Safety and SIL · Free Calculator
Spurious Trip Rate Calculator (MTTFS)
Estimate how often a voting architecture trips the plant falsely, from the safe failure rate of each channel and the repair time.
1oo2 vs 2oo3MTTFSSafe failure rateRepair time
h
Simplified equations without common cause or diagnostics; use them for comparing architectures.
1oo2 MTTFS11.42 years
2oo3 MTTFS95,130 years
1oo1 MTTFS22.83 years
Spurious trip rate by architecture
| Architecture | Spurious trip rate | MTTFS |
|---|---|---|
| 1oo1 | 5.00 × 10^−6 /h | 22.83 years |
| 1oo2 | 1.00 × 10^−5 /h | 11.42 years |
| 2oo2 | 4.00 × 10^−10 /h | 285,400 years |
| 2oo3 | 1.20 × 10^−9 /h | 95,130 years |
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Spurious Trip Equations
1oo1: STR = λS · 1oo2: STR = 2 λS
2oo2: STR = 2 λS² MTTR · 2oo3: STR = 6 λS² MTTR
MTTFS = 1 ÷ STR
2oo2: STR = 2 λS² MTTR · 2oo3: STR = 6 λS² MTTR
MTTFS = 1 ÷ STR
Worked example
With λS = 5 × 10⁻⁶ /h, 1oo2 trips about every 11 years, while 2oo3 with 8 h repair trips far less than once in a lifetime: it is both safe and available.
Key insight: 1oo2 is the safest voting but doubles spurious trips; 2oo3 is the usual choice where both safety and availability matter.
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Frequently Asked Questions
What is a spurious trip?
A shutdown caused by a safe failure of the SIS rather than a real process demand.
Why is 2oo3 voting popular?
It tolerates one failure in either direction: one dangerous failure still trips safely and one safe failure does not trip the plant.
What is MTTFS?
Mean time to fail spurious: the average time between false trips.
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