Patent Publication Number: US-11043339-B2

Title: Electrical switch

Description:
FIELD 
     The invention relates to an electrical switch. 
     BACKGROUND 
     There are a variety of electrical switches on the market with fixed and movable contacts. The movable contacts make connections between the stationary contacts. The electrical switch can, in its simplest case, only have two fixed contacts and one movable contact that performs coupling and disconnection between these two fixed contacts. On the other hand, the electrical switch can be a so-called transfer switch, which can include three fixed contacts. The transfer switch can comprise two switching states such that in the first switching state the first fixed contact is connected to the third fixed contact and in the second coupling mode, the second fixed contact is connected to the third fixed contact. The transfer switch can further comprise a third state i.e. a zero state in which all three fixed contacts are isolated from each other. The transfer switch can be used in a situation where it is necessary to connect the bad to a primary power supply or to a secondary power supply. Such a need is for example in hospitals where the primary power supply is the electrical grid and the secondary power supply is an emergency power plant. The load is thus coupled to the third fixed contact and the primary power source to the first or second fixed contact and the secondary power source respectively to the second or the first fixed contact. 
     The electrical switches can be provided with bumper contacts or knife contacts. The contact in the bumper contact structure is pressed to the fixed contacts. In the knife contacts, the movable contact consists of two blades hinged at one end to a fixed contact and the other end acts as a separating part. The knife contact construction can also be implemented with two openings so that the blades are connected to a rotating roller or so that the blades move straight up and down. The knife contacts are normally used in switches designed for a nominal current over 63 ampere and bumper contacts are used in switches designed for smaller currents. 
     CH 330629 discloses an electrical switch. The switch comprises a first fixed contact, a second fixed contact, a third fixed contact, and a rotatable knife contact comprising a pair of longitudinal blades flexibly connected to each other. The rotational axis of the rotatable knife contact is positioned at a second end of the rotatable knife contact. The rotational axis of the rotatable knife contact is positioned on the third fixed contact. A first end of the rotatable knife contact forms in a first switching event contact with the first fixed contact so that an electrical connection is formed between the first fixed contact and the third fixed contact. The first end of the rotatable knife contact forms in a second switching event contact with the second fixed contact so that an electrical connection is formed between the second fixed contact and the third fixed contact. The first fixed contact and the second fixed contact may comprise a bulge forming the contact portion in said fixed contacts. The area on the rotatable contact and on the first and the second fixed contact where a continuous contact is established may be provided with a silver coating. 
     U.S. Pat. No. 5,969,308 discloses a rotary switch including spring biased knife blade contacts. The rotary switch comprises a pair of curved or channel shaped conductive blades having free ends which engage a fixed line contact and a fixed load contact. The blades are resiliently supported on each other. The rotor assembly rotates between a switching event in which opposite ends of the blades contact the two fixed contacts simultaneously, whereby electrical contact is formed between the two fixed contacts and a non-switching event in which neither of the opposite ends of the blades contact the two fixed contacts, whereby the electrical contact between the two fixed contacts is broken. 
     SUMMARY 
     The invention relates to an improved electrical switch. 
     The electrical switch according to the invention is defined in claim  1 . 
     The electrical switch comprises: 
     a first fixed contact, 
     a second fixed contact, 
     a rotatable knife contact having a rotational axis and comprising at least one longitudinal pair of blades flexibly connected to each other, whereby the blades form, in a switching event, contact with contact portions of the first and/or the second fixed contact. 
     The electrical switch is characterized in that 
     the rotational axis (Z 1 -Z 1 ) of the rotatable knife contact ( 400 ) is positioned in a middle portion ( 450 ) of the rotatable knife contact ( 400 ), 
     a third fixed contact is positioned on an opposite side of the rotational axis of the rotatable knife contact in relation to the first and the second fixed contact being adjacent to each other, 
     the rotatable knife contact is electrically connected from the middle portion to the third fixed contact in all positions of the rotatable knife contact, whereby 
     the rotatable knife contact is rotatable between a first switching position in which a first outer end of the rotatable knife contact makes contact to the first fixed contact so that an electrical connection is formed between the first fixed contact and the third fixed contact and a second witching position in which a second opposite outer end of the rotatable knife contact makes contact to the second fixed contact so that an electrical connection is formed between the second fixed contact and the third fixed contact. 
     The electrical switch is a transfer switch in which both ends of the rotatable knife contact are utilized alternatively in the switching operation. 
     The third fixed contact may be connected with a permanent electrical connection to the middle portion of the rotatable knife contact. Both ends of the blades in the rotatable knife contact are thus free to form switching. The pole of the transfer switch may thus be realized with one rotatable knife contact and two separate contact chambers. 
     Several parallel pairs of blades may be provided in the rotating knife contact of the electric switch. 
     The blades in each pair of blades in the rotatable knife contact may be connected flexibly to each other so that the blades due to a separating force acting on either end of the blades can take a V-shape. An increase of the distance between the first end of the blades leads to a decrease of the distance between the second end of the blades and vice a versa. 
     An outwardly protruding area may be provided on an inner surface of each blade at a distance from an outer tip of the blade in the pair of blades of the rotatable knife contact. The outwardly protruding area may alternatively be provided on opposite outer surfaces of the contact portion of the first and the second fixed contact. The outwardly protruding areas will cause the separating force of the blades when the rotatable knife contact makes contact with the first or second fixed contact. 
     The nominal current in the continuous switching position flows thus only through the outwardly protruding areas between the blades of the rotatable knife and the first or second fixed contact. There are thus two separate functional areas in the blades of the rotatable knife contact and the first or second fixed contacts. A first area at the tip of the contacts for initial switching and a second area at a distance from the tip of the contacts for nominal current in the continuous switching position. 
     The third fixed contact may have a T-shape or an L-shape. This means that a free joint surface is formed on the horizontal arm of the third fixed contact within the housing. A braided cable may thus be used to connect the third fixed contact to the rotatable knife contact. One end of the braided cable may be attached to the free joint surface on the horizontal arm of the third fixed contact by welding, soldering or with a pressure joint and the other end of the braided cable may be attached to the middle portion of the rotatable knife contact by welding, soldering or with a pressure joint. 
     The attachment of the braided cable to the horizontal branch of the third fixed contact, makes it possible to use a longer braided cable which means that more play for the braided cable is achieved. A flexible joint that withstands well movement of the fourth contact is thus achieved between the third fixed contact and the fourth rotatable knife contact. Thus, the third fixed contact and the rotatable knife contact are electrically connected through the braided cable in all positions of the rotatable knife contact. 
     By forming the third fixed contact into a T-shape or L-shape, a support surface can be formed inside the housing against which the horizontal arm of the third fixed contact can be supported. The third fixed contact can thus be secured firmly to the housing. 
     The horizontal arm of the third fixed contact inside the housing also allows for temperature measurement of the horizontal arm. An opening can be provided in the housing wall from the outer surface of the housing wall through the support surface to the horizontal arm. The temperature sensor can thus be connected from the outside of the housing to the horizontal axis. The temperature of the horizontal arm inside the housing can be measured at the point where the braided cable is or the braided cables are attached to the horizontal arm i.e. from the most critical point. The temperature measurement thus indicates the condition of the joint between the third fixed contact inside the housing and the braided cable. 
    
    
     
       DRAWINGS 
       The invention will be described with reference to the accompanying drawings in which 
         FIG. 1  shows an axonometric view of an electrical switch, 
         FIG. 2  shows the electrical switch with the upper half of the housing being removed, 
         FIG. 3  shows an exploded view of a rotatable knife contact and a roller of the electrical switch, 
         FIG. 4  shows a rotatable knife contact of the electrical switch, 
         FIG. 5  shows a rotatable knife contact and a roller of the electrical switch, 
         FIG. 6  shows a fixed contact of the electrical switch, 
         FIG. 7  shows a blade of a rotatable knife contact of the electrical switch, 
         FIG. 8  shows a contact pin of a contact of the electrical switch, 
         FIG. 9  shows a fixed contact and a rotatable knife contact of the electrical switch, 
         FIG. 10  shows a non-modular three phase electrical switch. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an axonometric view of an electrical switch. 
