Abstract:
A rack assembly ( 1 ) includes cross-connecting bars ( 5 ) and guide bars ( 15 ) for printed circuit boards ( 59 ). These guide bars each include a fixing device ( 36 ) for the placement of a receiving element ( 17 ) which has a contact zone ( 37 ) and is placed into electric contact on a cross-connecting bar. The invention also provides for front elements ( 13 ) having centering contacts ( 25 ) that protrude into, and thereby come into contact with, the contact zone, in a low-ohmic, current-carrying manner, when the front element is set in place. The receiving element preferably has a contact blade ( 54 ) which extends into a guide groove ( 26 ) of the guide bar.

Description:
This is a Continuation of international Application PCT/DE99/03572, with an international filing date of Nov. 9, 1999, which was published under PCT Article 21(2) in German, and the complete disclosure of which is incorporated into this application by reference. 
    
    
     FIELD OF AND BACKGROUND OF THE INVENTION 
     The invention relates to modular systems that include a rack assembly for inserting electrical printed circuit boards. Such rack assemblies include at least opposite cross-connecting bars made of electrically conductive material, which are covered with guide bars situated opposite each other in pairs. Such rack assemblies thus provide volume elements suited for the insertion of electrical printed circuit boards. Such volume elements are also called mounting sites and are normally arranged parallel side by side. Electrical printed circuit boards can be inserted into such volume elements, which are then also arranged parallel side by side. 
     FIG. 1 shows one possible basic standard construction of a rack assembly  1 . The area for receiving printed circuit boards is bordered by cross-connecting bars made of electrically conductive material situated opposite each other. In the example shown in FIG. 1, there are two lower cross-connecting bars  5 , 7  and two opposite, upper cross-connecting bars  4 , 6 . The cross-connecting bars  4 , 5  form the boundaries of the front side of the rack assembly  1 , i.e., the insertion side  2  provided for positioning electrical printed circuit boards. The two cross-connecting bars  6 , 7  form the boundaries of the rear wall side  3  of the rack assembly  1 , which can be separated by a rear wall conductor plate. The cross-connecting bars  4 , 5 , 6 , 7  can be mounted by their ends, e.g., inside a control cabinet. In FIG. 1 there are vertical side wall metal plates  8 , 9 , to which the cross-connecting bars are screwed at their ends. Lastly, the upper and lower sides of the rack assembly  1  are closed by an upper and a lower cover plate  10 , 11 . 
     So-called front elements are provided for closing the insertion side  2  of the rack assembly  1 . Such front elements are assigned to the individual mounting sites on the interior of the rack assembly, to whose width they are adjusted and to which they can be connected or disconnected. This possibility of closure can be independent of whether or not the mounting site located behind a front element contains an electrical printed circuit board. Thus, empty mounting sites can be covered with a front element placed on the insertion side  2  of the rack assembly  1 , in the manner of an empty site cover. Alternatively, electrical printed circuit boards can be provided with a front element on the front side, so that, after the printed circuit board has been inserted into the rack assembly  1 , the front element covers the corresponding mounting site on the insertion side  2  in the same manner. 
     In FIG. 1, front elements  12 , 13  are provided for the two mounting sites  56 , 57  on the left side of the rack assembly  1 . These front elements are multisectional in design. Thus, the front element  13  contains a vertical face plate  19  made of an electrically conductive material. The lower and upper front faces of the face plate  19  are provided with an end piece  20 , 21 , which usually also consists of an electrically conductive material. As an example, these end pieces can be provided with so-called rotary lever pulling handles  22 , 23 . Such handles can be operated by a person through a pivot action so that a front element  12 , 13  can be easily attached or taken off, or an electrical printed circuit board connected to a front element can be easily inserted or removed. 
     In the example shown in FIG. 1, the front element  12  has already been pushed on and completely covers the mounting site  56  located behind it, and lies in the plane of the insertion side  2  of the rack assembly  1 . In this example, the front element  13  of the mounting site  57  situated behind it is affixed to the printed circuit board  59 . This connection is advantageously achieved with the help of the two end pieces  20 , 21 . The printed circuit board has for the most part already been inserted into the mounting site  57 . After complete insertion into the mounting site  57 , the front element  13  will come to lie flush against the front element  12  in the plane of the insertion side  2 . 
