The paper first describes the probable sphere of application of monocrystalline semiconductor rectifiers and states the problems which must be solved to ensure reliable operation under both normal and abnormal conditions. The basic characteristics of these rectifiers are then discussed. A brief description is given of the special test methods required to establish the characteristics, and it is shown that important properties can be entirely overlooked if test methods are inadequate.Operating conditions are examined under both normal conditions and during faults and overloads, and the protection requirements are deduced.The properties of h.r.c. fuses are briefly discussed, and by consideration of the rectifier properties and operating conditions the requirements are stated for the special fuses necessary. It is shown that these special fuses provide a basis for a protective system which enables full use to be made of the new semiconductor rectifiers.The paper concludes with a description of the practical application of these special fuses, and shows how different forms of protection can be co-ordinated to ensure satisfactory operation of semiconductor rectifiers under different conditions for a wide variety of duties.