to the further breaker 6 is 1 ms . However, the dimensioning should be made such that the breaker 6 is caused to break not until after the arrangement 5 having reduced the current flowing through the breaker 6 to at least a substantial degree .

It is illustrated in more detail in Fig 3 how the device may be realised. It is then pointed out that the invention is applicable in direct current (also HVDC = High Voltage Direct Current) and alternating-current connections. In the latter case, the line denoted 2 may be considered to form one of the phases in a multiphase alternating-current system. However, it should be kept in mind that the device according to the invention may be realised so that either all phases are subjected to the protection function according to the invention in case of a detected fault or that only that phase or those phases where a fault current occurs are subjected to current reduction. It appears from Fig 3 that the overcurrent reducing arrangement generally denoted 5 comprises an overcurrent diverter 7 for diverting overcurrents to earth 8 or otherwise another unit having a lower potential than the net- work 3. Thus, the overcurrent diverter may be considered as forming a current divider which rapidly establishes a short circuit to earth or otherwise a low potential 8 for the purpose of diverting at least a substantial part of the current flowing in the line 2 so that said current does not reach the object 1 to be protected. If there is a serious fault in the object 1, for instance a short circuit, which is of the same magnitude as the short circuit that the overcurrent diverter 7 is capable of establishing, it may be said that generally speaking a reduction to one half of the current flowing to the object 1 from the network 3 is achieved as a consequence of the overcurrent diverter 7 in case the fault is close to the latter. In comparison with Fig 2b, it appears, accordingly, that the current level i₂ illustrated therein and being indi- cated to amount to approximately half of i^ may be said to represent the worst occurring case. Under normal conditions, the purpose is that the overcurrent diverter 7 should be able to establish a short circuit having a better conductivity than the one corresponding to the short circuit fault in the object 1 to be protected so that accordingly a main part of the fault current is diverted to earth or otherwise a lower potential via the overcurrent diverter 7. It appears from this that, accordingly, in a normal fault case, the energy injection into the object 1 in case of a fault becomes substantially smaller than that which is indicated in Fig 2d as a consequence of lower current level i₂ as well as shorter time span t₂-t . It should be obvious that a certain protection is obtained also when a short-circuit, which has been established, has a somewhat lower conductivity than the one corresponding to the short-circuit fault in the object 1 to be protected.

It has been pointed out that the notation 8 not only in- eludes earth but another unit with a lower potential than the network/equipment 3. It is thereby to be noted that the unit 8 possibly could be formed by another power network or another equipment included in the electric power plant, said equipment having a lower level of voltage than the one which is effective for the network/equipment 3, to which the object 1, which is to be protected, is connected.

The over-current diverter 7 comprises at least one switch means connected between earth 8 or said lower potential and the line 2 between the object 1 and the network 3, and forming the first switch element 10. This switch element comprises a control member 9 and a switch member 10. This switch member is arranged to be open in a normal state, i.e. insulating in relation to earth. The switch member 10 may however be brought into a conductive state via the control member 9 in a very short time in order to establish current reduction by diversion to earth.

Fig 3 illustrates that an overcurrent conditions detecting arrangement may comprise at least one and preferably several sensors 11-13 suitable for detecting such overcurrent situations requiring activation of the protection function. As also appears from Fig 3, these sensors may include the sensor denoted 13 located in the object 1 or in its vicinity. Furthermore, the detector arrangement comprises a sensor 11 adapted to sense overcurrent conditions in the line 2 upstreams of the connection of the overcurrent reducing arrangement 5 and the line 2. As is also explained in the following, it is suitable that a further sensor 12 is provided to sense the current flowing in the line 2 towards the object 1 to be protected, i.e. the cur- rent which has been reduced by means of the overcurrent reducing arrangement 5. In addition, it is pointed out that the sensor 12, as well as possibly the sensor 13, is capable of sensing the current flowing in the line 2 in a direction away from the object 1, for instance in cases where energy magnetically stored in the object 1 gives rise to a current directed away from the object 1.

