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

Only when the process demand rate is documented as less than once per ten years

Answer

Never — IEC 61511 mandates redundant hardware for all SIL 2 functions

Answer

Only when the device SFF exceeds 99% as certified by a third-party body

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

Higher PTC increases the allowable interval between scheduled proof tests

Answer

Higher PTC eliminates the CCF beta factor from the PFD calculation

Answer

Higher PTC reduces the required diagnostic coverage percentage for the same SIL

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

The maximum SIL achievable through a specific hardware voting architecture

Answer

The fraction of systematic failures that can be detected by online diagnostics

Answer

The number of systematic failures a subsystem can tolerate before losing safety function

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

To allow each channel to be proof tested separately without shutting down the process

Answer

To reduce the total number of hardware components required in the safety system

Answer

To enable equipment from different vendors to interoperate within the same voting architecture

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

Increased probability of spurious trips caused by synchronous random failures

Answer

Excessive installation and ongoing maintenance cost compared to a 1oo1 system

Answer

Inability to achieve diagnostic coverage above 60% for the transmitter subsystem

Question 6

In SIS reliability calculations, what does Mean Time to Restore (MTTR) specifically represent?

Accepted Answer

The average time to detect, diagnose, and return a failed component to its operational state

Answer

The average time between consecutive failures of a complete safety system

Answer

The maximum permitted time between scheduled proof test intervals

Answer

The time required to complete a full safety integrity level verification assessment

Question 7

According to IEC 61508-2 architectural constraints, what is the maximum SIL claimable by a Type A subsystem with SFF < 60% and HFT = 0?

Accepted Answer

SIL 1

Answer

SIL 2

Answer

SIL 3

Answer

SIL 4