This work analyses the faults consequences in MSF plants. This analysis will help to design an optimal action plans in order to mitigate the faults effects. The nature of the faults consequences are studied at two levels: process level and mathematical level. At the first one, faults are grouped according its effects over the process state; at the second level, they are grouped according the mathematical features of its consequences analyzed as functions of time (i.e. signals). The study was carried out by using a dynamic simulator for MSF desalination plants. It takes into account the heaters and stages dynamic, hydraulic, standard instrumentation and control systems. This simulator was developed to study the effects of faults that may affect a MSF plant. in order to extend the results scope, the simulator allows the modification of topology and parameters into a wide range. Indeed, it is possible to change the number of stages belonging to the recovery and rejection sections, controllers' parameters (set point, integral time and gain), valves size, pumps characteristics, sea water conditions, stages and heater dimensions, etc. Since faults simulation is the main simulator goal, the model and its resolution were carefully designed to enhance stability and speed. The user can select the fault to simulate among a set of possible faults (fault in controllers, sensors, pumps, etc.), and can specify the activating time (at which the fault starts), the developing time (time elapsed from the fault start up until the fault reaches its maximum magnitude) and the fault magnitude. Thus, it is possible to simulate step and ramp perturbations. The simulator was tested with data from real plants and it has shown a good performance.