It is pointed out that the sensors 11-13 do not necessar- ily have to be constituted by only current and/or voltage sensing sensors. Within the scope of the invention, the sensors may be of such nature that they generally speaking may sense any conditions indicative of the occurrence of a fault of the nature requiring initiation of a protection function.

In cases where such a fault occurs that the fault current will flow in a direction away from the object 1, the device is designed such that the control unit 14 thereof will control the further breaker 6 to closing, in case it would have been open, and, in addition, the overcurrent reducing arrangement 5 is activated such that the short circuit current may be diverted by means of the same. When, for example, the object 1 is conceived to consist of a transformer, the function on occurrence of a short circuit therein could be such that the short circuit first gives rise to a violent flow of current into the transformer, which is detected and gives rise to activation of the arrangement 5 for the purpose of current diversion. When the current flowing towards the transformer 1 has been reduced in a required degree, the breaker 6 is caused to break, but, controlled by means of the control unit 14, not earlier than leaving time for the energy, in occurring cases, magnetically stored in the transformer 1 to flow away from the transformer 1 and be diverted via the arrangement 5. Furthermore, the device comprises a control unit generally denoted 14. This is connected to the sensors 11-13, to the overcurrent reducing arrangement 5 and to the further breaker 6. The operation is such that when the control unit 14 via one or more of the sensors 11-13 receives signals indicating occurrence of unacceptable fault currents towards the object 1, the overcurrent reducing arrangement 5 is immediately controlled to rapidly provide the re- quired current reduction. The control unit 14 may be arranged such that when the sensor 12 has sensed that the current or voltage has been reduced to a sufficient degree, it controls the breaker 6 to obtain operation thereof for breaking when the overcurrent is below a pre- determined level. Such a design ensures that the breaker 6 is not caused to break until the current really has been reduced to such a degree that the breaker 6 is not given the task to break such a high current that it is not adequately dimensioned for that purpose. However, the embodi- merit may alternatively also be such that the breaker 6 is controlled to break a certain predetermined time after the overcurrent reducing arrangement having been controlled to carry out current reduction.

The circuit breaker 4 may comprise a detector arrangement of its own for detection of overcurrent situations or otherwise the circuit breaker may be controlled via the control unit 14 based upon information from the same sensors 11-13 also controlling the operation of the overcurrent reducing arrangement.

It is illustrated in Fig 3 that the further breaker 6 comprises a switch 15 having metallic contacts. This switch 15 is operable between breaking and closing positions by means of an operating member 16, which in turn is controlled by the control unit 14. A shunt line 17 is con- nected in parallel over this switch 15, said shunt line comprising one or more components 18 intended to avoid arcs on separation of the contacts of the switch 15 by causing the shunt line 17 to take over the current conduc- tion from the contacts. These components are designed so that they may break or restrict the current. Thus, the purpose is that the components 18 normally should keep the conduction path in the shunt line 17 interrupted but close the shunt line when the switch 15 is to be opened so that accordingly the current is shunted past the switch 15 and in that way arcs do not occur or possibly occurring arcs are efficiently extinguished. The components 18 comprise one or more associated control members 19 connected to the control unit 14 for control purposes. According to one em- bodiment of the invention, said components 18 are controllable semiconductor components, for instance extinguish- able GTO thyristors, having necessary over-voltage arresters 30.

A disconnector 20 for galvanic separation in the current conduction path created by means of the shunt line 17 to the object 1 to be protected is arranged in series with said one or more components 18. This disconnector 20 is via an operating member 21 controlled by the control unit 14. The disconnector 20 is illustrated in Fig 3 as being placed in the shunt line 17 itself. This is of course not necessary. The disconnector 20 could also be placed in the line 2 as long as it ensures real galvanic separation, by series coupling with said one or more components 18, in the conduction path established by means of said series coupling so that accordingly there is not any possibility for current to flow through the components 18.

