The fault detection problem is studied in this paper for a class of linear networked control systems (NCS) with communication delays. Aim is to generate residual signals which, in the fault free case, are supposed to be identically zero. In practice, this condition is not satisfied due to various factors such as measurements noises, model uncertainties, and in particular for NCS communication induced delays. The aim of this paper is to design a fault isolation filter satisfying to directional properties and being robust against network induced delays as well.