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

IEC 61508-2 Table 3 specifies that a Type B subsystem with SFF in the 60–90% range requires HFT = 2 to satisfy SIL 3 architectural constraints.

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

CCF defeats redundancy by propagating a single failure mechanism — such as a design flaw, shared environment, or installation error — across multiple channels at once.

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

DU failures are the most critical category: they leave the safety system unable to respond to a demand, yet remain hidden until a proof test or an actual demand reveals them.

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 converts more dangerous failures into detected failures, reducing λDU and thus lowering the PFD calculated over the proof test interval.

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

Derating involves using a component at, for example, 50% of its rated power or voltage, which reduces internal stress and significantly improves long-term reliability.

(SIL) Safety Integrity Level Certified Practice Test

(SIL) Safety Integrity Level Certified Practice Test

SIL Hardware Architecture & Fault Tolerance 2