Safety Considerations Guide, Tricon v9.0 - Tuv-fs.com

Operating Modes 41Operating ModesEach input or output point is considered to operate in Triple Modular Redundant,dual, or single mode.The current mode indicates the number of channelscontrolling a point; in other words, the number of channels controlling the outputor having confidence in the input. System variables summarize the status of inputand output points. For safety reasons, system mode is defined as the mode of thepoint controlled by the fewest number of channels. When a safety-critical point isin dual or single mode, the application may need to shut down the controlledprocess within a pre-determined time.A user can further simplify and customize shutdown logic using special functionblocks provided by Triconex. By considering only faults in safety-critical modules,system availability can be improved. For more information, see Appendix A,“Peer-to-Peer Communication.”While operating in TMR mode, the process is protected each scan from the effectof a single safety-critical system fault. The system can also tolerate multiple faultsand continue to operate correctly unless the combined effects of multiple faultsaffects the same point on multiple channels. If a system fault occurs, the loss ofredundancy causes an increased probability-of-failure-on-demand. To keep thePFD within industry-acceptable guidelines, adherence with the recommendedmaximum operating period of 1500 hours in dual mode and 72 hours (SIL3/AK5)or 1 hour (SIL3/AK6) in single mode should be observed.A safety-critical fault is defined as a fault that can affect the ability of the systemto correctly control outputs, including:• Inability to detect a change of state on a digital input point.• Inability to detect a change of value on an analog input point.• Inability to change the state of a digital output point.• Inability of the system to:– Read each input point– Vote the correct value of each input– Execute the control program– Determine the state of each output point correctlyChapter 3Fault Management