SIL Hardware Architecture & Fault Tolerance 3

Question 1

Under IEC 61511, under what condition may a 1oo1 (simplex) architecture be acceptable for a SIL 2 safety instrumented function?

Accepted Answer

When sufficient diagnostic coverage detects failures and initiates a safe state before a demand occurs

Answer

IEC 61511 permits a 1oo1D approach where high diagnostic coverage detects failures quickly enough that the system is restored before a demand, effectively meeting the SIL 2 PFD target.

Question 2

How does Proof Test Coverage (PTC) influence the calculated PFD of a safety function?

Accepted Answer

Higher PTC reveals more DU failures during each test cycle, reducing the average PFD over time

Answer

PTC determines what fraction of DU failures are found and restored during each proof test; higher PTC reduces residual DU failures and lowers the average PFD across the proof test interval.

Question 3

What is 'systematic capability' (SC) as defined in IEC 61508 for hardware elements?

Accepted Answer

A measure of a component's ability to avoid and control systematic failures relative to a target SIL

Answer

Systematic capability (SC) is a rating assigned to a component indicating its design and manufacturing quality is sufficient to resist systematic failures up to that SIL; the SC must meet or exceed the target SIL.

Question 4

In a redundant safety system, what is the primary role of 'independence' between channels?

Accepted Answer

To prevent common cause failures by ensuring that different failure mechanisms affect each channel

Answer

Channel independence ensures that a single environmental, design, or installation failure cannot simultaneously defeat all redundant channels, which is the fundamental defense against CCF.

Question 5

A 1oo2 system uses two identical pressure transmitters from the same manufacturer installed on the same process tap. What is the primary safety concern?

Accepted Answer

Common cause failure due to shared design, manufacturing, and environmental vulnerabilities

Answer

Identical sensors from the same manufacturer sharing the same process tap are susceptible to the same design defects, material failures, and process blockages — a significant CCF risk that undermines the benefit of redundancy.

(SIL) Safety Integrity Level Certified Practice Test

(SIL) Safety Integrity Level Certified Practice Test

SIL Hardware Architecture & Fault Tolerance 3