The device as it has been described so far operates in the following manner: In absence of a fault, the circuit breaker 4 is closed just like the switch 15 of the further breaker 6. The components 18 in the shunt line 17 are in a non-conducting state. The disconnector 20 is closed. Finally, the switch means 10 of the overcurrent reducing arrangement 5 is open, i.e. it is in a non-conducting state. In this situation the switch means 10 must, of course, have an adequate electrical strength so that it is not inadvertently brought into a conducting state. Overvoltage conditions occurring in the line 2 as a consequence of atmospheric (lightning stroke) circumstances or coupling measures may, accordingly, not involve the voltage strength of the switch means 10 in its non-conducting state to be exceeded. For this purpose it is suitable to couple at least one over-voltage diverter 22 in parallel with the switch means 10. In the example such over-voltage arresters are illustrated on both sides of the switch means 10. Accordingly, the over-voltage arresters have the purpose to divert such overvoltages which otherwise could involve a risk for inadvertent breakthrough in the switch means 10.

The over-voltage diverters 22 are illustrated in Fig 3 to be connected to the line 2 itself on either sides of the connection of the switch means 10 to the line. It is in principle desirable that at least one over-voltage diverter has its connection as close as possible upstream in relation to the switch means 10. The over-current diverters 22 could instead, which is indicated in Fig. 3 with the dotted lines 26 be connected to the branch line forming electric connection between the switch means 10 and the line 2. Such a construc- tion enables integration of the switch means 10 and at least one over-voltage diverter 22 to one single electric apparatus, which apparatus may be brought in electric conducting connection with the line 2 via one single connection.

When an over-current state has been registered by means of some of the sensors 11-13 or the own sensor (it is of course realized that information from the own sensor of the circuit breaker 4 may be used as a basis for control of the. over-current reducing arrangement 5 according to the invention) of the circuit breaker 4 and this over-cur- rent state is of such magnitude that a serious fault of the object 1 is expected to be at hand, a breaking operation is initiated as far as the circuit breaker 4 is concerned. In addition, the control unit 14 controls the over-current reducing arrangement 5 to effect such reduc- tion, and this more specifically by bringing, via the control member 9, the switch means 10 into an electrically conducting state. As described before, this may occur very rapidly, i.e. in a fraction of the time required for breaking by means of the circuit breaker 4, for what rea- son the object 1 to be protected immediately is liberated from the full short-circuit current from the network 3 as a consequence of the switch means 10 diverting at least an essential part, and in practice the main part, of the current to earth or otherwise a lower potential . As soon as the current, which flows towards the object 1 via the further breaker 6, has been reduced in a required degree, which can be established on a pure time basis by a time difference between activation of the switch means 10 and operation of the breaker 6, or by sensing of the current flowing in the line 2 by means of, for instance, the sensor 12, the operating member 16 of the switch 15 is, via the control unit 14, controlled to open the contacts of the switch 15. For extinguishing or avoiding arcs, the components 18, e.g. GTO thyristors or gas switches, are via the control members 19 controlled to establish conductivity of the shunt line 17. When the switch 15 has been opened and, thus, provided galvanic separation, the component 18 is again controlled to bring the shunt line 17 into a non-conducting state. In that way the current from the network 3 towards the object 1 has been efficiently cut off. After having brought the shunt line 17 into a non-conducting state, galvanic separation may, in addition, be effected by means of the disconnector 20 by controlling the operating member 21 thereof from the control unit 14. When all these incidents have occurred, breaking by means of the circuit breaker 4 occurs as a last incident. It is important to note that the over-current reducing arrangement 5 as well as the further breaker 6 according to a first embodiment can be operated repeatedly. Thus, when it has been established by means of the sensors 11-13 that the circuit breaker 4 has been brought to cut off, the switch means 10 is reset to a non-conducting state and the switch 15 and the disconnector 20 are again closed so that when the circuit breaker 4 next time closes, the protection device is completely operable. Ac- cording to another embodiment, it is, however, contemplated that the over-current reducing arrangement 5 may require exchange of one or more parts in order to operate again.

