Patent Publication Number: US-6211478-B1

Title: Switching arrangement and method for its production

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a switching arrangement in particular for power switches such as those which are used in power plants, transformer substations and other electrical power supply facilities, in order to switch operating and excess currents on and off, and to a method for its production. 
     2. Discussion of Background 
     Switching arrangements of the generic type are known, in which in particular parts of switching contacts on which the roots of the arc which is formed during switching are situated consist of material which is particularly resistant to contact erosion. Such material is usually produced by sintering a mixture of metal powders e.g. tungsten as a high-melting component and copper as a lower-melting component. This sintered material is relatively difficult to process. In particular, it is extremely brittle and can only be shaped by means of processors which remove metal. It cannot be welded in a customary manner and can only be joined to other materials by comparatively complicated processes, for example by having copper cast behind it, by friction welding, by flush butt welding or electron beam welding or by soldering which, however, produced a comparatively weak joint, or may be removably joined by means of a screw connection, which, however, requires complicated machining. The provisions of parts or coatings which are resistant to contact erosion therefore generally requires a high level of outlay. 
     Since in practice the material which is resistant to contact erosion cannot be deformed, the possibilities for shaping a base body which is to be provided with a coating which is resistant to contact erosion are extremely limited. Owing to the brittleness of the material, it is also impossible to provide flexible parts with a coating which is resistant to contact erosion. For these reasons, it is generally deemed sufficient, for example, to use tips of switching pins, arcing rings and similarly simple parts which are made from material which is resistant to contact erosion. Other parts of the arcing chamber and areas which adjoin the latter, which are also exposed to the hot gases generated when opening the switch, on the other hand, remain unprotected. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the invention is to provide a novel switching arrangement of the generic type which is easy to produce, and a method for its production. 
     The advantages which are achieved by the invention are, in addition to the fact that the switching arrangement, in particular its parts which are resistant to contact erosion, are easier to produce, lie primarily in the fact that there are considerably wider options for the application of protective layers which are resistant to contact erosion. For example, according to the invention protective layers can be applied to parts of virtually any desired shape in a variable thickness and even with a variable composition. Due to their greater flexibility, protective layers can even be applied to flexible, in particular elastically deformable parts without the flexibility of these parts being significantly impaired and without there being any risk of crack formation in the protective layer. 
     The possibility of applying protective layers which are resistant to contact erosion to virtually any desired surfaces according to local requirements eliminates significant restrictions on the design of switching arrangements, in particular of the switching contacts, and makes it possible to implement designs which otherwise would have limited or no suitability for practical applications. As a result, there is considerably greater freedom when designing switches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 shows an axial longitudinal section through a power switch having switching arrangements according to the invention, in the switched-on position on the left and in the switched-off position on the right, 
     FIG. 2 a  shows an enlarged view of an axial longitudinal section through an arcing switch arrangement of the power switch in accordance with FIG. 1, 
     FIG. 2 b  shows an enlarged, slightly modified view of a detail from the arcing switch arrangement in accordance with FIG. 2 a,    
     FIG. 3 a  shows a diagrammatic, enlarged view of an excerpt from a rated-current switching arrangement of the power switch in accordance with FIG. 1, radially from the outside and in the switched-on position, 
     FIG. 3 b  shows, on a smaller scale, a section on  3   b — 3   b  in FIG. 3 a,    
     FIG. 3 c  shows an enlarged view of an excerpt from FIG. 3 b,    
     FIG. 4 a  shows an axial longitudinal section through a further switching arrangement according to the invention, in the switched-on position on the left and in the switched-off position on the right, 
     FIG. 4 b  shows an enlarged view of an axial longitudinal section through part of the arcing switch arrangement in accordance with FIG. 4 a , corresponding to a section on  4   b — 4   b  in FIG. 4 c,    
     FIG. 4 c  shows a section on  4   c — 4   c  in FIG. 4 b,    
     FIG. 5 a  shows an axial longitudinal section through a further switching arrangement according to the invention, in the switched-on position, 
     FIG. 5 b  shows the switching arrangement of FIG. 5 a  in the switched-off position, 
     FIG. 5 c  shows an enlarged view of an axial longitudinal section through part of the arcing switch arrangement in accordance with FIGS. 5 a ,  5   b , corresponding to a section on  5   c — 5   c  in FIG. 5 d , and 
     FIG. 5 d  shows a section on  5   d — 5   d  in FIG. 5 c.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the power switch which in FIG. 1 is shown in the switched-on position on the left and in the switched-off position on the right and can be used, for example, as a generator switch has housing  1 , which is essentially rotationally symmetrical. about a switching axis  2  and has an upper housing part  3  and a lower housing part  4 , both made from metal, which are connected by a cylindrical, central housing part  5  made from insulating material. The housing parts  3 ,  4  are each connected to the opposite voltage-carrying terminals of the power switch. 
