BUS BAR CONSTRUCTION AND METHOD

i0 The invention relates to fused residential panelboards of the kind using plug fuses and, in particular, to a bus bar and plug fuse contact assembly for such panelboards. As part of the wiring research program conducted by Ontario Hydro fused residential paDelboards using plug fuses were the subject of investigation as a result of reports that such panelboards, bave,on occasion, failed in such a way as to create a fire hazard. During the course of this inw stigation typical residential fused panelboards were purchased and tested at their rated capacities and measurements were made of the temperature rise at the contact point between plug fuse and bus bar interfaces. Such tests were conducted under carefully controlled conditions simulating actual cycling of such panelboards under load conditions when installed in normal service locations. Plug fuses were installed at predetermined torques so as to provide, from test to test, a relatively constant fuse to bus bar pressure. An analysis of the test results revealed that a number of failures were related to arcing at the fuse plug tips where contact is made with the bus bar and that such failures were most commonly associated with loads of a cyclic nature such as washing machines, water heaters, home heating units and other devices which, under automatic control, cycle between on and off conditions during their norma! operation. As a result of these conclusions» Canadian Standards Association established new standards which must be met by fused residential panelboards in order to be entitled to Canadian Standards Association approval. The criterion for these tests have been clearly set forth in electrical bulletin No. 1120 dated May 26, 1977 published by Canadian Standards Association and provide that such fused domestic panelboards, set up under the conditions set forth in the bulletin and cycled under simulated load for periods of one hour "on" and onehall hour "off" shall disclose no more than a 10 °C temperature rise between the fifth cycle and the two hundredth cycle and no more than a15°C temperature rise between the fiftieth cycle and the five hundredth cycle. Panelboards of the kind commercially available prior to the present invention have consistently failed such a test and it is the object of the present invention to provide a bus bar, fuse contact assembly which, when subjected to the test designated by Canadian Standards Association will meet these requirements and will enable a panelboard to achieve Canadian Standards Association approval. Although other factors may have a bearing upon the ability of the panelboard to meet the Canadian Standards Association requirements, it has been found that a significant factor resides in the fuse tip to bus bar contact and the principal object of the present invention is to provide an improved fuse tip to bus bar contact as will be described in detail with reference to the accompanying drawings in which like reference numerals denote like parts in the various views and in which: Figure 1 is a plan view, with the cover partly cut away, of fused residential panelboard embodying the present invention; Figure 2 is a section view taken along line 2-2 of Figure I Figure 3 is an exploded view, partly in section showing a detail of the fuse tip contact and an associated portion of a bus bar; and Figure 4 is a section view taken along line 4-4 of Figure 1 showing, in addition, in dotted lines the fuse in position in contact i0 with the fuse contact point° Referring now to Figure i, the fused domestic panelboard of a generally conventional kind is illustrated as being provided by an enclosure i0 which is formed of sheet metal and which is provided with known screw engaging deformations il by means of which a cover plate 12 may be secured in position by screws 13. Within the enclosure i0 the bus bars 14 and 15 are mounted in spaced, parallel relationship, insulated from the enclosure I0 by means of stand-off insulators 16 which are secured to the enclosure l0 and to which the bus bars are secured in a conventional way. Associated with the bus bars are cartridge fuse holders 17 of a conventional kind and plug fuse holders 18, also of a conventional kind all of which are provided with electrical terminals by means of which the panelboard may be wìred into the house or residential circuitry. Electrical contact means 19 are provided by means of which electrical energy from the service entrance lines may be connected to the bus bars and ground connections are provided by a convenient bank of terminals such as those illustrated by reference numeral So far, everything that has been described is entirely conventional and will be very familiar and well known to those skilled in the art. The essence of the invention is illustrated in Figures 3 and 4. Each bus bar 15 is provided, at spaced locations along its length corresponding fo the contact points for the plug fuses with silver plated copper rivets which are received in apertures formed in the bus bars. Referring now to Figure 3, a bus bar 15 is shown in cross section with the section