Top 25 MCQs on Safety Integrity Level (SIL) for Instrumentation and Control Engineers

Functional Safety Management (FSM) guarantees that every lifecycle stage of the SIF—from design to decommissioning is properly controlled so the function keeps meeting SIL and safety performance goals.

Risk Reduction Factor is the inverse of PFDavg, i.e., RRF = 1 / PFDavg. It quantifies how much the risk is reduced by the safety function. For example, a PFDavg of 10⁻³ yields an RRF of 1,000.

Components not SIL-certified raise questions about their failure modes and dependability, which could affect the system’s general SIL capacity and make validation more difficult.

Redundant architectures such as 2oo3 improve system dependability by letting the system operate even if one component fails, hence supporting greater SIL levels and availability.

IEC 61511, the process industry-specific adaption of the general functional safety standard IEC 61508, oversees the use of SIL in chemical, oil & gas, pharmaceutical, and related industries.

The reason for this is that dangerous undiscovered failures are the most worrisome since they hinder the system’s ability to react to a hazard and are not detected until a genuine need arises. These greatly affect the PFDavg computation.

A demand is a risky scenario. The purpose of the Safety Instrumented Function (SIF) is to detect and minimize. It’s a practical trigger that prompts systemic action.

The longer the proof test period, the more time hazardous undetected failures could lie unchecked. This raises the PFDavg, which could reduce the SIL compliance.

A HAZOP (Hazard and Operability) research finds possible process parameter deviations that could cause hazards. It provides the foundation for SIL allocation and risk analysis.

A 2oo3 (two out of three) setup requires two units to consent to act, hence balancing safety and availability. In many SIL applications, it is perfect since it allows for one failure without a false trip.

Diagnostic Coverage (DC) is the proportion of hazardous failures internal diagnostics automatically recognized. Higher DC affects the SIL-capable classification of a device by means of Safe Failure Fraction (SFF).

For SIL 3, the average probability of failure on demand (PFDavg) has to be between 10⁻³ and 10⁻⁴, indicating it permits 1 failure in 1,000 to 10,000 requests.

The SIF is engaged rarely in low-demand mode, which is common in many process industries (usually <1 time/year). PFDavg is computed differently in this mode.

Proof testing assists in identifying potentially harmful faults that go unnoticed in a SIF. By confirming the dependability of system components, regular proof testing guarantees ongoing SIL requirement compliance.

Systematic capability is a measure of how well a device or system is protected from systematic faults, such as design or procedural errors, and is critical in SIL certification per IEC 61508/61511.

The main measure for assessing the SIL of a SIF is PFDavg. It is the mean likelihood that the system will not function on demand.

A 1oo2 system allows any of the two channels to start a safety action or shutdown. Though it somewhat sacrifices safety in relation to a 2oo3 configuration, this improves availability.

SIL emphasizes Safety Instrumented Functions (SIFs). Although process control logic is crucial for process operation, it is not regarded as part of the safety function unless it directly affects a SIF.

The ratio of overall failures to safe and detected hazardous failures Used to grade hardware for SIL capabilities, Safe Failure Fraction (SFF) gauges the percentage of failures that are either safe or detected.

LOPA is a semi-quantitative tool for examining the efficacy of separate protection layers. It assists in determining the necessity of further risk reduction, such as a higher SIL-rated SIF.

SIL verification guarantees that the developed safety instrumented system satisfies the necessary SIL level; this is done during the realization phase of the safety lifecycle.

Designed to return a process to a safe state under pre-defined criteria, a Safety Instrumented Function (SIF) is a function. SIL compliance is evaluated on the fundamental unit.

The PFD ranges per SIL are:
SIL 1: 10⁻¹ to 10⁻²

SIL 2: 10⁻² to 10⁻³

SIL 3: 10⁻³ to 10⁻⁴

SIL 4: 10⁻⁴ to 10⁻⁵

The Safety Integrity Level (SIL) is a gauge of the degree of risk decrease a safety instrumented function (SIF) offers. A higher SIL means less likelihood of failure and therefore more risk reduction.

Functional safety across sectors is covered by the umbrella standard IEC 61508. For the process sector, however, IEC 61511, based on 61508, is ideally suited. Still, the basic standard is IEC 61508.