SIL Hardware Architecture & Fault Tolerance 2 — Questions and Answers
Question 1: Per IEC 61508-2, what minimum Hardware Fault Tolerance is required for a Type B subsystem with SFF between 60% and 90% targeting SIL 3?
- HFT = 0
- HFT = 1
- HFT = 2 (Correct answer)
- HFT = 3
Correct answer: HFT = 2
IEC 61508-2 Table 3 specifies that a Type B subsystem with SFF in the 60–90% range requires HFT = 2 to satisfy SIL 3 architectural constraints.
Question 2: What is a 'common cause failure' (CCF) in a redundant safety system?
- A failure event that causes multiple redundant channels to fail simultaneously from the same root cause (Correct answer)
- A failure that occurs at the exact moment a process demand is placed on the system
- A frequently recurring failure mode that appears across all SIL levels
- A repeated single-channel failure that eventually defeats the safety function
Correct answer: A failure event that causes multiple redundant channels to fail simultaneously from the same root cause
CCF defeats redundancy by propagating a single failure mechanism — such as a design flaw, shared environment, or installation error — across multiple channels at once.
Question 3: Which statement best describes a 'dangerous undetected' (DU) failure?
- A failure that immediately triggers a process demand on the safety system
- A failure that is detected but cannot be repaired within the allowed MTTR
- A failure that disables the safety function without being identified by online diagnostics (Correct answer)
- A failure discovered during a proof test that has already been self-restored
Correct answer: A failure that disables the safety function without being identified by online diagnostics
DU failures are the most critical category: they leave the safety system unable to respond to a demand, yet remain hidden until a proof test or an actual demand reveals them.
Question 4: How does increasing diagnostic coverage (DC) affect the average Probability of Failure on Demand (PFD)?
- Higher DC extends the allowable proof test interval without changing PFD
- Higher DC reduces the rate of dangerous undetected failures, thereby lowering the system PFD (Correct answer)
- Higher DC eliminates the need for hardware redundancy in SIL 2 applications
- Higher DC raises the achievable SIL target without any hardware changes
Correct answer: Higher DC reduces the rate of dangerous undetected failures, thereby lowering the system PFD
Higher DC converts more dangerous failures into detected failures, reducing λDU and thus lowering the PFD calculated over the proof test interval.
Question 5: In reliability engineering for safety systems, what does 'derating' mean?
- Applying a correction factor when ambient conditions exceed component rated specifications
- Reducing the SIL target after completing a risk assessment
- Lowering the proof test frequency to minimize operational disruption
- Operating components below their rated electrical, thermal, or mechanical limits to improve reliability (Correct answer)
Correct answer: Operating components below their rated electrical, thermal, or mechanical limits to improve reliability
Derating involves using a component at, for example, 50% of its rated power or voltage, which reduces internal stress and significantly improves long-term reliability.
Question 6: What is the Hardware Fault Tolerance of a 2oo2 (2-out-of-2) voting architecture?
- HFT = 0 (Correct answer)
- HFT = 1
- HFT = 2
- HFT = 3
Correct answer: HFT = 0
In 2oo2, both channels must function for the safety action to occur; a single channel failure defeats the safety function, so HFT = 0.
Question 7: For a SIL 2 safety function that must minimize spurious trips in a continuous process, which voting architecture is most appropriate?
- 1oo1
- 2oo2
- 1oo1D
- 2oo3 (Correct answer)
Correct answer: 2oo3
2oo3 achieves HFT = 1 (satisfying SIL 2 architectural requirements) and requires two channels to agree before activating, reducing spurious trips compared to 1oo2.
SIL Hardware Architecture & Fault Tolerance 2
Per IEC 61508-2, what minimum Hardware Fault Tolerance is required for a Type B subsystem with SFF between 60% and 90% targeting SIL 3?