line lying along a dìameter of an aperture 1O9861O 22 adapted to receive a rivet 21. The aperture 22 is formed by a punching operation with the punch being driven through the bus bar 15 in the direction of the arrow 23. Punching the bus bar in this manner provides an aperture 22 which, over a portion of its length, is provided with an essentially cylindrical wall 24 but, which is also provided, over at least a portion of its length with a outwardly tapering wall portion This formation is common to the punching operation and is not, in itself, a novel phenomenon but it is important in the successful i0 performance of the present invention. Each rivet 21 is provided with a cylindrical shank 26 which is of a length at least as great and, preferably, slightly greater than the thickness T of 'the bus bar and of a diameter that is a force fit within at least that portion of the aperture 22 defined by the substantially cylindrical wall portion 24. Each rivet 21 is provided with a disc-like head 27 of a diameter larger than the shank and, the free end of the shank is provided with a conical recess 26a having a maximum diameter slightly less than the diameter of the shank for reasons which will be explained below. Each rivet 21 is provided with a layer of fine silver 28 that is deposited thereon and which is of the order of ç024 inches in thickness. After the layer of fine silver 28 has been deposited, the entire rivet, silver layer and all is further silver plated. The rivets 21 are then driven into the apertures 22 from the side of the bus bar :remote from the side from which the punching operation was performed. By inserting or driving the rivets from this side of the bus bar, the seating of the rivets is somewhat facilitated in that the slightly outwardly flaring wall portion l0 provides the lead-in portion to enable the rivets to be properly seated. Furthermore, as the rivet is driven into the bus bar, some of the silver plating on the shank of the rivet illustrated at 29 in Figure 4 will be scraped off and will be forced into the small gap lying between the cylindrical shank of the rivet and the slightly outwardly flaring wall surface 25 so as to be compressed into intimate contact with both the copper rivet and the copper bus bar at a location indicated by reference character 30. Intimate electrical contact and heat exchange contact is, accordingly, thereby provided between the entire shank of the rivet and the wall portion of the aperture 22 and the forceable driving of the rivet!'into the bus bar also provides for intimate electrical and heat exchange contact between the underside 31 of the rivet head 27 as seen in Figure 4. Optionally, once the rivet has been seated in the aperture in the bus bar as seen in Figure 4, it may be staked at two diametrically opposed points 32 and 33 as indicated by the arrows in Figure 4. By this construction, a contact point is provided on the bus bar for each of the plug fuses shown in dotted lines at 34 in Figure 4 with the fuse tip contact 35 in intimate electrical and heat exchange contact with the silver plated face of the rivet head 27. Extensive testing of a bus bar constructed in accordance with the foregoing description in the environment of a fused domestic panelboard as illustrated in Figure 1 has enabled such a panelboard to meet the temperature rise requirements of the Canadian Standards Association bulletin No. 1120 and, accordingly, provides a useful and significant advance in the art. 10986Æ0 THE EMBODIMENTS OF THE INVENTION IN WHICH AN I OECIÆSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: I. A bus bar construction for a fused residential panelboard using plug fuses comprising an elongated bus bar having apertures therein at locations corresponding to the contact points for the plug fuses and a silver plated copper rivet seated in each aperture, each rivet having a cylindrical shank at least as long as the thoEckness of the bus bar and a disc-shaped head of a greater diameter than the shank lying on the side of the bus bar adjacent the plug fuses. 2. A bus bar construction as claimed in claim 1 wherein the bus bar is formed of copper and is tin plated. 3. A bus bar construction as claimed in claim 1 wherein each rivet has a layer of fine silver deposited on the exposed face of its disc-shaped head and is subsequently entirely silver plated. 4. A bus bar construction as claimed in claim 1 in which the shank of each rivet is slightly longer than the thickness of the bus bar and is provided, at the end of the shank remote from the head, with a conical recess of a maximum diameter slightly less than the diameter of the shank so as to facilitate the staking of the rivet to the bus bar. 5. A bus bar construction as claimed in claim 3 wherein the layer of fine silver deposited on the exposed face of the head of each rivet is of the order of .024 inches thick. 6. A method of making a bus bar and plug fuse contact assembly for a fused residential panelboard comprising the steps of: (a) punching apertures in the bus bar at spaced locations along its length corresponding to the contact points for plug fuses;