Safety systems are be used in a wide range of technical application. Beside the avail-ability of such systems the safety aspects, e. g. PFD and PFH figures, must be ob-served. Especially the calculation of these figures requires the use of standards. Worldwide are standards available for this calculation. The newest standard is IEC 61508. This standard is worldwide accepted. Another standard, which is used since years, is ISA-TR84.0.02. In this standard a safety calculation can be performed without using MTTR and common cause failure. Since the introduction of the standard IEC 61508 a lot of discussion concerning the PFD-number appears in the industry. The reason for that discussion is the way of calculation this numbers. This contribution will compare both calculation-methods.