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For hardwired I&C systems it was accepted that reliability analyses were purely based on random hardware failure. With the introduction of computers design deficiencies had to be taken into account additionally. As the impact of these deficiencies on system reliability could not be derived from physical models it had to be estimated by engineering judgement. During the last years a large number of computer based equipment had been installed in nuclear power plants. From cumulated operating experience of these installation it can be derived that design or manufacturing deficiencies do not degrade the reliability of digital I&C systems significantly.