Technical field

The invention relates to the field of medium and high voltage switching technologies and concerns a contact arrangement and an electrical switching device with such a contact arrangement according to the independent claims, particularly for a use as an earthing device, a fast-acting earthing device, a circuit breaker, a generator circuit breaker, a switch disconnector, a combined disconnector and earthing switch, or a load break switch in power transmission and distribution systems.

Background

Electrical switching devices are well known in the field of medium and high voltage switching applications. They are e.g. used for interrupting a current when an electrical fault occurs. As an example for an electrical switching device, circuit breakers have the task of opening contacts and keeping them far apart from one another in order to avoid a current flow, even in case of high electrical potential originating from the electrical fault itself. For the purposes of this disclosure the term medium voltage refers to voltages from 1 kV to 72.5 kV and the term high voltage refers to voltages higher than 72.5 kV. The electrical switching devices, like said circuit breakers, may have to be able to carry high nominal currents of 3000 A to 6300 A and to switch very high short circuit currents of 40 kA to 80 kA at very high voltages of 73 kV to 1200 kV.

During the closing and opening process of the electrical switching device an electric arc forms between the first and the second arcing contact. The electrical switching devices contain a fluid used to quench the electric arc as fast as possible. This is known and will not be described in more detail. Many types of such electrical switching devices, e.g. circuit breakers, exist, depending on the fluid used, the purpose, the voltage rating, the motion type of the contacts, etc. Some circuit breakers have a fixed first contact group consisting of a nominal contact and an arcing contact and a moving second contact group consisting of another nominal and arcing contact. For closing or opening the contact only the second contact group is moved towards or away from the first contact group, respectively. In other embodiments both contact groups can be moved towards each other or away from one another in order to open or close the contact, as is for example the case in so-called double-motion circuit breakers.

In embodiments the arcing contact of a moving contact group has a guiding serving the purpose of ensuring a stable mechanical movement and current conduction. As the moving arcing contact and the guiding are located in an atmosphere of said fluid, which can reach very high temperatures during the closing or opening process, it has been observed that metal erosion of the arcing contact results from touching the hot fluid. Furthermore, particles carried by the hot fluid tend to accumulate in the area of the guiding, thus increasing the friction between it and the arcing contact, when the arcing contact is moved. Such an increased friction is undesired, reduces the contact moving speed and shortens the lifetime of such a circuit breaker or increases the maintenance needs, respectively.

Because of this, a cover element has been developed for protecting said elements from the influence of the hot fluid. The implementation of the cover element has brought a certain relief with respect to said problem; however it has been observed that metal erosion on the arcing contact is still present in the area of its contacting extremity, which by definition cannot be covered by the cover element, and a reduced number of particles still accumulate in the vicinity of the guiding element, as still some particles are carried by the fluid into a gap between the cover element and the arcing contact .

DE 29 47 957 Al discloses a circuit breaker having on a fixed-contact side an exhaust gas volume equipped with cooling metal plates, that are axially extended, circumferentially folded in a skirt-like manner and that have radial or tangential exhaust gas openings. On the moveable-contact side, the arcing volume is connected via a blast-gas channel with a blast-gas pressurization chamber for actively blowing the arc.

Description of the invention

Thus, it is an objective of the present invention to further improve a contact arrangement and an electrical switching device with respect to the above mentioned disadvantages.

The objective is solved by the features of the independent claims. The dependent claims and their combinations relate to exemplary embodiments.

According to this, a contact arrangement having a longitudinal axis and comprising at least a first contact and at least a second contact is provided. The two contacts interact electrically and mechanically with each other for closing and opening the contact arrangement. The first contact is surrounded by a protecting cover element in such a way that a contacting extremity of the first contact protrudes beyond the cover element. The first contact and the cover element are arranged with respect to one another in such a way that a gap is provided between them along at least a portion of their elongation. The gap has a first opening towards a first volume surrounding the first contact. The cover element comprises at least one chamber connected at least to the gap.