It is pointed out that according to an alternative embodiment of the invention, the component or components 18 could be brought into a conducting state as soon as the over-current reducing arrangement 5 has been brought into a closing state and this independently of whether the switch 15 possibly is not opened thereafter. The control of the components 18 could then, as described before, occur via the control unit 14 or, alternatively, by means cf a control function involving a slavish following of the closing of the arrangement 5.

Fig 4 illustrates a first embodiment of the over-current reducing arrangement 5 with switch means denoted 10a. The switch means 10a has electrodes 23 and a gap 24 prevailing between these electrodes . The switch means as it has been described so far has means 25a in order to trigger the electrode gap 24 to form an electrically conducting path between the electrodes. A control member 9a is arranged to control the operation of the members 25a via the control unit 14a. The means 25a are in the example arranged for causing or at least initiating the electrode gap to assume electrical con- ductivity by means of causing the gap or part thereof to form a plasma. It is thereby essential that the means 25a are capable of realising a very rapid supply of triggering energy to the electrode gap. It is thereby preferred that the triggering energy is supplied in the form of radiative energy, which in turn is capable of effecting ionising/initiating of plasma in the electrode gap.

The means 25a comprises according to a particularly preferred embodiment of the invention at least one laser, which by means of energy supply to the electrode gap causes ionising/forming of plasma in at least a part of the electrode gap.

It is preferred in accordance with the invention to supply, with the aid of one or several lasers or other means 25a, energy to the electrode gap 24 in such a way that the complete electrode gap will be ionised and brought to the form of a plasma respectively, approximately momentarily in such a way that also the complete gap 24 immediately is brought to electrical conductivity. In order to spare with and optimize the use of the (normally) restricted available laser energy/-effeet , the means 25a may, in application of the invention, be arranged so that they can provide ioniza- tion/plasma formation in only one or more parts of the gap 24. In the embodiment according to fig 4, it is illustrated that the means 25a supply the radiant energy in one single spot or area 28. As will be described later, the invention also comprises application of the radiant energy in a plurality of spots or areas in the electrode gap, including also on one of or both of the electrodes, or in one or more rodlike areas extending continuously or substantially continuously between the electrodes.

By connecting the switch means 10a between the line 2 and earth 8 (or another unit with lower potential) as is dia- grammatically indicated in Fig 4, i.e. with one of the electrodes 23 connected to the line 2 and the other electrode connected to earth 8, there will be a voltage difference between the electrodes causing an electric field. The electric field in the gap 24 is intended to be utilised in order to convey or cause an electric breakdown between the electrodes as soon as the means 25a have been controlled to triggering, i.e. have given rise to ionising/forming of plasma in one or more parts of the electrode gap. The established ionis- ing/forming of plasma will be driven by the electric field to shunt the gap between the electrodes in order to in this way give rise to a low-resistant electrical conductive channel, i.e. an arc between the electrodes 23. It is pointed out that the invention is not intended to be restricted to use in connection with occurence of such an electric field. Thus, the intention is that the means 25a should be capable of establishing electrical conduction between the electrodes also without such a field.

Due to the demand on the switch means 10a to close very rapidly for current diversion, it is thus desirable when only a restricted part, e. g. a spot like part of the gap is ionised that the switch means is dimensioned in such a way that the strength of the electric field in the gap 24 will be sufficiently high for safe closing. It is however on the other hand a desire that the switch means 10a should have a very high electric strength against breakdowns between electrodes in its isolating rest position. The strength of the electric field in the gap 24 should therefore be proportion- ally low. This will on the other hand reduce the speed, with which the switch means may be caused to establish the current diverting arc between the electrodes. In order to achieve an advantageous relation between the desire for a safe trigging of the switch means and on the other hand high electric strength against undesired trigging, it is according to the invention preferred that the switch means is formed in such a way that regarding its complete operational environment the electric field in the gap 24 has a field strength which is not more than 30% of the field strength at which a spontaneous breakdown normally takes place, when the gap forms electric isolation. This causes a proportionally low probability of a spontaneous breakdown.

