[SystemSafety] SIL ratings to be scrapped?

[Peter Bernard Ladkin]

Chris,

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”.

When the system development is governed by IEC 61508 or its national equivalents (it is also an EN 
and these are taken by EU member countries as national standards), then there is no indication that 
SILs will disappear. The current edition of IEC 61508 is IEC 61508:2010 and the next maintenance 
action is due to start in 2014 and will likely take two years. Beyond that, it takes some time (up 
to a year) before a newly published standard becomes the only reference standard (there is a period 
of time in which both old and new editions are accepted). So, 2017 at the earliest.

I also see no movement, or even rumor, amongst my committee colleagues towards eliminating SILs from 
the next edition of IEC 61508.

Related, though, might be a concern about whether to eliminate the distinction between on-demand and 
continuous modes of operation. In contrast to some of my eminent colleagues, I think it is useful to 
distinguish between system functions which are on-demand and those which are continuous. However, an 
on-demand function usually requires a continuous-mode function to monitor the conditions whose 
status triggers the demand, and if this is built-in to the function itself, as it often is in E/E/PE 
systems, then the function becomes subject to continuous-mode SIL criteria. So there is much lower 
call for the demand-mode SIL criteria and it is plausible that they may disappear at some point.

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