In this paper we consider linear systems with faults. We present a structural analysis for sensor location in the Fault Detection and Isolation problem. We deal with this problem when the system under consideration is structured, that is, the entries of the system matrices are either fixed zeros or free parameters. With such structured systems one can associate a graph. We define a new set of separators (Irreducible Input Separators) which allows us to get sets of system variables in which additional sensors must be implemented to solve the considered FDI Problem.