     At the level of the central housing part  5 , a rated-current switching arrangement is arranged on the outside, which switching arrangement comprises circumferential, fixed rated-current contacts, which respectively adjoin the upper housing part  3  and the lower housing part  4  and are spaced apart from one another in the axial direction, namely an upper fixed rated-current contact  6  and a lower fixed rated-current contact  7 , as well as a movable rated-current contact  8  with contact fingers which follow one another in the circumferential direction and, in the switched-on position, in each case bridge the distance between the fixed rated-current contacts  6 ,  7 . The movable rated-current contact  8  is connected to a switching drive (not shown), by means of which it can be displaced in the axial direction between the switched-on position, in which it bridges the gap between the upper fixed rated-current contact  6  and the lower fixed rated-current contact  7 , and the switched-off position, in which it is at a distance from the upper fixed rated-current contact  6 . 
     At the bottom, the upper housing part  3  is closed off by a horizontal partition  9 . This partition supports the fixed part of an arcing switch arrangement  10 . In a central opening in the partition  9 , there is a tulip contact  11 , as first switching contact, with a plurality of elastic contact fingers which follow one another in the circumferential direction, are directed obliquely downward and toward the switching axis  2  and are separated by slots. A nozzle  12  which surrounds the switching axis  2 , is made from electrically insulating material and is in the shape of a funnel which tapers upward, is arranged opposite the tulip contact  11 . A switching pin  14 , which can be moved in the axial direction by means of the switching drive and, in the switched-on position, projects into the tulip contact  11  and on the outside is in contact with the contact fingers of said tulip contact, is mounted as the second switching contact in a slideway  13 , which is arranged in the lower housing part  4  and also produces a connection with good electrical conductivity. In the switched-on position, the contact fingers are elastically deformed slightly, so that they exert a comparatively high contact pressure on the switching pin  14 . The slideway  13  is secured to a partition  15  which closes off the lower housing part  4  at the top. The nozzle  12  is attached in a central opening in the partition  15 . 
     In the switched-off position, the switching pin  14  is pulled downward, so that its tip lies beneath the nozzle  12 . An arcing chamber  16  is then formed between the tulip contact  11  and the switching pin  14 , in which chamber an arc  17  has formed between the said switching contacts during the switching-off operation. The arcing chamber  16  is surrounded by a continuous annular heating volume  18  which is connected to the arcing chamber by the gap which separates the tulip contact  11  from the nozzle  12  and forms an encircling blowing slot  19 . On the outside, the heating volume  18  is closed off by an encircling wall  20  made from insulating material. A plurality of, for example four, blowing cylinders  21  are distributed over the circumference of the partition  15  and have blowing pistons  22 , which can be actuated by the switching drive and are connected to the heating volume  18 , in each case via blowing channels  23 . Nonreturn valves  24  are fitted at each of the openings of the blowing channels  23  leading into the heating volume  18 . 
     In order for the arrangement to be switched off, the movable rated-current contact  8  and the switching pin  14  are pulled downward. The movable rated-current contact  8  is disconnected from the upper fixed rated-current contact  6 , so that the current from the rated-current switching arrangement is switched to the arcing switch arrangement. When the switching pin  14  is disconnected from the tulip contact  11 , the arc  17  is then drawn which, when the switching pin  14  has reached the switched-off position, connects the tulip contact  11  to the tip of the switching pin  14  through the nozzle  12 . As a result of the heat which emanates from the arc  17  and the pumping action of the blowing cylinders  21 , the pistons  22  of which were moved downward together with the switching pin  14 , a high pressure builds up in the heating volume  18 , which pressure generates a strong flow of arc-extinguishing gas through the tulip contact  11  and the nozzle  12  and extinguishes the arc  17  at the next current zero crossing. 