The invention further relates to an electric switching device with a contact arrangement according to the invention as disclosed herein. Preferably, for closing and opening said electrical switching device at least the first contact is movable along the longitudinal axis towards the second contact.

Preferably, the contact arrangement according to the invention further comprises a guiding element for supporting the first contact in a sliding manner when the first contact is in motion. Preferably, the at least one chamber is arranged between the gap and the guiding element, and in particular is arranged in axial distance between the gap and the guiding element.

By providing such a chamber in the interior of the cover element, it is possible to reduce significantly the particle accumulation in the area of the guiding element of the first contact, because the hot fluid streaming from the first volume through the gap and containing particles carries them into the chamber. A vortex structure is formed in the chamber; hence, the particles are accumulated inside the chamber and not in the area of the guiding element. As a consequence, a circuit breaker using the invention is cheaper, because the requirements with respect to the contact and guiding element materials are lower.

An advantageous side effect of the invention is that the hot fluid streaming into the chamber is cooled therein and flows out through the gap again, thus additionally cooling the first contact.

In an advantageous embodiment of the invention the first contact is particularly a rod-type or a tube-type contact. In embodiments, the first contact is movable along the longitudinal axis and the cover element is stationary. Further embodiments relate to a contact arrangement, wherein

the at least one chamber is at least partially formed inside the cover element, and/or the at least one chamber comprises at least one cavity formed inside the cover element; and/or

- the at least one chamber is arranged down ^¬ stream from the gap and serves as a particle trap; and/or the gap extends at least partially, in particular completely, circumferentially around the first contact; and/or

the gap is provided between the first contact and the protecting cover element along at least a portion of their axial elongation along the longitudinal axis z; and/or

- the first contact is at least partially, in particular completely, surrounded by the protecting cover element; and/or

- the first contact is coaxially surrounded by the protecting cover element.

In embodiments of the invention the at least one chamber is further connected to the first volume by means of at least one second opening. This advantageously makes it possible to guide away the particles accumulated in the chamber, because the hot fluid flowing into the chamber via the gap flows out of the chamber through the second opening and therefore carries the particles out into the first volume, where their presence is far less critical with respect to the operation of the electrical switching device. For example, the second opening extends radially or axially outwards through the cover element.

In other embodiments two chambers connected at least to the gap, arranged successively with respect to the longitudinal axis and separated by a wall, are provided. Preferably, the one of the two chambers which is arranged farthest from the first opening is provided with the second opening and the one of the two chambers which is arranged closest to the first opening has no second opening. This advantageously combines the advantages of the already described embodiments of the invention. On the one hand a part of the hot fluid streaming into the gap accumulates in the first chamber, thus accumulating a part of the particles therein and partially flowing out again through the gap, thus cooling the first contact. On the other hand the portion of the hot fluid streaming into the second chamber and still containing particles can be evacuated through the second opening, thus carrying away the majority of the remaining particles contained therein.

In embodiments an edge of the cover element formed by a cover element wall facing the gap and an adjacent first chamber wall has a curvature ranging between 0.2 mm and 5 mm. This advantageously optimizes the fluid flow into the chamber with respect to a desired turbulence pattern.

In other embodiments at least a second chamber wall of the chamber, which is preferably located farthest from the first opening, is coated with a Teflon layer. Particularly, all chamber walls can be coated with a Teflon layer. The Teflon layer is advantageously chosen to be thicker than 2 mm. The advantages resulting from the Teflon layer will be described in connection with the description of the drawings.

In other embodiments a plurality of projections extending transversally with respect to the longitudinal axis is provided on a cover element wall facing the gap. The advantage of providing projections is that a cooling effect of the first contact is increased. This will also be described in more detail in connection with the drawings.

The electrical switching device according to the invention can be used as an earthing device, a fast- acting earthing device, a circuit breaker, a generator circuit breaker, a switch disconnector, a combined disconnector and earthing switch, or a load break switch. In embodiments, the cover element serves in addition as a further nozzle of the switching device, i.e. serves for guiding (and in particular concentrating) the flow of hot gases escaping from the arcing zone towards or along or aside the first contact 4a.