The strength of the electric field in the electrode gap 24 in its isolating state is suitably not more than 20% and preferably not more than 10% of the field strength at which a spontaneous breakdown normally takes place. In order to on the other hand achieve an electric field in the electrode gap 24, which promotes forming of an arc at initiation of ionising/forming of plasma in a part of the electrode gap in a relatively rapid way, it is preferred that the strength in the electric field is at least 0,1% and suitably at least 1% (E₄) , and preferably at least 5% of the field strength, at which a spontaneous breakdown normally takes place.

The electrode gap 24 is, as may be seen in Fig. 4, enlcosed in a suitable casing 32. A vacuum as well as a suitable medium in the form of gas or even fluid may for this purpose be present in the gap 24. In the case of a gas/fluid the medium in the gap is intended to be formed in such a way that it might be ionised and brought to plasma by trigging. It would in such a case be suitable to initiate ionisa-. tion/forming of plasma in the gap 24 at a point somewhere between the electrodes 23. It is however in Fig 4 illustrated the conceived case where there either is a vacuum or a suitable medium in the gap 24. It is then preferred that initiation of closing takes place by way of making the laser 25a, which is illustrated in Fig 4, to focus the emitted radiative energy in at least one area 28 on or in the vicinity of one of the electrodes via a suitable optical system 27. This implies that the electrode will operate as an electron and ion emitter for establishing an ionised environment/a plasma in the electrode gap 24 in such a way that thus an arc will be formed between the electrodes. One of the electrodes 23 may according to Fig 4 have an opening 29, through which the laser 25a is arranged to emit the radiative energy to the area 28 with support of the optical system 27.

Fig 5 illustrates a variant 10b of the switch means, where instead the system laser 25b/optics 27b focus the radiative energy in a triggering area 28b, which is situated between the electrodes and in a medium between these electrodes . Plasma is accordingly, on triggering, intended to be developed from this area to bridging of the electrodes.

Fig 6 illustrates an embodiment based upon an optical system

27e comprising a lens system 35, via which arriving laser pulses are conveyed to a diffractive optical phase element 36, a kinoform. This element is designed to have a plurality of focal points or spots 28e generated starting from a sin- gle incoming laser pulse. These focal spots 28e are distributed along the axis of symmetry between the electrodes 23e. As a consequence of the focal spot 28e being distributed along a line between the electrodes 23e, a more safe establishment of an electrical conduction path between the elec- trodes is achieved, meaning as high a probability for triggering as possible at a voltage/electrical field strength as low as possible and with a time delay as short as possible.

The kinoform 36 is low absorbing and may, accordingly, resist extremely high optical energy densities. The kinoform is, accordingly, produced from a dielectrical material so that it will not disturb the electrical field between the electrodes in any serious degree .

It appears from fig 7 that the light may be focused in an elongated focal area 28i located between the electrodes 23i by means of an axicone 36i. This elongated focal area may according to one embodiment of the invention extend continuously all the way between the electrodes but could also adopt only a part of the gap therebetween.

Fig 8 illustrates an embodiment where a generator lb is connected to a power network 3a via a transformer la. The objects which are to be protected are therefore represented by the transformer la and the generator lb. The over-current reducing arrangement 5a and the further breaker 6a as well as the ordinary circuit breaker 4a are apparently arranged in resemblance with what is evident from Fig 1 in the case that the object 1 in Fig 1 is conceived to form the object la according to Fig 8 It is therefore in this regard re- ferred to the descriptions in connection to Fig 1. The same is true for the protection operation of the over-current reducing arrangement 5c and the further breaker 6c in relation to the generator lb. The generator lb should therefore in this case be equivalent to the object 1 in Fig 1 while the transformer la should be equivalent to the equipment 3 in Fig 1. The over-current reducing arrangement 5c and the further breaker 6c will therefore in combination with the conventional circuit breaker 4b be able to protect the generator lb against a violent current flow in the direction from the transformer la.

