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Safety and SIS · Glossary

What is Voting Architecture?

Symbol: MooN1oo1, 1oo2, 2oo3Safety and SIS

Voting architecture describes how many of N redundant channels must act to trip, such as 1oo2 or 2oo3.

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Voting Architecture Explained

1oo2 trips if either of two channels demands it, giving high safety but more spurious trips. 2oo2 needs both, giving fewer spurious trips but lower safety. 2oo3 balances both.

Common cause failures limit the benefit of redundancy, so they are included in the calculation.

Voting Architecture Formula

1oo2: PFDavg ≈ (λDU TI)² ÷ 3 + β λDU TI ÷ 2
βcommon cause factor
TIproof test interval
Example: Simplified IEC 61508-6 form, ignoring repair time and diagnostics.

Voting Architecture: Typical Values

1oo1HFT 0
1oo2HFT 1, safety oriented
2oo2HFT 0, availability oriented
2oo3HFT 1, balanced
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Calculators for Voting Architecture

CalculatorPFDavg CalculatorFree online calculator with formula and worked example.CalculatorSpurious Trip Rate CalculatorFree online calculator with formula and worked example.CalculatorFault Tree CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadSIL PFDavg Calculation Excel SheetFree download in Excel and PDF.

Frequently Asked Questions

What is Voting Architecture?

Voting architecture describes how many of N redundant channels must act to trip, such as 1oo2 or 2oo3.

Which voting is used for transmitters?

2oo3 is common for critical trips; 1oo2 where spurious trips are acceptable.

Can voting be degraded?

Yes, on a detected fault 2oo3 can switch to 1oo2 or trip, as defined in the SRS.

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Related Terms

Safety termHardware Fault ToleranceHardware fault tolerance is the number of hardware faults a subsystem can have while still performing its…Safety termCommon Cause FailureCommon cause failure is a single cause that makes redundant channels fail together, such as a shared…Safety termSpurious TripA spurious trip is an unwanted shutdown caused by a safety system fault rather than a real process hazard.

More Safety and SIS Terms

Safety termProof Test IntervalProof test interval is the time between periodic tests that reveal dangerous undetected failures in a…Safety termProof Test CoverageProof test coverage is the fraction of dangerous undetected failures that a proof test actually reveals.Safety termMean Time Between FailuresMean time between failures is the average operating time between failures of a repairable item.Safety termLayers of Protection AnalysisLayers of protection analysis estimates the frequency of a hazardous event and checks whether independent…Safety termIndependent Protection LayerAn independent protection layer is a safeguard that can prevent a hazardous scenario on its own and is…Safety termHAZOPHAZOP is a structured team review that uses guide words to find hazards and operability problems in a…

All safety and sis terms · Full glossary A to Z

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