[SystemSafety] ARRL: A Criterion for Composable Safety and Systems Engineering

[Braband, Jens]

Thanks for sharing your preprint. I read it with great interest but IMHO it contains a lot of unfounded statements and also some obvious errors, of which I state only the most important here:


-        Table 1 is completely wrong. SIL does NOT correspond directly to consequence only. Part 5 of IEC 61508 shows different ways of SIL allocation, none of them corresponds to table 1. It is also wrong that SIL depends on average statistical values, see part also 5.

-        Also table 2 is oversimplified, e. g. neither does ASIL-D correspond completely to SIL 3 or DAL B nor does SIL 4 to DAL A

-        A SIL is not a system property, but relates to a safety function. A system may deliver many different functions, of which only a subset may be safety-relevant. E. g. a monocoded processor system may run safety and non-safety functions.

-        A SIL level alone is not the top level safety requirement. More important are the functional safety requirements

-        If a residual risk is low and its probability is high then its severity should also be low.  So where's the beef in your Murphy example?

-        No justification on the definition of ARRL and its relation to SIL is given. The "rule" is not justified

-        Quality cannot be equaled with reliability alone and even less with MTBF. Have a closer look into Nancy's books ...

Best regards

Jens Braband

PS These are my personal views and not necessarily that of my employer or other organizations that I sometimes represent.


Von: systemsafety-bounces at lists.techfak.uni-bielefeld.de [mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] Im Auftrag von Vincenzo De Florio
Gesendet: Freitag, 13. September 2013 12:45
An: systemsafety at techfak.uni-bielefeld.de
Betreff: [SystemSafety] ARRL: A Criterion for Composable Safety and Systems Engineering

Dear Madams, dear Sirs,

I'd like to draw your attention to the following paper: "ARRL: A Criterion for Composable Safety and Systems Engineering", which will be presented at the SASSUR (Next Generation of System Assurance Approaches for Safety-Critical Systems) workshop<http://conf.laas.fr/SAFECOMP2013/?q=node/26> of SAFECOMP2013 on 24th September in Toulouse, France. The paper is authored by Eric Verhulst and Bernhard Sputh (Altreonic, Belgium), Jose Luis de la Vara (the Simula Research Lab, Norway), and Vincenzo De Florio (University of Antwerp, Belgium). The abstract is as follows:

"While safety engineering standards define rigorous and controllable
processes for system development, safety standards' differences in distinct
domains are non-negligible. This paper focuses in particular on the aviation,
automotive, and railway standards, all related to the transportation market.
Many are the reasons for the said differences, ranging from historical reasons,
heuristic and established practices, and legal frameworks, but also from the
psychological perception of the safety risks. In particular we argue that the
Safety Integrity Levels are not sufficient to be used as a top level requirement
for developing a safety-critical system. We argue that Quality of Service is a
more generic criterion that takes the trustworthiness as perceived by users better
into account. In addition, safety engineering standards provide very little
guidance on how to compose safe systems from components, while this is the
established engineering practice. In this paper we develop a novel concept
called Assured Reliability and Resilience Level as a criterion that takes the
industrial practice into account and show how it complements the Safety
Integrity Level concept."
Kind regards,
Vincenzo De Florio


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