Key SIL (Safety Integrity Levels) Terms

Safety Integrity Levels

The global value of SIL (Safety Integrity Levels) to the process industries has increased significantly over the years. For many companies, SIL is still an elusive term sometimes misunderstood and implemented incorrectly. To fully understand SIL and its consequences, it is essential to comprehend the necessary words, acronyms, and phrases often used and how they relate to the pursuit of functional safety.  The following are some of the most commonly used:

Dangerous failure

Failure with the potential to bring the safety instrumented system into a dangerous or non‐functional state.


Failure Modes Effects and Diagnostic Analysis


Hardware Fault Tolerance, ability of a hardware to continue to perform a required function in the presence of faults or errors.


Mean Time Between Failures


Probability of Failure on Demand, Probability of hazardous failures for a safety function on demand.

Safety Function

The ability of a system to carry out actions necessary to maintain a defined safe state for a process, equipment, or a plant.

Safety‐Related System

A safety‐related system performs the safety functions that are required to maintain a safe condition (for example, a flow meter, a burner, and a PLC).


Safe Failure Fraction, percentage of failures that do not have the potential to put the safety‐related system in a hazardous state.


Safety Integrity Level, IEC 61508 defines four Safety Integrity Levels (SIL1 through SIL4). Each level corresponds to a level of probability for the failure of a safety function.


Safety Instrumented System, implementation of one or more safety instrumented functions.

Piping Specialties / PSI Controls

Terms and definitions courtesy of Kurz Instruments