Fault Detection in Redundant Circuits

It is shown that a set of diagnostic tests designed for a redundant circuit under the single-fault assumption is not necessarily a valid test set if a fault occurrence is preceded by the occurrence of some (undetectable) redundant faults. This is an additional reason (besides economy) for trying to eliminate certain kinds of redundancy from the circuit. However, single-fault analysis may remain valid for some types of redundancy which serve a useful purpose, such as the elimination of logic hazards in two-level circuits.