     The electric switch  500  comprises a housing  10  having a longitudinal direction Y-Y, a height direction X-X perpendicular to the longitudinal direction Y-Y, and a thickness direction Z-Z perpendicular to the longitudinal direction Y-Y and to the height direction X-X. The height direction X-X and the thickness direction Z-Z form transverse directions in relation to the longitudinal direction Y-Y of the housing  10 . 
     The housing  10  consists of two halves  10 L and  10 U. The first half  10 L of the housing  10  is placed against the second half  10 U of the housing  10  so that a substantially closed space is formed within the two halves  10 L,  10 U. Each half  10 L of the housing  10  comprises a side panel  10 E,  10 F and side walls  10 A,  10 B,  10 C,  10 D extending perpendicularly from the peripheral edges of the side panels  10 E,  10 F. The outer edges of the side walls  10 A,  10 B,  10 C,  10 D of the halves  10 L,  10 U of the housing  10  are placed against each other when the halves  10 L,  10 U of the housing  10  are joined together. The outer edges of the side walls  10 A,  10 B,  10 C,  10 D of the halves  10 L,  10 U of the housing  10  may comprise nested projections, whereby the joint between the two halves  10 L,  10 U of the housing  10  can be made to sustain the pressure caused by arcs within the housing  10 . 
     A first side wall  10 A and a second side wall  10 B of the housing  10  are positioned spaced apart from each other in a longitudinal direction Y-Y of the housing  10 . The first side wall  10 A and the second side wall  10 B are positioned opposite to each other. The first and the second side walls  10 A,  103  extend in the height direction X-X and in the thickness direction Z-Z of the housing  10 . 
     A third and a fourth side wall  10 C,  10 D connect the edges of the first side wall  10 A and the second side wall  103 . The third side wall  10 C and the fourth side wall  10 D are positioned opposite to each other. The third and the fourth side wall  10 C,  10 D extend in the longitudinal direction Y-Y and in the thickness direction Z-Z of the housing  10 . 
     The side panels  10 E,  10 F are positioned spaced apart from each other in the thickness direction Z-Z of the housing  10 . The side panels  10 E,  10 F connect the opposite edges of the side walls  10 A,  10 B,  10 C,  10 D. The side panels  10 E,  10 F extend in the longitudinal direction Y-Y and in the height direction X-X of the housing  10 . 
     Each half  10 L,  10 U of the housing  10  is also provided with mounting holes  21 ,  22 ,  23 ,  24  extending through the housing  10 . The two halves  10 L,  10 U of the housing  10  may be secured to each other with mounting bolts and nuts extending through these fastening openings  21 ,  22 ,  23 ,  24 . The first half  10 L and the second half  10 U of the housing  10  may further have adjustment means or adjustment surfaces for adjusting the two halves  10 L,  10 U in a correct position in relation to each other. 
     A first fixed contact  100 , a second fixed contact  200 , and a third fixed contact  300  is provided in the housing  10 . Each of these three fixed contacts  100 ,  200 ,  300  is connectable to an external electrical circuit with respect to the housing  10 . The housing  10  is further provided with a rotatable knife contact  400  positioned wholly in the interior of the housing  10 . The rotatable knife contact  400  is mounted on a roller  80  having a second end protruding out from an opening  19  in the side plane  10 F of the housing  10 . The rotatable knife contact  400  is shown in  FIG. 2 . 
     The cross section of the housing  10  may be substantially rectangular. 
       FIG. 2  shows the electrical switch with the upper half of the housing being removed. 
     The figure shows the position of the first fixed contact  100 , the second fixed contact  200 , the third fixed contact  300 , and the rotatable knife contact  400  fitted into the housing  10  is seen. The figure shows further a longitudinal center line Y 1 -Y 1  and a transverse center line X 1 -X 1  of the housing  10 . 
     The first fixed contact  100  comprises a substantially straight connection portion  110  and a contact portion  120  within the housing  10 . The connection portion  110  of the first fixed contact  100  extends along a first connecting channel from the inside to the outside of the housing  10  through a first opening  11 A in the first side wall  10 A of the housing  10 . The first connecting channel is constituted of a groove half in each half  10 L,  10 U of the housing  10 , which groove halves are positioned opposite to each other to form the first connecting channel when the halves  10 L,  10 U of the housing  10  are joined together. The connection portion  110  of the first fixed contact  100  can thus be connected to an external electrical circuit with respect to the housing  10 . The contact portion  120  of the first fixed contact  100  acts as a plate-like contact surface. The contact is formed from both opposing surfaces of the contact portion  120 . 
     The second fixed contact  200  comprises in a similar way a substantially straight connection portion  210  and a contact portion  220  within the housing  10 . The connection portion  210  of the second fixed contact  200  extends along a second connecting channel from the inside to the outside of the housing  10  through a second opening  11 B in the first side wall  10 A of the housing  10 . The second connecting channel is formed of a groove half in each half  10 L,  10 U of the housing  10 , which groove halves are positioned opposite to each other to form the second connection channel when the halves  10 L,  10 U of the housing are joined together. The connection portion  210  of the second fixed contact  200  can thus be connected to an external electrical circuit with respect to the housing  10 . The contact portion  220  of the second fixed contact  200  acts as a plate-like contact surface. The contact is formed from both opposing surfaces of the contact portion  220 . 
     The first fixed contact  100  and the second fixed contact  200  are positioned adjacent to each other within the housing  10  on opposite sides of the longitudinal center line Y 1 -Y 1  of the housing  10 . The connection portion  110  of the first fixed contact  100  and the connection portion  210  of the second fixed contact  200  may be parallel and extend essentially in the longitudinal direction Y-Y of the housing  10 . 
     The third fixed contact  300  is, in this embodiment, formed by two L-shaped pieces  310 ,  320  forming together a T-shaped body. The horizontal arms  312 ,  322  of the two L-shaped bodies  310 ,  320  point in opposite directions and the vertical arms  311 ,  321  of the two L-shaped bodies  310 ,  320  are clamped against each other back to back. The horizontal arm  312  of the first L-shaped body  310  may be seated against a first support surface  12 A inside the housing  10 . The horizontal arm  322  of the second L-shaped body  320  may be seated against a second support surface  12 B inside the housing  10 . The vertical arms  311 ,  321  of the L-shaped bodies  310 ,  320  extend along a third connection channel from the inside to the outside of the housing  10  through a third opening  11 C in the second side wall  10 B of the housing  10 . The third connection channel is formed of a groove half in each half  10 L,  10 U of the housing  10 , which groove halves are positioned opposite to each other to form the third connection channel when the halves  10 L,  10 U of the housing  10  are joined together. 
     The third fixed contact  300  is thus located on the opposite side of the housing  10  in respect of the first and the second fixed contacts  100 ,  200 . The first support surface  12 A and the second support surface  128  may extend in the height direction X-X of the housing  10 , The horizontal arm  312 ,  322  of each of the L-shaped bodies  310 ,  320  has an outwardly from the housing  10  directed face, which seats against the respective support surface  12 A,  12 B of the housing  10  and a free surface directed to the interior of the housing  10 . 
     The vertical arms  311 ,  321  of each of the L shaped bodies  310 ,  320  of the third fixed contact  300  may extend substantially in the longitudinal direction Y-Y of the housing  10 . 
     The horizontal arm  312  of the first L shaped body  310  of the third fixed contact  300  is connected by at least one first braided cable  31  to a middle portion  450  of the rotatable knife contact  400 . The horizontal arm  322  of the second L-shaped body  320  of the third fixed contact  300  is connected by at least one second braided cable  32  to the middle portion  450  of the rotatable knife contact  400 . The braided cables  31 ,  32  are attached to the free surface directed towards the interior of the housing  10  of the horizontal arm  312 ,  322  of the L-shaped body  310 ,  320 . The middle portion  450  of the rotatable knife contact  400  may be provided with a protrusion for attaching the braided arms  31 ,  32 . The braided cables  31 ,  32  form an electrical connection between the third fixed contact  300  and the rotatable knife contact  400 . 