Fig 8 also illustrates the further over-current reducing arrangement 5b with the associated further breaker 6b. Apparently, over-current-reducing arrangements 5a and 5b will therefore be arranged on either sides of the transformer la. It is to be noted that the further breakers 6a and 6b, re- spectively are positioned in the connections between said over-current reducing arrangements 5a and 5b and the transformer la. The further over-current reducing arrangement 5b is intended to protect the transformer la from violent cur- rent flows towards the transformer from the generator lb. The circuit breaker 4b will apparently be capable of breaking independently of in which direction between the objects la and lb a safety function is desired.

Fig 9 illustrates diagrammatically that a switching arrangement 5r is coupled in series in the line 2r previously discussed between the network 3r and the object lr. The switching arrangement 5r comprises, suitably, a switch element lOr with the previously described character, i.e. a switch ele- ment having an electrode gap adapted to be brought into electrically conducting closing by means of radiant energy. However, this is not shown more closely in fig 9. As appears from fig 9, the switching arrangement 5r is intended to have a purely switching function, i.e. the feeding of the object lr or possibly feeding in the opposite direction may occur via the switch element lOr when this is in a conducting state. When there is a need, the switch element lOr may be made to inhibit current passage relatively rapidly, e.g. for protection of the object lr or possibly even the network 3r from current flow from the object Id. In order to achieve switching-off by means of the switch element lOr in alternating current connections, it is sufficient that the means for energy supply to the electrode gap are caused to cease with such energy supply. On the following passage through zero, extinguishing of the arc in the switch element lOr is intended to take place so that the current feeding ceases . In direct current applications, it is probably necessary to support the turning-off function by taking measures to reduce or eliminate the voltage difference over the switch element lOr. Such means may consist in a switch element 40 coupled parallel to the switch element lOr. Closing of the switch element 40 means that the current is shunted passed the switch element lOr, a fact which causes the arc in the switch element lOr to be extinguished. ^"In case such a measure would not be sufficient, further switches could as a complement thereto be arranged on either sides of the switch element lOr in line 2r to totally disconnect the switch element lOr from the line 2r.

The purpose with Fig 9 is to illustrate that the switch ar- rangement 5r according to the invention may find general switching applications, in which it may be the question of protecting various apparatus but also of switching power in various load situations in a more general sense.

In Fig 10 the curve 38 shows how the voltage depends on the current according to the curve 37 in the laser switch means 10, for instance according to Fig 3. In a DC-situation the voltage is in principle independent of the current and reaches a certain level determined by the geometry of the discharging chamber, gas, pressure, electrode material, etc. In an AC-situation the conditions are different. At zero current, the discharge is extinguished and a de-ionization of the gas is initiated. If the operating net is sufficiently strong, it is able to create an over-voltage large enough to restart the discharge, and the current will continue. This results in the steep voltage increase and the high top value shortly after the zero cross-over of the current. It should be stated that, in normal cases, it is very difficult to make the plasma extinguish, compare with the situation in a normal high current breaker. Thus, when the switch means 10 is lightened and burns, the current -voltage characteristic is described by Fig 10. This sequence should also describe the situation by the fault in the object in question. In order to accomplish a switching arrangement having a very low voltage loss, a switch element 40 which brings at least two contact members into galvanic contact with each other may be used, according to the illustration of Fig 11. The second switch element 40, designed as a mechanical switch means, is normally substantially slower than the first switch element 10, constituted by a switch means suitably triggered by means of a laser. Fig 11 illustrates how, in a similar way, a control unit 14 controls a control member 9, which in its turn executes ignition of the switch means 10. The control unit 14 is also connected to a maneuvering member for bringing the mechanical switch means 40 between a closed and an open position.

The two switch means 10, 40 are arranged in parallel in a branch line between the line 2 and earth 8.