For the purposes of this disclosure the fluid used in the gas insulated switchgear can be SF ₆ gas or any other dielectric insulation medium, may it be gaseous and/or liquid, and in particular can be a dielectric insulation gas or arc quenching gas. Such dielectric insulation medium can for example encompass media comprising an organofluorine compound, such organofluorine compounds being selected from the group consisting of: a fluoroether, a fluoroamine, a fluoroketone, an oxirane, a fluorolefin (in particular hydrofluorolefin) , and mixtures thereof; and preferably being a fluoroketone and/or a fluoroether, more preferably a perfluoroketone and/or a hydrofluoroether . Herein, the terms "fluoroether", "oxirane", "fluoroamine", "fluoroketone" and "fluoroolefin refer to at least partially fluorinated compounds. In particular, the term "fluoroether" encompasses both hydrofluoroethers and perfluoroethers , the term "oxirane" encompasses both hydrofluorooxiranes and perfluorooxiranes , the term "fluoroamine" encompasses both hydrofluoroamines and perfluoroamines , the term "fluoroketone" encompasses both hydrofluoroketones and perfluoroketones , and the term "fluoroolefin" encompasses both hydrofluoroolefins and perfluoroolefins . It can thereby be preferred that the fluoroether, the fluoroamine, the fluoroketone and the oxirane are fully fluorinated, i.e. perfluorinated .

In embodiments, the dielectric insulation medium is selected from the group consisting of: a (or several) hydrofluoroether ( s ) , a (or several) perfluoroketone ( s ) , a (or several) hydrofluoroolefin ( s ) , and mixtures thereof. In particular, the term "fluoroketone" as used in the context of the present invention shall be interpreted broadly and shall encompass both fluoromonoketones and fluorodiketones or generally fluoropolyketones . The term shall also encompass both saturated compounds and unsaturated compounds including double and/or triple bonds between carbon atoms. The at least partially fluorinated alkyl chain of the fluoroketones can be linear or branched and can optionally form a ring.

In particular, the fluoroketone can be a fluoromonoketone and/or may also comprise heteroatoms, such as at least one of a nitrogen atom, oxygen atom and sulphur atom, replacing one or more carbon atoms. More preferably, the fluoromonoketone, in particular perfluoroketone, shall have from 3 to 15 or from 4 to 12 carbon atoms and particularly from 5 to 9 carbon atoms. Most preferably, it may comprise exactly 5 carbon atoms and/or exactly 6 carbon atoms and/or exactly 7 carbon atoms and/or exactly 8 carbon atoms.

In embodiments, the dielectric insulation medium comprises at least one compound being a fluoroolefin selected from the group consisting of: hydrofluoroolefins (HFO) comprising at least three carbon atoms, hydrofluoroolefins (HFO) comprising exactly three carbon atoms, trans-1, 3, 3, 3-tetrafluoro-l-propene (HFO- 1234ze) , 2, 3, 3, 3-tetrafluoro-l-propene (HFO-1234yf) , trans-1, 2, 3, 3, 3 pentafluoroprop-l-ene (HFO-1225ye (E- isomer) ) , cis-1, 2, 3, 3, 3 pentafluoroprop-l-ene (HFO-1225ye (Z-isomer) ) , and mixtures thereof.

The dielectric insulation medium can further comprise a background gas or carrier gas different from the organofluorine compound, in particular different from the fluoroether, the fluoroamine, the fluoroketone, the oxirane and the hydrofluorolefin and preferably can be selected from the group consisting of: air, N2, 02, C02, a noble gas, ¾; O ₂ , NO, ₂ O, fluorocarbons and in particular perfluorocarbons and preferably CF ₄ ; CF ₃ I, SF ₆ , and mixtures thereof. Short description of the drawings

Embodiments, advantages and applications of the invention result from the dependent claims and from the now following description by means of the figures. It is shown in:

Fig. 1 a sectional side view of an exemplary high voltage circuit breaker with a prior art cover element ;

