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Although there has been active interest in static relays for more than 20 years, it is only with the advent of modern semiconductors that appreciable progress has been made. The increasing performance required of modern protection is reaching the limits of capability of electromechanical relays, and practical development of static protection with higher performance and greater potentiality is becoming increasingly necessary. The paper surveys the broad approach to the development of static protection, the bases of design, the selection of components and the assessment of environmental conditions, all of which are important in producing equipment suitable for manufacture and having the performance and reliability associated with power-system protection. Typical examples of practical equipment are given to illustrate the various approaches and views expressed in the paper. The possible influence of static protection on associated equipment is discussed, and some future trends are predicted. Finally, the present state of development is assessed as being reached mainly by the work of manufacturing organisations, and the view is expressed that further development will require increasing participation by prospective users.