A fault model is an engineering model of something that could go wrong in the construction or operation of a piece of equipment. From the model, the designer or user can then predict the consequences of this particular fault. Fault models can be used in almost all branches of engineering. Basic fault models Basic fault models in digital circuits include: Static faults, which give incorrect values at any speed and sensitized by performing only one operation: the stuck-at fault model. A signal, or gate output, is stuck at a 0 or 1 value, independent of the inputs to the circuit. the bridging fault model. Two signals are connected together when they should not be. Depending on the logic circuitry employed, this may result in a wired-OR or wired-AND logic function. Since there are O(n^2) potential bridging faults, they are normally restricted to signals that are physically adjacent in the design. the transistor faults. This model is used to describe faults for CMOS logic gates. At transistor level, a transistor maybe stuck-short or stuck-open. In stuck-short, a transistor behaves as it is always conducts (or stuck-on), and stuck-open is when a transistor never conducts current (or stuck-off). Stuck-short will produce a short between VDD and VSS. The open fault model. Here a wire is assumed broken, and one or more inputs are disconnected from the output that should drive them. As with bridging faults, the resulting behavior depends on the circuit implementation. Dynamic faults, only at-speed and are sensitized by performing multiple operations sequentially: The transition delay fault (or transition fault) model, where the signal eventually assumes the correct value, but more slowly (or rarely, more quickly) than normal. Small-delay-defect model Fault assumption A fault model, falls under one of the following assumptions: single fault assumption: only one fault occur in a circuit. if we define k possible fault types in our fault model the circuit has n signal lines, by single fault assumption, the total number of single faults is k×n. multiple fault assumption: multiple faults may occur in a circuit. Fault collapsing There are two main ways for collapsing fault sets into smaller sets. Equivalence collapsing It is possible that two or more faults produce same faulty behavior for all input patterns. These faults are called equivalent faults. Any single fault from the set of equivalent faults can represent the whole set. In this case, much less than k×n