ISA CAP Risk Management & Mitigation 2 — Questions and Answers
Question 1: In a Safety Instrumented System (SIS), what does the term 'spurious trip rate' measure?
- The frequency of undetected failures
- The rate of false shutdowns not caused by actual process hazards (Correct answer)
- The probability that a SIS fails to respond on demand
- The number of scheduled proof tests per year
Correct answer: The rate of false shutdowns not caused by actual process hazards
Spurious trip rate measures how often a SIS trips (shuts down) when no actual hazard exists, causing unnecessary downtime.
Question 2: Which IEC 61511 concept refers to the fraction of time a safety function is unavailable due to failure?
- Safety Integrity Level (SIL)
- Probability of Failure on Demand Average (PFDavg) (Correct answer)
- Risk Reduction Factor (RRF)
- Proof Test Interval (PTI)
Correct answer: Probability of Failure on Demand Average (PFDavg)
PFDavg quantifies the average probability that a safety function will fail to operate when a demand occurs over a defined interval.
Question 3: A HAZOP study uses guide words to identify deviations. Which guide word specifically addresses the situation where a process variable is absent?
- More
- Less
- No/None (Correct answer)
- Reverse
Correct answer: No/None
The guide word 'No/None' identifies the complete absence of the intended condition, such as no flow or no temperature.
Question 4: In risk matrix methodology, what happens to the residual risk after applying safeguards?
- It is eliminated completely
- It remains unchanged because risk is inherent
- It is reduced but must still be compared to the tolerable risk criterion (Correct answer)
- It is transferred to another party automatically
Correct answer: It is reduced but must still be compared to the tolerable risk criterion
After applying safeguards, residual risk is reduced but must be evaluated against the organization's tolerable risk criterion to determine if further action is needed.
Question 5: What is the primary purpose of a 'proof test' in a safety instrumented system?
- To verify the process is operating within design limits
- To detect latent dangerous failures that do not reveal themselves during normal operation (Correct answer)
- To calibrate the final control element
- To confirm the SIS logic solver firmware version
Correct answer: To detect latent dangerous failures that do not reveal themselves during normal operation
Proof tests are performed to reveal dangerous undetected (latent) failures that cannot be identified through self-diagnostics during normal operation.
Question 6: In the context of ISA-84 / IEC 61511, what does 'Mode of Operation: Demand Mode' mean for a SIS?
- The SIS continuously controls the process
- The SIS only activates when a process demand (hazardous condition) occurs (Correct answer)
- The SIS is in manual override mode
- The SIS performs diagnostic checks on demand from the operator
Correct answer: The SIS only activates when a process demand (hazardous condition) occurs
In demand mode, the SIS is dormant and only activates to bring the process to a safe state when a hazardous condition (demand) is detected.
Question 7: Which risk reduction technique involves redesigning a process to eliminate the hazard rather than controlling it?
- Administrative controls
- Passive safeguards
- Inherently safer design (Correct answer)
- Procedural mitigation
Correct answer: Inherently safer design
Inherently safer design eliminates or significantly reduces hazards at the source by changing chemistry, process conditions, or materials rather than adding controls.
ISA CAP Risk Management & Mitigation 2
In a Safety Instrumented System (SIS), what does the term 'spurious trip rate' measure?