This paper presents an evaluation of the impact in frequency and loadabilities of include Demand Response on a power system. First, a theoretical background is presented related to Demand Response. Next, Time of Use is selected as the study Demand Response strategy and IEEE-39 is used as a test power system. IEEE-39 is modified to produce similar features to the Colombian electricity system. A Demand Response penetration model is applied and evaluated at three levels. The electrical variables used for the evaluation of the impact are the frequency, the rotor angle, and lines and transformer loadabilities under contingency. The results lead to a flattening in the demand curve, producing a flattening and an improvement in the electrical variables profile.