After a review of the aims and merits of protective devices for low-voltage installations, the joint use of h.r.c. fuses and miniature circuit-breakers is shown to be functional in many installations. The hypothesis that the 0.01 sec fusing current forms a criterion for discrimination is subjected to experimental tests and the statistical nature of discrimination is analysed. Practical conclusions for installation design are drawn, and a new fuse series and various other methods for backing-up miniature circuit-breakers are suggested.