     The tulip contact  11  (FIG. 2 a ), as the first switching contact of the arcing switch arrangement, comprises individual contact fingers  25  which surround the switching axis  2 . They are elastically deformable to a slight extent and their tips, in the switched-on position, are deflected outward slightly by the contact with the switching pin  14 , thus ensuring sufficient contact pressure. The tulip contact  11  has a base body  26  which is made from copper or another suitable material and the surface of which, in the area of the contact fingers  25 , lies below the final dimension of the surface and, in the area, bears a protective layer  27  which is resistant to contact erosion and fills up the difference with respect to the final dimension. The protective layer  27  is produced by means of the plasma-spraying process which is well known from other technical fields. The composition of the protective layer may substantially correspond to that of the conventional material which is resistant to contact erosion. 
     The switching pin  14 , which is the second switching contact of the arcing switch arrangement, also comprises a base body  28 , which is made, for example, from a copper alloy or some other known material which is suitable for the purpose, and the surface of which, at the tip and in the adjoining area which lies below the final dimension of the surface, said final dimension only being produced by means of a protective layer which is again applied to the base body  28  by means of plasma spraying. At the tip of the switching pin  14 , the protective layer  29  forms a relatively solid cap which tapers into a somewhat thinner casing. In accordance with FIG. 2 a , the casing extends beyond the contact area with which the contact fingers  25  of the tulip contact  11  are in contact in the switched-on position. 
     FIG. 2 b  shows a slightly different design of the protective layer  29 , according to which it stops in front of this contact area. Since the conductivity of the protective layer  29  is less than that of the base body  28 , the contact resistance is consequently lower and it is easier to switch the current to the arcing switch arrangement. Since, in the arcing switch arrangement outlined above, hot gases flow through the tulip contact  11  while the arrangement is being switched off, it is recommended for the protective layer of said tulip contact to extend at least onto the front area and the inside. In the case of the switching pin  14 , on the other hand, the areas which lie slightly further behind the tip are not subjected to high loads and generally do not require a protective layer. In the case of switching arrangements in which those parts of the two switching contacts which lie behind the front areas are not subjected to relatively high loads, it is conceivable for the protective layers not to extend as far as the contact area of both sides, thus reducing the contact resistance further. 
     The extent and thickness of the protective layers  27 ,  29  can be adapted with great accuracy to the requirements which are dependent on the load on the switching contacts. In general, it is sufficient for protective layers to be applied to the disconnection areas of the switching contacts, at which they detach from one another when the arrangement is being switched off and where arc roots are first formed, and at the front areas which lie opposite one another in the switched-off position and between which, consequently, the arc burns and which are subjected to particularly high loads from radiation and hot gases. However, it is also quite possible and, under certain circumstances, sensible for other parts of the arcing chamber, such as for example wall sections, also to be protected from arc-heated gases by means of a protective layer which is resistant to contact erosion and is applied by means of plasma spraying. 
     The rated-current switching arrangement of the power switch in accordance with FIG. 1 comprises the upper fixed rated-current contact  6  as the first switching contact and the movable rated-current contact  8  as the second switching contact. The latter has (FIGS. 3 a ,  3   b ) several hundred parallel contact fingers which are distributed over the circumference of the rated-current switching arrangement and, combined to form groups of a plurality of contact fingers, are in each case mounted on an axially displaceable support ring  31  by means of a pressure spring  30 . In this case, a plurality of groups of contact fingers  32  are in each case followed by one group of slightly longer power-switch fingers  33 . The upper fixed rated-current contact  6  is designed as a contact ring  34 , against the outside of which the contact fingers  32  and the power-switched fingers  33  bear in the switched-on position. 
     In turn (FIG. 3 c ), the power-switch fingers  33  comprise a base body  35  which, on the spherical cap facing toward the contact ring  34 , has a protective layer  36  which is resistant to contact erosion and is again applied by means of plasma spraying. The same applies to the contact ring  34  which, on its switched-off-side edge has a protective layer  37  which on the outside is drawn slightly upward. Above the protective layer  37 , the contact ring  34  has a silver-coated contact zone  38  which, in the switched-on position, is in contact not only with the power-switch fingers  33  but also with the slightly shorter contact fingers  32 , which are likewise silver-coated. The rated-current switching arrangement has a very high continuous current-carrying capacity and a very low contact resistance. 
