PLC Safety & Functional Safety Flashcards
6 cards from real PLC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 PLC Safety & Functional Safety flashcards as text
Which document defines the safety requirements for a specific safety instrumented system during its lifecycle?
Answer: Safety Requirements Specification (SRS)
The Safety Requirements Specification (SRS) formally defines the functional and integrity requirements for each safety instrumented function.
What is the purpose of a HAZOP (Hazard and Operability Study) in relation to safety PLCs?
Answer: To systematically identify process hazards that safety instrumented functions may need to mitigate
A HAZOP identifies potential process deviations and hazards, which then define the safety functions that the safety PLC must implement.
In safety PLC programming, why is the use of global variables and indirect addressing generally restricted?
Answer: They can create unpredictable program behavior that is difficult to verify and validate for functional safety compliance
Global variables and indirect addressing introduce complexity and potential unintended interactions that make safety software harder to verify, violating functional safety best practices.
What is 'common cause failure' (CCF) in the context of redundant safety PLC systems?
Answer: A single failure event that defeats multiple redundant channels simultaneously
Common cause failure (CCF) occurs when one root cause (e.g., a power surge or software bug) causes multiple redundant channels to fail at the same time.
Which practice helps prevent common cause failures in redundant safety PLC installations?
Answer: Physical separation and using diverse hardware or software from different manufacturers for redundant channels
Diversity (different hardware/software) and physical separation reduce the chance that a single event, such as an EMI burst or firmware bug, can disable all redundant channels.
During the safety lifecycle, what activity immediately follows the identification of required Safety Instrumented Functions?
Answer: Determining the required SIL for each Safety Instrumented Function
After identifying the SIFs, the next step is to determine the required Safety Integrity Level (SIL) for each function through risk assessment methods.