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

HFT = N means the subsystem can tolerate N faults and still perform its required safety function.

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

Type A subsystems have well-defined, fully enumerable failure modes for all components, distinguishing them from the more complex Type B.

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

SFF = (λS + λDD) / (λS + λDD + λDU), combining safe and detected dangerous failure rates as a fraction of all failures.

Question 4

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

Accepted Answer

HFT = 1

Answer

In 1oo2, one channel can fail while the other still initiates the safety action, so the system tolerates one fault — HFT = 1.

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

2oo3 requires agreement from two channels for the safety action, reducing spurious trips versus 1oo2, while a single failure still leaves two channels able to act (HFT = 1).

(SIL) Safety Integrity Level Certified Practice Test

(SIL) Safety Integrity Level Certified Practice Test

SIL Hardware Architecture & Fault Tolerance