     When the arrangement is being switched off, firstly, the contact fingers  32  are disconnected from the contact ring  34 , after which the current switches entirely onto the power-switch fingers  33 . When the latter are also disconnected from the contact ring  34 , an arc is formed between the disconnection areas at the ends of the power-switch fingers  34  and at the edge of the contact ring  34  before the current switches completely to the arcing switch arrangement, the protective layers  36  and  37  ensuring that the contact erosion is kept within tight limits. 
     The following text describes two further examples of arcing switch arrangements, in which particular options which the invention opens up are employed. In particular, they comprise switching contacts which, in order to utilize electromagnetic forces so as to increase the contact pressure, are of complex form and are to some extent flexible, but at the same time are provided with a protective layer which is resistant to contact erosion and substantially meets the requirements. In particular, they have switching pins, each with a resistant section which adjoins the tip, has a protective layer on the outside and is divided into two or more parallel or antiparallel conductor elements. 
     A second embodiment according to the invention of an arcing switch arrangement of a power switch, which is illustrated in FIGS. 4 a - 4   c  in the switched-on position on the left and in the switched-off position on the right, and, e.g. in the power switch in accordance with FIG. 1, can replace the first embodiment of an arcing switch arrangement according to the invention illustrated in FIGS. 1 and 2 a ,  2   b —the corresponding components therefore bear the same reference numerals—again has an annular heating volume  18  inside a housing which is rotationally symmetrical about a switching axis  2  and is made from insulating material, which heating volume surrounds a first switching contact, which is connected to the first electrical terminal, and a second switching contact. The first switching contact is designed as a first switching ring  39  which is attached to the underside of the partition  9 , and the second switching contact is designed as a switching pin  14 . A second switching ring  40 , which is attached to the top side of the partition  15 , is arranged opposite at a distance from the first switching ring  39 , in such a manner that an arcing chamber  16 , which is connected to the heating volume  18  via an encircling blowing slot  19 , is located between these switching rings, which are arranged concentrically with respect to the switching axis  2 . 
     Further down the switching pin  14  is surrounded by a tulip slide  41  which, like the second switching ring  40 , is connected to the second electrical terminal. The switching pin  14  has a carrier, which is designed as a central mandrel  42  and into the tip of which a cap  43  is screwed, which cap is made from material which is resistant to contact erosion and clamps a sleeve  44  made from highly conductive spring-elastic material, in particular a ring  45 , at its front end. A group of eight elongate contact fingers  46 , which are arranged at the same level on the switching pin  14 , are separated by slots and project toward the rear, so as to surround the mandrel  42  approximately parallel to the latter, starts from the ring  45 . The mandrel  42  is surrounded, from the cap  43  to beyond the ends of the contact fingers  46 , by an insulation sleeve  47 , which is overlapped by a thicker insulation ring  48 . 
     In the switched-on position, the contact surfaces  49  which lie just before the ends of the contact fingers  46  are in contact with the inside of the first switching ring  39 . The switching pin  14  substantially fills the opening of said first switching ring, as well as that of the second switching ring  40 , in which the insulation ring  48  is located. The current path runs from the first switching ring  39 , via the contact surfaces  49 , into the contact fingers  46  and, through the latter, to the ring  45  and on through the mandrel  42  and the tulip slide  41 . The front part of the mandrel  42 , which is surrounded by the contact fingers  46 , forms one conductor element which carries a current which is antiparallel to the currents in the contact fingers  46 , which likewise form conductor elements and to which it is electrically conductively connected by the ring  45 . The resultant electromagnetic repelling force between the mandrel  42  and the contact fingers  46  spreads the latter apart and presses their contact surfaces  49  against the inside of the first switching ring  39 . The resultant contact forces, as well as the opposite contact-disconnection forces which are oppositely directed thereto, intensify as the current intensity increases, resulting in a compensation of the forces which is independent of the current intensity. 
     During the first phase of the switching-off movement, that area of the switching pin  14  which is in contact with the first switching ring  39  moves toward the cap  43 , so that the length of the antiparallel current paths decreases relatively quickly as, at the same time, do the contact forces. When the switching pin  14  is pulled out of the first switching ring  39 , an arc forms between the latter and the cap  43 . When the cap  43  moves past the second switching ring  40 , the arc root jumps from the cap onto the second switching ring, so that the arc then burns between the first switching ring  39  and the second switching ring  40 . The air is blown out of the heating volume  18  and is extinguished at the next zero crossing of the current. 
