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Protective systems for nuclear power reactors have assumed a high degree of development. The primary motive has been the emphasis given to making reactors safe and more recently the desire to offset the site distance requirement through engineered safeguards. Increased reactor operating experience and improved safety analyses have contributed to a better definition of the events and conditions requiring protective systems. The judicious use of such philosophies as redundancy and coincidence in system designs has led to both greater safety and reactor operating continuity. Reliability of protective systems has been enhanced by parallel efforts in system and individual component design. Techniques and procedures for checking and testing protective systems have been developed and adopted at many installations to offset the inherent difficulty of assessing reliability of systems which experience little or no use under actual stressed conditions. Design practices have been effected to provide greater assurance that systems are independent although this remains as one of the outstanding problems. Protective systems in the context of monitoring devices are being developed by the application of noise analyses, digital computer control and the use of transistorized or solid state circuitry. Finally, the actual performance of protective systems is being manifested through analysis of existing operating records.