How “Protection System” Reliability Differs from “System” Reliability and Why It's Important

There is an important operational characteristic that is nearly unique to protection systems: Protection systems are specifically designed to respond to initiating events, whereas most other types of systems are not. We expose some analytical details needed to characterize protection system reliability. We appeal to well established results from martingale calculus for counting processes to explain why protection system reliability often defies numerical quantification. We will conclude that caution must be exercised when studying protection system reliability for regulatory and/or engineering design purposes.