Fig. 2 a sectional partial side view of the first contact of the circuit breaker of Fig. 1 with a cover element according to an embodiment of the invention and with schematized stream lines and vortex structure of an insulation fluid streaming therethrough;

Fig. 3 the sectional partial side view according to Fig. 2 with relevant dimensions indicated in the figure;

Fig. 4 the sectional partial side view according to Fig. 2 or 3 with a first embodiment of the contact arrangement according to the invention;

Fig. 5 the sectional partial side view according to Fig. 2 or 3 with a second embodiment of the contact arrangement according to the invention;

Fig. 6 the sectional partial side view according to Fig. 2 or 3 with a third embodiment of the contact arrangement according to the invention;

Fig. 7 the sectional partial side view according to Fig. 2 or 3 with a fourth embodiment of the contact arrangement according to the invention;

Fig. 8 a sectional partial side view according to a fifth embodiment of the contact arrangement according to the invention;

Fig. 9 the sectional partial side view according to Fig. 2 or 3 with a sixth embodiment of a chamber of the contact arrangement according to the invention, containing a Teflon layer; and Fig. 10 the sectional partial side view according to Fig. 2 or 3 with a seventh embodiment of a chamber of the contact arrangement according to the invention with projections.

Ways of carrying out the invention

The invention is described for the example of a high voltage circuit breaker, but the principles described in the following also apply for the usage of the invention in other switching devices, e.g. of the type mentioned at the beginning.

In the following same reference numerals denote structurally or functionally same or similar elements of the various embodiments of the invention.

A sectional partial view refers to the illustration of only a section of the elements of the figure for reasons of simplicity. It is assumed that said elements are rotational symmetric to a longitudinal axis z of the contact arrangement.

In the context of this disclosure the terms "cross section" and "width" relate to an area located in a plane which is perpendicular to the longitudinal axis z. Accordingly, the term "elongation" or "elongated" relates to an elongation in the direction of the longitudinal axis z.

Fig. 1 shows a sectional side view of an embodiment of a high voltage circuit breaker 1 with a cover element 2a according to the prior art during an opening process with an electric arc 20 between the contacts 4a, 4b. The circuit breaker 1 is rotationally symmetric about the longitudinal axis z. Only the elements of the circuit breaker 1 which are related to the present invention will be described in the following, other elements present in the figures are not relevant for understanding the invention and are known by the skilled person in high voltage electrical engineering. The circuit breaker 1 comprises a first contact group 3a, 4a and a second contact group 3b, 4b. The contacts 3a, 3b are nominal contacts of the circuit breaker 1 and will not be described in more detail. The contacts 4a, 4b are arcing contacts of the circuit breaker 1. In the following the arcing contact 4a is more generally called first contact 4a and the arcing contact 4b is more generally called second contact 4b. For the purposes of explaining the invention it is assumed that the first contact 4a and the second contact 4b are movable along the longitudinal axis z and cooperate with one another for closing or opening an electrical circuit. The first contact 4a is actuated by an arrangement comprising a gear 23 or auxiliary gear 23 which links a connection rod 22, which itself is connected to a nozzle 21, with the first contact 4a. The nozzle 21 can be actuated to move back and forth in the direction of the longitudinal axis z by a movement of the second contact group 3b, 4b. Other embodiments with the second contact group 3b, 4b being stationary are readily possible. In such a case the first contact is actuated by an external actuator (not shown) .

The contact arrangement described above is enclosed in a shell or enclosure 16 of the circuit breaker 1. An insulating fluid of the type mentioned above is present inside the circuit breaker 1 in a first volume 15. The first volume 15 also comprises a volume around the first and the second contacts 4a, 4b, as indicated in the figure, and in particular is radially outside of or radially around the first contact 4a. The fluid flow for said opening process is indicated by arrows 6. In the following the arrangement of the first contact 4a with some additional elements will be described .

The first contact 4a is partially surrounded by the protecting cover element 2a in order to protect it, at least partly, from contacting the hot fluid streaming around the first contact 4a (arrows 6) . A guiding element 17 for supporting the first contact 4a in a sliding manner, when the first contact 4a is in motion, is provided. A current flowing through the first contact 4a is guided via one or more current fingers 19 and a current carrier 18 to the exterior of the circuit breaker 1.