     A measurement aperture  17  may be provided in the second side wall  10 B of the housing  10 . The measurement aperture  17  extends through the second side wall  10 B of the housing  10  and through the second support surface  123  to the horizontal arm  322  of the third fixed contact  320 . A temperature sensor may be attached from the measurement aperture  17  to the surface of the horizontal arm  322  of the third fixed contact  320  that is seated against the second support surface  12 B. The temperature of the portion of the third fixed contact  320  that is inside the housing  10 , i.e. the horizontal arm  322 , can thus be measured from the point where the joint between the horizontal arm  322  of the third fixed contact  320  and the braided cable  32  is located. At a specified load current, the temperature in the joint remains stable, whereby a change in the temperature indicates a problem at the joint. In the case of two L-shaped pieces or one T-shaped piece, it is sufficient to measure the temperature only in one of the second horizontal arms, because also a problem in the joint in the opposite horizontal arm is displayed at the measuring point when a larger part of the current attempts to pass through the intact joint. 
     The rotatable knife contact  400  comprises at least one longitudinal pair of blades  410 ,  420  with two opposite outer ends  401 ,  402 . The rotatable knife contact  400  rotates in respect of the housing  10  around a rotational axis Z 1 -Z 1 . The rotatable knife contact  400  may be turned between a first switching position and a second switching position. A first outer end  401  of the rotatable knife contact  400  makes, in the first switching position, contact to the first fixed contact  100 , whereby an electrical connection is formed between the first fixed contact  100  and the third fixed contact  300 . A second outer end  402  of the rotatable knife contact  400  remains open in the first switching position. A second outer end  402  of the rotatable knife contact  400  makes, in the second switching position, contact to the second fixed contact  200 , whereby an electrical connection is formed between the second fixed contact  200  and the third fixed contact  300 . A first outer end  401  of the rotatable knife contact  400  remains open in the second switching position. The outer ends  401 ,  402  ends of the rotatable knife contact  400  may thus be utilized alternatively in the switching operation. 
     The rotatable knife contact  400  may further have a zero position between the first and the second switching position in which the first, the second, and the third fixed contacts  100 ,  200 ,  300  are electrically isolated from each other. 
     The rotational axis Z 1 , Z 2  of the rotatable knife contact  400  may be located at a middle portion of the blades  410 ,  420  in the rotatable knife contact  400 . The opposite outer ends  401 ,  402  of the blades  410 ,  420  are thus free to make contact with the contact portion  120 ,  220  of the first and the second fixed contact  100 ,  200 . 
     The rotational axis Z 1 , Z 2  of the rotatable knife contact  400  may be located at the intersection of the transverse center line X 1 -X 1  passing in the height direction X-X of the housing  10  and the longitudinal center line Y 1 -Y 1  passing in the longitudinal direction Y-Y of the housing  10 . The rotational axis Z 1 -Z 1  of the rotatable knife contact  400  extends in the embodiment of the figure perpendicularly to the plane of the paper i.e. perpendicular to the longitudinal direction Y-Y and perpendicular to the height direction X-X of the housing  10 . The rotatable knife contact  400  may be supported on a roller  80  positioned within the housing  10 . The roller  80  may rotate around the rotational axis Z 1 , Z 1  of the rotatable knife contact  400 . 
     The housing  10  may comprise a first chamber  13 A and a second chamber  13 B. The first chamber  13 A and the second chamber  13 B may be on opposite sides of the longitudinal center line Y 1 -Y 1  of the housing  10 . The contact portion  120  of the first fixed contact  100  and a first arc extinguishing apparatus  14 A may be positioned in the first chamber  13 A. The contact portion  220  of the second fixed contact  200  and a second arc extinguishing apparatus  14 B may be positioned in the second chamber  13 B. The first end  401  of the blade pair of the rotatable knife contact  400  may, in a switching event, move within the first chamber  13 A and the second end  402  of the blade pair may, in a switching event, move within the second chamber  138 . 
     When the first end  401  of the blades  410 ,  420  of the rotatable knife contact  400 , after having been in contact with the contact portion  120  of the first fixed contact  100 , rotates counterclockwise, the contact between the blades of the rotatable knife contact  400  and the contact portion  120  of the first fixed contact  100  is disconnected and an arc illuminates between them through the gas (air) in the interior of the housing  10 . This arc is cut off when the blade pair passes through the first arc extinguishing apparatus  14 A. 
     When the second end  402  of the blades  410 ,  420  of the rotatable knife contact  400 , after having been in contact with the contact portion  220  of the second fixed contact  200  rotates clockwise, the contact between the blades of the rotatable knife contact  400  and the contact portion  220  of the second fixed contact  200  is disconnected and an arc illuminates between them through the gas (air) in the interior of the housing  10 . This arc is cut off when the pair of blades passes through the second arc extinguishing apparatus  14 B. 
     The first and second arc extinguishing apparatus  14 A,  14 B may be formed of plates  15 A,  15 B extending in the thickness direction Z-Z of the housing  10 . Each plate  15 A,  15 B may be provided with a slit in which the end  401 ,  402  of the blades of the rotatable knife contact  400  can pass during a switching event. The plates  15 A,  15 B may extend substantially in the radial direction relative to the rotational axis Z 1 -Z 1  of the rotatable knife contact  400 . The plates  15 A,  15 B may be made of metal, preferably of steel. 
     The combustion gases generated by the arc may be driven out of the housing  10  through the first chamber  13 A or the second chamber  13 B and finally through a first exhaust opening  18 A or a second exhaust opening  18 B in the second side wall  10 B of the housing  10 . The combustion gases may mainly discharge from the region of the first contact portion  120  of the first fixed contact  100  within the first chamber  13 A towards the third side wall  10 C of the housing  10 . The combustion gases may pass through a first discharge channel provided in the first chamber  13 A between the third side wall  10 C of the housing  10  and an outer perimeter of the first extinguishing apparatus  14 A and further via an outward portion of the first chamber  13 A to the first exhaust opening  18 A. The combustion gases may be discharged in a corresponding manner mainly from the region of the contact portion  220  of the second fixed contact  200  within the second chamber  133  towards the fourth side wall  10 D of the housing  10 . The combustion gases may pass through a second discharge channel provided in the second chamber  13 B between the fourth side wall  10 D of the housing  10  and an outer perimeter of the second extinguishing apparatus  143  and further via an outward portion of the second chamber  133  to the second exhaust opening  18 A. 
     The combustion gases may be driven out of the housing  10  through the exhaust openings  18 A,  18 B in the second side wall  10 B of the housing  10 , i.e. the combustion gases may be directed to the same side of the housing  10  where the third fixed contact  300  is located. A possible deposition of conductive particles in the combustion gases on the second side wall  10 B of the housing  10  cannot make a short circuit between two fixed contacts, since this second side wall  10 B of the housing  10  is provided with only one fixed contact, i.e. the third fixed contact  300 . 
     The housing  10  may also comprise two stoppers  16 A,  16 B limiting the rotational movement of the rotatable knife contact  400 . When the rotatable knife contact  400  is turned clockwise into the first contact position, the first end  401  of the rotatable knife contact  400  may come into contact with the contact portion  120  of the first fixed contact  100 . At the same time, the opposite second end  402  of the rotatable knife contact  400  may turn against a second stopper  16 B which may stop the clockwise turn of rotatable knife contact  400 . When the rotatable knife contact  400  is rotated counterclockwise into the second contact position, the second end  402  of the rotatable knife contact  400  may come into contact with the contact portion  220  of the second fixed contact  200 . At the same time, the opposite first end  401  of the rotatable knife contact  400  may turn against a first stopper  16 A which may stop the counterclockwise turn of the rotatable knife contact  400 . These stoppers  16 A,  16 B may also limit the chambers  13 A,  13 B formed in the housing  10 . The stoppers  16 A,  16 B may also protect the braided cables  31 ,  32  from the combustion gases and from metal vapor. There may further be walls  16 C,  16 D between the fixed contacts  100 ,  200  which also may limit the rotational movement of the rotatable knife contact  400 . 