     Guide bars for guiding the printed circuit boards are advantageously arranged in opposite pairs for each mounting site in the mounting zone between the cross-connecting bars. In the example shown in FIG. 1, the entire mounting zone between the upper and lower cross-connecting bars  4 , 6  and  5 , 7 , i.e., the sum of all mounting sites, is already provided with such pairs of guide bars. Thus, the printed circuit board  59  is guided on the upper and lower front side by the guide bars  14 , 15  situated opposite one another. 
     When inserting front elements or units made of front elements and electrical printed circuit boards, such elements should be centered in the insertion direction, particularly immediately before the final inserted position is achieved. This prevents an accidental lateral shift of, in particular, the front elements, which could at least hinder complete insertion thereof. This centering is especially necessary if adjoining mounting sites are already covered with front elements. 
     Finally, it is preferable to form a secure electrically conductive contact between the front elements and the rack assembly, particularly if the front elements and their components largely consist of electrically conductive materials. Electrically conductive contacts may be classified into the following different types. 
     One first type of contact can be described as a protective contact or ground contact. Through such a contact, high stray currents can be diverted from front elements to the rack assembly and then interrupted by downstream network protection devices, e.g., load separating switches or stray current line protection switches, at a speed ensuring the protection of persons. A protective contact should be especially effective if all the printed circuit boards in a rack assembly are electrically activated. 
     Another type of contact is used for diverting electrostatic voltage and can be described as an ESD (electro static discharge) contact. In this way, static potentials can be diverted from front elements to the rack assembly and its ground connection. An ESD contact should be especially effective when printed circuit boards are inserted into or removed from a rack assembly. 
     European patent 0 579 859 B1 describes a printed circuit board with guide pins for exact positioning during insertion into a rack assembly. In that patent, two guide pins are provided for centering a printed circuit board on the outsides of the corner assembly bodies. These guide pins support a longitudinal guide bar on the side flanks of the opposite head during insertion. In addition, a ground contact pin is arranged between the guide pins. This ground contact pin drops into an encoding chamber containing a ground contact spring on the front side of the opposite head of the longitudinal guide bar when the printed circuit board is inserted. 
     The reference WO 96/42187 describes a front system for a printed circuit board with an active/passive switch. For centering, the end piece of the printed circuit board is provided with a guide pin designed to enter into a corresponding guide bore. In that reference, no elements are provided for producing a protective electrical contact between the front element and the cross-connecting bars. 
     The reference WO 97/49271 describes a rack assembly for electrical printed circuit boards. The printed circuit boards are provided on one longitudinal side with a contact strip designed as a conductor track, which is interconnected to a metallic face plate of the printed circuit board via an electric resistor. 
     When the printed circuit board is inserted into the rack assembly, the contact strip comes into an electrically conductive contact loop with a contact spring, which is integrated in a guide bar, particularly at the head area, and is connected to the corresponding cross-connecting bars in an electrically conductive manner. The face plate of the printed circuit board is also provided with a screw, through which a direct electrically conductive connection to the rack assembly is produced once the circuit board is in the inserted state, while circumventing the resistor. Implementations of guide bars with loop contact springs are known, e.g., from DE 36 24 883 and DE GM 295 09 185. 
     Lastly, DE 36 24 839 C2 describes a guide bar for printed circuit boards. Its guide groove is bordered by side walls, whereby one side wall is separated from the groove base in a section thereof, and is designed as a bell curve shaped plate spring in such a way that the width of the guide groove is tapered. 
     OBJECTS OF THE INVENTION 
     One object of the present invention is to provide a modular system with a rack assembly for inserting electrical printed circuit boards, which enables the simple subsequent installation of contact parts for protective contacts of front elements. Another object of the present invention is to provide an especially simple integration as well as simple subsequent installation of devices for redirecting static voltage potentials. 
     SUMMARY OF THE INVENTION 
     According to one formulation of the invention, these and other objects are solved by a modular system for electrical printed circuit boards having a rack assembly and electrically conductive front elements. The rack assembly is provided with mounting sites for insertion of the electrical printed circuit boards on a module side of the rack assembly, and includes opposite cross-connecting bars made of electrically conductive material, and guide bars for insertion of the printed circuit boards into the mounting sites. The guide bars are affixed to the cross-connecting bars and include fixing devices for insertion of locating contacts made of electrically conductive material, which are placed into low-ohmic contact on the cross-connecting bars and have respective contact zones. The electrically conductive front elements are each assigned to at least one of the mounting sites and each have at least one centering contact protruding in a direction of the module side of the rack assembly. When the front element is placed on the at least one mounting site, it comes into low-ohmic and high current-carrying contact with the contact zone of at least one of the locating contacts. 
