The Spurious Trip Level™ Concept For Safety Functions
Safety Integrity Level (SIL) measures safety. But what about availability? The Spurious Trip Level (STL) fills this critical gap by quantifying the impact of unnecessary trips on your business.
The Missing Metric in Functional Safety
Functional safety standards such as IEC 61508 and IEC 61511 define how to achieve safety through Safety Integrity Levels (SIL). These standards focus on reducing the probability that a safety function fails when it is needed.
However, they do not address the opposite problem: safety functions activating when they are not needed.
The key question is: how do you measure the business impact of safety functions that trip too often?
Introducing the Spurious Trip Level (STL)
To address this gap, Risknowlogy introduced the Spurious Trip Level (STL)—a measurable metric that quantifies the performance of safety functions in terms of process availability.
STL complements SIL by focusing on asset protection, not just risk reduction.
Risknowlogy Insight: SIL protects people. STL protects your business. You need both.
SIL vs STL: Two Sides of the Same Coin
- SIL (Safety Integrity Level): Measures the probability that a safety function fails on demand (PFD)
- STL (Spurious Trip Level): Measures the probability that a safety function activates without demand (PFS)
A system designed only for high SIL may result in frequent unnecessary trips, causing downtime and financial losses. STL ensures that availability is also considered.
Understanding STL Quantitatively
STL is expressed through the Probability of Fail Safe (PFS), which represents the likelihood of a spurious trip caused by internal system failure.
| STL Level | Probability of Fail Safe (per year) |
|---|---|
| 5 | ≥ 10E-6 to < 10E-5 |
| 4 | ≥ 10E-5 to < 10E-4 |
| 3 | ≥ 10E-4 to < 10E-3 |
| 2 | ≥ 10E-3 to < 10E-2 |
| 1 | ≥ 10E-2 to < 10E-1 |
Linking STL to Business Impact
The higher the cost of a spurious trip, the higher the required STL level should be. STL enables organizations to align safety design with financial risk.
| STL Level | Typical Financial Impact of Spurious Trip |
|---|---|
| 6 | > €20M |
| 5 | €10M – €20M |
| 4 | €5M – €10M |
| 3 | €1M – €5M |
| 2 | €500k – €1M |
| 1 | €100k – €500k |
| None | < €100k |
Why STL Matters
In real operations, unnecessary trips:
- Stop production and reduce revenue
- Introduce risk during shutdown and startup phases
- Increase operational and maintenance costs
- Reduce confidence in safety systems
STL provides a structured way to balance safety availability and process availability.
Conclusion
Functional safety is not just about preventing dangerous failures—it is also about avoiding unnecessary interruptions.
SIL ensures that safety functions work when needed. STL ensures they do not act when they are not needed.
Together, they provide a complete framework for designing systems that are both safe and economically viable.
Go deeper — STL & Functional Safety
Learn how to balance safety and availability using SIL and STL in real-world system design.
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