Electric fuses having one or more fusible elements, in particular helically wound fusible elements, have generally supports of insulating material for supporting the fusible element or elements. In the past such supports have been the source of great difficulties. It is generally desirable to make such supports of a high grade ceramic material and, as a result, the cost of such supports is high. A further serious limitation consists in that their bulk either limits seriously the amount of arc energy absorbing filler that can be placed into a fuse casing of given size, or precludes size limitation or reduction which is often an essential requirement.
This situation gave rise to the problem of evolving fuses having one or more helically wound fusible elements which would not require a mandrel-like insulating support and yet allow provision of extremely thin, extremely fragile fusible elements wound according to any desired diameter. The process disclosed and claimed in my U.S. Pat. No. 3,848,214; Nov. 12, 1974 for METHOD OF ASSEMBLING ELECTRIC HIGH VOLTAGE FUSES AND SUBASSEMBLY THEREFOR makes it possible to manufacture fuses wherein the helically wound fusible element or elements are supported only by the granular arc-quenching filler by which they are surrounded in the absence of any mandrel-like fusible element support.
The performance of fuses manufactured in accordance with the above patent has proven to be satisfactory. However, the manufacture thereof may require under certain circumstances particular care. In a fuse structure wherein the healically wound fusible element or elements are permanently supported by mandrel-like structure the process of filling the casing of the fuse with granular arc-quenching filler is less critical than in a fuse lacking a permanent mandrel-like supporting structure wherein the function of supporting the fusible element or elements is ultimately solely assigned to the granular arc-quenching filler or quartz sand by which they are surrounded.
In fuses manufactured in accordance with U.S. Pat. No. 3,848,214 the helical windings of the fusible element or elements are initially formed on a squirrel-cage-like structure whose element supporting rods are subsequently withdrawn from the windings at a point of time of the manufacturing process when the windings have gained sufficient support from the granular arc-quenching filler by which they are surrounded. Rod withdrawal is an additional process step not normally encountered in the manufacture of highvoltage fuses. If badly performed rod withdrawal may result in damage to extremely fragile fusible elements. Rod withdrawal results in the formation of voids which voids must not exceed a certain critical volume and timely be filled with granular arc-quenching filler.
It is, therefore, a prime object of this invention to provide electric fuses having fusible elements, in particular one or more helically wound fusible elements, which fuses are not subject to the limitations of prior art fuses having mandrel-like fusible element supports, and which fuses can be manufactured at smaller cost than fuses manufactured in accordance with U.S. Pat. No. 3,848,214.
Another object of the invention is to provide electric fuses having one or more helically wound fusible elements that are supported by a low cost mandrel-like support combining a minimum of bulk with sufficient dimensional stability.
Other objects and advantages of the invention will become more apparent as this specification proceeds.
USPTO Backgrounds
USPTO Backgrounds