     According to a further formulation of the invention, the modular system for electrical printed circuit boards includes, again, a rack assembly and electrically conductive front elements. The rack assembly is provided with mounting sites for insertion of the electrical printed circuit boards on a module side of the rack assembly. The rack assembly includes (i) opposite cross-connecting bars made of electrically conductive material, (ii) guide bars for insertion of the printed circuit boards into the mounting sites, wherein the guide bars include respective guide grooves and are affixed to the cross-connecting bars, and (iii) locating contacts made of electrically conductive material. These locating contacts are respectively assigned to the mounting sites, are placed into low-ohmic contact on said cross-connecting bars, have respective contact zones, and include respective contact blades that respectively extend into the guide grooves of the guide bars. Each of the electrically conductive front elements for the printed circuit boards include at least one centering contact protruding in a direction of the module side of the rack assembly. Upon insertion of the printed circuit board into the respective mounting site, the centering contact comes into low-ohmic, high current-carrying contact with the contact zone of a respective one of the locating contacts of the respective mounting site. In addition, the contact blades of the locating contacts respectively come into contact with the printed circuit boards to draw off electrostatic potentials. 
     According to yet another formulation, the invention provides a rack assembly including a guide bar, a receiving element and a front element. The guide bar supports a printed circuit board within the rack assembly. The receiving element is mounted to the guide bar, is composed of a different material than the guide bar, and has a contact cavity. The front element is fastened to the printed circuit board and includes a contact pin configured to mate with the contact cavity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and further advantageous refinements of the invention according to the features of the dependent claims are explained in more detail below with the aid of diagrammatic, exemplary embodiments in the drawing, in which: 
     FIG. 1 shows a perspective lateral view of a modular system with a rack assembly and front elements, which was described in detail above. 
     FIGS. 2 a ,  2   b ,  2   c  show a perspective top view of a first preferred embodiment of a guide bar and locating contact with a means of producing a ground contact in accordance with the invention, 
     FIGS. 3 a ,  3   b ,  3   c ,  3   d  show a perspective top view of a second preferred embodiment of a guide bar and locating contact like those shown in FIG. 2, with a contact means and provided with an additional means for connection to the cross-connecting bars, 
     FIG. 4 shows a perspective top view of a section of a front element, a cross-connecting bar and a guide bar into which a locating contact has been inserted in accordance with FIGS. 3 a ,  3   b ,  3   c ,  3   d,    
     FIG. 5 shows a cross-section through a locating contact with a means of contact according to FIGS. 3 b ,  3   c,    
     FIG. 6 shows a perspective lateral view of another embodiment of a locating contact, 
     FIG. 7 shows a perspective lateral view of yet another embodiment of a locating contact, 
     FIGS. 8 a ,  8   b  show a perspective top view of an exemplary guide bar and a locating contact, which, according to another embodiment of the present invention, are also provided with a contact blade for an ESD contact, and 
     FIG. 9 shows a perspective top view of a guide bar with a locating contact inserted, in accordance with FIGS. 8 a ,  8   b.    
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, which was already described in detail above, the front element  13  on the mounting site  57 , which as an example is connected to the printed circuit board  59 , is already provided with a centering contact  24 , 25  extending towards the insertion side  2  of the rack assembly  1  in accordance with the present invention. These centering contacts are each respectively attached to upper and lower end pieces  20 , 21 , whereby the end pieces  20 , 21  are affixed to the front sides of the face plate  19  of the front element  13 . The end pieces are also used for mounting upper and lower lever pulling handles  22 , 23 , which enable the simple insertion or removal of the unit made up of the front element  13  and printed circuit board  58 . Like the face plate and cross-connecting bars, the end pieces with the centering contacts also consist of electrically conductive material. The upper and lower cross-connecting bars  4 , 6  and  5 , 7  are provided with guide bars on the inner sides, which face the intermediately situated mounting sites. Here, each mounting site is provided with a pair of guide bars situated opposite one another, into which the edges of an intermediate printed circuit board can be guided. In the example shown in FIG. 1, guide bars  14 , 15  are assigned to mounting site  57 . 
