In this article a brief critical description of the models presented/displayed by standard IEC 61000-3-7 appears to evaluate the impact of the fluctuating loads in systems of medium power of and high voltage. The basic concepts are explained and a discussion becomes of the analytical procedures that allow, from the knowledge of the source and the characteristics of the load, to consider of way expeditious (and approximated) its effect on the quality of the feeding voltage, using like defect acceptance number the methodology based on the obtaining of severities of flicker, applied in many other standards IEC. Expressions set out that give better approaches, and like support, they present/display simple examples in which computer simulations confirm the simple analytical methods.