The core principle being tested here is the systematic approach to determining the appropriate Safety Integrity Level (SIL) for a safety function, specifically focusing on the impact of common cause failures (CCF) on the overall safety integrity. IEC 61508-5 provides guidance on the methods for determining the SIL, including the risk-graph method and the safety-parameter-based method. When considering the architectural constraints for achieving a target SIL, particularly for higher SILs (SIL 3 and above), the concept of independence between redundant elements is paramount. A common cause failure is a failure that affects multiple safety-related elements simultaneously. To mitigate the impact of CCF and achieve a higher SIL, architectural constraints are imposed, such as requiring a higher degree of independence between redundant components. This independence is often quantified by a diagnostic coverage factor or by specifying a maximum allowable CCF rate. For a safety function requiring SIL 3, and assuming the initial safety-related elements (e.g., two independent sensors) are designed to meet the target failure rates for random hardware failures, the architectural constraints related to CCF become critical. If the diagnostic coverage for CCF is insufficient, or if the independence between redundant elements is not adequately demonstrated, the system may not be able to achieve the target SIL 3. The question posits a scenario where the initial assessment suggests SIL 3 is achievable based on random hardware failure rates, but the architectural constraints, particularly concerning CCF, are not met. This implies that the system, as designed, cannot reliably achieve SIL 3 due to the potential for common cause failures to compromise the redundancy. Therefore, the safety function must be assigned a lower SIL, which is SIL 2, as this is the highest SIL that can be demonstrably achieved given the architectural limitations. The explanation of the calculation involves understanding that achieving a higher SIL requires not only meeting random hardware failure rate targets but also satisfying stringent architectural constraints, especially regarding CCF. If these constraints are not met, the effective SIL is reduced. The transition from a potential SIL 3 to an actual achievable SIL 2 is a direct consequence of failing to meet the architectural requirements for CCF mitigation at SIL 3.

The core principle being tested here is the systematic approach to safety integrity level (SIL) determination for a safety instrumented function (SIF) when a higher-level risk assessment has already established the required SIL. IEC 61508-1:2010, Clause 7.4.2.2, outlines the process for determining the SIL of an SIF. When the overall risk reduction required for a hazardous event is known (e.g., from a HAZOP or LOPA), this directly translates to the required SIL for the SIF intended to mitigate that hazard. The subsequent steps involve architectural constraints and diagnostic coverage, but the initial determination of the target SIL is driven by the risk assessment outcome. Therefore, the most direct and correct approach to establishing the SIL for a specific SIF is to derive it from the quantified risk reduction requirements identified during the overall safety lifecycle, specifically from the hazard and risk analysis phase. This ensures that the SIF is designed to achieve the necessary level of safety to reduce the risk to an acceptable level, as mandated by the standard. The other options represent activities that occur *after* the target SIL is established or are not the primary drivers for its initial determination. For instance, assessing hardware fault tolerance occurs after the SIL is known, and defining the safety requirements specification is a subsequent step in the design process.

The fundamental principle guiding the selection of a Safety Integrity Level (SIL) for a safety function is the potential severity of harm that could occur if the safety function fails to perform its intended safety action. IEC 61508 mandates a risk-based approach. This involves identifying hazardous events, assessing the likelihood and consequences of these events, and then determining the necessary risk reduction. The SIL is a discrete level corresponding to a range of risk reduction. When considering a safety function designed to prevent a catastrophic event with a high probability of fatalities, the required risk reduction will be substantial. This necessitates a higher SIL. Conversely, if the potential harm is minor or the probability of occurrence is very low, a lower SIL might be acceptable. The process involves a systematic analysis of potential hazards, the exposure of individuals to these hazards, and the controllability of the hazardous event by the people involved. The objective is to achieve a tolerable level of risk. Therefore, the most critical factor in determining the SIL for a safety function is the quantified risk associated with the hazardous event it is intended to mitigate, specifically focusing on the potential severity of harm and the likelihood of its occurrence.

The core principle being tested here is the relationship between the Safety Integrity Level (SIL) and the required diagnostic coverage (DC) for random hardware failures in a safety function. IEC 61508-2:2010, specifically Table 4, provides the minimum diagnostic coverage requirements for different SILs for safety-related systems. For a Type A element (which is assumed for a generic safety-related system component unless otherwise specified) operating in a high demand or continuous mode of operation, the diagnostic coverage required to achieve a certain SIL is defined. Specifically, to achieve SIL 3, the diagnostic coverage for random hardware failures must be at least \(99\%\). This is because SIL 3 represents a significantly reduced probability of dangerous failure per hour, and achieving this requires a high level of fault detection and mitigation. The other options represent diagnostic coverage levels associated with lower SILs (SIL 1 and SIL 2) or a level that is insufficient for SIL 3. Therefore, a diagnostic coverage of \(99\%\) is the minimum requirement for a Type A element to contribute to a safety function with SIL 3.

The scenario describes a situation where a safety-related system, designed to achieve a Safety Integrity Level (SIL) 3, is being modified. The modification involves replacing a legacy component with a new one that has a higher diagnostic coverage (DC) for random hardware failures. Specifically, the original component had a DC of 60%, and the new component has a DC of 90%. The question asks about the impact of this change on the overall system’s SIL.

According to IEC 61508-2:2010, the target for the Probability of Failure on Demand (PFD) for a low-demand mode of operation for SIL 3 is between \(10^{-3}\) and \(10^{-2}\). The diagnostic coverage (DC) is a critical parameter in calculating the Safe Failure Fraction (SFF) and subsequently the PFD of a safety function. The SFF is a measure of the proportion of failures that are detected by diagnostics. For a Type A element (a safety-related element whose behavior is completely specified by the safety requirements specification and whose failures are not necessarily detected by a safety-mechanism), the SFF is calculated as \(SFF = \frac{FF + (SBF \times DC)}{FF + SBF}\), where FF is the number of failures that are not detected and SBF is the number of safely detected failures.

