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Safe startup and operation of the nuclear reactor require neutron-sensing instruments to cover a range of about 109. This coverage is obtained by a counting rate meter, a period meter, and a high level power indicator. At normal operating levels, the reactor is regulated by the high level power controller and protected by the high level trips. Emergency shutdown of the reactor can be initiated by any one of 54 different conditions. Since these safety circuits are almost 100% “fail safe”, careful design, good maintenance, and cautious operation have been necessary to avoid accidental shutdowns. The period trips and high level trips are provided in triplicate for safety and are coincidence-connected in pairs to guard against accidental shutdown. The use of suppressed-zero circuits has proved helpful. The theory of coincidence-connected trips is briefly discussed.