SIL Hardware Architecture & Fault Tolerance

Question 1

What does Hardware Fault Tolerance (HFT) represent in a safety-related subsystem?

Accepted Answer

The number of faults a subsystem can withstand while maintaining its safety function

Answer

The fraction of failures that result in a safe state

Answer

The required interval between proof tests

Answer

The maximum acceptable probability of failure on demand

Question 2

According to IEC 61508, how is a 'Type A' subsystem characterized?

Accepted Answer

A subsystem in which failure modes of all components are well defined and can be fully enumerated

Answer

A complex subsystem containing components whose failure modes are not fully defined

Answer

A software-only safety controller with no hardware element

Answer

A subsystem requiring HFT ≥ 1 regardless of Safe Failure Fraction

Question 3

Which statement correctly describes Safe Failure Fraction (SFF) per IEC 61508?

Accepted Answer

The fraction of total failures that are either safe failures or dangerous but detected failures

Answer

The probability a system fails to reach a safe state when a demand occurs

Answer

The percentage of operating time without any recorded failure

Answer

The ratio of detected dangerous failures to all dangerous failures

Question 4

What is the Hardware Fault Tolerance of a 1oo2 (1-out-of-2) voting architecture?

Accepted Answer

HFT = 1

Answer

HFT = 0

Answer

HFT = 2

Answer

HFT = 3

Question 5

Why is a 2oo3 voting architecture commonly preferred over 1oo2 in process safety applications?

Accepted Answer

It reduces spurious trips compared to 1oo2 while still maintaining HFT = 1

Answer

It requires fewer installed components than a 1oo2 system

Answer

It completely eliminates common cause failure contributions to PFD

Answer

It removes the need for periodic proof testing once commissioned

Question 6

What is the primary purpose of architectural constraints defined in IEC 61508-2?

Accepted Answer

To set minimum hardware redundancy requirements based on target SIL and subsystem type

Answer

To specify proof test intervals based on device failure rates

Answer

To define minimum software complexity for each SIL level

Answer

To establish maximum response times for safety instrumented functions

Question 7

In safety architecture notation, what does the 'D' suffix signify in a 1oo1D configuration?

Accepted Answer

Diagnostics are incorporated to detect and respond to dangerous failures

Answer

The system uses dual redundant power supplies

Answer

Direct wiring to the final element without intermediary relays

Answer

Deterministic response timing is guaranteed by the controller