As mentioned, contacting of the first contact 4a with the hot fluid is undesired, because it causes higher metal erosion on the first contact 4a. It has been observed that this erosion of the first contact 4a is particularly pronounced in the area just before the tip of the cover element 2a, because a certain stagnation of the hot fluid streaming in the direction of the cover element 2a occurs there. Furthermore, particles present in the fluid stream into a gap between the cover element 2a and the first contact 4a and accumulate there, particularly at the location of the guiding element 17. This results in a higher friction between the guiding element 17 and the first contact 4a. A consequence of this accumulation is that the speed of the first contact 4a may decrease and therefore the contacting with the mating contact 4b cannot be carried out in the prescribed time. Another consequence is that more heat is produced because of the higher friction, which is undesired as well. The invention will be explained in connection with the now following figures, which all show detail views of a part of the first contact 4a, a cover element 2 according to embodiments of the invention, the guiding element 17 and in Fig. 2 the current finger 19.

Fig. 2 shows a sectional partial side view of the first contact 4a of the circuit breaker 1 of Fig. 1 with a cover element 2 according to the invention and with schematized stream lines 6 of an insulation fluid streaming therethrough. The cover element 2 replaces the prior art cover element 2a of Fig. 1. The first contact 4a and the cover element 2 are arranged with respect to one another in such a way that a gap 7 is provided between them along a portion of their elongation. The gap 7 has a first opening 10 towards the first volume 15 surrounding the first contact 4a. The cover element 2 comprises one chamber 9 fluidly connected to the gap 7. Furthermore, in the shown embodiment the chamber 9 has an optional second opening 8 connecting it with the first volume 15. As can be seen, a part of the fluid streams through the first volume 15 and another part of it enters the gap 7 through the first opening 10 and streams into the chamber 9. From there it is evacuated into the first volume 15 through the second opening 8. By guiding into the chamber 9 the part of the fluid streaming in the vicinity of the first contact 4a an accumulation of hot gas at the tip of the cover element 2 is reduced, thus also reducing an erosion of the first contact 4a at that location. Furthermore, fewer particles within the fluid stream are deposited on the first contact 4a in said area and/or in the area of the guiding element 17. The majority of particles are transported into the chamber 9, where they can accumulate without causing damage to the first contact 4a. In this embodiment, the particles are evacuated from the chamber 9 through the second opening 8, thereby further improving said effect.

In an embodiment of the chamber 9 without the second opening 8 (see e.g. Fig. 4), the fluid streaming into the chamber 9 stagnates there and is mixed with colder fluid which was already present in the chamber 9. The cooled down fluid then streams back out of the chamber 9 through the gap 7. By this an additional positive effect of an increased cooling of the first contact 4a is achieved and the metal erosion is decreased because of an overall lower temperature of the fluid in the area at the tip of the cover element 2.

Fig. 3 shows the sectional partial side view according to Fig. 2 with relevant dimensions indicated in the figure.

Rl is a radius of the cover element 2 from the longitudinal axis z to a cover element wall 12c facing the gap 7. R2 is a radius of the first contact 4a, which in the context of the present disclosure is preferably a rod-type or a tube-type contact, from the longitudinal axis z to the outer surface of the first contact 4a. Al denotes an average cross section of the gap 7 and A2 an average cross section of the chamber 9. A ratio between the average cross section area A2 of the chamber 9 and the average cross section area Al of the gap 7 is preferably greater than or equal to 1.2. In particular, the cross section areas Al, A2 are measured transversely to the longitudinal axis z.

In embodiments, a width of the gap 7 ranges between 0.2 mm and 2 mm. The minimum width in the mentioned range is chosen in such a way that a high enough amount of the fluid can enter the gap and therefore the stagnation at the tip of the cover element 2 can be effectively reduced. The maximum width in said range is chosen to be small enough to ensure a high enough fluid speed in the gap 7, or in other words a strong enough stream, capable of carrying the particles into the chamber 9.