     The arc is an electrical discharge which is generated when the voltage between two contacts exceed the dielectric strength of the material (air) between the contacts. The resistance between the contacts increases when the contacts open and the contact pressure reduces resulting in an arc between the contacts. The contacts will thus heat up and a portion of the contact material may melt and eventually evaporate. The breakthrough occurs when the metal vapor and air molecules between the contacts break down into atoms and further into ions increasing the electrical conductivity of the gas. The arc may be extinguished by increasing the arc voltage, i.e. by transferring energy away from the arc. The energy of the arc may be reduced by prolonging, cooling or braking the arc with perpendicular extinguishing plates of metal. 
       FIG. 3  shows an exploded view of a rotatable knife contact and a roller of the electrical switch. 
     The rotatable knife contact  400  comprises, in this embodiment, a single blade pair formed of two longitudinal blades  410 ,  420 . 
     The blades  410 ,  420  in the pair of blades may be attached to each other with a spring structure  460 ,  470 . The spring structure  460 ,  470  may comprise a spring guide  461 ,  471 , a spring  462 ,  472  and a tensing bar  480 . 
     The spring guide  461 ,  471  may be formed of a longitudinal plate extending in the longitudinal direction of the blade  410 ,  420  and positioned against the outer surface of the blade  410 ,  420 . Both ends of the plate may comprise arms  461 A,  471 A extending in a transverse direction over the edges of the blade  410 ,  420 . The inner surface of the plate may comprise pins  461 B,  471 B that may be seated in a groove  416 ,  426  in the outer surface of the blade  410 ,  420 . The groove  416 ,  426  in the outer surface of the blade  410 ,  420  may be in the same position as the protrusion  415 ,  425  in the inner surface of the blade  410 ,  420 , which is seen in  FIG. 4 . The groove  416 ,  426  and the protrusion  415 ,  425  may be made in one step by punching the blade  410 ,  420  from the outer surface. The pins  4618 ,  4713  may lock the spring guide  461 ,  471  to the blade  410 ,  420  in the transverse direction of the blade  410 ,  420  and may allow a small movement in the longitudinal direction of the blade  410 ,  420 . 
     The spring  462 ,  472  may be formed of a spring  462 ,  472  extending in the longitudinal direction of the blade  410 ,  420  and being adapted into the outer surface of the spring guide  461 ,  471 , Opposite ends of the spring  462 ,  472  may comprise a groove  462 A,  472 A having the form of a half circle and being seated against a pin  461 C,  471 C protruding from the outer surface of the spring guide  461 ,  471 . A middle portion of the spring  462 ,  472  may comprise a groove  462 B,  472 B, which may receive a tensing bar  480 . 
     The pins  461 B,  471 B in the inner surface of the spring guide  461 ,  471  and the pins  461 C,  471 C in the outer surface of the spring guide  461 ,  471  may be made by punching from the opposite side of the spring guide  461 ,  471 . 
     The tensing bar  480  may be formed of a U-formed piece, which may compress the blades  410 ,  420  together at a desired force. The pressing force of the tensing bar  480  may be adjusted by changing the dimensions of the tensing bar  480 . The tensing bar  480  may extend over one edge of the blades  410 ,  420 . The cross section of the tensing bar  480  may be round and it extends in a transverse direction in view of the longitudinal direction of the blade pair  410 ,  420 . The tensing bar  480  may be positioned substantially at a longitudinal middle point of the blades  410 ,  420 . 
     The figure shows also the protrusions  83 ,  84  protruding from the cylindrical portion  85  of the roller  80 . One of the protrusions  83  may be formed of a separate part, which is pushed with the blade pair into the roller  80 . This removable protrusion  83  may be attached to the roller  80  with quick coupling means. The figure shows further the third fixed contact  300  and the braided cable  31  with which the third fixed contact  300  is connected to the rotatable knife contact  400 . 
     The magnetic field caused by a current passing in the same direction in each blade  410 ,  420  in the rotatable knife contact  400  will produce a force between the blades  410 ,  420 . The force will pull the blades  410 ,  420  towards each other. The spring guides  461 ,  471  will restrict the leakage of the magnetic field from the blades  410 ,  420 , whereby a strong magnetic field is maintained between the blades  410 ,  420  especially in a short circuit situation with strong currents. The spring guides  461 ,  471  are of metal, preferably of steel. 
       FIG. 4  shows a rotatable knife contact of the electrical switch. 
     The rotatable knife contact  400  comprises at least one pair of blades  410 ,  420 . Each blade  410 ,  420  may be formed as one single piece. Each blade  410 ,  420  may be formed of a substantially straight solid bar having a length, a width and a thickness. The bar may have a substantially rectangular cross section. The length of the blade  410 ,  420  may correspond to the length of the rotatable knife contact  400 . The protrusion  415 ,  425  in the middle portion of the blade  410 ,  420  may then be made by punching the bar from the opposite side. 
     Each blade  410 ,  420  in the pair of blades may comprise a protruded middle portion  415 ,  425 . The protruded middle portions  415 ,  425  may seat against each other when the blades  410 ,  420  are connected to each other. The blades  410 ,  420  in the pair of blades may thus become supported at each other through the protruded middle portions  415 ,  425 . The width of the protruded middle portion  415 ,  425  may be only a portion of the width of the blade  410 ,  420 . 
     The blades  410 ,  420  in the rotatable knife contact  400  may comprise two opposite outer ends  401 ,  402 . A first contact gap A 1  may be formed between the two opposite blades  410 ,  420  at the first end  401  of the blades  401 ,  402  and a second contact gap A 2  may be formed between the two opposite blades  410 ,  420  at the second end  402  of the blades  401 ,  402 . 
     The two blades  410 ,  420  in each pair of blades may be flexibly supported at each other with the spring structure  460 ,  470  described earlier. Due to the flexible support of the blades  410 ,  420 , the blades  410 ,  420  may when a separating force F 1 , F 2  is acting on either end  401 ,  402  of the blades  410 ,  420 , take a V-shape. When the distance between the blades  410 ,  420  in the first end  401  of the pair of blades is increased, then the distance between the blades  410 ,  420  at the second end  402  of the pair of blades will decrease and vice a versa. The separating force F 1 , F 2  may be caused by the contact portion  120 ,  220  of the first and the second fixed contacts  100 ,  200  penetrating into the contact gap A 1 , A 2  between the ends  401 ,  402  of the blades  410 ,  420 . 
     The protruded middle portions  415 ,  425  of the blades  410 ,  420  may act as a sort of a pivot point P 1 , P 2  between the blades  410 ,  420 . The pivot points P 1 , P 2  may be formed at opposite longitudinal ends of the protruded middle portions  415 ,  425  of the blades  410 ,  420 . 
     A separating force F 1 , F 2  acting between the blades  410 ,  420  at the first end  401  of the pair of blades may result in pivoting of the blades  410 ,  420  around the second pivot point P 2 . The distance between the blades  410 ,  420  at the first end  401  increases and the distance between the blades  410 ,  420  at the second end  402  decreases. A separating force F 1 , F 2  acting between the blades  410 ,  420  at the second end  402  of the pair of blades may result in pivoting of the blades  410 ,  420  around the first pivot point P 1 . The distance between the blades  410 ,  420  at the second end  402  increases and the distance between the blades  410 ,  420  at the first end  401  decreases. 
     The spring structure  460 ,  470  may produce a counter force to the separating force F 1 , F 2  so that the blades  410 ,  420  may be returned to a substantially parallel position when no separating force F 1 , F 2  is acting on the blades  410 ,  420  in either end  401 ,  402  of the pair of blades. 
