Fault modeling is a fundamental element for several activities, ranging from off- and on-line testing, to fault tolerance and dependability-aware design. These activities are carried out during various design phases, dealing with specifications at different abstraction levels. Therefore, modeling faults across abstraction levels is of paramount importance to introduce dependability-related issues from the early phases of design. This paper analyzes how faults can be modeled at the different levels of abstraction with respect to Transaction Level Models, and how these models are related across levels. The work focuses on soft errors and aims at providing support to dependability analysis. A case study of a Transaction Level specification of a Network-on-Chip switch is used to evaluate the methodology and its applicability.