This tool calculates the effective failure rate of two active on-line units, where "1 of 2" units are required for success. Each unit may have a different failure and/or repair rate. The calculation implements Equation 2 shown on page 90 of the United States Air Force Rome Laboratory Reliability Engineer's Toolkit (1993). It takes into account repair of any failed units and assumes that these units are repaired within the unit mean corrective maintenance time (Mct) entered below. See this page for additional details on time inputs (Mct) for system restoration.