SACA Safety Systems and Functional Safety 4 — Questions and Answers
Question 1: What does the term 'safe state' mean in the context of functional safety?
- A state where the system is powered off completely
- A condition in which the system does not pose an unacceptable risk (Correct answer)
- A state where all diagnostics have passed
- A mode with maximum operational speed enabled
Correct answer: A condition in which the system does not pose an unacceptable risk
A safe state is a defined operating condition in which a system no longer poses an unacceptable risk, even if it cannot continue normal operation.
Question 2: In IEC 61511, what is the primary purpose of a Safety Instrumented Function (SIF)?
- To optimize process efficiency
- To bring a process to a safe state upon demand (Correct answer)
- To log operational data for audits
- To control normal process variables
Correct answer: To bring a process to a safe state upon demand
A Safety Instrumented Function is designed to bring a process to a safe state when a predetermined condition is detected, preventing or mitigating a hazardous event.
Question 3: Which failure mode is addressed by 'deenergize-to-trip' design philosophy in safety systems?
- Common cause failures
- Dangerous undetected failures
- Power supply failures causing loss of safe actuation (Correct answer)
- Systematic failures in software
Correct answer: Power supply failures causing loss of safe actuation
Deenergize-to-trip ensures that loss of power causes the safety actuator to move to the safe position, preventing power failure from leaving the system in a dangerous state.
Question 4: What is the purpose of proof testing in a Safety Instrumented System?
- To verify the system meets production throughput targets
- To detect dangerous undetected faults and restore the PFD to its initial value (Correct answer)
- To calibrate process transmitters for accuracy
- To test cybersecurity controls on the SIS network
Correct answer: To detect dangerous undetected faults and restore the PFD to its initial value
Proof testing exercises the full safety function to reveal dangerous undetected faults and reset the probability of failure on demand (PFD) accumulation.
Question 5: In a redundant 2oo3 voting architecture, how many channels must agree on a fault for a trip to occur?
- 1 out of 3
- 2 out of 3 (Correct answer)
- 3 out of 3
- Any single channel independently
Correct answer: 2 out of 3
In a 2oo3 (two-out-of-three) voting system, at least two channels must detect the hazardous condition before the system initiates a protective action.
Question 6: Which IEC standard specifically addresses functional safety of electrical/electronic/programmable electronic safety-related systems in general industry?
- IEC 61511
- IEC 62061
- IEC 61508 (Correct answer)
- ISO 13849
Correct answer: IEC 61508
IEC 61508 is the foundational standard for functional safety of E/E/PE safety-related systems across general industries, from which sector-specific standards like IEC 61511 are derived.
Question 7: What does 'spurious trip rate' measure in a Safety Instrumented System?
- The frequency of dangerous process deviations detected
- The rate at which the system initiates unintended safe shutdowns (Correct answer)
- The number of proof tests performed per year
- The probability that a hazardous event reaches the environment
Correct answer: The rate at which the system initiates unintended safe shutdowns
Spurious trip rate (STR) quantifies how often a SIS activates unnecessarily, causing production losses without an actual hazardous demand.
SACA Safety Systems and Functional Safety 4
What does the term 'safe state' mean in the context of functional safety?