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A study of a distribution system is presented where the effect of distributed generation (DG) presence is analyzed. The system is typical of a rural, industrial, and low-power (weak) distribution scheme. The concept of specific energy for voltage sag immunity of sensitive equipment (SE) under the new scenario created by the DG presence is applied, especially studying the double magnitude/duration sag that is caused by the change in source impedance due to the response of dissimilar protection devices. The new protection coordination concepts of the not-anymore-radial distribution systems are described. A new methodology for the study of SE behavior under voltage sag when connected to a weak distribution system with DG is proposed.