Functional Safety Principles & Standards Flashcards
7 cards from real SIL practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Functional Safety Principles & Standards flashcards as text
A SIS operates in low-demand mode with a proof test interval of 1 year. If one proof test reveals a failure that has been present since the last test, what does this indicate about the PFD calculation?
Answer: The average PFD (PFDavg) accounts for this by averaging the probability of being in a failed state over the full test interval
PFDavg integrates the probability of an undetected dangerous failure existing over the entire proof test interval, accounting for failures that could persist until found by the periodic proof test.
Under IEC 61511, when must a Management of Change (MOC) procedure be applied to a SIS?
Answer: Whenever any modification is made to the SIS hardware, software, or operating procedures
MOC applies to any change—hardware, software, configuration, or procedures—that could affect the functional safety of the SIS.
Which IEC 61508 software safety integrity level (SSIL) technique is categorized as 'Highly Recommended' for SSIL 3 and 4 to reduce systematic faults?
Answer: Formal methods
Formal methods (e.g., model checking, theorem proving) are Highly Recommended at SSIL 3/4 to rigorously prove the absence of systematic design errors.
What is the meaning of 'demand mode of operation' in the context of a safety instrumented function?
Answer: The SIS is normally passive and activates only when a demand (hazardous condition) is detected
In demand mode the SIF remains dormant until a process demand occurs, at which point it must perform its safety function to prevent a hazardous event.
Which concept in IEC 61508 addresses the ability of a system to avoid bringing itself or the process into a hazardous state due to component failures?
Answer: Fail-safe design
Fail-safe design ensures that upon component failure the system moves to a pre-defined safe state rather than a hazardous one.
In a layer of protection analysis (LOPA), what value is typically assigned as the initiating event frequency for a 'basic process control system (BPCS) failure causing high pressure'?
Answer: 10⁻² per year
A generic BPCS control loop failure initiating event frequency is typically taken as 10⁻¹ per year, but specific failures like high-pressure initiating causes are often assigned 10⁻² per year per industry guidance.
According to IEC 61511, which activity must be completed before the detailed SIS design phase begins?
Answer: Completion of the Safety Requirements Specification (SRS)
The SRS must be completed and approved before detailed SIS design, as it defines what the SIS must do and the integrity required.