Probability analysis of unrevealed faults in nuclear power plant protection systems

The author develops basic probability models for unrevealed faults of single and redundant equipment within protection systems. Results reveal that high safety system availability can be achieved by improved test methods, equipment redundancy, and the use of majority voting trip logic. It is concluded that if tests are performed on the equipment individually, this provides some continuous protection to the system when dealing with unrevealed faults. Unrevealed faults of redundant equipment can be detected by staggered testing where the lowest fractional dead time of the redundant combination is achieved when the test is carried out at times alternately T_c/2 apart where T_c is the test-proof period