The Oak Ridge National Laboratory second generation of protection systems as applied to the High Flux Isotope Reactor (HFIR) has three independent channels, each with seven inputs; some of the inputs are calculated automatically and continuously. General coincidence logic was selected to permit on-line testing of complete channels and on-line maintenance. Although some designers consider local coincidence systems less likely to cause spurious scrams due to equipment failures, operating experience with the HFIR system has been entirely satisfactory. With the reactor operating at full power for 13 months prior to October 1967, there has been one safe (scram), but no unsafe, equipment failure. Sufficient confidence in the equipment has been gained to increase the test interval from 8 to 24 h. On-line maintenance has advantages over off-line maintenance, in addition to conservation of downtime, and tends to enhance safety.