An electrical power system includes power conversion apparatus for receiving electric power from a source and supplying the electric power in converted form to a load and control apparatus for automatically stopping the operation of the power conversion apparatus in response to the departure of a selected operating parameter, such as input voltage to the power conversion apparatus, from a preestablished range having upper and lower limits. A signal representative of the operating parameter is sensed and utilized to produce an out-of-range signal only when the operating parameter departs from the preestablished range, the out-of-range signal being utilized in turn to stop the operation of the power conversion apparatus.