1oo2 vs 2oo3
Side by side comparison: how they work, where each fits, and which to choose.
Quick answer: 1oo2 trips if either of two sensors demands it, maximising safety but doubling spurious trips; 2oo3 needs two of three, giving high safety and fewer spurious trips.
1oo2 vs 2oo3: Comparison Table
| Aspect | 1oo2 | 2oo3 |
|---|---|---|
| Channels | 2 | 3 |
| Hardware fault tolerance | 1 | 1 |
| Spurious trips | Higher (either channel) | Low |
| Online maintenance | Trip or bypass needed | Can degrade to 1oo2 or 2oo2 |
| Cost | Lower | Higher |
| PFDavg (simplified) | (λDU TI)² ÷ 3 plus common cause | about (λDU TI)² plus common cause |
Choose 1oo2 when
- High safety needs where trips are acceptable
Choose 2oo3 when
- Critical units where spurious trips are costly
Key Differences Explained
Both tolerate one dangerous failure. Common cause failures limit how much redundancy helps, so separate taps, diverse sensors and staggered testing matter.
Detected faults can change voting automatically, as defined in the safety requirements specification.
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Frequently Asked Questions
What is the difference between 1oo2 and 2oo3?
1oo2 trips if either of two sensors demands it, maximising safety but doubling spurious trips; 2oo3 needs two of three, giving high safety and fewer spurious trips.
What is 2oo2?
Both must demand a trip: fewest spurious trips but lowest safety (HFT 0).
Does 2oo3 always beat 1oo2?
For availability yes; for PFDavg 1oo2 is slightly better.
Where is voting done?
In the safety PLC logic.
Learn more on the blog
What is SIL (safety integrity level)
A practical guide on instrumentationblog.in that goes deeper into this topic with field examples.
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