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For several components in the power transmission system, the probability distributions of the scheduled outage durations were found to be significantly different from the conventionally assumed exponential, Weibull, or log-normal distributions. Based on statistical data, a unique distribution was obtained for these durations. New equations were derived for the frequency and mean duration of the system failures due to coincident forced and scheduled outages. Computations for a sample system show that the application of a Markov model with the implied assumption of exponentially distributed scheduled down times may lead to significant errors in the system reliability indices.