What is voting logic in safety instrumented system?
What is voting logic in safety instrumented system?
Voting logic (MooN or k-out-of-n) tells you how many channels out of n need to agree for a safety action to happen. For example, 1oo1, 1oo2, 2oo2, or 2oo3. It controls the balance between availability, safety integrity (PFD), and excursions that are annoying.
Engineering implications
- PFD vs. spurious trips: More needed votes usually mean fewer unintentional trips, but they can change the overall PFD calculation. Choose voting to fulfill SIL while keeping spurious trip rates low.
- When treating CCF, always do a common-cause failure analysis (using β-factor methodologies or IEC guidelines). To make things less likely to happen, employ a variety of sensor types, locations, and power sources.
- 1oo1 is for simple, low-availability vital functions; 1oo2/2oo2 and 2oo3 are for better availability and redundancy when continuity is very important.
Implementation & validation
- Use a certified logic solver or hardware voting unit to hold the vote. Check the voting windows, the sample alignment, and the diagnostic coverage.
- Fault injection (sensor short, drift, communication loss) and log verification of vote outcomes must be included of FAT/SAT and proof tests.
- For improvements to brownfields, estimate the effects of current PFD and false trips, and then update the SRS and maintenance plans before starting up.
Troubleshooting
- Use vote logs to find channels that are only sometimes working, verify loop response times, look at filters and delays, and make sure that sensor calibration and signal conditioning are correct.