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Reliability and availability (R&A) studies have been made a part of the IEEE Recommended Practice For Industrial and Commercial Power System Analysis (IEEE Standard 399-1980). R&A studies should improve the selection of system configurations and components by basing decisions on quantitative information rather than on intuitive and qualitative decisions. The R&A indices for any electric power system design can be computed from knowledge of the statistical performance of individual components within the system. One of the important indices is the system overall downtime, which has a direct cost impact due to plant process interruption. In addition to R&A computation of electrical components in simple series or parallel configurations, other commonly encountered electrical power systems situations are considered such as a) scheduled maintenance of system components, b) manual switching of system components, c) transformer overloading practices, d) parallel redundant connections within a system, e) spare components available on short notice, and f) local generator units. These situations will be discussed, and the methods of calculation required to include them in R&A studies will be presented.