To assess operator errors contribution to major accidents likelihood, an estimation of human errors probabilities during a credible process emergency is necessary. In this paper, the human error probability (HEP) is the likelihood of failure to perform a corrective task in Control Room in order to prevent the hazardous event. Since human performance in diagnosing emergency situations is strongly dependent of the available time for performing correctly the requested actions, the estimations were performed for different cases, corresponding to different capabilities of the process plant to cope with the upset without loss of containment over a specified interval of time. The quantification was made by the means of THERP and HEARTH. As a result of the study it is evident that Human Factors Engineering as well as process risk assessment comprising the quantification of human error contribution should include a study of the process physics and the determination of the operators mental burden in case of credible emergency situations Independent shut-down logics should be taken into consideration in case of very quick and credible accidental sequences. Credit on the human safety barrier can be assigned only if the plant equipment is able to withstand upsets without losing equipment mechanical integrities. Although THERP is not a very recent method, it is still helpful due to its comprehensive estimation procedure and reproducibility. HEARTH is useful for quick human reliability estimations and when a specific approach is missing for the specific case study (i.e. field different to alarms handling).