This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.
Common terms used to quantify dangerous failures include which of the following?
a) probability of failure on demand (PFD) and nuisance trip rates
b) probability of failure on demand (PFD), risk reduction factor (RRF), and safety availability (SA)
c) mean time between failure, spurious (MTBFsp); nuisance trip rates; and safety availability (SA)
d) mean time between failure, spurious (MTBFsp) and risk reduction factor (RRF)
e) none of the above
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PFD, RRF, and SA are extremely important in quantifying dangerous failures.
- Probability of failure on demand: A value that indicates the probability of a system failing to respond to demand when a failure occurs.
- Risk reduction factor: RRF = 1/PFD
- Safety availability: SA = 1 – PFD
The correct answer is B, probability of failure on demand (PFD), risk reduction factor (RRF), and safety availability (SA). Spurious trips and nuisance trips are indicative of “safe” failure modes, not “dangerous” failures. This makes answers A, C, and D incorrect.
Reference: ANSI/ISA-84.00.01-2004 standard