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 in digital circuits include:
- 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 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.
- The delay fault model, where the signal eventually assumes the correct value, but more slowly (or rarely, more quickly) than normal.
A fault model in an Aerospace context is a set of structured information which helps users or systems to identify and isolate a problem that occurs on an engine, Line-replaceable unit (LRU), or Auxiliary power unit (APU) during a flight. Associated with this fault model may be a suggested repair procedure along with references to Aircraft maintenance manuals (~ Light maintenance manual ).