Re: [SystemSafety] SIL ratings to be scrapped?

From: Peter Bernard Ladkin < >
Date: Wed, 21 Aug 2013 16:57:42 +0200

> 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

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systemsafety_at_xxxxxx Received on Wed Aug 21 2013 - 16:57:53 CEST

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