SIL Verification Calculator
Does this safety function reach its target SIL? Work out PFDavg from the failure rates.
Data: 2oo2 uses the IEC 61508-6:2010 simplified formula as published by the 61508 Association. Other formulas and clause numbers not yet confirmed. About standard editions
For reference only. Always verify calculations against AS 61511 / IEC 61511 and consult a certified functional safety engineer (TUV/CFSE) for SIL verification of safety instrumented functions.
Target SIL
SIF Elements
Sensor
Logic Solver
Final Element
Test & Common Cause
8,760 hours
Typical: 0.05 (good), 0.1 (average), 0.2 (poor)
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About This SIL Verification Calculator
This calculator computes PFDavg for safety instrumented functions and verifies the achieved SIL level. Built for instrument technicians and safety engineers working on process plants in Australia and internationally.
Use it when assessing SIF architectures during design or proof test planning. Enter failure rates, proof test intervals, and voting architecture to see the PFDavg for each subsystem and the overall SIF.
Calculations follow the methods in IEC 61511 (AS 61511). This is a reference tool. Always verify results with your facility safety engineer and against the current edition of the standard.
Questions
- What is SIL verification and when is it needed?
- SIL verification confirms that a safety instrumented function achieves the required Safety Integrity Level by calculating its probability of failure on demand (PFDavg). It is required by IEC 61511 / AS 61511 for all safety instrumented systems in the process industry.
- What is PFDavg and how does it relate to SIL?
- PFDavg is the average probability of failure on demand over a proof test interval. SIL 1 requires PFDavg between 0.01 and 0.1, SIL 2 between 0.001 and 0.01, and SIL 3 between 0.0001 and 0.001. Lower PFDavg means higher integrity.
- What voting architectures does this calculator support?
- This calculator supports 1oo1, 1oo2, 2oo2, and 2oo3 architectures with common cause failure (beta) factors. It calculates PFDavg for the sensor, logic solver, and final element subsystems, then sums them for the overall SIF.
- What is PFDavg in safety instrumented systems?
- PFDavg (average Probability of Failure on Demand) is the average likelihood that a safety instrumented function will fail to respond when a dangerous condition occurs, calculated over the proof test interval. For SIL 1 the PFDavg must be between 0.01 and 0.1 (1 in 10 to 1 in 100 demands), for SIL 2 between 0.001 and 0.01, and for SIL 3 between 0.0001 and 0.001. AS 61511 requires PFDavg to be calculated for each subsystem and summed for the overall SIF. This calculator computes PFDavg from your failure rate, proof test interval, and architecture inputs.
- How does voting architecture affect the achievable SIL level?
- Redundant voting architectures dramatically lower PFDavg and increase the achievable SIL. A 1oo2 (one-out-of-two) architecture, where either channel can trip the system, can achieve SIL 2 or SIL 3 depending on failure rates. A 2oo3 (two-out-of-three) architecture provides high availability with SIL 3 capability. Use this calculator to compare how different architectures affect your PFDavg.
- What proof test interval is required for SIL 2?
- There is no single fixed proof test interval for SIL 2. It depends on the PFDavg calculation for your specific equipment failure rates and architecture. Typical proof test intervals for SIL 2 range from 1 to 5 years. Doubling the proof test interval roughly doubles the PFDavg, so a sensor with a dangerous failure rate of 2.0E-6 per hour in a 1oo2 architecture might achieve SIL 2 with a 2-year test interval but fail with a 5-year interval. AS 61511 requires the proof test interval to be documented in the safety requirements specification. Use this calculator to find the maximum proof test interval that keeps your SIF within the SIL 2 PFDavg target of less than 0.01.