A higher DC directly increases the SFF. For a system to achieve SIL 3, the SFF must be greater than or equal to 90% and less than 99%. If the SFF is 90% or greater, the element can be considered to have a high level of diagnostic coverage, contributing positively to the overall SIL. The change from 60% DC to 90% DC significantly improves the diagnostic capabilities of the component. This improvement in diagnostic coverage, assuming other factors remain constant and the component’s architecture is suitable, will likely increase the SFF of the safety function. If the original system was at the lower bound of SIL 3 (e.g., PFD just below \(10^{-3}\)) and the SFF was borderline for SIL 3 (e.g., around 90%), increasing the DC to 90% would further strengthen the system’s ability to detect failures. This enhancement in diagnostic coverage is a key strategy for maintaining or improving the SIL of a safety-related system, especially when components are replaced or upgraded. The question probes the understanding of how improved diagnostic coverage directly contributes to the robustness of the safety function and its ability to meet the stringent PFD targets required for SIL 3. The correct approach is to recognize that increased diagnostic coverage directly enhances the SFF, which is a primary factor in achieving and maintaining a given SIL, particularly for SIL 3 where high diagnostic coverage is essential.

The core principle being tested here is the appropriate selection of a safety integrity level (SIL) for a safety function based on the risk reduction required. IEC 61508-1:2010, Clause 7.4.2.2, outlines the process for determining the SIL. The risk assessment for the hazardous event of uncontrolled steam release due to a faulty pressure relief valve indicates a high potential for severe harm to personnel and significant environmental damage. The initial risk, without any safety function, is estimated to be at a level requiring substantial reduction. The proposed safety function, a pressure monitoring and shutdown system, is intended to reduce this risk to an acceptable level.

The required risk reduction factor (RRF) is calculated by dividing the initial risk level by the target acceptable risk level. Assuming the initial risk is assessed as “high” and the target acceptable risk is “low,” a significant risk reduction is needed. IEC 61508 defines SILs based on the required risk reduction: SIL 1 (RRF 10 to 100), SIL 2 (RRF 100 to 1000), SIL 3 (RRF 1000 to 10,000), and SIL 4 (RRF 10,000 to 100,000). Given the severity of potential consequences and the need for a substantial reduction in the probability of the hazardous event occurring, a SIL 3 rating is the most appropriate choice. This level of integrity ensures that the safety function is sufficiently reliable to achieve the necessary risk reduction for this high-consequence scenario. SIL 2 might not provide sufficient risk reduction, and SIL 4 would likely be overly conservative and economically unfeasible for this specific hazard. SIL 1 is clearly insufficient given the described risk.

The core principle being tested here is the distinction between the Safety Integrity Level (SIL) of a safety function and the SIL of individual elements within that safety function. IEC 61508:2010 mandates that the SIL of a safety function is determined by the required risk reduction. While individual components contribute to the overall reliability, the safety function’s SIL is not simply the average or highest SIL of its constituent parts. Instead, the architecture and the way these elements are combined (e.g., redundancy, diversity) are crucial. A safety function with a high SIL (e.g., SIL 3) can be achieved using elements with lower SILs (e.g., SIL 1 or SIL 2) if the architectural design and diagnostic coverage are sufficient to meet the overall target risk reduction. Conversely, using only SIL 3 components does not automatically guarantee a SIL 3 safety function if the architecture is flawed or diagnostic coverage is inadequate. The safety lifecycle activities, particularly the safety requirements specification and the system design, are where the SIL of the safety function is established and then allocated to the elements. The explanation of how the safety function’s SIL is achieved through the combination of elements, considering architectural constraints and diagnostic capabilities, is fundamental to the standard.

The core principle being tested here is the systematic approach to determining the appropriate Safety Integrity Level (SIL) for a safety function, specifically when considering the impact of diagnostic coverage and common cause failures on the overall reliability of a safety-related system. IEC 61508-1:2010, Annex D, provides guidance on calculating the Probability of Failure on Demand (PFD) for different architectural configurations and failure modes. For a Type B element (which is assumed for the purpose of this question, as it’s a common scenario in complex systems and requires more nuanced consideration than Type A), the PFD calculation involves accounting for both random hardware failures and systematic failures. The diagnostic coverage (\(DC\)) is a critical parameter that reduces the effective failure rate of a component by detecting and mitigating certain failures. Common cause failures (CCF) are failures that affect multiple elements simultaneously, and their impact is often quantified by a common cause factor (\(\beta\)).

For a low demand mode of operation, the PFD can be approximated. A key aspect of IEC 61508 is the iterative process of safety lifecycle management. When a system is designed, the target SIL must be achieved. If the initial architecture, even with diagnostic coverage, does not meet the required PFD for the target SIL, architectural changes or redundancy might be necessary. The question implies a scenario where the initial assessment indicates a potential shortfall. The correct approach involves re-evaluating the system architecture, considering the effectiveness of diagnostics against specific failure modes, and potentially increasing redundancy or improving diagnostic capabilities to achieve the required PFD. The explanation focuses on the *process* of achieving SIL, not a specific numerical calculation, as the question asks about the *most appropriate action*. The concept of architectural constraints and the need to demonstrate compliance with the target SIL through appropriate safety mechanisms and their effectiveness is paramount. This involves understanding how diagnostic coverage, failure rates, and common cause failures interact to determine the overall safety performance of the system. The process of safety validation and verification, as outlined in IEC 61508, would also play a role in confirming the effectiveness of any implemented measures.

