This paper deals with the problem of Fault Detection and Isolation (FDI) in Networked Control Systems (NCS). The effects of unknown networked induced delays on conventional observed based residual generator are studied. It is shown that the detection performances may be reduced due to the sensitivity of the residuals signals to the delays. With the assumption that the network delays belong to a given bounded set, a threshold is defined on the basis of the network characteristic, in order to enhance the robustness of the decision making process. The detection thresholds, which depend on the bounded values of the network delays, are determined from the network calculus theory.