SIL Functional Safety Principles & Standards 5 — Questions and Answers
Question 1: Which international standard provides guidance specifically for the application of functional safety in the machinery sector?
- IEC 61511
- IEC 62061 (Correct answer)
- IEC 61513
- ISO 13849
Correct answer: IEC 62061
IEC 62061 is the IEC 61508-derived standard for safety-related control systems in the machinery sector, defining SILCL requirements.
Question 2: A functional safety plan (FSP) required by IEC 61511 must document which of the following?
- Only the hardware architecture selected for the SIS
- The functional safety activities, responsibilities, and resources for all lifecycle phases (Correct answer)
- The proof test procedures and schedules only
- The HAZOP worksheets and action items
Correct answer: The functional safety activities, responsibilities, and resources for all lifecycle phases
The FSP defines the overall approach to managing functional safety, covering activities, responsibilities, resources, and schedules across all lifecycle phases.
Question 3: When applying the risk graph method for SIL determination, which parameter accounts for the ability of people to avoid the hazard after it has been initiated?
- Consequence parameter (C)
- Exposure/frequency parameter (F)
- Probability of avoiding the hazard parameter (P) (Correct answer)
- Demand rate parameter (W)
Correct answer: Probability of avoiding the hazard parameter (P)
The avoidance parameter (P) in the risk graph reflects whether people can avoid the hazard after initiation, such as through warning or escape routes.
Question 4: Under IEC 61508, what does 'hardware fault tolerance (HFT)' of 1 mean?
- The system can tolerate one unsafe failure before losing its safety function
- The system requires redundancy such that one fault does not cause loss of the safety function (Correct answer)
- The system has one diagnostic test channel for detecting faults
- The system must be proof tested once per year
Correct answer: The system requires redundancy such that one fault does not cause loss of the safety function
HFT = 1 means the subsystem can tolerate one dangerous hardware fault without losing its ability to perform the safety function, typically requiring redundancy.
Question 5: What is the significance of the 'ALARP' (As Low As Reasonably Practicable) principle in functional safety risk assessment?
- It defines the exact SIL required for every safety function
- It requires that residual risk be reduced further if the cost of doing so is not grossly disproportionate to the benefit gained (Correct answer)
- It exempts safety functions from proof testing if risk is already low
- It replaces quantitative risk assessment in all cases
Correct answer: It requires that residual risk be reduced further if the cost of doing so is not grossly disproportionate to the benefit gained
ALARP requires demonstrating that residual risk has been reduced to the lowest level practicable, with further reduction only waived if costs are grossly disproportionate to safety benefit.
Question 6: Which of the following lifecycle activities is associated with the term 'pre-startup safety review (PSSR)' within the IEC 61511 framework?
- Verifying that the installed SIS meets the SRS before placing it into service (Correct answer)
- Generating the initial hazard and risk assessment
- Developing the safety requirements specification
- Performing the final five-year periodic functional safety assessment
Correct answer: Verifying that the installed SIS meets the SRS before placing it into service
A PSSR verifies that the installed SIS has been built, commissioned, and tested in accordance with the SRS before being placed into operation.
Question 7: According to IEC 61511, what is the recommended maximum SIL for a single safety instrumented function using off-the-shelf non-PE (non-programmable) safety instrumented equipment?
- SIL 1
- SIL 2
- SIL 3 (Correct answer)
- SIL 4
Correct answer: SIL 3
IEC 61511 limits a single SIF to SIL 3 maximum, as SIL 4 requires extraordinary justification and is rarely applied in the process industry.
SIL Functional Safety Principles & Standards 5
Which international standard provides guidance specifically for the application of functional safety in the machinery sector?