SIL Functional Safety Principles & Standards 2 — Questions and Answers
Question 1: Which IEC 61511 clause specifically addresses the competency requirements for personnel involved in functional safety activities?
- Clause 5 (Correct answer)
- Clause 6
- Clause 8
- Clause 12
Correct answer: Clause 5
IEC 61511 Clause 5 defines organizational and functional safety management requirements including competency of personnel.
Question 2: In the context of IEC 61508, what does the term 'proven-in-use' (also called prior-use justification) allow an engineer to do?
- Skip the entire safety lifecycle
- Use existing components without full V&V if sufficient field history demonstrates reliability (Correct answer)
- Assign a higher SIL than calculated
- Replace functional safety management with quality management
Correct answer: Use existing components without full V&V if sufficient field history demonstrates reliability
Proven-in-use allows components with sufficient documented field history to be credited without full V&V, provided failure data supports the required failure rate.
Question 3: Which of the following best describes the relationship between IEC 61508 and IEC 61511?
- IEC 61511 is a sector-specific standard derived from IEC 61508 for the process industry (Correct answer)
- IEC 61508 was developed after IEC 61511 to provide a generic framework
- IEC 61511 supersedes IEC 61508 entirely
- IEC 61511 applies to machinery while IEC 61508 applies to process plants
Correct answer: IEC 61511 is a sector-specific standard derived from IEC 61508 for the process industry
IEC 61511 is the process-sector application standard derived from the generic IEC 61508 framework.
Question 4: A safety function is classified as SIL 2. What is the required probability of failure on demand per hour (PFH) range for a high-demand mode SIS?
- ≥10⁻⁴ to <10⁻³
- ≥10⁻⁷ to <10⁻⁶
- ≥10⁻⁶ to <10⁻⁵ (Correct answer)
- ≥10⁻⁵ to <10⁻⁴
Correct answer: ≥10⁻⁶ to <10⁻⁵
For high-demand (continuous) mode, SIL 2 requires PFH ≥10⁻⁶ to <10⁻⁵ dangerous failures per hour.
Question 5: What is the primary purpose of a Functional Safety Audit (FSA) as defined in IEC 61511?
- To replace the hazard and risk assessment
- To verify that functional safety activities are being carried out in accordance with plans and procedures (Correct answer)
- To calculate the SIL target for each safety function
- To perform final acceptance testing of the SIS
Correct answer: To verify that functional safety activities are being carried out in accordance with plans and procedures
An FSA is an independent review to confirm that functional safety management activities conform to established plans, procedures, and the standard.
Question 6: Under IEC 61508, which architectural constraint methodology allows credit for diagnostic coverage when determining hardware integrity?
- Route 1H (Hardware architectural constraints)
- Route 2H (Hardware assessment) (Correct answer)
- Route 3H (Software assessment)
- Route 4H (Systematic capability)
Correct answer: Route 2H (Hardware assessment)
Route 2H allows a full quantitative assessment including diagnostic coverage credit, unlike Route 1H which uses look-up tables.
Question 7: Which lifecycle phase in IEC 61511 produces the Safety Requirements Specification (SRS)?
- Hazard and risk assessment
- SIS design
- SIS conceptual design and allocation of safety functions (Correct answer)
- Installation and commissioning
Correct answer: SIS conceptual design and allocation of safety functions
The SRS is produced during conceptual design, after the LOPA/HAZOP, when safety functions and their SIL targets are allocated to the SIS.
SIL Functional Safety Principles & Standards 2
Which IEC 61511 clause specifically addresses the competency requirements for personnel involved in functional safety activities?