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This paper describes modeling of power system reliability including protection failures from a new perspective. For a specific component, called focus-component, the undesired-tripping mode failure of its protection has no interaction with other components and the influence on itself can be represented by a two-state process. The fail-to-operate mode of its protection is represented by its unreadiness probability, which does not effect its own up and down states. However, the unreadiness probabilities of protection devices of the adjacent faulted components can cause external undesired-tripping mode failures of the focus-component, given its protection is providing backup protection to those adjacent faulted ones. After decoupling complex interactions among related components, these impacts on the component under focus can be represented by parallel independent two-state processes according to sources of impact. Finally, the complete Markov model of focus-component is derived and a new concept named power unit zone is proposed.