This paper analyses the impacts of credible contingency and an event of cyber attack on the dynamic performance of a real large-scale interconnected power grid. Any credible contingency, for example, short circuit fault or unnatural behaviour of protective devices due to cyber intrusion could create catastrophic consequences and even complete blackout to the power systems. In order to protect power systems against cyber events, it is necessary to analyse the impacts of both faults and cyber attacks on the dynamic behaviour of the power system to identify cyber events from credible contingencies. In this paper, a simplified model of an Australian 14-generator interconnected system is considered as a testbed and MATLAB/Simulink Simpowersystems Toolbox is used for the analyses. A real-life incident of faults has considered as case study and an event of a cyber attack on protection relay function is simulated to explore the possible similar impacts on the same page. The systematic analyses of different properties of the system will help to design the detection and counter measure techniques to ensure the system is protected from cyber threats.