Abstract: The voltage collapse is a dynamic process that may take a few minutes which is called the slow voltage collapse or that may take a few seconds which is called the fast voltage collapse. These collapses can be avoided through preventives and curative measures. The load shedding activated by voltage criterion is a defensive mode that allows limiting the degradation of the voltage at unusable values. This load shedding by undervoltage must consider the dynamics of the voltage drop and irreversible degradation. In other cases, the voltage collapses may occur following the failure of the transmission network, related to the maximum power transmissible, to ensure the transfer of energy. Thus, a phenomenon of static stability loss is generally initiated. The study of voltage stability of the Tunisian network showed that the lines with highest index voltage stability, in normal situation, engender highest index voltage stability in N-1 situation. Therefore, these lines should be treated with caution during the planning phases of the electrical system.