L denotes an average elongation of the chamber 9 from a first chamber wall 12a to a second chamber wall 12b. A volume of the chamber 9 is chosen in order to satisfy the equation:

V > 300 -R/P (1) with V being the volume of the chamber 9, R being a specific gas constant of the insulating gas arranged in the interior of the circuit breaker 1 and P being a filling pressure value of the insulating gas inside the circuit breaker 1. The volume of the chamber 9 can be calculated depending on its average elongation and its average cross section, taking into account that the chamber 9 is a substantially annular volume around the longitudinal axis z. This volume can be calculated according to known mathematical formulae. The dimensions A2, L in the calculated volume formula are given such values that the above equation (1) is satisfied. An inclination angle between the cover wall 12c facing the gap 7 (and being virtually elongated in axial direction) and the first chamber wall 12a is denoted by a. In other words the angle a describes an inclination angle of the first chamber wall 12a with respect to the longitudinal axis z and away from the longitudinal axis z when moving to the right (substantially downstream) . The angle a between the cover element wall 12c facing the gap 7 and the adjacent first chamber wall 12a preferably ranges between 30° (obtuse angle) and 150° (acute angle), and shown are exemplarily 90°. Please note that the inclination of the first chamber wall 12a can equivalently be described by the complementary angle 90°- inside the cover material between cover wall 12c facing gap 7 and first chamber wall 12a, or as the outside angle +180° between cover wall 12c facing gap 7 and first chamber wall 12a. Furthermore, an edge 11 of the cover element 2, formed by the cover element wall 12c facing the gap 7 and the adjacent first chamber wall 12a can have a curvature ranging between 0.2 mm and 5 mm. By these measures the turbulence of the fluid stream may be influenced. For example, the lower the value of the angle a is, meaning that the wall 12a is less inclined, and the sharper the curvature of the edge 11 is, the higher is the fluid turbulence in the area following the edge 11. A higher turbulence is desired such that vortices are created in the chamber 9, which "suck" the particles of the fluid stream inside the chamber 9 instead of allowing them to travel further towards the guiding element 17 and to deposit there. Said edge curvature and said wall inclination may also be varied depending on the volume and overall shape of the chamber 9. Optimum chamber shapes, inclinations and curvatures have been simulated on a computer using fluid dynamics simulation models and taking into account the dimensional constraints of the chamber 9 with respect to the equation (1) . Furthermore, another degree of freedom is given by the consideration of implementing a second opening 8 or not. If implemented, the second opening 8 allows further variables like its diameter, if the opening is tubular, or its tapering, if it is conical, to be taken into account in said simulations.

By varying the parameters described above it is possible to produce an optimum fluid flow into the chamber 9 with a maximum effect to achieve the already mentioned advantages of the invention.

The following figures show a non-exhaustive plurality of embodiments of the invention. The shapes of the chambers 9 in each figure is described taking into account a body of rotation about the longitudinal axis z of the cover element "slice" shown in the figures.

Fig. 4 shows the sectional partial side view according to Fig. 2 or 3 with a first embodiment of a chamber 9 of the contact arrangement according to the invention. In this embodiment the chamber 9 is ring- shaped and has no second opening.

Fig. 5 shows the sectional partial side view according to Fig. 2 or 3 with a second embodiment of a chamber 9 of the contact arrangement according to the invention. In this embodiment the chamber 9 is shaped as a truncated cone ring and has no second opening.

Fig. 6 shows the embodiment of Fig. 4 as a third embodiment of a chamber 9 according to the invention with a second opening 8 which is oriented perpendicularly to the longitudinal axis z.

The choice of the location and orientation of the second opening 8 depends, amongst others, on the circuit breaker design.

