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Safety Integrity Level(SIL)

Standards & Compliance
instrumentationelectricalprocess

Definition

A discrete level (SIL 1 through SIL 4) for specifying the safety integrity requirements of safety instrumented functions (SIFs). Each SIL level corresponds to a target probability of failure on demand (PFD): SIL 1 = 0.1-0.01, SIL 2 = 0.01-0.001, SIL 3 = 0.001-0.0001, SIL 4 = 0.0001-0.00001. Higher SIL levels require more rigorous design, testing, and maintenance.

Context

Applied to safety instrumented systems (SIS) in process industries to ensure that safety functions meet required risk reduction targets. SIL determination is a key output of process hazard analysis (LOPA) and directly influences the design, testing, and maintenance requirements for safety-critical instrumentation.

Examples

  1. A high-integrity pressure protection system (HIPPS) is rated SIL 3, requiring redundant sensors, redundant logic solvers, and proof testing at intervals calculated to maintain the target PFD.
  2. A SIL 2 emergency shutdown function requires proof testing every 12 months to maintain the probability of failure on demand within the 0.01-0.001 range.

Applicable Standards

  • IEC 61508
  • IEC 61511
  • ISA 84

Related Terms

Criticality Classification

The assignment of a criticality ranking (typically A, B, or C) to each equipment item based on the consequences of its failure across safety, environmental, production, and maintenance cost dimensions. Criticality classification determines the level of engineering attention, inspection rigor, maintenance strategy, and spare parts investment appropriate for each asset.

Hazard Rate

The instantaneous rate of failure at time t, given that the item has survived to time t. Hazard rate (also called the instantaneous failure rate or hazard function) describes how the risk of failure changes over the item's life. Unlike failure rate (which is an average), hazard rate is a time-dependent function that captures the changing nature of failure risk as equipment ages.

Failure Rate(Lambda)

The average number of failures per unit of time for a population of items, typically expressed as failures per hour, per year, or per million hours. Failure rate (lambda) is the reciprocal of MTBF for constant-failure-rate systems. It is a statistical average measure, distinct from the instantaneous hazard rate, and is most accurate when the failure rate is approximately constant (beta near 1).

Where this appears in Reliatic

This concept is used in the Reports module.