The aim of this thesis is to supply a software tool, that would meet the requirements of Comission regulation (EU) 2017/1485 about establishing a guideline on electricity transmission system operation (abrv. System operation guideline – SO GL) and CSA methodology (abrv. methodology for coordinating operation security analysis - CSAM). The two regulations instruct that each TSO of synchronous area must determine an observability area for its transmission system and a contingency list, which consists of the elements whose unplanned outages could lead to the unwanted operational states of transmission system. Both demands can be fulfilled by finding the influence factor IPF, which provides information about which internal elements are most affected by an outage combination of two other elements. First element of the outage combination can be located either inside or outside the concerned TSO's transmission system and the second element must be located outside the TSO's transmission system. The supplied software tool is performing a N-1-1 security assessment, where the outage combination is in compliance with the above stated rules. The result of the thesis is a new software tool, which provides its user the IPF factors for all elements within the 3-rd ring i.e. 3 nodes outside the Slovenian power system.