     According to the present invention, each guide rail is provided with at least one locating contact, which in turn is assigned to a centering contact on the corresponding front element. In FIG. 1, the locating contacts  16 , 17  are placed in the guide bars  14 , 15  and assigned to the centering contacts  24 , 25  on the front element  13 . When the printed circuit board  59  is inserted into the mounting site  57  and as a result the front element  13  is placed on the insertion side  2  of the rack assembly  1 , the centering contacts  24 , 25  on the front element  13  come into an electric, low-ohmic and high current-carrying contact with the locating contacts  16 , 17  on the guide bars  14 , 15 . This produces a protective contact between the front element  13 , printed circuit board  59  and rack assembly  1 . 
     Due to the two-piece, separate design of the guide bars and the locating contacts, the present invention has the advantage under certain circumstances that guide bars that are already situated in a rack assembly can later be provided with locating contacts. One particular advantage is also that the guide bars and locating contacts can consist of different materials. Thus, the guide bars can be inexpensively manufactured from electrically non-conductive material, e.g., plastic, while at least the surface of the drop-in locating contacts consist of an electrically conductive material, particularly a metal. 
     A first exemplary embodiment of the present invention is described in detail below with the aid of FIGS. 2 a ,  2   b  and  2   c . FIG. 2 a  shows a perspective top view of the head area of a guide rail, FIG. 2 b  shows a locating contact and FIG. 2 c  shows an advantageous, additional means of contact for insertion into the locating contact. 
     The guide bars  15 , segments of which are shown in FIG. 2 a , have a head area  31 , to which a bar-shaped extension area  32  is attached. On their upper ends is a guide groove  26  for receiving the edge of a printed circuit board. This edge can be inserted via a funnel-shaped insertion area  26 A at the front  39  of the head area  31  in the guide groove  25 , which is bordered by a left, right groove wall  27 , 28 . In the example shown in FIG. 2, one segment of the groove wall  27  is separated from the groove base and designed as a pressed-in spring area  29 . Such an arrangement is described in DE 36 24 839. In addition, there is a break in groove walls  27 , 28  and in the groove base in the area  30 . A flexible ESD contact can be inserted here, such as those described in DE 3624883 or DE GM 295 09 185. Finally, in the example shown in FIG. 2, the extension area  32  merges into a separate, and under certain circumstances multi-sectional extension  33 , which can be attached to the extension area  32 . Such a design is described in WO 96/39013. In addition, at the front  39  of the head area  31 , below the insertion area  26 A, there are encoding chambers  34  for the insertion of encoding pins. 
     Advantageously, each guide bar is provided with at least one fixing device for the insertion of a locating contact  17  made of electrically conductive material. Such a fixing device is preferably designed as a trough-shaped locating chamber  36 , and is preferably arranged in the head area  31  of the guide bars and in spatial proximity to the insertion side  2  of the rack assembly  1 . The locating contact can be placed into low-ohmic contact on the cross-connecting bars and is also provided with a contact zone, which serves to come into contact with a preferably pin-shaped centering contact on a front element. In accordance with another embodiment not specifically shown here, the fixing device can be provided with notch springs, which extend from the head area of the guide bars and clasp onto the flanks of a locating contact. 
     The contact zone can preferably be implemented in the form of a contact cavity integrated into the locating contact. For simplicity&#39;s sake, the term contact cavity will be used for the term contact zone in the following explanation. Of course, the contact zone or area can also be of a constructively different design. 
     In the embodiment shown in FIG. 2, the locating chamber  36  is also bordered by one area of the front face  39  of the head area  31 . In order for a centering contact to be able to extend down into the locating chamber  36  and into any locating contact  17  that may be inserted there, the front face  39  is also provided with a locating opening  35  for a centering contact. The advantage of this design is that at least a centering effect is achieved when a centering contact enters the locating opening  35 , even if no locating contact  17  has been inserted in the locating chamber  36 . 
     FIG. 2 b  shows a first embodiment of a locating contact  17 . This contact, symbolized here by an arrow, can be inserted into locating chamber  36 . This contact can be designed as a block-shaped contact body  18 , which on its underside, not shown here, is provided with a means of contact  38  for producing a low-ohmic contact with a cross-connecting bar, especially a clamp pin, and on the front is provided with a contact cavity  37  for receiving a preferably pin-shaped centering contact. In the inserted state, the opening of the contact cavity  37  corresponds to the locating opening  35  in the head area  31  of the guide bar  15 . 
