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The need for faster and more sensitive protection on expanding interconnected power systems and the advantages to be gained by the use of high-speed protection in conjunction with autoreclosing in increasing system-stability limits and power transfer are discussed. Advantages to be gained in the use of static relays in satisfying these needs, while at the same time improving reliability and lowering maintenance costs, are considered. In particular, environmental conditions affecting relays employing static circuits and the safeguards employed to minimize the effects are discussed. Backup protection requirements, in terms both of selectivity and of speed, and some recent developments in static relays for both primary and backup protection are briefly described.