The paper describes the construction of the two-element powder-filled cartridge fuse. Two recently introduced special features are described in detail, namely the provision for adapting the fuse for oil-immersion, and the striker-pin mechanism which ensures tripping on all three phases should only one fuse be blown. Discrimination in operation between fuses and other protective gear in circuit is discussed.Experiments are described which show the ability of powder to prevent corona formation on the fusible conductor. Transient voltages produced on fuse operation are important, and it is shown that a reduction in the magnitude of the transient can be obtained by changing the section of the fusible conductor in one or more places. A series of tests are described illustrating the behaviour of h.v. fuses on short-circuit in an ordinary 50-c./s. system. Typical oscillograms (ordinary and cathode ray) obtained during these tests are given. A test is also described which reproduces the operating conditions obtained when a rectifier backfires, and oscillograms are given showing the behaviour of the fuse when subjected to such a test. Fuses such as those described have been developed to have a rupturing capacity up to 1 000 MVA at 66 kv.The next section of the paper deals with the effect of these fuse developments on protection technique. It is suggested that H.R.C. fuses can be used (1) to improve fuse-switches; (2) as a back-up to safeguard old circuit-breakers in cases where the short-circuit burden has been increased; (3) in combination with a simple load-breaking switch, the switch dealing with small overloads only, all short-circuits and heavy overloads being taken care of by the fuse. A simple air-blast switch is described, suitable for use in the load-breaking switch and fuse combination.