The core principle being tested here is the systematic approach to hazard and risk analysis as mandated by IEC 61508. Specifically, it relates to the selection of appropriate safety integrity levels (SIL) and the subsequent derivation of safety requirements. The process begins with identifying hazards associated with a safety-related system. For each identified hazard, the potential consequences are evaluated, considering factors like the severity of harm, the frequency or duration of exposure, and the probability of the hazardous event occurring. This evaluation leads to the determination of a target SIL for the safety function designed to mitigate that hazard. IEC 61508 outlines that the SIL is determined based on the risk reduction required. A higher SIL signifies a greater need for risk reduction. Once the SIL is established, this informs the required architectural constraints and the fault tolerance of the safety-related system. The explanation for the correct option emphasizes the iterative nature of this process, where the initial hazard identification and SIL determination are foundational, guiding subsequent design and verification activities. The other options represent incomplete or misapplied aspects of the safety lifecycle, such as focusing solely on fault detection without considering the initial risk assessment, or conflating diagnostic coverage with the overall SIL determination process, or prioritizing system availability over the necessary risk reduction for the identified hazards. The correct approach involves a comprehensive risk assessment to establish the SIL, which then dictates the necessary safety measures.

The core principle being tested here is the systematic approach to achieving functional safety, specifically how the Safety Integrity Level (SIL) requirements influence the selection and verification of safety-related system elements. IEC 61508 mandates a lifecycle approach, where the safety requirements specification (SRS) is a foundational document. This SRS details the safety functions, performance requirements, and the required SIL for each function. The subsequent design and implementation phases must then ensure that the chosen hardware and software elements, when integrated, can meet these specified SIL requirements. This involves considering architectural constraints, fault tolerance, diagnostic coverage, and the overall reliability of the components. The verification and validation activities are crucial to confirm that the implemented system indeed achieves the intended safety performance. Therefore, the process begins with a clear definition of safety needs and their associated SILs, which then drives all subsequent technical decisions and verification efforts. The concept of safety integrity is not an afterthought but a primary driver from the initial stages of safety lifecycle management.

The core of this question lies in understanding the IEC 61508:2010 requirement for the Safety Integrity Level (SIL) determination process, specifically concerning the selection of techniques and measures for the safety lifecycle phases. Part 1, Clause 7.4.3.2.2 outlines the need to select appropriate techniques and measures for each safety lifecycle phase. For the concept phase (Clause 7.4.3.2.2.2), the standard emphasizes the importance of defining the safety requirements specification (SRS) and the safety plan. The SRS is a foundational document that details the functional and safety requirements, including performance criteria, environmental conditions, and operational modes. The safety plan, on the other hand, outlines the activities, responsibilities, and resources required to achieve and maintain functional safety throughout the lifecycle. Therefore, the most critical initial step in the concept phase, as per the standard’s intent for establishing a robust safety case, is the thorough definition of these two documents. Without a well-defined SRS and a comprehensive safety plan, subsequent phases of the safety lifecycle, such as system design and verification, would lack the necessary direction and rigor to ensure the achievement of the target SIL. The other options, while important in later stages, are not the primary focus of the initial concept phase’s foundational activities as mandated by the standard. For instance, detailed hardware fault tolerance calculations are typically performed during the system design phase, and the development of a fault injection test strategy is a verification activity.

The core principle being tested here is the relationship between the Safety Integrity Level (SIL) and the required diagnostic coverage (DC) for random hardware failures in a safety function. IEC 61508-2:2010, Table 4 specifies the minimum diagnostic coverage required for different SILs for elements that are not inherently fault-tolerant. For a Type A element (which is assumed for a standard component unless otherwise specified) operating in a low demand mode of operation, the diagnostic coverage requirements are as follows:

* SIL 1: \(DC \ge 60\%\)
* SIL 2: \(DC \ge 80\%\)
* SIL 3: \(DC \ge 90\%\)
* SIL 4: \(DC \ge 99\%\)

The question describes a safety function with a target SIL of 3. It then states that the implemented safety-related system achieves a diagnostic coverage of 85% for random hardware failures. To determine if this meets the requirement for SIL 3, we compare the achieved DC with the minimum required DC for SIL 3. Since 85% is less than the required 90% for SIL 3, the system does not meet the target SIL. Therefore, the safety function’s achieved SIL is lower than the target SIL of 3. Specifically, an 85% diagnostic coverage for a Type A element in low demand mode typically corresponds to a SIL 2 capability. This is because the probability of failure on demand (PFD) for SIL 2 is between \(10^{-2}\) and \(10^{-3}\), and achieving 85% DC for a Type A element is generally sufficient to fall within this range, whereas 90% DC is needed for SIL 3. The explanation focuses on the direct comparison of the achieved diagnostic coverage against the standard’s requirements for the specified SIL, highlighting the gap and its implication on the overall safety integrity level achieved.

The core principle being tested here is the systematic approach to determining the appropriate Safety Integrity Level (SIL) for a safety function, particularly when considering the impact of common cause failures (CCF) on redundant elements within a safety-related system. IEC 61508-2:2010, specifically Annex D, provides guidance on architectural constraints and the calculation of the Safe Failure Fraction (SFF) and the diagnostic coverage (DC) for different SIL levels. For a Type B element to achieve SIL 3, it must meet specific architectural constraints. A key constraint for Type B elements at SIL 3 is that the SFF must be at least \(90\%\). The SFF is calculated as \(SFF = 1 – (\text{PFD}_{\text{total}} / \text{PTI}_{\text{total}})\), where \(\text{PFD}_{\text{total}}\) is the total probability of failure on demand and \(\text{PTI}_{\text{total}}\) is the total probability of the input signal being present. Alternatively, and more practically for architectural assessment, SFF can be considered as the proportion of failures that are detected by diagnostics or are inherently safe.

