Effect of Human Error on The Availability of Periodically Inspected Redundant Systems

The engineered safeguards of nuclear power plants contain many redundant systems which are not operating under normal conditions but they are called upon to act when emergency conditions develop. To ensure their operability, the systems are periodically tested (usually once a month). In this paper, unavailability formulae for several common logic configurations are developed, taking into account the probability of failure of components due to human error in the simultaneous testing scheme. The probability of the component being down due to human error is assumed to depend on the number of components which have gone through the tests consecutively prior to the inspection of the component under consideration. The system unavailability is the sum of unavailability due to hardware failure alone and the unavailability contribution due to human error.