[SystemSafety] SIL ratings to be scrapped?

[Peter Bernard Ladkin]

> On 8/21/13 4:25 PM, Chris Hills wrote:
>> I have just been taking to one of my customers who do control systems for industrial systems.  They
>> have been told by someone they are doing a system for that SIL ratings will “soon be scrapped”.
>> Apparently “It will be replaced by ALARP”.

I should probably have mentioned something about ALARP as well.

ALARP is a principle of English law and derivatives. ALARP is often illustrated in engineering 
texts, as in the informational part of IEC 61508, by a layered triangle diagram, however this does 
not change its status as a legal principle. Whereas SIL is a purely technical concept. You can't 
replace a technical engineering concept by a legal principle; to suggest you could makes a category 
mistake.

Besides, no one in Continental Europe uses ALARP as a means of judging if sufficient attention has 
been paid to reducing risks. French and Germans use "at least as good" (GAMAB, "globalement au moins 
aussi bon", resp. MGS "mindestens gleiche Sicherheit"), meaning you compare the new system with its 
installed predecessor to show that it does not increase any risk compared with its predecessor. 
Variation comes through categorising risk: if you throw all risk into one basket, then you can 
increase one type of risk as long as it is counterbalanced by a stronger reduction elsewhere; 
whereas if you require that the risk is as low in each and every fine-grained risk category you 
cannot do this.

PBL

Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany
Tel+msg +49 (0)521 880 7319  www.rvs.uni-bielefeld.de