A Type B element with a diagnostic coverage of \(99\%\) for random hardware failures, and assuming no systematic failures are present in the diagnostics themselves, would have a significant portion of its potential failures detected. The standard implies that for Type B elements, a higher diagnostic coverage contributes to a higher SFF. While the exact calculation of SFF involves considering various failure modes and their probabilities, a high diagnostic coverage directly translates to a high SFF, provided the detected failures are indeed prevented from causing a hazardous event. For a Type B element to be considered for SIL 3, it must achieve an SFF of at least \(90\%\). A diagnostic coverage of \(99\%\) for random hardware failures is a strong indicator that the element can meet or exceed this SFF requirement, assuming the diagnostics are effective and the remaining \(1\%\) of undetected failures do not collectively push the overall failure rate beyond the SIL 3 threshold. Therefore, the most appropriate conclusion, based on the principles of IEC 61508-2 regarding architectural constraints for Type B elements, is that it can be used for SIL 3 if the SFF requirement is met, which is highly probable with \(99\%\) diagnostic coverage. The other options represent either a misunderstanding of the SFF concept, an incorrect application of diagnostic coverage, or an assumption about the SIL level that is not directly supported by the provided information without further analysis of the entire safety function.

The core principle being tested here is the appropriate selection of a safety integrity level (SIL) for a safety function based on the identified risk reduction required. IEC 61508-1:2010, Clause 7.4.2.2 outlines the process for determining the SIL. The scenario describes a hazardous event with a high frequency of occurrence (10 occurrences per year) and a severe consequence (death or irreversible injury). The target risk reduction required is substantial, aiming to reduce the risk to an acceptable level.

To determine the SIL, one must consider the risk associated with the hazardous event without the safety function and compare it to the acceptable risk level. The standard provides guidance on mapping risk levels to SILs. A high frequency of occurrence combined with a severe consequence indicates a high initial risk. To achieve a sufficiently low residual risk, a significant reduction factor is needed.

Let’s consider a hypothetical initial risk of \(10^{-2}\) per year (representing 10 occurrences per year, each with a probability of consequence of 1, leading to a total risk of 10 per year, which is then normalized to a per-year basis for comparison with target risk levels). The target risk reduction factor for SIL 3 is typically in the range of \(10^3\) to \(10^4\). For SIL 4, it’s \(10^4\) to \(10^5\). Given the severity and frequency, a SIL 3 safety function would aim to reduce the risk by a factor of at least \(10^3\). If the initial risk is \(10^{-2}\) per year, a SIL 3 would aim for a residual risk of \(10^{-2} / 10^3 = 10^{-5}\) per year or lower. A SIL 4 would aim for a residual risk of \(10^{-2} / 10^4 = 10^{-6}\) per year or lower.

The question implies a need for a high level of risk reduction due to the severe consequences and frequent occurrence. While a precise numerical calculation of the initial risk is not provided, the qualitative description strongly suggests that a SIL 3 or SIL 4 is warranted. However, the question asks for the *most appropriate* SIL. SIL 3 is generally considered adequate for situations with severe consequences and high frequency when the required risk reduction is substantial but not extreme. SIL 4 is reserved for the most critical applications where the residual risk must be exceptionally low. Without more precise quantitative risk assessment data, selecting SIL 3 represents a robust and commonly applied level of safety integrity for such scenarios, balancing the need for safety with the practicalities of implementation. The other options represent lower levels of safety integrity that would not provide sufficient risk reduction for the described hazard.

The core principle being tested here is the systematic approach to determining the appropriate Safety Integrity Level (SIL) for a safety function, specifically focusing on the impact of diagnostic coverage and the resulting Safe Failure Fraction (SFF). The standard IEC 61508-1:2010, Annex D, provides guidance on calculating the SFF.

Let’s assume a hypothetical safety-related system where the following failure rates are estimated for the safety function’s elements:
– \( \lambda_{SD} \) (dangerous detected failures) = \( 10^{-5} \) failures per hour
– \( \lambda_{SI} \) (dangerous undetected failures) = \( 5 \times 10^{-6} \) failures per hour
– \( \lambda_{SU} \) (safe undetected failures) = \( 2 \times 10^{-6} \) failures per hour

The total dangerous failure rate \( \lambda_D \) is the sum of dangerous detected and dangerous undetected failures:
\( \lambda_D = \lambda_{SD} + \lambda_{SI} \)
\( \lambda_D = 10^{-5} + 5 \times 10^{-6} = 1.5 \times 10^{-5} \) failures per hour

The Safe Failure Fraction (SFF) is calculated as:
\( \text{SFF} = \frac{\lambda_{SD}}{\lambda_{SD} + \lambda_{SI}} \)
\( \text{SFF} = \frac{10^{-5}}{10^{-5} + 5 \times 10^{-6}} = \frac{10^{-5}}{1.5 \times 10^{-5}} = \frac{1}{1.5} \approx 0.667 \)

According to IEC 61508-1:2010, Table D.4, for Type A elements (hardware elements with a well-defined failure behavior and well-understood failure modes), the following SFF ranges correspond to diagnostic coverage:
– SFF \( \ge \) 99%: High diagnostic coverage (achieves SIL 3 or 4)
– 90% \( \le \) SFF \( < \) 99%: Medium diagnostic coverage (achieves SIL 2 or 3)
– 60% \( \le \) SFF \( < \) 90%: Low diagnostic coverage (achieves SIL 1 or 2)
– SFF \( < \) 60%: Very low diagnostic coverage (achieves SIL 0 or 1)

In this scenario, the calculated SFF is approximately 0.667, or 66.7%. This falls within the range of 60% \( \le \) SFF \( < \) 90%. For Type A elements, this SFF range typically supports achieving up to SIL 2, assuming other parameters (like the required Probability of Failure on Demand or Average Failure Time) are met. The question asks about the *maximum* SIL that can be achieved based on this SFF value for a Type A element. Therefore, the maximum achievable SIL is 2. The explanation must focus on the calculation of SFF and its interpretation against the standard's tables for Type A elements to determine the maximum SIL. It is crucial to understand that SFF is a key metric for assessing the effectiveness of diagnostic measures in mitigating dangerous failures and thus determining the achievable SIL. The calculation involves identifying and summing relevant failure rates and then applying the SFF formula. The subsequent step is to map this SFF value to the corresponding SIL capability based on the element type and the standard's guidelines.

