Increasing the Reliability of Electronic Equipment by the Use of Redundant Circuits

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Abstract
Electronic equipments which employ large numbers of tubes and components are often unreliable because a large number of parts may fail. A means of countering the undesirable effects of increasing equipment complexity is by introducing even more tubes and components as redundant circuitry. A requirement for obtaining the conditions, in which redundancy is helpful is for the causes of failure of parts to be independent. This calls for circuits to be designed so that as parts fail, their failure will not cause others to fail. The equations relating reliability to the number of circuit elements in the redundant and nonredundant cases are derived and applied to examples which show the degree of improvement which can be achieved. It is shown that equipment having hundreds of tubes could be made sufficiently reliable to satisfy most requirements. This greatly increased reliability can be maintained indefinitely by providing maintenance periods during which replacements are made for the parts which have failed while the operation has continued satisfactorily, the faulty condition having been obscured by the redundancy. This latter property of redundant circuit must be considered when establishing maintenance doctrines for equipment using these principles, since operational tests do not usually indicate the presence of faulty components or tubes. Rather, each tube and component must be temporarily disconnected from its redundant counterpart for the purposes of the test.

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