     FIG. 2 c  shows an advantageous implementation for an additional means of contact  40  between a locating contact and a centering contact. This can advantageously be designed as a cage spring or cylinder spring, which can be integrated into the contact cavity  37  of the locating contact  17 . The cage spring  40  is preferably provided with a front, slotted, cylindrical hull  41  and a rear, slotted, cylindrical hull  43 , which are connected by inwardly bent spring contact guard plates  42 . These hulls create a narrowed contact channel, which in an electrically conductive manner lies atop a centering contact extending down into the means of contact  40  through the front guide area  44  in the hull  41 . 
     A second embodiment is explained in more detail below with the aid of FIGS. 3 a ,  3   b ,  3   c  and  3   d . As in FIG. 2 a , FIG. 3 a  shows a perspective top view of the head area of a guide bar, FIG. 3 b  a locating contact and FIG. 3 c  an additional advantageous means of contact for insertion in the locating contact, analogous to the one shown in FIG. 2 c . Finally, FIG. 3 d  shows an additional means of connecting the locating contact to a cross-connecting bar. 
     In comparison to the locating contact in FIG. 2 b , the locating contact  17 ′ shown in FIG. 3 b  has an additional fastening bracket  46  on the block-shaped contact body  18 ′. Said bracket is provided with a reach-through opening  47  for a fastener  45 , which is shown in FIG. 3 d  as a screw. The fastener  45  serves to permanently fasten the locating contact  17 ′ to the cross-connecting bars. At the same time, this produces an electrically low-ohmic, high current-carrying contact that is especially favorable in terms of meeting the requirements for protective contact. 
     It is especially advantageous if the head area  31 ′ of the guide bar  15 ′ is arranged as shown in FIG. 3 a . In particular, the bracket  46  of a locating contact  17 ′ inserted in the locating trough  36  lies on top of the upper surface  60  of the head area  31 ′. The fastener  45  passes through the reach-through opening  47  of the bracket  46  as well as through the corresponding reach-through opening  48  in the head area  31 ′, down to a cross-connecting bar situated below. This ensures an extremely compact mounting of the guide bar and locating contact on a cross-connecting bar. 
     Such an arrangement is shown in the mounted state in FIG.  4 . The lower guide bar  15 ′ is laid on the upper surface  50  of a front, lower cross-connecting bar  5 , a section of which is shown here, in the area of its preferably enlarged head area  31 ′ and is preferably coupled onto the cross-connecting bar  5  via mounting holes  51 . 
     A locating contact  17 ′ is inserted into the head area  31 ′ of the guide bar  15 ′ in the manner described above. A screw  45 , serving as a connector, passes through the locating contact  17 ′ as well as through the head area  31 ′ of the guide bar  15  and is screwed into a mounting hole  51  in the cross-connecting bar  5 . 
     In addition, FIG. 4 shows a cross-section of the front element  13  for the mounting site  57 , which for the sake of a better overview is not equipped with an electrical printed circuit board here. One longitudinal side of the front element is provided with an additional flexible contact strip  49 . The front element  13  contains a face plate  19  of an electrically conductive material, on whose lower end is found a lower end piece  21  with a lower lever pulling handle  23  and a lower centering contact  25 . When the front element  13  is placed on the insertion side  2  of the rack assembly, the end piece corresponds to the locating contact  17 ′ in such a manner that it extends down into the locating opening  35  to the contact cavity  37 . This produces both a ground contact of the front element  13  and of any printed circuit board interconnected therewith, as well as a mechanical mechanism for centering during the insertion process. 
