liminfoSafety SIL Reference
Free reference guide: Safety SIL Reference
About Safety SIL Reference
The Safety SIL Reference is a structured quick-lookup guide for functional safety engineering. It covers Safety Integrity Levels (SIL 1 through SIL 4) as defined by IEC 61508, along with PFDavg and PFH probability targets for both low-demand and continuous/high-demand modes of operation. The reference also maps SIL levels to ISO 13849 Performance Levels (PL a through PL e) and safety categories (B through 4).
Beyond SIL definitions, this reference provides practical calculation formulas for PFD in 1oo1, 1oo2, and 2oo3 voting architectures, including common cause failure (CCF) beta-factor adjustments. It covers SIS lifecycle phases per IEC 61511, SIL allocation methods such as LOPA and risk graph analysis, Safe Failure Fraction (SFF) requirements, Hardware Fault Tolerance (HFT) tables, and proof test coverage values for full stroke, partial stroke, and disassembly inspections.
The reference also includes entries on Safety Instrumented Function (SIF) architecture, safety PLC products from Siemens, Rockwell, HIMA, ABB, and Yokogawa, emergency stop design per IEC 60204-1, safety relay modules, Functional Safety Management (FSM), and failure rate data sources including OREDA and Exida SERH databases. All content is organized by category and searchable in your browser with no installation required.
Key Features
- Complete SIL 1-4 PFDavg and PFH threshold tables for low-demand and continuous modes
- ISO 13849 Performance Level (PL) to SIL mapping with Category B through 4 structural requirements
- PFD calculation formulas for 1oo1, 1oo2, and 2oo3 voting architectures with CCF beta-factor
- SIL allocation methods including LOPA calculation steps and IEC 61508 risk graph parameters
- SFF and HFT requirement matrices for Type A and Type B subsystems
- SIS lifecycle phases, SIF architecture diagrams, and proof test interval guidance
- Safety PLC and safety relay product references from major vendors
- IEC 61508, IEC 61511, ISO 13849, and IEC 62061 standard summaries with part breakdowns
Frequently Asked Questions
What is Safety Integrity Level (SIL)?
SIL is a measure of the risk reduction provided by a safety function, defined in IEC 61508. It ranges from SIL 1 (lowest) to SIL 4 (highest). Each level specifies target failure probability ranges: for continuous mode, SIL 1 requires PFH between 10^-6 and 10^-5 per hour, while SIL 4 requires PFH between 10^-9 and 10^-8 per hour.
What is the difference between PFDavg and PFH?
PFDavg (Probability of Failure on Demand average) is used for low-demand mode safety functions where demand occurs no more than once per year. PFH (Probability of dangerous Failure per Hour) is used for continuous or high-demand mode. The choice between them depends on the demand rate relative to the proof test interval.
How does SIL relate to ISO 13849 Performance Level (PL)?
SIL 1 corresponds approximately to PL c, SIL 2 to PL d, and SIL 3 to PL e. IEC 62061 uses SIL for machine safety, while ISO 13849 uses PL. Both standards can be applied in parallel for machine safety applications, and the reference includes the detailed mapping between them.
What voting architectures are covered in this reference?
The reference covers 1oo1 (single channel), 1oo2 (one-out-of-two, safety priority), 2oo2 (both must actuate, availability priority), and 2oo3 (two-out-of-three, optimal balance). Each entry includes PFD formulas and explains the trade-off between safety (low PFD) and spurious trip rate.
What is LOPA and how is it used for SIL allocation?
LOPA (Layer of Protection Analysis) is a semi-quantitative method for determining the required SIL of a Safety Instrumented Function. It calculates residual risk by multiplying the initiating event frequency by the PFD of each Independent Protection Layer (IPL), then compares the result against a tolerable risk target.
What is the Safe Failure Fraction (SFF) and why does it matter?
SFF is the ratio of safe failures plus detected dangerous failures to total failures. IEC 61508 requires minimum SFF values depending on the target SIL and the Hardware Fault Tolerance (HFT). For example, a Type A subsystem at SIL 2 with HFT=0 needs SFF >= 90%, but this relaxes to SFF >= 60% with HFT=1.
How do I determine proof test intervals?
Proof test intervals are back-calculated from the target PFDavg using the formula PFDavg = lambda_DU * TI / 2 for a 1oo1 architecture. Typical intervals range from 1 to 5 years. Test coverage also matters: partial stroke tests achieve about 60% coverage, full stroke tests about 90%, and full disassembly inspections about 99%.
What failure rate databases does this reference mention?
The reference covers four major sources: manufacturer SIL certificates with PFD/SFF data, OREDA (Offshore Reliability Data) for offshore platform equipment, Exida SERH (Safety Equipment Reliability Handbook), and IEC 61508 SN 29500. Typical values are provided, such as lambda_DU of 5e-7/h for pressure sensors and 1e-6/h for shutdown valves.