Normally, the mechanical switch means 40 will close after that the switch means 10 triggered by means of radiation has been brought to an electrically conducting state. When the mechanical switch means 40 closes, there is a complete connection to earth, i.e. zero voltage loss across the switch means 10 triggered by means of laser, as long as the mechanical switch means 40 may have its contacts in contact with each other with a contact pressure sufficient enough to counteract the repelling effect of the current forces.

In other words, the embodiment illustrated in Fig 11 should be incorporated in the embodiment shown in Fig 3 , such that the two switch means 10 and 40 will be positioned in parallel.

As Fig 9 presents a serial embodiment, the conditions there are somewhat different from those in Fig 11. In Fig 12, there is illustrated how the voltage across the combination of the first and second switch element 10, 40 in Fig 11 depends on time during closure. The voltage level Ul is determined by the system. When, at the time t2 , the plasma switch means 10 closes the current connection, the voltage is reduced to the level U2. When subsequently, at t4 the mechanical switch means 40 closes, the voltage loss across the plasma switch means 10 is totally eliminated.

It should be noted that the description presented here- inabove only should be considered as exemplifying for the inventive idea, on which the invention is built. Thus, it is obvious for the men skilled in the art that detailed modifications may be made without leaving the scope of the inven- tion. As an example, it may be mentioned that according to the invention, it is not necessary to use a laser for supply of ionising/plasma forming energy to the gap 24. Also other radiative sources, for example electron guns, or other energy supply solutions may be applied as long as the rapid- ness and reliability demands according to the invention are fulfilled. It should be observed that the switch means 10 may according to the invention be applied for protection of electric objects against fault-related over-currents also in other operative cases than the ones illustrated in Figs 1, 3 and 8, where the device according to the invention is arranged in order to reduce the negative effects of the relatively slow breaking time of the circuit breaker 4. Thus, the switch means according to the invention does not necessarily need to be operation-related to such a circuit breaker 4. It should finally be observed that the invention is well suited for alternating current as well as direct current .

Claims

1. A device for switching electric power comprising at least one electric switching arrangement (5) , characterized in that the switching arrangement (5) comprises at least one first switch element (10) , which comprises an electrode gap (24) , which is convertible between an electrically substantially isolating state and an electrically conducting state, means (25) for causing or at least initiating the electrode gap or at least a part thereof to assume electrical conductivity, said means (25) for causing or at least initiating the electrode gap to assume conductivity being adapted to supply energy to the electrode gap to bring the gap or at least a part thereof to the form of a plasma, and at least one second switch element (40) adapted to establish a galvanic contact between its contact members, said first and second switch elements being connected in parallel.

2. A device according to claim 1, characterized by said means (25) for causing or at least initiating the electrode gap or a part thereof to assume electrical conductivity comprising at least one laser (25) .

3. A device according to claim 1 or 2 , characterized in that the means (25) for supplying triggering energy to the electrode gap are arranged to apply the radiative energy on or at least in the vicinity of at least one of the electrodes

(23) .

4. A device according to any preceding claim, characterized in that the means (25) for supplying triggering energy to the electrode gap are arranged to locate the radiative energy in one spot or area in the gap (24) between the electrodes (23) .

5. A device according to any preceding claim, characterized in that the members (25, 27) for supplying the triggering energy to the electrode gap are arranged to apply the radi- ant energy in two or more spots or areas (28) between the electrodes .

6. A device according to claim 5, characterized in that the means for supplying triggering energy to the electrode gap are arranged to locate said two or more spots or areas of radiant energy along a line extending between the electrodes, said line corresponding to the extent of an electric conduction path desired between the electrodes.

7. A device according to any preceding claim, characterized in that the means (25) for supplying triggering energy to the electrode gap are arranged to apply the radiant energy in one or more elongated areas (28i,k, m, n) , the longitudinal axes of which extend substantially along the direction of the electric conduction path which is intended between the electrodes.

8. A device according to claim 7, characterized in that the means for supplying triggering energy to the electrode gap are adapted to shape the elongated area so that it bridges, entirely or substantially entirely, the space between the electrodes .

