CAS CAS Automation Specialist Safety & Standards 2 — Questions and Answers
Question 1: What is the definition of Probability of Failure on Demand (PFD) in a safety system?
- The frequency at which a safety function activates unnecessarily
- The average probability that a safety function fails to respond when a demand occurs (Correct answer)
- The percentage of time a system is offline for maintenance
- The rate of spurious trips per year
Correct answer: The average probability that a safety function fails to respond when a demand occurs
PFD is the average probability that a safety instrumented function will fail to perform its intended function when called upon.
Question 2: Which safety architecture uses redundancy where all channels must agree before a protective action is taken?
- 1oo1 (1-out-of-1)
- 1oo2 (1-out-of-2)
- 2oo2 (2-out-of-2) (Correct answer)
- 2oo3 (2-out-of-3)
Correct answer: 2oo2 (2-out-of-2)
2oo2 (2-out-of-2) requires both channels to vote for a shutdown, reducing spurious trips but lowering fault tolerance.
Question 3: In NFPA 70E, arc flash boundaries are established to protect workers from which specific hazard?
- Chemical exposure
- Thermal energy from an electrical arc (Correct answer)
- High-pressure hydraulic release
- Radio-frequency interference
Correct answer: Thermal energy from an electrical arc
NFPA 70E arc flash boundaries define safe working distances based on incident energy (cal/cm²) from an electrical arc event.
Question 4: What does the term 'proof test interval' mean in the context of safety instrumented systems?
- The time between scheduled software updates
- The periodic interval at which a safety function is tested to detect dangerous undetected failures (Correct answer)
- The maximum allowed response time of a safety sensor
- The interval between operator certification renewals
Correct answer: The periodic interval at which a safety function is tested to detect dangerous undetected failures
The proof test interval is the time period between full functional tests of a SIS to reveal and correct hidden dangerous failures.
Question 5: Which standard specifically addresses cybersecurity for industrial automation and control systems (IACS)?
- IEC 61508
- ISA/IEC 62443 (Correct answer)
- NFPA 70E
- IEC 61131-3
Correct answer: ISA/IEC 62443
ISA/IEC 62443 provides a comprehensive framework for cybersecurity in industrial automation and control systems.
Question 6: In a risk matrix used during PHA, which two dimensions are typically evaluated?
- Cost and schedule
- Probability of occurrence and severity of consequence (Correct answer)
- Operator skill level and equipment age
- Detection time and repair time
Correct answer: Probability of occurrence and severity of consequence
Risk matrices plot the likelihood (probability) of a hazardous event against the severity (consequence) of its outcome.
CAS CAS Automation Specialist Safety & Standards 2
What is the definition of Probability of Failure on Demand (PFD) in a safety system?