SIL, Safety Integrity Level – Basic Definitions

Safety Integrity Level (SIL): is a measure of the safety system performance, related to the Probability of Failure on Demand (PFD). Four numbered levels has been defined from 1 to 4, the higher the SIL number, the higher the cost and complexity of the safety system, and the better the performance is expected. Each SIF is assigned a SIL, As per IEC 61511, each SIF shall have an associated SIL.

SIL	PDF avg	Availability %	Consequences
1	0.01 – 0.01	90 - <99	minor injury
2	0.001 – 0.01	99 – <99.9	major injury
3	0.0001 – 0.001	99.9	multiple fatality
4	0.00001 – 0.0001	>99.99	community fatality

It has been observed that 95% of implemented SIF have SIL 1, while 5% of implemented SIF have SIL 2, and only near 1% of SIF have SIL 3. SIL 4 is for nuclear plant SIF, and SIL 3 for off-shore SIF applications.

Probability of Failure on Demand (PFD avg): is the probability of a functional unit failing to respond to a demand for action upon hazardous condition. It is measured in failures per hour.

Availability: probability that a functional unit will perform its task.

Hardware Failure Tolerance (HFT): the ability of a hardware to continue to perform a safety function in presence of failures or errors. An HTF of N, means that N + 1 faults will cause loss of safety function.

Safe Failure Fraction (SFF): ratio of average failure rates

To what extent can a device or process be expected to perform safely, and, in the event of a failure, to what extent can the process be expected to go to the safe state? These issues are addressed by the term safety integrity level (SIL). Therefore SIL gives a measure of safety risk or risk reduction to a tolerable limit for a given process.

See previous article Differences between SIF and SIS

References
IEC 61508/IEC 61511 Safety Integrity Level, Functional safety in the process
industryerisk reduction with safety instrumented systems, Endress þ Hauser catalog