The core principle being tested here is the appropriate selection of a safety integrity level (SIL) for a safety function when the required risk reduction is achieved by multiple independent safety-related systems, each contributing to the overall risk reduction. IEC 61508-1:2010, Clause 7.4.3.2, addresses the situation where a safety function is implemented by several independent safety-related systems. It states that the SIL of the overall safety function should be the highest SIL assigned to any of the individual safety-related systems contributing to that safety function, provided that the independence requirements are met. In this scenario, the safety function requires a risk reduction factor (RRF) of 1000, which translates to a target SIL of 3 (since SIL 3 corresponds to an RRF of 1000 to 10,000). System A has a target SIL of 3, and System B has a target SIL of 2. When combining independent systems to achieve a single safety function’s risk reduction, the overall SIL is determined by the highest SIL of the contributing systems. Therefore, if System A (SIL 3) and System B (SIL 2) are independently contributing to the same safety function, and the overall requirement is SIL 3, the system with the highest SIL (SIL 3) dictates the overall SIL for that safety function. The explanation focuses on the principle of “highest SIL wins” for independent contributions to a single safety function, as per the standard’s guidance on combining safety-related systems. This approach ensures that the overall risk reduction meets or exceeds the target, as the more capable system’s integrity level is maintained.

The core principle being tested here is the appropriate selection of a safety integrity level (SIL) for a safety function based on the risk reduction required. IEC 61508-1:2010, Clause 7.4.2.1, specifies that the SIL is determined by the risk assessment, which quantifies the required risk reduction. The risk reduction factor (RRF) is calculated as the ratio of the tolerable risk to the residual risk. In this scenario, the tolerable risk is given as \(10^{-5}\) per hour, and the estimated risk without the safety function is \(10^{-3}\) per hour.

The required risk reduction factor (RRF) is calculated as:
\[ \text{RRF} = \frac{\text{Tolerable Risk}}{\text{Residual Risk}} \]
\[ \text{RRF} = \frac{10^{-5} \text{ per hour}}{10^{-3} \text{ per hour}} = 10^{-2} \]
This calculation seems incorrect as risk reduction factor should be greater than 1. Let’s re-evaluate the definition of RRF in the context of IEC 61508. The RRF is the factor by which the safety function reduces the risk. Therefore, the residual risk is the risk without the safety function divided by the RRF.

\[ \text{Residual Risk} = \frac{\text{Risk without safety function}}{\text{RRF}} \]
We want the residual risk to be less than or equal to the tolerable risk.
\[ \frac{10^{-3} \text{ per hour}}{\text{RRF}} \le 10^{-5} \text{ per hour} \]
Rearranging to solve for RRF:
\[ \text{RRF} \ge \frac{10^{-3} \text{ per hour}}{10^{-5} \text{ per hour}} \]
\[ \text{RRF} \ge 100 \]

Now, we need to map this RRF to a Safety Integrity Level (SIL). IEC 61508-1:2010, Table 2, defines the ranges of RRF for each SIL:
* SIL 1: RRF from 10 up to \(10^2\) (i.e., \(10 \le \text{RRF} < 100\))
* SIL 2: RRF from \(10^2\) up to \(10^3\) (i.e., \(100 \le \text{RRF} < 1000\))
* SIL 3: RRF from \(10^3\) up to \(10^4\) (i.e., \(1000 \le \text{RRF} < 10000\))
* SIL 4: RRF from \(10^4\) up to \(10^5\) (i.e., \(10000 \le \text{RRF} < 100000\))

Since the required RRF is \(100\), this falls into the lower bound of the SIL 2 range. Therefore, the safety function must be designed to achieve SIL 2. The explanation focuses on the calculation of the required risk reduction factor based on the tolerable and estimated risks, and then mapping this factor to the appropriate SIL as defined in the standard. This process is fundamental to the safety lifecycle and ensures that the safety function provides adequate risk reduction for the identified hazard. The selection of SIL is a critical step in determining the necessary integrity requirements for the safety-related system.

The core principle being tested here relates to the systematic fault avoidance and control measures mandated by IEC 61508 for achieving functional safety. Specifically, it addresses the transition from the Safety Integrity Level (SIL) determination phase to the subsequent design and implementation phases. When a safety requirement specification (SRS) is developed, it defines the necessary safety functions and their associated SILs. The subsequent safety-oriented design specification (SODS) is where the architectural design and specific safety mechanisms are detailed to meet these requirements. A critical aspect of this transition is ensuring that the architectural design choices directly address the identified safety requirements and SIL targets. This involves selecting appropriate hardware and software architectures, implementing diagnostic coverage, and defining fault tolerance strategies. The process of verifying that the SODS adequately reflects and enables the achievement of the SRS’s safety goals is a fundamental part of the safety lifecycle. Therefore, the most appropriate activity to ensure this alignment is the review and approval of the SODS against the SRS, confirming that the proposed architecture and mechanisms are capable of meeting the specified SIL. This review ensures that the foundational safety requirements are translated into a technically sound and verifiable design, preventing the introduction of systematic faults early in the lifecycle.

