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

Which portion of the PFD calculation is usually the least significant (i.e., has the least impact)?

A. The dangerous automatically detected portion
B. The dangerous manually detected portion
C. The dangerous never detected portion
D. The common cause portion

CAP Answer

The answer is A: "The dangerous automatically detected portion".

PFD is the probability that a system will fail dangerously and not be able to perform its safety function when required. The dangerous automatically detected failures are identified through automatic testing and diagnostic coverage, reducing the probability of failure on demand due to those types of failures.

The other terms in the PFD are usually quite significant, due to longer test intervals, imperfect manual testing, and smaller diagnostic coverage.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, 2nd Ed., 2006