9. A device according to any of claims 7 or 8 , characterized in that the means (27) for supplying triggering energy to the electrode gap are adapted to form two or more elongated focal areas (28) in the electrode gap, said focal areas being located longitudinally after each other along the electric conduction path intended between the electrodes.

10. A device according to any of claims 1 and 3, characterized in that the means for supplying triggering energy to the electrode gap are adapted to apply the radiant energy on at least one of the electrodes as well as between them.

11. A device according to any of the claims 3-10, characterized in that at least one of the electrodes at the electrode gap has an opening (29) , through which the means (25) for supplying triggering energy are arranged to direct the radiative energy.

12. A device according to any of claims 3-11, characterized in that the means for supplying triggering energy to the electrode gap comprise a system for controlling electromagnetic wave energy.

13. A device according to claim 12, characterized in that the control system comprises at least one refractive, re- flective and/or diftractive element.

14. A device according to claim 13, characterized in that the element is formed by an axicone .

15. A device according to claim 13, characterized in that the element is formed by a kinoform.

16. A device according to any preceding claim, characterized in that the first switch element 10 is adapted to protect an electric object against over-currents and/or over-voltages by being brought into an electrically conducting state.

17. A device according to claim 16, wherein the electric object (1) is connected to an electric power network (3) or another equipment included in the electric power plant by means of a line (2) , characterized in that the first switch element (10) is connected to the line (2) between the object (1) and the electric power network/equipment.

18. A device according to claim 17, characterized in that a switching device (4) is connected to the line (2) between the network/equipment (3) and the spot where the first switch element (10) is connected to the line.

19. A device according to claim 18, characterized in that it comprises a further breaker (6) which is arranged between the spot where the first switch element (10) is connected to the line (2) and the object (1) .

20. A device according to claim 19, characterized in that the further breaker (6) is adapted to break lower voltages and currents than the switching device (4) , and therefore it is able to achieve a shorter breaking time than the latter, and that the further breaker is adapted to break only when the over-current towards or from the object (1) has been re- duced by means of the first switch element (10) but substantially prior to the switching device.

21. A device according to claim 10 or 20, characterized in that it comprises a control unit (14) connected to the de- tecting arrangement (11-13) and to the further breaker (6) in order to achieve actuation of the further breaker for breaking purposes when the over-current towards or away from the object (1) is indicated, by means of the detecting arrangement, to be below a predetermined level.

22. A device according to claim 16, characterized in that it comprises a control unit (14) connected to the first switch element (10) and to the over-current conditions detecting arrangements ^"(11-13), said control unit (14) being arranged to control the first switch element to closing based upon information from the over-current conditions detecting arrangement when required for reasons of protection.

23. A device according to claim 22 and one or more of the claims 19-21, characterized in that one and the same control unit (14) is arranged to control, based upon information from the over-current conditions detecting arrangement (Ills) , the first switch element (10) and the further breaker (6) .

24. A device according to any preceding claim, characterized in that the first switch element (10) has a shorter breaking period than the second switch element (40) .

25. Use of a device according to any preceding claim for protection of an object against fault-related over-currents .

26. A method of switching electric power by means of at least one electric switching arrangement (5) comprising first and second switch elements (10, 40) , characterized in that, when a closure of the switching arrangement is desired, initially the first switch element (10) is brought to close by converting an electrode gap (24) included in the first switch element from an electrically generally insulat- ing state to an electrically conducting state by means of a means (25) for bringing or at least initiating the electrode gap or at least a part thereof to assume electrical conductivity, that, by means of the means (25) for bringing or at least initiating the electrode gap to assume conductivity, energy is supplied to the electrode gap in the form of radiant energy in order to bring the gap or at least a part thereof to the form of a plasma by means of this radiant energy, and that the second switch element (40) , which is connected in parallel with the first switch element (10) , is activated to establish a galvanic contact between its contact members .

27. A method according to claim 26, characterized in that the first and second switch elements (10, 40) of the switching arrangement (5) are activated to close in order to pro- tect an electric object (1) from over-currents or overvoltages .