INTRODUCTION In recent years great research effort has been directed toward fault diagnosis of Electrical Power Generation Systems. Many papers have been published on this subject [1, 2], and even if we restrict ourselves to the theoretical treatment of faults, there is a variety of problem formulations depending on fault models, measurement conditions, the final object of the fault diagnosis, and so forth. Key factors in the classification of diagnostic problems are the following. 1. Subsystems under diagnosis: a. Elements contained in the subsystem: 1) “RLC or RC”, 2) “controlled sources, i. e. by definition, a two-port element which describes the energy transference or energy conversion between two subsystems. b. Excitation of the system: 1) DC Power Supply. 2) AC Power Supply – Single-frequency or multi-frequency. c. Number of exciting sources (independent sources or inputs): 1) Single exciting source. 2) Multiple exciting sources. 2. Fault models: a) Short circuits or open circuits (hard faults). b) Malfunction of an element or subsystem that occurs at intervals, usually irregular, in an element or subsystem that functions normally at other times (intermittent faults). c) Element or parameter-value deviations outside the tolerance bounds (soft faults) 3. Measurements: a) Voltage, b) Current, c) Frequency, d) Phase. 4. Final object of fault diagnosis: a) To determine voltages and currents, b) To identify the faulty elements, c) to locate and isolate the faults within a subsystem. In addition to the problem formulation directly related to electrical and mechanical elements, attempts have been made to apply techniques of system diagnosis to Electrical Power Diagnosis (EPD). In diagnosis of large- scale system, use of the computers is inevitable. One special feature of EPD strategy which is quite different from those of Energy System analysis is that the information for diagnosis or knowledge about the subpart of the EP system is very restricted. Therefore diagnosability, or whether or not the diagnostic problem formulated is solvable under the specific requirements and the expected performance indexes are achievable, must be preliminary researched in order to discriminate instability intrinsic to the problem from that due to the computational method and errors. Since there are several problem formulations, diagnosability must be described as structural as functional according to them In the following sections, the diagnosability of soft faults in a EP system which is in a “Fuzzy MLD Fault Model” is consider. An advantage of assuming soft faults only is that the EP model structural and functional description is known, and the equation derived for the Mixed Logical Dynamic (MLD) definition included simple, directed and co-directed subsystem, relationship can be inferred. It is possible to assume that the system contains as active elements, controlled-transferring conversion subsystems and several element-values relationship in terms of passive and active energy conversion in the different steps of the model. Most elements and function conversion can be represented by their equivalent equation conversion model and at last, a complete dynamic and recursive equation. Therefore, this approach not suffers loss of generality. For simplicity of discussion we further assume that the system contains direct transfer energy conversions in just one direction. Either if a cross-coupling effect is required to be expressed then an inverse transfer function will be used to describe it. Computability of the parameters in the system from measured and known values is developed first, since it is the basis of fault diagnosis. Then the result is applied to fault location by the fault verification or assume-and-check method. It is shown that the diagnosability of an EP system, like the system diagnosability, depends on the connectivity of the system under test. Finally, a brief discussion is given on diagnosis by multiple taps of evaluation mode.