Background & objective: Occupational accidents may occur in any types of activities. Carrying out daily activities such as repairing and maintaining are one of the work phases that have high risck. Despite the issuance of work permits or work license systems for controling the risks of non-routine activities, the high rate of accidents during activity indicates the inadequacy of such systems. A main portion of this lacking is attributed to the human errors. Then, it is necessary to identify and control the probable human errors during issuing permits.

Methods: In the present study, the probable errors for four categories of working permits were identified using SHERPA method. Then, an expert team analyzed 25500 issued permits during a period of approximately one year. Most of frequent human errors and their types were determined.

Results: The “Excavation” and “Entry to confined space” permit possess the most errors.  Approximately, 28.5 present of all errors were related to the excavation permits. The implementation error was recognized as the most frequent error for all types of error taxonomy. For every category of permits, about 40% of all errors were attributed to the implementation errors.

Conclusion: The results may indicate the weakness points in the practical training of the licensing system. The human error identification methods can be used to predict and decrease the human errors.