United States Patent [191 [11] 4,081,853 Wickson [45] Mar. 28, 1978 {75154 IOVERCURRENT PROTE CT ION ISYSTEM ' FOREIGN PATENTI DOCUMENTS nventor: Arthur K. Wickson Pa os Verdes Estates, Calif‘ ’ 476,607 8/1915 France ............................... .. 361/104 - _ . Primary Examiner-Harry E. Moose, Jr. [73] Asslgnee‘ it: gzgr%glicforporanon’ LOS Attorney, Agent, or Firm—Joel D. Talcott; Frank J. g ' ' Kowalski; Albert J. Miller [21] Appl. No.: 669,790 [57] ABSTRACI. [22] Flled: Mar‘ 23’ 1976 A plurality of fuses are connected in a general parallel [51] Int. C13z ................................... .. H02H 3/08 con?guration for protecting a circuit, the fuses being [52] US. Cl. .......................................... .. 361/104 serially connected with inductances of varying magni [58] Field of Search ................... .. 317/15, 14 B, 40 A, tude such that steady state current may be divided 317/50; 337/4, 6, 161, 162, 293; 361/104 equally among the fuses permitting the use of fuses of [5 6] References Cited lower rating. During an overcurrent surge, substantially the entire current overload is applied to each fuse in US PATENT DOCUMENTS turn so that the total elapsed time for opening the circuit 835,388 11/1906 Berg ........ .. 317/40 AX is less than with fuses alone 2,284,1l4 5/1942 Weichsel 317/40 A 2,672,540 3/1954 Dewey ............................... .. 337/293 11 Claims, 4 Drawing Figures Over current Protection US. Patent March 28, 1978 4,081,235 3 Fig. 1. Over current Protection 3s 42 32 BW/40 4e 48 14%,50 34 I g . 54 56 52 : 20 so 30 ‘0A 29 24 / ,4 / 74 42 68\ M /76 62 Flg- 3 662% so Zea/‘65o 64 7o __rY4\72 £56 2e /20 L4“ - {F2 Fig. 4. 4,081,853 , 1 2 protection system is greatly accelerated so that the time OVERCURRENT PROTECTION SYSTEM for opening the circuit is substantially reduced. Thus, This invention is directed to overcurrent protection by the use of inexpensive components, a sure and reli systems and more particularly to a novel system for use able fuse system is provided which is capable of rapidly in protecting sensitive components, such as semi-con disconnecting a load from the power source on occur ductors, against damage caused by excessive current, rence of a sudden overcurrent fault. the fuse system being designed to respond more quickly The advantages of the overcurrent protection system than fuses alone to a sudden increase in current above of this invention will be more readily apparent when the the continuous rated value of current for the circuit. following speci?cation is read in conjunction with the It is generally desirable in electrical systems to open appended drawings wherein: the circuit very quickly to interrupt an overcurrent FIG. 1 is a generally schematic view of an electrical condition. In some cases, however, it is critical that circuit in which the overcurrent protection system of circuit interruption occur in a very short time period. this invention may be used; For example, substantial advances have been made in FIG. 2 is a schematic view of a preferred embodiment controlling the application of power to loads, such as of overcurrent protection system of this invention; electrical motors, by using thyristors either singly or in FIG. 3 is a schematic view of an alternate embodi bridge or other combinations to control the application ment of the overcurrent protection system of this inven of power to the load. While the use of power thyristors tion; and is highly desirable, these semiconductor devices are FIG. 4 is a schematic view similar to FIG. 2 for illus destroyed by excessive current overloads of sufficient 20 trating the operation of the overcurrent protection sys duration and are expensive to replace. tem of this invention. Accordingly, fuses and other protective devices are Referring now to the drawings, FIG. 1 illustrates in connected in series with these power thyristors to pro simple schematic form an example of a circuit in which vide protection against damage due to overload cur an overcurrent protection system 10 in accordance with rents. However, the degree of protection provided by 25 this invention might be used. It will be understood, any protective device in response to the occurrence of however, that the overcurrent protection system 10 an overcurrent fault is dependent upon its ability to may be utilized in a wide variety of circuit applications. respond with suf?cient speed to prevent damage to the In this speci?c example, it is desired to utilize a three semiconductor device. phase alternating current power source 12 to provide a When a given overload current is suddenly applied to direct current output through terminals 14 for energiz a fuse, the