     FIG. 5 shows a cross-section of a locating contact  17 ′, situated in the head area  31 ′ of the guide bar  15 ′, with an inserted contact element  40 . The embodiment corresponds to the examples shown in FIGS. 3 a ,  3   b ,  3   c ,  3   d  and  4 . Below the head area  31 ′, the guide bar  15 ′ is provided with at least one grip knob  52 , which extends into a mounting hole  51  on a cross-connecting bar. Furthermore, below the transition between the head area  31 ′ and the extension area  32  of the guide bar  15 ′ there is a notch handle  53 , which serves to grip the rear of an edge of the cross-connecting bar. Moreover, FIG. 5 shows the thread of a screw  45  that simultaneously serves as a fastener for locating contact  17 ′ and for the head area  31 ′ of the guide bar. Finally, the cross-section shows a cylinder spring  40  placed in the contact cavity of the locating contact  17 ′. Specifically, the cylinder spring  40  is placed between the locating contact  17 ′ and a centering contact, to provide additional contact. The underside of the locating contact  17 ′ is provided with an additional means of contact  38 ′, preferably in the form of a peg, which also locks into a mounting hole  51  on the cross-connecting bar. Finally, from the cross-section shown in FIG. 5 it is clear that the locating opening  35  on the front  39  of the head area  31 ′ fits exactly into the opening of the contact cavity  37  in the locating contact  17 ′ situated behind it. 
     FIG. 6 shows a perspective lateral view of another implementation of a locating contact  17 ″, in which the means of contact  38 ″ has additional barbs or blade edges on the underside. This improves the electric contact between the locating contact and a cross-connecting bar in a mounting hole  51 . FIG. 7 shows another perspective lateral view of another implementation of a locating contact  17 ′″, in which the contact body  18 ′″ is provided with both a contact cavity  37  and a reach-through opening  47  as fasteners. 
     FIGS. 8 a ,  8   b  show perspective top views of other embodiments of a guide bar  15 ″″ and a locating contact  17 ″″. As far as these embodiments correspond to those shown in FIGS. 3 a ,  3   b ,  3   c ,  3   d  and  4 , reference is made to the corresponding explanations above. However, the embodiment of the locating contact  17 ″″ shown in FIG. 8 b  is also provided with a contact blade  54  in accordance with the present invention. In the example of FIG. 8 b , this contact blade is preferably designed as one piece connected to the fastening bracket  46 ″″ extending in the direction of the guide groove  26  on the guide bar. In another embodiment not shown here, said contact blade can also be a separate part that is mechanically interconnected to the locating contact  17 ″″. 
     In the locating contact  17 ″″ shown in FIG. 9 in its inserted position in the locating chamber  36 , the contact blade  54  lies on the upper surface  60 ″″ of the head piece  31 ″″ of the guide bar  15 ″″ and extends down into its guide groove  26 . Advantageously, the right groove wall  28  has a break  55 . If a printed circuit board is then inserted into the guide groove  26 , the printed circuit board produces a type of ESD contact via the contact blade  54  of the locating contact  17 ″″ with a cross-connecting bar, the rack assembly and its ground contact means. The connection is preferably achieved through a conductor track on the edge of the printed circuit board, which, especially during the insertion or removal process, comes into scraping contact with the contact blade. This allows electrostatic potentials, especially from a front element  13  of the printed circuit board, to be diverted. The particular advantage of the embodiments of the present invention shown in FIGS. 8 a ,  8   b  and  9  is that, with help of a single locating contact inserted into a guide bar, a ground as well as an ESD contact can be achieved between the front elements and the rack assemblies of the modular system. Separate scraping contact springs in the base of the groove of the guide groove in accordance with the systems described in references DE 3624883 and DE GM 295 09 185 are thus eliminated. 
     Advantageously, one of the groove walls of a guide groove (as shown in the examples in FIGS. 8 a ,  9  the left groove wall  27  of the guide groove  26  on the guide bar  15 ) is separated from the base of the groove in one area  55  situated opposite the contact blade  54 . This section of the groove wall is designed as a bell curve shaped plate spring so that the width of the guide groove  26  is tapered in the area where the contact blade  54  intervenes. Thus the edge of a printed circuit board can be pressed onto the contact blade and, by way of example, a conductor track situated on the printed circuit board is brought into scraping contact with the contact blade. Furthermore, in the embodiment shown in FIGS. 8 a ,  8   b , an additional means of connection can also be provided, preferably in the form of a screw, through which the locating contact  17 ″″ along with the contact blade  54 , as well as the guide bar  15 ″″, particularly via the head area  31 ″″, can be mechanically and electrically connected to a cross-connecting bar situated under it. In the mounted position, shown in FIG. 9, such a coupling screw is placed in the reach-though openings  47  and  48  of the locating contact and the guide bar  15 ″″. 
     The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.