     The blades  410 ,  420  may, in a non-deflected situation, rotate in parallel planes. The figure shows a central rotation plane X 1 -X 1  between the blades  410 ,  420 . 
       FIG. 5  shows a rotatable knife contact and a roller of the electrical switch. 
     The blades  410 ,  420  in the pair of blades in the rotatable knife contact  400  may be supported on a cylinder-like roller  80  so that opposing ends  401 ,  402  of the rotatable knife contact  400 , which also constitute the opposing ends of the blade pair  410 ,  420 , protrude from the roller  80 . The roller  80  may comprise a cylindrical portion  85  provided with two side protrusions  83 ,  84  extending radially outwards in opposite directions from the cylindrical portion  85 . A center axis of the two side protrusions  83 ,  84  pass through the rotational axis Z 1 -Z 1  of the roller  80 . Each of the two side protrusions  83 ,  84  may comprise two spaced apart walls extending perpendicular to the rotational axis Z 1 , Z 1  of the roller  80 . A first edge of the walls in each side protrusion  83 ,  84  may comprise a guide part perpendicular to the wall. The guide parts may extend from the edge of the wall towards each other and terminate at a distance from each other. There are no guide parts in the second opposite edge of the walls in each side protrusion  83 ,  84 . A first edge of each blade  410 ,  420  in the rotatable knife contact  400  may be supported on a respective guide part in the side portions  83 ,  84  of the roller  80 . A second opposite edge of each blades  410 ,  420  in the rotatable knife contact  400  is thus free. The free edge of the blades  410 ,  420  faces towards the conduct portions  120 ,  220  of the first and second fixed contact  100 ,  200 . The contact portion  120 ,  220  can thus be received between the blades  410 ,  420  from the free edge of the blades  410 ,  420 . The pair of blades  401 ,  420  is centralized in the roller  80  with stoppers  87 A,  87 B in the roller  80 . 
     The roller  80  that is positioned within the housing  10  may be rotatable in respect of the housing  10 . The roller  80  may comprise an end portion  81 ,  82  at each longitudinal opposite end of the roller  80 . Each end portion  81 ,  82  of the roller  80  may be supported in a circular opening  19  formed in each side panel  10 E,  10 F of the housing  10 . The end portions  81 ,  82  of the roller  80  rotate against the circumference of the circular opening  19  in each side panel  10 E,  10 F of the housing  10 . The rotatable knife contact  400  thus rotates with the roller  80  around the rotational axis Z 1 -Z 1  directed in the thickness direction Z-Z of the housing  10 . 
       FIG. 6  shows a fixed contact of the electrical switch. 
     The first and second fixed contacts  100 ,  200  may be identical or mirror images of each other. The contact portion  120 ,  220  of the first and second fixed contacts  100 ,  200  may be formed as a plate-like piece. In this embodiment, the contact portion  120 ,  220  of the fixed contact  100 ,  200  is formed of two similar, spaced apart branches. The connection portion  110 ,  210  of the first and the second fixed contact  100 ,  200  may terminate in a U-shaped portion extending in a direction perpendicular to the longitudinal direction of the connection portion  110 ,  210 . Each branch of the U-shaped portion may comprise the actual contact portion  120 ,  220 , which in a switching event may seat between the blades  410 ,  420  of the moving contact  400 . The contact portion  120 ,  220  in each branch may receive one blade pair  410 ,  420  of the rotatable knife contact  400 . The fixed contact  100 ,  200  in the figures is aimed to work with such a rotatable knife contact  400 , which has two parallel pairs of blades  410 ,  420 . The contact portion  120 ,  220  of each branch of the fixed contact  100 ,  200  may, in a switching event, seat between one pair of blades  410 ,  420 . 
     The opposite surfaces in each branch of the contact portions  120 ,  220  of the first and the second fixed contact  100 ,  200  may comprise a roughened area  130 ,  230 . The roughened area  130 ,  230  may drag against the inner surface of the blades  410 ,  420  of the knife contact  400  in a switching event. The contact portions  120 ,  220  may become sooty in the switching event, which may increase the contact resistance and the heating of the contacts. 
     Each branch of the contact portions  120 ,  220  of the first and the second fixed contact  100 ,  200  may further comprise a first contact pin  140 ,  240 . The first contact pin  140 ,  240  may extend only along a portion of the area of the contact portion  120 ,  220 . The first contact pin  140 ,  240  may extend on the opposite contact surfaces of the contact portion  120 ,  220 . The first contact pin  140 ,  240  may also extend over a front edge  155 ,  255  of the contact portion  120 ,  220 . The first contact pin  140 ,  240  may be positioned in a recess in the contact portion  120 ,  220 . The first contact pin  140 ,  240  may be firmly attached in the recess to the contact portion  120 ,  220 . The material of the first contact pin  140 ,  240  may be selected so that it withstands the erosion of the arc better than the actual contact portion  120 ,  220 . The first contact pin  140 ,  240  may protect the actual contact portions  120 ,  220  from the wearing effect of the arc and may thereby increase the lifetime of the contacts in the electrical switch. 
     Each branch of the contact portions  120 ,  220  of the first and the second fixed contact  100 ,  200  may further comprise a protruded nose  150 ,  250 . The protruded nose  150 ,  250  may be positioned in a front edge  155 ,  255  of the contact portion  120 ,  220  and it will make the initial contact with the blade  410 ,  420  of the knife contact  400  when the knife contact  400  is closed. The protruded nose  150 ,  250  may protect the contact portion  120 ,  220  when the rotatable knife contact  400  is closed in a short circuit situation i.e. the output of the electrical switch  500  is short circuited. A main portion of the short circuit current may pass through the protruded nose  150 ,  250 . This may protect the contact pins  140 ,  240  in the contact portion  120 ,  220  from heating up too much. An excessive heating of the contact pins  140 ,  240  might cause melting of the soldering material of the contact pins  140 ,  240  resulting in disconnection of the contact pins  140 ,  240  from the contact portion  120 ,  220 . The contact resistance and the resistivity of the contact pin  140 ,  240  may be higher than that of the contact portion  120 ,  220 , which causes a greater heating of the contact pin  140 ,  240  especially when subjected to a high short circuit current. The contact oscillations are much smaller when the contact portion  120 ,  220  and the blades  410 ,  420  make contact compared to the situation when the contact pins  140 ,  240 ,  440  make contact. This is due to the fact that the contact pins  140 ,  240 ,  440  are much harder than the contact portion  120 ,  220  and the blades  410 ,  420 . The contact oscillations generate arcs causing wear to the contact portions  120 ,  220  and the blades  410 ,  420  and this may result in that opposite contact surfaces weld to each other. Also some combination of materials have a tendency to weld to each other during a switching event. 
     The protruded nose  150 ,  250  which extends a little bit further outwards than the first contact pin  140 ,  240  will make the first contact with the blades  410 ,  420  when the rotatable knife contact  400  closes against the contact portion  120 ,  220  of the fixed contact  100 ,  200 , The first contact may be established between the second contact pins  440  in the blades  410 ,  420  and the protruded nose  150 ,  250  in the contact portion  120 ,  220  of the fixed contact  100 ,  200  when the rotatable knife contact  400  closes. 
       FIG. 7  shows a blade of a rotatable knife contact of the electrical switch. 
     The opposite ends of the blade  410 ,  420  of the rotatable knife contact  400  may comprise contact portions  411 ,  421 ,  412 ,  422  through which the contact to the fixed contacts  100 ,  200  may be formed. An outwardly protruding area  430  may be provided on an inner surface of each blade  410 ,  420  at a distance from an outer tip of the blade  410 ,  420  in the pair of blades of the rotatable knife contact  400 . The outwardly protruding area  430  at one end  401 ,  402  of the blades  410 ,  420  of the rotatable knife contact  400  will at the end of the switching event, when the rotatable knife contact  400  has reached its permanent contact position, seat on a corresponding roughened area  130 ,  230  of the contact portion  120 ,  220  of the corresponding fixed contact  100 ,  200 . The other opposite end  401 ,  402  of the blades  410 ,  420  will be free. 
