After a summary of the advantages of the high-rupturing-capacity cartridge fuse, the breaking-capacity requirements for fuses for use in low- and medium-voltage circuits, as set forth in British Standard No. 88—1939, are enumerated. The short-circuit operation of cartridge fuses is discussed, at first in a general way and then in the light of detailed investigations of the phenomena during the melting and arcing periods. Conclusions are drawn, both analytically and from test results, as to the effect of circuit conditions on the duty imposed, and these are compared with the requirements of B.S. No. 88.Following a Section dealing with the choice of materials, the various types of fuse for low- and medium-voltage circuits and for high-voltage circuits are considered. The paper concludes with notes on the application of these fuses.