     The first switching ring  39 , the second switching ring  40  and the cap  43 , since these are simple rigid parts, comprise solid material which is resistant to contact erosion and has been produced in a known way by sintering. However, they could also, in a similar manner to the tip of the switching pin in accordance with FIG. 2 b , in each case comprise a base body which is made, for example, from a copper alloy and bears a protective layer of material which is resistant to contact erosion applied by plasma spraying. 
     The flexible sleeve  44 , which is of more complicated shape, is, at any rate, composed of a base body  50  and a protective layer  51  which forms its outer surface and is made from material which is resistant to contact erosion, applied to the base body  50  by plasma spraying. The base body  50  comprises an elastic material of good conductivity. The protective layer  51  is sufficiently flexible to follow the elastic deformations to which the contact fingers  46  are subjected. It is thus possible to protect not only those parts which are subjected to particularly high loads, such as the tip of the switching pin  14 , the first switching ring  39  and the second switching ring  40 , on which arc roots are formed, but also the entire outside of a resistant section of the switching pin  14 , which section is formed by the cap  43 , the sleeve  44  and the front part of the mandrel  42  located inside them, and is likewise subjected to high loads from the hot gases which flow out. 
     In the third embodiment according to the invention of an arcing switch arrangement, which is illustrated in FIGS. 5 a - 5   d  and otherwise corresponds to the second embodiment, the attraction between parallel currents is used to apply the necessary contact forces. The switching pin  14  has two parallel, elastically flexible extensions  52   a ,  52   b , which adjoin the end of the carrier which is again designed as a mandrel  42  and are separated from one another by a slot  53 . At the end, each of the extensions  52   a ,  52   b  has a contact piece  54   a  or  54   b  respectively with a contact surface  49  for making electrical contact with the inner surface of the first switching ring  39 , to which it is connected, by means of a connection piece  55   a  or  55   b , in such a manner that each of the contact pieces  54   a ,  54   b  is offset through 180° from the respective extension  52   a  or  52   b  with reference to a switching-pin axis which coincides with the switching axis  2 . The connection pieces  55   a, b  are designed as short screw sections which form half a turn. 
     The contact pieces  54   a ,  54   b  are separated from one another by a continuation  53 ′ of the slot  53 . Together, they have a polygonal cross section, in this example a dodecagonal cross section. The first contact piece  54   a  runs out into the hemispherical tip of the switching pin  14 . Apart from this difference, the parts of the switching pin  14 , which each comprise an extension  52   a  or  52   b , a connection piece  55   a  or  55   b  and a contact piece  54   a  or  54   b  and are formed integrally with the mandrel  42  made from highly conductive spring-elastic material, completely correspond to one another. 
     In the switched-on position illustrated in FIG. 5 a , in which the contact pieces  54   a ,  54   b  are pressed slightly toward one another as a result of contact with the first switching ring  39  and the extensions  55   a ,  55   b  are correspondingly spread apart, so that the contact surfaces  49  are pressed against the inside of the first switching ring  39  even by elastic restoring forces, the current path runs through the first switching ring and the contact surfaces  49 , into the contact pieces  54   a ,  54   b , through the latter and the connection pieces  55   a ,  55   b , the extensions  52   a ,  52   b  and a section of the mandrel  42  and on across the tulip slide  41 . The second switching ring  40  is not in contact with the switching pin  14 . The two comparatively long extensions  52   a ,  52   b  carry parallel currents and are therefore attracted to one another. As a result, the contact pieces  54   a ,  54   b  which are connected thereto and are offset by 180° with respect thereto are pressed apart, and their contact surfaces  49  consequently press more strongly against the inside of the first switching ring  39 . As a result of the polygonal cross section of the switching pin  14  in the area of the contact surfaces  49 , it is always in contact with the first switching ring  39  at at least four points. 
     Shortly after the switching-off movement begins, the contact pieces  54   a ,  54   b  also come into contact with the second switching ring  40 , thus partly short-circuiting the current path described above. As a result, the electromagnetic attraction between the extensions  52   a  and  52   b , and likewise the contact forces generated by this attraction, are reduced. Consequently, the further retraction of the switching pin  14  is not impeded by excessively high friction forces. When the tip of the switching pin  14  is pulled out of the opening of the first switching ring  39 , an arc forms between these parts. When the tip of the switching ring  14  then moves past the opening of the second switching ring  40 , the arc switches to the latter. It then burns between the first switching ring  39  and the second switching ring  40 , is subjected to a blowing action from the heating volume  18  and is extinguished at the next current zero crossing. 
