Fault detectors have a main role in protection algorithms and should have reliable performance in both dependability and security approaches. In this paper, the robustness of a cumulative sum-based (CUSUM) fault detector against network's transient conditions such as lightning and line energizing is investigated. The results have shown that CUSUM-based fault detection unit is completely secure i.e. it can properly discriminate faults from transients such as lightning and line energizing.