     The outwardly protruding areas  430  of the contact blades  410 ,  420  of the rotatable knife contact  400  may be positioned towards the edge  455  of the blade  410 ,  420  that will first become into contact with the first or second fixed contact  100 ,  200  in a switching situation. The sweep of the protruding areas  430  over the roughened areas  130 ,  230  of the contact portion  120 ,  220  of the corresponding fixed contact  100 ,  200  mat thus become as long as possible in the switching event. 
     These outwardly protruding areas  430  of the contact blades  410 ,  420  of the rotatable knife contact  400  may pass over the roughened area  130 ,  230  of the fixed contact  100 ,  200  whenever the coupling is formed and cut off. The outwardly protruding areas  430  and the roughened areas  130 ,  230  remain thus clean. The roughened area  130 ,  230  also collects more silver and grease to the area and new silver is revealed when the roughened area  130 ,  230  wears. 
     Second contact pins  440  may be provided on the ends  401 ,  402  of the blades  410 ,  420  of the rotatable knife contacts  400 . The second contact pin  440  may extend only along a portion of the area of the contact portions  411 ,  421 ,  412 ,  422  of the blade  410 ,  420  of the rotatable knife contact  400 . The second contact pin  440  may extend on the opposite contact surfaces of the blades  410 ,  420 . The second contact pin  440  may also extend over a front edge of the blade  410 ,  420 . The second contact pin  440  may be positioned in a recess in the blade  410 ,  420 . The second contact pin  440  may be firmly attached in the recess to the blade  410 , 420 . The material of the second contact pin  440  may be such that it withstands the erosion of the arc better than the rest of the blade  410 ,  420 . The second contact pin  440  may protect the blade  410 ,  420  from the wearing effect of the arc and may thereby increase the lifetime of the contacts in the electrical switch. The second contact pin  440  may pass over the first contact pin  140 ,  240  in a switching event. The second contact pin  440  may overlap only partly the first contact pin  140 ,  240  when passing over the first contact pin  140 ,  240  in a switching event. The second contact pin  440  may not make direct contact with the first contact pin  140 ,  240  in a situation in which the contact pins  140 ,  240 ,  440  are slightly below the outer surface of the contact. 
     The outwardly protruding areas  430  of the blades  410 ,  420  may produce a separating force F 1 , F 2  between the blades  410 ,  420  in the pair of blades when the contact portion  120 ,  220  of the fixed contact  100 ,  200  is received between the blades  410 ,  420 . The flexible support of the blades  410 ,  420  to each other will result in the V-shape of the blades. The ends  401 ,  402  of the blades  410 ,  420  that extend beyond the outwardly protruding areas  430  will thus be separated from the contact portion  120 ,  220  of the fixed contacts  100 ,  200 . Only the outwardly protruding areas  430  will make contact to the contact portion  120 ,  220  of the fixed contact  100 ,  200 . The area for switching-on and switching-off and the area for nominal continuous current are separated in the blades  410 ,  420  of the rotatable knife contact  400  and in the contact portions  120 ,  200  of the first and second fixed contact  100 ,  200 . The nominal continuous current flows through the outwardly protruding areas  430 . 
     When closing the rotatable knife contact  400  to the first or to the second fixed contact  100 ,  200 , the outer tip of the blades  410 ,  420  makes first contact with the contact portion  120 ,  200  of the fixed contact  100 ,  200 . The contact area moves slightly towards the middle of the blades  410 ,  420  as the rotatable knife contact  400  turns further in the closing direction, whereby the tip of the blades  410 ,  420  loses contact with the fixed contacts  100 ,  200 . The tip of the rotatable knife contact  400  wears when the rotatable knife contact  400  is closed and opened, whereby the base portion of the rotatable knife contact  400  is saved for conducting the nominal current. 
     The middle portion  450  of the blade  410 ,  420  and the protruded middle portions  415 ,  425  of the blades  410 ,  420  are also shown in the figure. 
       FIG. 8  shows a contact pin of a contact of the electrical switch. 
     The contact pin  140  shown in the figure may be used in the fixed contacts  100 ,  200  and in the rotatable knife contact  400 . The first contact pins  140 ,  240  of the two fixed contact contacts  100 ,  200 , and the second contact pins  440  of the rotatable knife contact  400  may thus be identical. The contact pin  140  preferably has a P-shape, thereby protecting the two sides of the contact  100 ,  200 ,  400 . The contact pin  140  may comprise a front section  141  and a rear section  142 . The contact pin  140  may further comprise a first inner surface  143 A and a second inner surface  1438  coming into contact with the recess in the contact  100 ,  200 ,  400 . The contact pin  140  may further comprise a first outer surface  144 A, a second outer surface  1448  and a front surface  144 C. The first and the second outer surfaces  144 A,  1448  may form contact surfaces to the opposite contact in a switching event. The arcs between the fixed contacts  100 ,  200  and the rotatable knife contact  400  may thus pass through opposite contact pins  140 . 
     The contact pin  140  may be attached to the contact  100 ,  200 ,  400  from the inner surfaces  143 A,  1438  of the contact pin  140 . The first inner surface  143 A may have a rough structure in order to facilitate the fastening on the contact pin  140  to the contact  100 ,  200 ,  400 . A soldering material may be applied on the inner surfaces  143 A,  143 B of the contact pin  140 . The contact pin  140  may then be heated with welding, whereby a firm joint may be formed between the contact pin  140  and the contact  100 ,  200 ,  400 . The first outer surface  144 A may form a contact surface with the respective contact or blade surface of the contact. The second outer surface  144 B may form a contact surface on the opposite surface of the contact or the blade. 
     The first and/or the second contact pin  140 ,  240 ,  440  may be positioned in the recess so that an outer surface of the contact pins  140 ,  240 ,  440  is substantially flush with the outer surface of the contact  100 ,  200 ,  400 . This might be difficult to achieve due to manufacturing tolerances. The first and/or the second contact pin  140 ,  240 ,  440  may therefore be positioned in the recess so that the outer surface of the contact pin  140 ,  240 ,  440  is slightly below the outer surface of the contact  100 ,  200 ,  400 . The slightly inward position of the contact pins  140 ,  240 ,  440  may be advantageous when the contact is closed in a short circuit situation. The first and/or the second contact pin  140 ,  240 ,  440  may on the other hand be positioned in the recess so that the outer surface of the contact pin  140 ,  240 ,  440  it is slightly above the outer surface of the contact  100 ,  200 ,  400 . Such a slightly outward position might also work due to contact vibrations and a suitable form of the contact. The size of the nominal current of the electrical switch might also influence the choice between these three possibilities. 
     The first and/or the second contact pin  140 ,  240 ,  440  extend in the figures on both surfaces of the contact  100 ,  200 ,  400  and over the front edge  155 ,  255 ,  455  of the contact  100 ,  200 ,  400 . This is an advantageous embodiment. The first and/or the second contact pin  140 ,  240 ,  440  could, however, instead extend only on one surface of the contact  140 ,  240 ,  440  or there could be contact pins  140 ,  240 ,  440  on both opposite surfaces of the contact  100 ,  200 ,  400  without a connection portion extending over the edge of the contact  100 ,  200 ,  400 . 
     The first and/or the second contact pin  140 ,  240 ,  440  is in the figures shown as one entity. The first and/or the second contact pin  140 ,  240 ,  440  could, however, also be composed of several entities. Two or more contact pins could be positioned adjacent to each other in the recess or in adjacent recesses. The contact pin may thus be formed of two or more entities forming together the contact pin. The recess may have the form of a groove. 
       FIG. 9  shows a fixed contact and a rotatable knife contact of the electrical switch. 
