I am actually working with the functional safety project where the customer does not have sufficient knowledge about this and they don't come with SRS (safety requirement spec)that mentioned about mission time. Whereas our company has strict requirement to understand every aspect in PFDavg calculation including mission time, i am trying to elaborate to the customer what mission time is, and they may know the answer, but myself also still have a doubt about it.

@Noah Tibasiima thanks all, really appreciate, yes correct that PTC, mission time are directly affecting PFDavg calculation, and over time the danger failure is accumulated due to imperfect of PTC. Unfortunately even for PTC they don't define. This actually greenfield project, new plant (geothemal power plant) and this is basically for ESD system, LOPA study shows that SIFs are non-SIL, but they still go with SIS. That's why maybe they don't have further study to develop the SRS. But their RFQ to us is SIL3 logic solver and align with IEC61511 lifecycle, so we go with Functional Safety Management in executing this project. If i can resume for what you have described, mission time shall be defined considering the achievable proof test interval, and the PTC as such in the end of mission time the PFDavg/RRF value is still in tolerable region, typically every major overhaul. That actually i have asked, they mentioned major overhaul will be conducted every 4 years, shall it be defined as the mission time? And how should end user do after mission time?should they replace all the equipment in the SIF in order to reset the mission time?