Safety Integrity Levels (SIL)

The SIL concept is a way of categorizing safety functions into five discrete levels: SIL0 - SIL4. The SIL determination follows a complex, although systematic, process as shown below (Figure A.5 from EN 50129).

However, for many purposes the quantitative SIL value can be substituted with a more straightforward qualitative approach when categorizing safety functions. For example:
Safety critical functions kind of compare to SIL3/4 (as e.g. the emergency brake in a train or the logic in an interlocking system.)
Safety related functions kind of compare to SIL 1/2 (as e.g. emergency announcement speakers in a train or warning lamps for track crossing)


Interpretation

At high SIL, the heaviest measures to avoid random, systematic errors and common cause failures have to be used at all phases in the V-model.

The SIL determination often ends up in complicated mathematical discussions among risk analytics (e.g. is the human failure rate 1e-3 or 2e-4 [pr. action]).

These types of discussions narrow the number of persons, who participates in the safety discussions; which again might decreases the safety awareness among the other staff groups: Implementation engineers, maintenance staff, train drivers and sub suppliers as an unfortunate side effect.

In an operating organization, with many small projects and a few major projects, it can therefore be advantageously to simplify the categorization of the safety functions into the above described categories e.g. "Safety related" and "Safety critical".

Such a concept is easier to communicate to the staff groups and integrate into the used procedures and documents.

Note 1; the used categorization method should be described in the Safety plan and agreed upon by the Safety Authority.

Note 2; for product developers and suppliers it will most likely be necessary to make quantitative risk calculations and common cause analysis, see examples in e.g. TR 50451:2007.

Next chapter >> 5.1 What is the task of the Assessor?

Focus on the Source

The SIL levels are explained in EN 50129 in the normative Annex A, "Safety Integrity Level".
Related concepts like Systematic and random failures, Tolerable Hazard rates (THR), Common cause Failures (CCF), process independence and safety targets are also explained.

Annex B of EN 50129 explains about Detailed technical requirements to e.g. redundancy and CCF.

Annex C of EN 50129 explains about Identification of hardware component failure modes.

TR 50451:2007, "Railway applications. Systematic allocation of safety integrity requirements" explains how to calculate the needed SIL of a new product.