     The figure shows the contact portion  220  of the second fixed contact  200  and a blade  410  in the rotatable knife contact  400 . The blade  410  comprises a second contact pin  440  at each outer end of the blade  410  and a protruded area  430  at a distance from the outer tip of the blade  410 . The protruded area  430  may be positioned radially inside the second contact pin  440  on the blade  410 . The longitudinal center line Y 1 -Y 1  and the transverse center line X 1 -X 1  as well as the rotational axis Z 1 -Z 1  of the blade  410  are shown in the figures. A circle having the rotational axis Z 1 -Z 1  as center point and a radius extending to the middle of the second contact pin  440  is also shown in the figure. 
     The blade  410  is shown in solid lines in a neutral position B 1 , in dashed lines in an intermediate position B 2 , and in dashed lines in a final switching position B 3 . 
     The figure shows that the blade  410  may when it is turned counterclockwise, first make contact with the protruded nose  250  of the contact portion  220  in the front edge  255  of the contact portion  220 . When the blade  410  is turned further counterclockwise, the second contact pin  440  will pass over the first contact pin  240 . The second contact pin  440  and the first contact pin  240  may overlap each other only partly during the switching event i.e. they may not be fully aligned in respect of each other. The second contact pin  440  may or may not make a direct contact with the first pin  240  in the switching event depending on the position of the contact pins  240 ,  440  on the recesses. There may also be a direct contact beyond the contact pins  140 ,  240 ,  440  between the contacts  100 ,  200 ,  400  already at this stage. This may reduce the contact resistance and thereby the thermal stress on the contact pins  140 ,  240 ,  440 . This may reduce the risk of the contact pins  140 ,  240 ,  440  becoming detached from the contacts  100 ,  200 ,  440 . 
     The outer end portions of the blades  410 ,  420  containing the second contact pins  440  may be lifted from the surface of the contact portion  120 ,  220  of the first and second fixed contact  100 ,  200  immediately when the protruded portion  430  of the blade  410 ,  420  makes contact with the contact portion  120 ,  220  of the first and second fixed contact  100 ,  200 . 
     In the final position B 3  of the blade  410 , the contact between the blade  410  and the contact portion  220  may be established only through the protruded area  430  on the blade  410  and the roughened area  230  on the contact portion  220 . There may be no contact between the first contact pin  240  and the second contact pin  440  in this position. The outer end portion of the blades  410 ,  420  of the rotatable knife contact  400  may thus be out of contact with the contact portion  120 ,  220  of the first and the second fixed contact  100 ,  200 . 
     The contact points between the blade  410  and the contact portion  220  may be opened in the reverse order when the contact opens i.e. the blade  410  is turned clockwise from the contact portion  220 . The arc may start between the protruded nose  250  and the rotatable knife contact  400 , but it may quickly move outwards between the first and the second contact pins  140 ,  240 ,  440 . This is due to current forces in the arc and magnetic forces caused by the extinction plates  15 A,  15 B. 
     The first and second fixed contacts  100 ,  200  are located in the housing  10  as shown in  FIG. 2 . A center plane of the contact portions  120 ,  220  may coincide with a center plane positioned in the center between the blades  410 ,  420  of the rotatable knife contact  400 . 
     The first contact pins  140 ,  240  and the second contacts pins  440  may act together. The arc may be mainly directed through the first contact pins  140 ,  240  and the second contact pins  440  when the rotatable knife contact  400  is opened. 
     A continuous contact between the rotatable knife contact  400  and the first or second fixed contact  100 ,  200  may be achieved through the roughened area  130 ,  230  on the opposite surfaces of the contact portion  120 ,  200  of the first or the second fixed contact  100 ,  200  and the outwardly protruding areas  430  on the inner surface of the blades  410 ,  420  of the rotatable knife contact  400 . At this end position there may no longer be any contact between the first contact pin  140 ,  240  of the first or second fixed contact  100 ,  200  and the second contact pin  440  of the rotatable knife contact  400 . 
       FIG. 10  shows a non-modular three phase electrical switch. 
     All three phases may be positioned adjacent to each other in a common casing  50  comprising two halves  50 U,  50 L. The casing  50  may be divided with intermediate walls  51  into three compartments, whereby each compartment may form a housing  10 . Each compartment i.e. each housing  10  may comprise a roller  80  and a rotatable knife contact  400 . The first fixed contact  100  may be positioned in the upper half  50 U of the casing  50  and the second fixed contact  200  may be positioned in the lower half  50 L of the casing  50 . The third fixed contact  300  may be L-shaped, whereby the vertical branch may protrude from the casing  50  through the junction between the upper half  50 U and the lower half  50 L of the casing  50 . The rollers  80  may be connected to each other through the intermediate walls  51  in the casing  50 . The invention may also be applied to such a non-modular electrical switch. 
     In the embodiment in the figures, the roughened areas  130 ,  230  are positioned on the outer surfaces of the contact portions  120 ,  220  of the first and the second fixed contact  100 ,  200  and the outwardly protruding areas  430  are positioned on the inner surface of the ends  401 ,  402  of the blades  410 ,  420  of the rotatable knife contact  400 . The situation could also be reversed. The outwardly protruding areas would then be positioned on the outer surfaces of the contact portions  120 ,  220  of the first and the second fixed contact  100 ,  200 . The roughened area would in a corresponding way be positioned on the inner surface of the blades  410 ,  420  of the rotatable knife contact  400 . 
     The roughened area  130 ,  230  may be formed of a raster crossing extending outwards or inwards from the surface on which the roughened area is formed. The outwardly protruding area  430  may have a spherical form. 
     In an embodiment in which the rotatable knife contact  400  comprises only one pair of blades  410 ,  420 , only one branch is needed in the contact portion  120 ,  220  of the first and second fixed contacts  100 ,  200 . This single branch forms a shaped sheet like part seating between the pair of blades  410 ,  420 . The upper blade  410  forms contact with an upper surface of the contact portion  120 ,  220  and the lower blade  420  forms contact with the opposite lower surface of the contact portion  120 ,  220 . 
     The amount of blade pairs  410 ,  420  in the rotatable knife contact  400  may be increased in a situation where a greater current-carrying capacity through the electrical switch  500  is required. The blade pairs  410 ,  420  may be superimposed on each other in the roll  80 . The blade pairs  410 ,  420  will then act synchronously with respect to each other, i.e., the superimposed blade pairs  410 ,  420  are parallel. 
     In a situation where the rotatable knife contact  400  comprises two superimposed blade pairs  410 ,  420 , the contact portion  120 ,  220  of the first and second fixed contacts  100 ,  200  may comprise a bifurcated structure in which each branch is plate-like. A lower blade pair  410 ,  420  of the rotatable knife contact  400  may receive the lower branch of the contact portion  120 ,  220  of the first or the second fixed contact  100 ,  200 . An upper blade pair  410 ,  420  of the rotatable knife contact  400  may receive the upper branch of the contact portion  120 ,  220  of the first or the second fixed contact  100 ,  200 . The contact surface between the contact portions  120 ,  220  of the first and the second fixed contacts  100 ,  200  and the rotatable knife contact  400  may thus be increased, whereby the current-carrying capacity may be increased. 
     The electrical switch  500  shown in the figures is intended for relatively high currents. The third fixed contact  300  is therefore formed of two L-shaped bodies  310 ,  320 . The current is thus distributed from the middle portion  450  of the rotatable knife contact  400  to each of the braided cables  31 ,  32  and further to each of the L-shaped bodies  310 ,  320  of the third fixed contact  300 . 
     The third fixed contact  300  may instead of two single L-shaped bodies  310 ,  320  be made of a single T-shaped body. The third fixed contact  300  can also instead of two L-shaped bodies  310 ,  320  be made of only one L-shaped body  310 ,  320 . Either of the two bodies  310 ,  320  shown in the figure could be used, but the second  320  is preferable due to the measurement opening  17  being positioned in connection with the horizontal branch  312  of the second body  320 . 
     The first and the second fixed contact  100 ,  200  as well as the blades  410 ,  420  of the rotatable knife contact  400  are formed of a first electrically conductive material composition. The first and the second contact pins  140 ,  240 ,  440  are formed of a second electrically conductive material composition. 
     The second material composition may be different from the first material composition. 
