ISA Certified Automation Professional (CAP) program

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Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

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)

CAP Answer

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.

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

Reference: ANSI/ISA-84.00.01-2004 standard