A method is provided of operating a reactor trip system, including: determining whether an automated shutdown of the nuclear plant has been demanded; energizing an interface relay to de-energize an undervoltage coil of a circuit breaker when the automated shutdown has been demanded; energizing an interface relay to energize a shunt trip coil of the first circuit breaker when the automated shutdown has been demanded; de-energizing the undervoltage coil when the manual shutdown has been demanded; energizing the shunt trip coil when the manual shutdown has been demanded; performing a check of a cyclical execution of processors in first and second divisions using separate watchdog timers in each division and determining whether watchdog timer signals have been de-energized in the first and second divisions; and de-energizing the undervoltage coil if the first and second watchdog timer signals have both been actuated.