The core principle being tested here is the appropriate selection of a safety integrity level (SIL) for a safety function based on a risk assessment. IEC 61508 mandates that the SIL determination process must consider the severity of potential harm, the frequency or duration of exposure to the hazard, and the probability of the hazardous event occurring, taking into account any existing risk reduction measures. A SIL 3 assignment is justified when the risk reduction required is substantial, typically corresponding to a situation where the probability of a dangerous failure leading to a hazardous event is between \(10^{-3}\) and \(10^{-2}\) per hour, assuming the safety function is required to prevent or mitigate severe harm (e.g., fatality or irreversible injury) and the exposure to the hazard is frequent. The other options represent lower SILs, which would be assigned if the risk reduction requirements were less stringent, meaning the tolerable risk level is higher. For instance, SIL 1 is for lower risk reduction, SIL 2 for moderate, and SIL 4 for the highest risk reduction. The scenario described, involving frequent exposure to a hazard that could cause severe injury or death, necessitates a high level of risk reduction, aligning with the requirements for SIL 3. The process of determining the SIL is iterative and involves detailed hazard and risk analysis, ensuring that the chosen SIL is commensurate with the identified risks. This aligns with the overall objective of IEC 61508 to achieve the necessary risk reduction for safety-related systems.

The core of this question lies in understanding the relationship between the Safety Integrity Level (SIL) and the required diagnostic coverage (DC) for common cause failures (CCF) in a safety instrumented function (SIF) implemented with a single hardware component. IEC 61508-2:2010, Annex D, Table D.3 provides guidance on the architectural constraints and diagnostic coverage requirements for hardware. Specifically, for a Type A element (a component whose failure modes are well understood and predictable, such as a relay or a transistor), the required diagnostic coverage for CCF is directly linked to the target SIL.

For SIL 1, the required diagnostic coverage for CCF is typically \( \ge 60\% \).
For SIL 2, the required diagnostic coverage for CCF is typically \( \ge 70\% \).
For SIL 3, the required diagnostic coverage for CCF is typically \( \ge 80\% \).
For SIL 4, the required diagnostic coverage for CCF is typically \( \ge 90\% \).

The question describes a scenario where a SIF requires SIL 2 and is implemented using a single hardware component of Type A. Therefore, the minimum diagnostic coverage required for common cause failures for this component to meet the SIL 2 target is \( \ge 70\% \). This diagnostic coverage is crucial for mitigating the impact of common cause failures, which are failures that affect multiple components simultaneously due to a shared cause. The lead implementer must ensure that the diagnostic mechanisms implemented within the hardware are sufficient to achieve this level of coverage for CCF. Failure to do so would mean the SIF does not meet the specified SIL, potentially leading to an unsafe condition. The selection of appropriate diagnostic techniques, such as redundant sensing, diverse sensing, or built-in self-tests, is paramount in achieving the required diagnostic coverage.

The core of this question lies in understanding the relationship between the Safety Integrity Level (SIL) and the required diagnostic coverage (DC) for random hardware failures, specifically for a Type A element in a safety function. According to IEC 61508-2:2010, Table 7, for a Safety Integrity Level 3 (SIL 3) requirement, the target diagnostic coverage for a Type A element (which is typically a simple element with a well-understood failure mode, like a relay or a basic logic gate) to achieve a Safe Failure Fraction (SFF) of at least 90% is 90%. The SFF is calculated as \(SFF = \frac{FF + (1-F_{D}) \times(\text{other failures})}{(\text{total failures})}\), where FF is the fraction of failures that are detected and lead to a safe state, and \(F_D\) is the diagnostic coverage. For a Type A element, the diagnostic coverage directly contributes to the SFF. To achieve SIL 3, the SFF must be at least 90%. The diagnostic coverage required to meet this SFF target for a Type A element is 90%. Therefore, the correct diagnostic coverage for random hardware failures for a Type A element intended for SIL 3 is 90%. This ensures that the probability of dangerous failure per hour is within the range specified for SIL 3. The selection of the appropriate diagnostic coverage is a critical step in the hardware design process to ensure that the safety function can achieve its required integrity level. It directly impacts the reliability of the safety-related system and its ability to detect and mitigate potential hardware failures.

The core principle being tested here is the systematic approach to determining the appropriate Safety Integrity Level (SIL) for a safety function, specifically focusing on the impact of common cause failures (CCF) when considering hardware fault tolerance. IEC 61508-1:2010, Clause 7.4.4.3, outlines the requirements for achieving a target SIL. When a safety function requires a certain SIL, and hardware fault tolerance is employed to meet the architectural constraints for that SIL, the effectiveness of the fault tolerance mechanisms against CCF is paramount. The standard specifies that for higher SILs (SIL 3 and SIL 4), specific quantitative targets for the probability of common cause failure must be met. These targets are typically expressed as a fraction of the total failure rate of the safety-related system. For example, to achieve SIL 3, the probability of a common cause failure leading to the loss of the safety function, given that at least one element has failed, must be less than or equal to \(10^{-3}\). This is often referred to as the \( \beta \) factor or \( q \) factor depending on the specific context and calculation method. The architectural constraints for SIL 3, as detailed in Annex A of IEC 61508-2:2010, require that the diagnostic coverage for random hardware failures is sufficiently high, and importantly, that the probability of common cause failures does not prevent the achievement of the target SIL. This involves assessing the effectiveness of segregation, independence, and diversity in the design to mitigate CCF. Therefore, the most critical consideration when a safety function requires SIL 3 and hardware fault tolerance is implemented is the quantitative assessment of common cause failure probability to ensure it aligns with the architectural constraints for that SIL.

The core of this question lies in understanding the IEC 61508:2010 requirement for the validation of safety functions. Part 2, Clause 7.4.12, specifically addresses the validation of the safety-related system. This clause mandates that validation activities must demonstrate that the safety-related system achieves the required safety integrity level (SIL) and meets the overall safety requirements. The validation process should confirm that the system, as implemented, is capable of achieving the specified safety functions under all reasonably foreseeable operating conditions and fault conditions. This involves a comprehensive review of the design, implementation, and testing performed throughout the safety lifecycle. The objective is to provide sufficient evidence that the system will perform its intended safety functions safely and reliably, thereby reducing the risk to an acceptable level. This is distinct from verification, which confirms that the system has been built according to its specifications, and from the initial risk assessment, which identifies the hazards and determines the necessary SIL. The validation is the final confirmation that the system, as a whole, is fit for purpose from a functional safety perspective.

