Hardware Architecture & Fault Tolerance Flashcards
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Read the first 7 Hardware Architecture & Fault Tolerance flashcards as text
What does Hardware Fault Tolerance (HFT) represent in a safety-related subsystem?
Answer: The number of faults a subsystem can withstand while maintaining its safety function
HFT = N means the subsystem can tolerate N faults and still perform its required safety function.
According to IEC 61508, how is a 'Type A' subsystem characterized?
Answer: A subsystem in which failure modes of all components are well defined and can be fully enumerated
Type A subsystems have well-defined, fully enumerable failure modes for all components, distinguishing them from the more complex Type B.
Which statement correctly describes Safe Failure Fraction (SFF) per IEC 61508?
Answer: The fraction of total failures that are either safe failures or dangerous but detected failures
SFF = (λS + λDD) / (λS + λDD + λDU), combining safe and detected dangerous failure rates as a fraction of all failures.
What is the Hardware Fault Tolerance of a 1oo2 (1-out-of-2) voting architecture?
Answer: HFT = 1
In 1oo2, one channel can fail while the other still initiates the safety action, so the system tolerates one fault — HFT = 1.
Why is a 2oo3 voting architecture commonly preferred over 1oo2 in process safety applications?
Answer: It reduces spurious trips compared to 1oo2 while still maintaining HFT = 1
2oo3 requires agreement from two channels for the safety action, reducing spurious trips versus 1oo2, while a single failure still leaves two channels able to act (HFT = 1).
What is the primary purpose of architectural constraints defined in IEC 61508-2?
Answer: To set minimum hardware redundancy requirements based on target SIL and subsystem type
Architectural constraints in IEC 61508-2 Tables 2 and 3 mandate minimum HFT values based on the target SIL and whether the subsystem is Type A or Type B.
In safety architecture notation, what does the 'D' suffix signify in a 1oo1D configuration?
Answer: Diagnostics are incorporated to detect and respond to dangerous failures
The 'D' in 1oo1D denotes that diagnostic coverage is used to detect dangerous failures and initiate a safe response, compensating for the lack of hardware redundancy.