     In this case too, the first switching ring  39  and the second switching ring  40  comprise, in a conventional way, solid sintered material which is resistant to contact erosion. However, the switching pin  14  again comprises a base body  56  made from highly conductive elastic material which, in the area of the resistant section which is formed by the connection pieces  55   a ,  55   b  and the adjoining contact pieces  54   a ,  54   b  and is divided into two conductor elements, bears a protective layer  57  which is resistant to contact erosion and has been applied by means of plasma spraying. In the area of the tip of the switching pin  14 , which tip is subjected to particularly high loads as a result of the arc root, the protective layer is relatively thick, while on the outer surfaces of the contact pieces  54   a ,  54   b  and of the connection pieces  55   a ,  55   b  it is somewhat thinner. It is also conceivable for the hemispherical tip of the switching pin to be designed as a cap made from solid material which is resistant to contact erosion and has been sintered in a customary manner. In any case, the entire outside of the complex-shaped, resistant section of the switching pin  14 , along which some of the emerging hot gases flow, is formed by the surface of the protective layer  57  which is resistant to contact erosion. 
     Since the application of a protective layer which is resistant to contact erosion is considerably simplified by the invention, it is practicable to protect not only parts of switching contacts but also other parts which are subjected to loads from the hot gases generated during the switching-off operation. For example, the encircling wall surfaces of the widening exhaust openings adjoining the arcing rings  39 ,  40  are likewise formed by protective layers  58 ,  59  produced by plasma spraying on base bodies  60 ,  61  of the partitions  9 ,  15 . 
     During the plasma-spraying process which is used to apply a protective layer which is resistant to contact erosion to a base body, a high electric field is used to generate, from a suitable plasma gas, a plasma into which a powder mixture is introduced by means of a carrier gas. The powder mixture is liquefied and, together with the gas, is accelerated through the electric field toward the base body, onto the surface of which it is sprayed, forming a layer which rapidly solidifies. To avoid oxidation, the plasma spraying is preferably carried out in vacuo. 
     The resistance to contact erosion of the resultant protective layers is in no way inferior to that of parts which are resistant to contact erosion which have been produced in a conventional manner. The protective layers are also relatively flexible, so that any deformation of the base body is not impeded. The thickness of the layer applied by plasma spraying can be set accurately and variably. Therefore, metal-removing machining is therefore usually only necessary to a slight extent, mainly in order to adjust the surface properties. Above all, it is often useful to reduce the surface roughness by grinding or polishing. The removal of greater volumes of material, for example by milling, is also possible but not usually necessary. 
     With regard to the composition of the powder mixture which is used in each case to produce the protective layer which is resistant to contact erosion on the base body and which therefore also corresponds to the composition of the protective layer, there are numerous possibilities. The mixture can substantially be adapted to meet the particular requirements. Generally, as with known sintered materials which are resistant to contact erosion, the mixture will normally contain a high-melting component, in order to obtain good resistance to contact erosion, and a lower-melting component which, due to evaporation, contributes to cooling. In most cases, it ought to be advantageous for high-melting metals with a melting point of at least 2000° C., such as W, Mo or Ir, to form at least 10% (by weight), preferably by at least 50% (by weight), while Cu, Ag, Ti, Fe can be used as lower-melting material with a melting point of below 2000° C. As with conventional sintered material which is resistant to contact erosion, a mixture of tungsten and copper has proven particularly successful, particularly at levels of 80% (by weight) tungsten and 20% (by weight) copper. Other copper alloys, in particular with Mo, are also advantageous. 
     In addition, it is also possible to use protective layers which comprise exclusively high-melting material or—in particular for applications in which the loads are comparatively low—protective layers which do not contain any high-melting components, but rather comprise, for example, only copper with an addition, for example chromium. In fact, very many compositions are possible, provided that the resistance to contact erosion is sufficient for the particular application. For example, in addition to those mentioned above, other suitable constituents of the powder mixture are Au, Ru, Pd, Os, Pt and, in addition, Ni, Cd, Sn, C. 
     There are also many suitable materials for the base body, which can be selected depending on requirements, such as for example Cu, Ag, Fe, steel, Al or, if high conductivity and, at the same time, elasticity of the material are required, a flexible copper alloy, such as CuBe, CuCr or CuCrZr. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.