The requirement for reliable electrical energy supply increases continuously because of its vital role in our lives. However, events due to various factors in the power grid can cause energy supply to be interrupted. One of these factors is human error and thus human reliability analysis is a serious element in the industry. The first step is to identify the roots of human error, on which there has been limited research in this area. In this paper, the potential and actual causes of human error in maintenance teams of power transmission system protection are identified and predicted within a framework of human factors analysis and classification system method. Then, human error factors are ranked to help improve human reliability. The proposed method is implemented in the Fars Electricity Maintenance Company.