Fig. 7 shows the embodiment of Fig. 4 as a fourth embodiment of a chamber 9 according to the invention with a second opening 8 oriented parallel to the longitudinal axis z. Fig. 8 shows the sectional partial side view according to a fifth embodiment of the contact arrangement according to the invention. In this embodiment two chambers 9a, 9b, connected to the gap 7 and arranged successively with respect to the longitudinal axis z and separated by a wall 9c, in particular by a circumferential wall 9c, are provided. The chamber 9a has no second opening, whereas the chamber 9b has a second opening 8. The dimensions of the two chambers 9a, 9b are also chosen in such a way that the equation (1) described in the context of Fig. 3 is fulfilled. It is understood that a varying configuration with respect to the presence and/or location of the second opening 8 and/or the shape of the chambers 9a, 9b is possible.

Fig. 9 shows the sectional partial side view of Fig. 2 or 3 with a sixth embodiment of a chamber 9 of the contact arrangement according to the invention, containing a Teflon layer 13. The Teflon layer 13 coats the walls of the chamber 9 and is in direct contact with the hot fluid streaming into the chamber 9. In other embodiments only certain walls of the chamber 9 may be coated with the Teflon layer 13, e.g. the second chamber wall 12b (see Fig. 3), as this wall has the highest impact with the fluid streaming into the chamber 9. The hot fluid causes an evaporation of Teflon when coming into contact with it. By this, a gas is generated which helps increasing the pressure in the chamber 9 and also helps the cooling process of the fluid located in the chamber 9, thus increasing the cooling effect of the first contact 4a when the cooled fluid streams again out of the chamber 9 through the gap 7. The Teflon layer 13 is particularly preferred in an embodiment of the chamber 9 without a second opening 8. The thickness of the Teflon layer is chosen depending on the ratings of the electrical switching device 1, it is e.g. thicker than 2 mm. Fig. 10 shows the sectional partial side view according to Fig. 2 or 3 with a seventh embodiment of the contact arrangement according to the invention with projections 14 arranged on the cover element wall 12c facing the gap 7. In this way a plurality of projections 14 extending transversally to the longitudinal axis z is provided in the travel path of the fluid. By providing projections 14 and arranging them in said way in the travel path of the fluid a heat transfer enhancement (i.e. improved cooling of the fluid or hot gases) is attained. The projections 14 produce extremely stable vortices which increase the rate of convective heat transfer from the fluid to the projection surface. This is on the one hand due to the fact that the heat exchange area between the fluid and the ambient is considerably increased by the projection surfaces and on the other hand, because the flow pattern and the turbulence of the hot fluid are changed, resulting in a convective heat flow from the first contact 4a towards the cover element wall 12c, then into the cover element 2 and finally out into the first volume 15.

By providing a chamber inside a cover element of a rod-type or tube-type contact of a high-voltage electrical switching device it is possible to reduce the metal erosion of the contact and to prevent, particularly in embodiments with a moving contact, the friction between a guiding element of the contact and the contact. The consequences are a longer lifetime of the switching device, reduced maintenance, more compact and cheaper switching devices as well as a higher switching reliability due to an improved constancy of the closing speed or opening speed of the contact.

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may otherwise variously be embodied and practised within the scope of the following claims. Therefore, terms like "preferred" or "in particular" or "particularly" or "advantageously", etc. signify optional and exemplary embodiments only. List of reference numerals

1 = circuit breaker

2 = cover element according to the invention

2a = prior art cover element

3a = first nominal contact

3b = second nominal contact

4a = first arcing contact

4b = second arcing contact

6 = fluid flow

7 = gap

8 = second opening

9 = chamber

9a = first chamber

9b = second chamber

9c = wall between the first and the second chamber

10 = first opening

11 = edge

12a = first chamber wall

12b = second chamber wall

12c = cover element wall facing gap

13 = Teflon layer

14 = projections

15 = first volume

16 = shell, enclosure

17 = guiding element

18 = current carrier

19 = current finger

20 = electric arc

21 = nozzle

22 = connecting rod

23 = gear, auxiliary gear

Al = average cross section of gap

A2 = average cross section of chamber

L = average elongation of chamber

Rl = cover element radius

R2 = radius of first contact

z = longitudinal axis

a = angle between cover wall facing the gap and first chamber wall