elapsed time before the fuse link vaporizes is ing a suitable load (not shown). To accomplish this, a determined by Fr value or rating of the fuse. This rating thyristor bridge circuit 16 is used. Conductors 18 are is roughly a constant for the fuse and the vaporization connected between the AC power source 12 and the time for any current level through a particular fuse can thyristor bridge 16 to provide input voltage to the be determined by equating Ft to this rating. Any means 35 bridge. This three phase power output is converted in a that reduces this rating reduces the time for the link to well known manner to a direct current output fed vaporize and, accordingly, increases the protection of ' through output conductors 20 and 22 to the DC output the components in series with the fuse. Individual fuses terminals 14. have been designed, and are available, with low rating The overcurrent protection system 10 is interposed in values to provide the highest degree of protection, but 40 the conductor 20. It will be seen that only one overcur the minimum ratings that can be attained are limited by rent protection system 10 is required for protection of the electrical and thermal properties of materials that all six thyristors of the bridge 16 unlike prior art fuse are available for the construction of the fuse. systems which are generally only capable of protecting It is known in the prior art that a single fuse can be one or two thyristors due to insuf?cient sensitivity. replaced by a plurality of smaller fuses connected in 45 For optimum operation, the overcurrent protection parallel, e.g., a 500 amp fuse may be replaced by ?ve system 10 must have a very small resistance so as to not 100 amp fuses connected in parallel. However, the fuse utilize signi?cant power when the bridge 16 is being rating of the combination turns out to be approximately used to power a DC load. However, in the event of a the same as the rating of the single fuse because, al sudden overcurrent condition which could otherwise though the rating of each fuse is smaller, the overcur 50 damage one or more of the thyristors in the bridge 16, rent fault is divided among the fuses, e.g. a 1000 amp the overcurrent protection system 10 must open thev fault current would be divided 200 amps to each fuse, so circuit completely in a short enough time period to that the total time required for vaporizing all the fuses prevent any thyristor damage. to open the circuit is roughly the same'as the time re The overcurrent protection system 10 of this inven quired to vaporize a single 500 amp fuse. ‘ 55 tion. fully accomplishes these requirements and the pre In accordance with this invention, a single fuse is ferred embodiment is illustrated in FIG. 2. The overcur replaced by a plurality of smaller fuse connected in rent protection system 10 is connected to the conductor parallel. An inductor is connected in series with each 20 at junctions 24 and 26. A conductor 28 connects the fuse but one, the total inductance connected in series junction 24 through a fuse 30 and junctions 32 and 34 to with each fuse being of differing values so that upon the junction 26. The junction 24 is also connected by a occurrence of an overcurrent fault, the inductors intro conductor 36 through an inductor 38, a junction 40 and duce a high reactance into the circuit causing substan a fuse 42 to the junction 32. A conductor 44 runs from tially the entire overcurrent fault to be directed through the junction 40 through an inductor 46, a junction 48 the fuse having no series inductor associated therewith. and a fuse 50 to the junction 34. The junction 48 is in After this fuse rapidly vaporizes, substantially the entire 65 turn connected to the junction 26 by a conductor 52 overcurrent is directed through the fuse having the least through an inductor 54 and a fuse 56. ' value of inductive reactance in series therewith. In this In the preferred embodiment, the fit ratings of the manner, vaporization of all fuses in the overcurrent fuses 30, 42, 50 and56 are generally equal and the induc 4,081,853 3 4 tors 38, 46 and 54 have substantially equal values of if the inductor 74 had an inductance L, the inductor 80 inductance and a very small value of DC resistance. would have an inductance 2L and the inductor 72 When the overcurrent protection system 10 of FIG. 2 would have an inductance 3L. It will be vunderstood, is operating in a circuit under steady state conditions, however, that great variations in the values of these the ?ow of current through each of the fuses 30, 42, 50 inductances are possible. Further, as in the circuit of and 56 is about equal, differing only by the practically FIG.