SIL Hardware Architecture & Fault Tolerance 2

Question 1

Per IEC 61508-2, what minimum Hardware Fault Tolerance is required for a Type B subsystem with SFF between 60% and 90% targeting SIL 3?

Accepted Answer

HFT = 2

Answer

HFT = 0

Answer

HFT = 1

Answer

HFT = 3

Question 2

What is a 'common cause failure' (CCF) in a redundant safety system?

Accepted Answer

A failure event that causes multiple redundant channels to fail simultaneously from the same root cause

Answer

A failure that occurs at the exact moment a process demand is placed on the system

Answer

A frequently recurring failure mode that appears across all SIL levels

Answer

A repeated single-channel failure that eventually defeats the safety function

Question 3

Which statement best describes a 'dangerous undetected' (DU) failure?

Accepted Answer

A failure that disables the safety function without being identified by online diagnostics

Answer

A failure that immediately triggers a process demand on the safety system

Answer

A failure that is detected but cannot be repaired within the allowed MTTR

Answer

A failure discovered during a proof test that has already been self-restored

Question 4

How does increasing diagnostic coverage (DC) affect the average Probability of Failure on Demand (PFD)?

Accepted Answer

Higher DC reduces the rate of dangerous undetected failures, thereby lowering the system PFD

Answer

Higher DC extends the allowable proof test interval without changing PFD

Answer

Higher DC eliminates the need for hardware redundancy in SIL 2 applications

Answer

Higher DC raises the achievable SIL target without any hardware changes

Question 5

In reliability engineering for safety systems, what does 'derating' mean?

Accepted Answer

Operating components below their rated electrical, thermal, or mechanical limits to improve reliability

Answer

Applying a correction factor when ambient conditions exceed component rated specifications

Answer

Reducing the SIL target after completing a risk assessment

Answer

Lowering the proof test frequency to minimize operational disruption

Question 6

What is the Hardware Fault Tolerance of a 2oo2 (2-out-of-2) voting architecture?

Accepted Answer

HFT = 0

Answer

HFT = 1

Answer

HFT = 2

Answer

HFT = 3

Question 7

For a SIL 2 safety function that must minimize spurious trips in a continuous process, which voting architecture is most appropriate?

Accepted Answer

2oo3

Answer

1oo1

Answer

2oo2

Answer

1oo1D