The subject of high-power fusible cut-outs is one entitled to the greatest respect, but it has undoubtedly been neglected in the past. The author has endeavoured to tackle the subject from a logical point of view in the present paper, and, in view of its fundamental importance, the problem of rupturing capacity has been selected as that to which attention is primarily confined.The paper includes a consideration of the factors influencing the rupturing capacity of a cut-out, and a description of an extensive series of tests carried out on various types of cut-outs, including many of those most widely used to-day. A selection is given from the records of tests on fundamental forms of elementary construction, on plain cut-outs and on various commercial forms, and this shows clearly the inadequacy of such cut-outs for high-power work. Certain recently developed forms of cartridge cut-outs which operated satisfactorily are referred to and their sphere of usefulness is indicated. A description is given of an ironclad oilimmersed cut-out, which appears from the tests to satisfy nearly all requirements for high-power work. A new design of cut-out for small-current high-pressure services is also described.An attempt has been made, by carrying out actual rupturing-capacity tests, to indicate how far the various types of cut-outs can be safely employed, and a chart has been drawn up to show a comparison between the rupturing capacities of these types.A striking feature made apparent by the tests is that there are remarkably few efficient cut-outs available. This is all the more to be regretted when it is realized that a fusible cut-out can be made into a most accurate and efficient device—consistency of calibration and consistency of rupturing performance being possible to a high degree of excellence.