     The housing  10  and the roller  80  of the electric switch  500  may be made of an electrically insulating material, e.g., of plastic. 
     The second material composition may comprise at least one material that has a higher resistivity to the wearing effect of an arc acting between the fixed contacts and the rotatable knife contact in a switching event compared to any of the materials in the first material composition. 
     The first, second and the third fixed contact  100 ,  200 ,  300  as well as the rotatable knife contact  400  may be of electrically conductive material, e.g. pure copper (Cu). The copper in these contacts may be coated with silver (Ag). The silver coating may reduce the contact resistance and protect the copper from oxidation. Copper and silver may form the first material composition. 
     The copper in the third fixed contact  300  may be coated with tin (Sn). Tin is cheaper than silver and there is no need for the low contact resistance provided by silver in the third fixed contact  300 . The third fixed contact  300  is continuously connected to the rotatable knife contact  400 . Tin may also function as an intermediate material when the braided cables  31 ,  32  are welded to the third fixed contact  300 . 
     The first braided cable  31  and the second braided cable  32  may also be of electrically conductive material, e.g. copper. The braided cables  31 ,  32  can be made of very thin strands so that the braided cables become elastic. Each horizontal arm  311 ,  321  of the third fixed contact  300  may be coupled with one or several braided cables to the middle portion  450  of the rotatable knife contact  400 . The braided cable  31 ,  32  becomes elastic when the thickness thereof i.e. the number of strands therein is not too high. The braided cable  31 ,  32  must, however, have a certain cross section area in order to have a sufficient current-carrying capacity. By using very thin strands, a smooth movement is achieved, but the number of strands increases. 
     The contact pins  140 ,  240 ,  440  may be of copper-tungsten e.g. 25% copper and 75% tungsten (Cu/W). The contact pins  140 ,  240 ,  440  may have a high thermal conductivity and the thermal properties may be such that melting and vaporizing of the material requires a lot of thermal energy. Copper-tungsten withstands the wearing effect of the arc better than silver. The contact resistance of copper-tungsten is higher than that of silver, but this is not critical in this application as the continuous contact in the end position of the rotatable knife contact  400  is not established through the contact pins  140 ,  240 ,  440 . Copper and tungsten may form the second material composition. 
     The melting point of tungsten is more than three times higher than the melting point of copper and silver. The hardness of tungsten is more than two times the hardness of copper and silver. 
     The melting point of at least one material in the second material composition may be at least two times higher than the melting point of each of the materials in the first material composition. The melting point gives an indication of the materials suitability for the contact pins  140 ,  240 ,  440 , but it is not the only decisive criteria. The ability of a material to withstand the wearing effect of the arc is a more complex question and it cannot be determined based on only one criteria. The second material composition should also be compatible with the first material composition. 
     Examples of other possible materials that may be used in the first and the second contact pins  140 ,  240 ,  440  are Copper-Wolfram (CWW), Silver-Wolfram (Ag/W), Silver-Tungsten Carbide (Ag/WC), Silver-Tungsten Carbide-Carbon (Ag/WC/C) and Silver-Molybdenum (Ag/Mo). The list contains only examples of suitable materials for the contact pins  140 ,  240 ,  440  i.e. it is by no means an exclusive list of possible materials. 
     A multiphase electrical switch may be formed by placing several electrical switches  500  together to form a modular package of electrical switches  500 , The rotational axis Z 1 -Z 1  of each rotatable knife contact  400  will coincide in such a solution. The electrical switches  500  may be connected to each other through the roller  80  of the rotatable knife contact  400 . A first end of the roller  80  may extend at a distance from the surface plane of the housing  10 , and the other opposite end of the roller  80  may substantially remain in the surface plane of the housing  10 , The first end of the roller  80  may comprise a cylindrical outer end with a first tooth engagement on the outer circumference. The other end of the roller  80  may in a corresponding way comprise a cylindrical recess with a second tooth engagement on the periphery of the recess. When two adjacent electrical switches  500  are coupled together, the first protruding end of the roller  80  in the first electrical switch  500  is positioned in the second recess of the roller  80  of the second electrical switch  500  so that the teeth engage with each other. The rollers  80  of both electrical switches  500  are thus interconnected so that they rotate synchronously. 
     A multiphase electrical switch may on the other hand be formed in a common casing being divided with intermediate walls into adjacent housings  10  as shown in  FIG. 10 . The adjacent housings  10  form compartments in the casing. The rotational axis Z 1 -Z 1  of each rotatable knife contact  400  may coincide also in such a solution. 
     An electrical switch  500  according to the invention may be an automatic electric switch, the fourth rotatable knife contact  400  being rotated through an actuator. The actuator may be, for example, a solenoid whose linear movement is converted into a rotational motion by means of a power transmission apparatus. The power transmission apparatus may rotate the roller  80  from the zero position clockwise or counterclockwise and thereby move the rotatable knife contact  400  between the contact positions. The actuator may also comprise a spring for returning the rotatable knife contact  400  to the zero position. 
     An electrical switch  500  according to the invention can act as a transfer switch e.g. in a hospital environment. In the hospital, there is a need to connect a load to a primary power supply or to a secondary power supply, whereby the primary power supply is an electrical network and the secondary power supply is a backup power plant. The load is thus coupled to the third fixed contact and the primary power source to the first or second fixed contact and the secondary power source correspondingly to the second or first fixed contact. Depending on the position of the electrical switch  500 , the load can be supplied either from the electrical network or from the backup power supply. By connecting a sufficient number of electrical switches  500  in parallel, a multi-phase transfer or changeover switch is provided. In a hospital, the load may be formed, for example, by the power needed in an operating room, where breaks in the power supply cannot be accepted. 
     The third fixed contact  300  is in the embodiment shown in the figures connected with one or several braided cables to the middle portion  450  of the rotatable knife contact  400 . This is an advantageous solution. The third fixed contact  300  may, however, instead of braided cables be connected by a pivot connection to the middle portion  450  of the rotatable knife contact  400 . The pivot connection in the rotatable knife contact  400  may be realized by attaching the pair of blades  410 ,  420  to each other with a shaft extending along the rotational axis Z 1 -Z 1  of the blades  410 ,  420 . The blades  410 ,  420  and the shaft form a fixed construction. A bushing is further arranged on the shaft, whereby the shaft and the blades  410 ,  420  are rotatable in respect of the bushing. The bushing may be provided with a connection protrusion extending perpendicular to the rotational axis Z 1 -Z 1 . The connection protrusion forms a middle portion  450  of the blades  410 ,  420 . The third fixed contact  300  may extend from the opening  11 C in the second side wall  10 B of the housing  10  to the connection protrusion of the bushing i.e. the middle portion  450  of the blades  410 ,  420 . The inner end of the third fixed contact  300  may be attached to the connection protrusion of the bushing with a pressure joint e.g. a bolt and a nut. The third fixed contact  300  may be straight in such an embodiment. The electrical contact is formed between the shaft and the interior of the bushing and/or between the blades  410 ,  420  and the ends of the bushing. The blades  410 ,  420  may be slightly flexible also in this solution allowing for the V-form. In a situation in which the rotatable knife contact  400  comprises several pairs of blades  410 ,  420 , the bushing in each pair of blades  410 ,  420  may be coupled with a connection bar to the third fixed contact  300 . The third fixed contact  300  may thus split into branches within the housing  10 . 
     The blades  410 ,  420  in the rotatable knife contact  400  are in the embodiment shown in the figures connected flexibly to each other through pivot points P 1 , P 2 , Another possibility would be to connect the blades  410 ,  420  flexibly to each other so that the vertical distance between blades  410 ,  420  may change uniformly along the length of the blades  410 ,  420 . The blades  420 ,  420  would thus move in parallel from each other and towards each other. 
     An electrical switch  500  according to the invention may be manufactured for a nominal current range of 100 to 1600 amperes. 
     The invention and its embodiments are not limited to the examples shown in the figures, but the invention may vary within the scope of the protection defined by the claims.