The core principle being tested here is the appropriate selection of safety integrity levels (SIL) for safety functions within a system, specifically considering the impact of diagnostic coverage and common cause failures. IEC 61508 mandates a systematic approach to SIL determination and allocation. When a safety function’s architectural design relies on redundancy or diversity to achieve a target SIL, the effectiveness of these measures against common cause failures (CCF) becomes paramount. A higher SIL requirement necessitates more robust measures to mitigate CCF. If the diagnostic coverage of the safety function’s elements is insufficient to meet the required risk reduction for a given SIL, or if the chosen architecture is susceptible to CCF that cannot be adequately addressed, the safety function may need to be assigned a lower SIL than initially targeted, or additional, more effective mitigation strategies must be implemented. The question focuses on the consequence of failing to adequately address CCF, which directly impacts the achievable SIL. A failure to implement sufficient measures against CCF means the system cannot reliably achieve the risk reduction associated with a higher SIL, forcing a re-evaluation and potential reduction of the assigned SIL to reflect the actual achievable safety performance. This is a fundamental aspect of the safety lifecycle, particularly during the system design and architecture phases, ensuring that the safety integrity of the implemented safety functions aligns with the identified risks.

The core of IEC 61508’s approach to achieving functional safety in complex systems lies in the systematic management of safety lifecycle activities. When considering the transition from the design phase to the implementation phase, a critical aspect is the verification of the safety requirements specification (SRS) against the architectural design. This verification ensures that the proposed architecture adequately addresses all identified safety functions and their associated integrity levels (SILs). Specifically, Part 3 of IEC 61508, which deals with the software aspects, emphasizes the importance of rigorous verification activities throughout the lifecycle. The architectural design must be demonstrably capable of realizing the safety functions defined in the SRS, including considerations for fault tolerance, diagnostic coverage, and the avoidance of systematic failures. A key verification activity at this stage is the review of the architectural design documentation against the SRS to confirm that all safety requirements have been correctly translated into design elements, and that the chosen architecture supports the required safety integrity. This ensures that the subsequent implementation will be based on a sound and verifiable foundation, minimizing the risk of introducing new systematic faults. The process involves checking for completeness, consistency, and feasibility of the architectural solutions in meeting the specified safety goals.

The core principle being tested here is the relationship between the Safety Integrity Level (SIL) and the required diagnostic coverage (DC) for random hardware failures in a safety function. IEC 61508-2:2010, Table 12, specifies the target diagnostic coverage for safety-related systems. For a Type A element (which is assumed for a general safety function unless specified otherwise) operating in a low demand mode of operation, the required diagnostic coverage for achieving a specific SIL is as follows:

* SIL 1: \(DC \ge 60\%\)
* SIL 2: \(DC \ge 80\%\)
* SIL 3: \(DC \ge 90\%\)
* SIL 4: \(DC \ge 99\%\)

The question describes a safety function that has been assigned SIL 2. Therefore, the diagnostic coverage for random hardware failures must be at least 80%. The explanation of why this is the correct approach involves understanding that SIL is a measure of the risk reduction achieved by a safety function. Higher SILs demand more robust safety mechanisms, including more comprehensive diagnostics to detect and mitigate random hardware failures. Diagnostic coverage quantifies the effectiveness of these mechanisms in detecting such failures. Achieving SIL 2 necessitates a significant level of fault detection, hence the requirement for at least 80% diagnostic coverage for the relevant hardware elements. This ensures that the probability of a dangerous failure per hour is within the target range for SIL 2.

The core principle being tested here is the appropriate selection of a safety integrity level (SIL) for a safety function based on the risk assessment and the concept of risk reduction. IEC 61508 mandates that the SIL is determined by the required risk reduction factor (RRF). The RRF is calculated as the ratio of the tolerable risk to the actual risk. In this scenario, the initial risk is assessed as \(10^{-3}\) per hour, and the tolerable risk is \(10^{-5}\) per hour. Therefore, the required risk reduction factor is \( \text{RRF} = \frac{\text{Tolerable Risk}}{\text{Actual Risk}} = \frac{10^{-5} \text{ per hour}}{10^{-3} \text{ per hour}} = 10^{-2} \). However, the RRF is typically expressed as a factor greater than 1, representing how much the risk needs to be reduced. So, the required reduction is by a factor of \( \frac{10^{-3}}{10^{-5}} = 100 \).

IEC 61508 defines SILs based on ranges of RRFs and corresponding target failure rates for safety functions.
– SIL 1: RRF of 10 to 100 (or PFDavg of \(10^{-1}\) to \(10^{-2}\))
– SIL 2: RRF of 100 to 1000 (or PFDavg of \(10^{-2}\) to \(10^{-3}\))
– SIL 3: RRF of 1000 to 10000 (or PFDavg of \(10^{-3}\) to \(10^{-4}\))
– SIL 4: RRF of 10000 to 100000 (or PFDavg of \(10^{-4}\) to \(10^{-5}\))

A required risk reduction factor of 100 falls squarely within the range for SIL 2 (100 to 1000). The safety function must be designed to achieve a level of risk reduction that meets or exceeds this requirement. Therefore, the appropriate SIL for this safety function is SIL 2. This selection ensures that the residual risk after the safety function is implemented is within the tolerable limits. The process of determining the SIL is a critical step in the safety lifecycle, directly influencing the design, implementation, and verification of safety-related systems to prevent systematic failures and control random hardware failures.

