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Hardware Architecture & Fault Tolerance Flashcards

7 cards from real SIL practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Hardware Architecture & Fault Tolerance flashcards as text

  1. Under IEC 61511, under what condition may a 1oo1 (simplex) architecture be acceptable for a SIL 2 safety instrumented function?

    Answer: When sufficient diagnostic coverage detects failures and initiates a safe state before a demand occurs

    IEC 61511 permits a 1oo1D approach where high diagnostic coverage detects failures quickly enough that the system is restored before a demand, effectively meeting the SIL 2 PFD target.

  2. How does Proof Test Coverage (PTC) influence the calculated PFD of a safety function?

    Answer: Higher PTC reveals more DU failures during each test cycle, reducing the average PFD over time

    PTC determines what fraction of DU failures are found and restored during each proof test; higher PTC reduces residual DU failures and lowers the average PFD across the proof test interval.

  3. What is 'systematic capability' (SC) as defined in IEC 61508 for hardware elements?

    Answer: A measure of a component's ability to avoid and control systematic failures relative to a target SIL

    Systematic capability (SC) is a rating assigned to a component indicating its design and manufacturing quality is sufficient to resist systematic failures up to that SIL; the SC must meet or exceed the target SIL.

  4. In a redundant safety system, what is the primary role of 'independence' between channels?

    Answer: To prevent common cause failures by ensuring that different failure mechanisms affect each channel

    Channel independence ensures that a single environmental, design, or installation failure cannot simultaneously defeat all redundant channels, which is the fundamental defense against CCF.

  5. A 1oo2 system uses two identical pressure transmitters from the same manufacturer installed on the same process tap. What is the primary safety concern?

    Answer: Common cause failure due to shared design, manufacturing, and environmental vulnerabilities

    Identical sensors from the same manufacturer sharing the same process tap are susceptible to the same design defects, material failures, and process blockages — a significant CCF risk that undermines the benefit of redundancy.

  6. In SIS reliability calculations, what does Mean Time to Restore (MTTR) specifically represent?

    Answer: The average time to detect, diagnose, and return a failed component to its operational state

    MTTR covers the full restoration cycle after a failure is identified — detection, diagnosis, repair, and return to service — and affects the exposure time for dangerous detected failures in PFD calculations.

  7. According to IEC 61508-2 architectural constraints, what is the maximum SIL claimable by a Type A subsystem with SFF < 60% and HFT = 0?

    Answer: SIL 1

    Per IEC 61508-2 Table 2, a Type A subsystem with SFF below 60% and no fault tolerance (HFT = 0) is limited to SIL 1 by architectural constraints.