Today’s electricity systems need to be more resilient, not only in the face of ever-increasing loads, but also with the accelerating penetration of renewable energy. However, due to environmental constraints and ever-increasing loads, these systems are often operated beyond their thermal capacity. When operating at the limit of their capacity, electrical systems can be affected by voltage and line instabilities. Various stability indices have been proposed in the literature for monitoring and predicting network stability, but their effectiveness is not always satisfactory. The present work has been devoted to the development of a relevant and effective analytical tool for the prediction of network collapse. The effectiveness and applicability of this tool were validated using standard IEEE 14 and 30 bus systems. In fact, this tool, known as the Complex Stability Index for Transmission Line (CSITL), performs better than traditional line voltage stability indices. At base load, line 1-5 of the IEEE 14 bus system is the most critical, carrying an apparent power of 78 MVA, i.e. 78% of the limit. For this line, the CSITL, NCPI and FVSI indices are 0.9963, 0.1826 and 0.0629 respectively. For the most critical line of the IEEE 30 bus system at base load, these indices are 0.7229, 0.0849 and 0.0419 respectively. The application of CSITL to the 14-bus and 30-bus test networks shows that it remains a powerful and excellent tool for effective and efficient management of transport networks, even in a disrupted context.