The core principle being tested here is the systematic approach to determining the appropriate Safety Integrity Level (SIL) for a safety function, specifically focusing on the impact of diagnostic coverage on the required hardware fault tolerance (HFT). IEC 61508-1:2010, Clause 7.4.4.2, outlines the relationship between SIL, architectural constraints (including HFT), and the probability of failure on demand (PFD) or probability of failure per hour (PFH). For a Type A element (low-demand mode, safety instrumented function), the target PFD is derived from the SIL. For a SIL 3 function, the target PFD is typically in the range of \(10^{-3} \le PFD < 10^{-2}\).

The question describes a scenario where the initial architectural assessment suggests a need for HFT = 1 (meaning two independent elements are required for redundancy). However, the introduction of a diagnostic coverage (DC) of 90% for random hardware failures in the safety-related elements significantly impacts the effective failure rates. IEC 61508-2:2010, Annex D, provides methods for calculating the PFD considering diagnostic coverage. A common approach for a 1oo2 (one out of two) architecture with a diagnostic coverage of 90% on each element, assuming a common cause failure factor (CCF) and a failure rate per hour (\(\lambda\)) for a single element, would lead to a reduced effective PFD.

Let's assume a base failure rate \(\lambda\) for a single element. For a 1oo2 architecture without diagnostics, the PFD is approximately \(\frac{\lambda^2 T}{2}\) for a low-demand mode with a proof test interval T, or more generally, it relates to the probability of both elements failing. With diagnostics, the probability of a dangerous failure that is not detected by diagnostics is reduced. If we consider the probability of a dangerous failure on demand for a single element to be \(p\), and the diagnostic coverage is DC, the probability of an undetected dangerous failure is \(p \times (1 – DC)\).

For a 1oo2 architecture, the system fails if both elements fail. If we consider the probability of a dangerous failure on demand for a single element, \(p_{elem}\), and the diagnostic coverage is 90% (\(DC = 0.9\)), the probability of an undetected dangerous failure for a single element becomes \(p_{elem} \times (1 – 0.9) = 0.1 \times p_{elem}\). The system fails if both elements have an undetected dangerous failure. The probability of this occurring in a 1oo2 architecture is approximately \((0.1 \times p_{elem})^2\), assuming independence and no common cause failures.

However, the question is about the *required* HFT. IEC 61508-2:2010, Table 4, specifies the HFT requirements for different SILs and architectural styles. For a SIL 3 safety function, if the architectural constraint is that the element is not suitable for the required SIL (e.g., it's a Type A element with a certain failure rate), then HFT=1 is typically required for a 1oo1 architecture to achieve SIL 3. If the diagnostic coverage of the element is high enough to meet the SIL 3 PFD target even with HFT=0 (1oo1), then HFT=0 would be sufficient. The question implies that the initial assessment led to HFT=1. The introduction of a 90% diagnostic coverage on the *elements* themselves, when considering a 1oo1 architecture, would mean that the effective probability of a dangerous failure on demand from that single element is significantly reduced. If the diagnostic coverage of 90% is sufficient to bring the PFD of a single element (with 1oo1 architecture) below the SIL 3 threshold (e.g., \(PFD < 10^{-3}\)), then HFT=0 would be permissible for that element.

The key is that the diagnostic coverage is applied to the *elements*. If a single element with 90% diagnostic coverage can meet the PFD requirements for SIL 3 (which is a very stringent requirement, \(PFD < 10^{-3}\)), then a 1oo1 architecture (HFT=0) would be sufficient. The calculation would involve determining the PFD of a single element with 90% DC and comparing it to the SIL 3 target. Without specific failure rates and proof test intervals, we rely on the general principles of IEC 61508. A 90% diagnostic coverage on a single element is substantial and, in many cases, can allow a 1oo1 architecture to meet SIL 3 requirements, effectively reducing the HFT requirement from 1 to 0. Therefore, the most appropriate action is to re-evaluate the HFT requirement based on the improved diagnostic capabilities of the elements.

The correct approach is to re-evaluate the hardware fault tolerance requirement for the safety function, considering that the increased diagnostic coverage of the individual safety-related elements may allow for a lower hardware fault tolerance (specifically, from HFT=1 to HFT=0) while still meeting the target Safety Integrity Level (SIL) 3. This re-evaluation is a critical step in the safety lifecycle, as mandated by IEC 61508, to ensure that the system design is optimized and cost-effective without compromising safety. The standard emphasizes that diagnostic coverage directly influences the probability of dangerous failures, and thus the required architectural constraints. If the diagnostics are sufficiently effective, the need for redundancy (HFT=1) might be eliminated, allowing for a simpler and potentially less expensive 1oo1 architecture. This aligns with the principle of achieving the required safety integrity through appropriate measures, whether they be architectural redundancy or effective diagnostics.

The core principle being tested here is the appropriate selection of safety integrity levels (SIL) for safety functions based on the risk reduction required. IEC 61508 mandates a systematic approach to determining SIL, which involves hazard and risk analysis. The target SIL for a safety function is derived from the acceptable risk level for a specific hazardous event. If a hazardous event has an unacceptable risk level of \(10^{-3}\) per hour and the safety function is intended to reduce this risk to an acceptable level of \(10^{-5}\) per hour, the required risk reduction factor (RRF) is calculated as the ratio of the unacceptable risk to the acceptable risk.

Calculation:
Required Risk Reduction Factor (RRF) = Unacceptable Risk / Acceptable Risk
RRF = \( \frac{10^{-3} \text{ per hour}}{10^{-5} \text{ per hour}} \)
RRF = \( 100 \)

According to IEC 61508-1:2010, Table 1, the SIL corresponding to a required risk reduction factor of 100 is SIL 2. SIL 2 requires an RRF between 10 and 100. Therefore, the safety function must be designed to achieve SIL 2. This involves specifying appropriate safety requirements for the safety-related system, including architectural constraints and diagnostic coverage, to ensure the required level of risk reduction is met. The selection of SIL is a critical step in the safety lifecycle, influencing all subsequent design, implementation, and verification activities.