This paper proposes six performance indices (PIs) for efficient detecting the most weakness load buses which may be subjected to voltage collapse occurrence at normal and emergency conditions. The classification of the suggested ranking PIs are dependent on their severity on the voltage collapse. The suggested PIs are based on the following physical quantities: equivalent system impedance and the related load impedance at each load bus, equivalent system admittance and the related load admittance at each load bus, load bus voltages and loading levels at different loading buses. These PIs derived based on circuit's theory and tested using one standard system, namely IEEE 14-bus. In addition, the capability of these PIs is demonstrated using the west Delta network (WDN) as a real part of the Unified Egyptian Network (UEN). The results detect efficiently the most sever load buses which may be subjected to voltage collapse occurrence.