Safety integrity level time dependency

SIL-based approach uses obtained time-dependent PFH functions for a monosystem and for a system with applied redundancy architecture to calculate the corresponded safety integrity level at any period of time. This approach can be implemented in an intelligent system that will periodically recalculate PFH value of a system and related SIL. An intelligent system will notify an operator about a close border between SILs. In addition, based on introduced method, it can be suggested two modes of maintenance for a braking system of moving walks full and economical. In a full maintenance mode the system has to be completely recovered till the initial value of reliability. In an economical mode SIL will be kept on the required level, but the system will be recovered partially by means of replacement/repair of the elements with the worst reliability characteristics. Time of periodical maintenance t is different in these two modes. In case of full mode is constant. In case of economical mode, this time is reduced every maintenance period. 

It is obvious that the requirement for the PFD depends directly on the intensity of requests for the supervisory function induced by other parts of the safety-related system, the EUC or the EUC control device. If the intensity of the requests changes with time, the requirement for the PFD would also change, leading to a changed safety integrity level. 

It can be observed that the PFD, which is important for determination of the safety integrity level, depends on proof test intervals and reaction times. 

This concept is further supplemented in die TIDE proposal by a speed cut-off directly at the prime movers if the zero velocity control command fails to act within a certain time span. Safety integrity is further enhanced by the use of safety monitoring, both of the central control functions and at the sub-system power module levels. The safety monitors would remove power from different areas, depending on the fault, and report back to the central display unit. In addition to the continuous action of closing a DMS, there is also need for a large red button, prominently displayed, as a final emergency off switdi. 

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