SACA Safety Systems and Functional Safety 5 — Questions and Answers
Question 1: What is a 'common cause failure' (CCF) in the context of redundant safety systems?
- A failure that occurs only in a single channel
- A single event that causes multiple redundant channels to fail simultaneously (Correct answer)
- A failure mode exclusive to software components
- A fault detected by the diagnostic self-test
Correct answer: A single event that causes multiple redundant channels to fail simultaneously
Common cause failure is a single root cause (e.g., a shared environment, design flaw, or maintenance error) that defeats multiple redundant channels at the same time.
Question 2: In the context of the Safety Lifecycle, what activity immediately follows the Hazard and Risk Assessment?
- Installation and commissioning of the SIS
- Definition of overall safety requirements (Correct answer)
- Selection of safety integrity level
- Design and engineering of the safety function
Correct answer: Definition of overall safety requirements
After identifying hazards and assessing risks, the next step in the IEC 61508/61511 safety lifecycle is to define the overall safety requirements that the system must meet.
Question 3: Which metric quantifies the average probability that a safety function will fail to respond when a demand occurs?
- SIL (Safety Integrity Level)
- PFD (Probability of Failure on Demand) (Correct answer)
- MTTFd (Mean Time to Dangerous Failure)
- DDC (Dangerous Detected Coverage)
Correct answer: PFD (Probability of Failure on Demand)
PFDavg (average Probability of Failure on Demand) is the key metric used to quantify SIS reliability for low-demand mode safety functions.
Question 4: Under IEC 62061, which term describes the portion of dangerous failures detected by the system's own diagnostics?
- Safe Failure Fraction (SFF)
- Diagnostic Coverage (DC) (Correct answer)
- Beta factor
- Hardware Fault Tolerance (HFT)
Correct answer: Diagnostic Coverage (DC)
Diagnostic Coverage (DC) is the fraction of dangerous failures that are detected by automatic diagnostic tests within the safety-related system.
Question 5: What is the role of a 'final element' in a Safety Instrumented Function?
- To measure the process variable and transmit it to the logic solver
- To perform the physical action that brings the process to a safe state (Correct answer)
- To execute the safety logic and make trip decisions
- To provide operator interface and alarm management
Correct answer: To perform the physical action that brings the process to a safe state
The final element (e.g., a shutdown valve or contactor) is the actuator component that physically implements the safety action commanded by the logic solver.
Question 6: What does 'Hardware Fault Tolerance' (HFT) of 1 mean for a safety subsystem?
- The subsystem can tolerate zero faults before losing the safety function
- The subsystem can still perform its safety function with one fault present (Correct answer)
- The subsystem requires one channel to be active at all times
- The subsystem has a 1% probability of hardware failure per year
Correct answer: The subsystem can still perform its safety function with one fault present
HFT of 1 means the subsystem can experience one hardware fault and still correctly perform its intended safety function, requiring at least two channels in a redundant configuration.
Question 7: In functional safety management, what is the primary purpose of a 'functional safety assessment' (FSA)?
- To calculate the SIL target for each safety function
- To provide an independent judgement that safety requirements have been correctly implemented (Correct answer)
- To update the process hazard analysis after a near-miss event
- To train operators on emergency shutdown procedures
Correct answer: To provide an independent judgement that safety requirements have been correctly implemented
A functional safety assessment is an independent, systematic review performed to verify that the safety lifecycle activities and their outputs satisfy the applicable functional safety requirements.
SACA Safety Systems and Functional Safety 5
What is a 'common cause failure' (CCF) in the context of redundant safety systems?