Re: [SystemSafety] Separating critical software modules from non-critical software modules

From: René Senden < >
Date: Thu, 25 Jul 2013 22:22:58 +0200

Dear Myriam,  

In your example, are we talking about sw-modules
(safety-critical/non-safety-critical) which are part of the same

For example do these modules correspond to (or are they part of) CSCs within the same CSCI… ?

Also, in both cases (critical vs non-critical), are we talking about newly developed modules, reuse with(out) modifications, or perhaps all of the “above”?  

It always remains an evaluation on a case-by-case-basis, at the same time the criteria for such evaluations are simply not very clear or perhaps not very generic..

One could probably argue successfully/convincingly for and against “sufficient independence” or “freedom from interference” in a significant amount of cases..

The efforts involving thorough analyses, as far as this is possible, are often quite significant to the point that they rival the efforts following the conclusion that

the non-critical modules are not quite so non-critical… Unfortunately, some consider that such efforts are to be commensurate with the “level of criticality”

involved, which is not defendable in my view… still it turns out to be deceptively rational in practice…  

Should we be talking about critical CSCIs vs non-critical CSCIs, this may offer some advantages since the corresponding interfaces (dependencies) are
(or will be)

assumed to be transparent (appropriately documented) and under configuration/change-control, which of course facilitates the required analyses for independence…

The higher up in the hierarchy the separation of critical from non-critical can be made, the better…but I suppose that probably goes without saying…  

I am not so sure this is helpful to you…

Best regards,


From: systemsafety-bounces_at_xxxxxx [mailto:systemsafety-bounces_at_xxxxxx Sent: dinsdag 23 juli 2013 10:22
To: systemsafety_at_xxxxxx Subject: [SystemSafety] Separating critical software modules from non-critical software modules  

Dear All,

For any software development project, many software modules are involved, where some are defined as safety critical, others are not. For example, in railway signaling, communications modules are likely to be defined as critical, whereas other modules such as those involving data storage or other basic functions are not. An analysis may be performed with the objective of demonstrating that the safety critical modules are entirely independent from the non critical modules, leading to the conclusion that the application of a programming standard for safety critical software is only required for those modules defined as safety critical (note the phrase “with the objective of demonstrating…”; I would hesitate before drawing the conclusion that the analysis really demonstrates what it is supposed to demonstrate).

In my field the EN 50128 would be applied, however, it could be any standard for safety critical software. Thus, the software is developed applying the standard only to the modules which have been defined as “safety critical”. In order to supposedly save time/money, etc., the rest of the modules are developed as non-critical software, either as SIL 0 functions or according to a standard programming standard. My question is whether such an approach is really valid, given that the application of a safety critical standard does not only involve the application of specific language features, it involves an entire development life cycle, and I find it difficult to see how the modules defined as “non-critical” then do not form part of that life cycle. I’m not saying it is not valid, but I would like to know how others see this.

Additionally, if the same programmers are involved in the programming of both critical and non-critical modules, does it really make sense that they only pay attention to the features required for safety critical software when programming the critical modules, and modify their programming style for the rest of the modules (or revert back to their “usual” style)? These questions also depend on what you consider as critical, for example, for a control system with a HMI, you could only consider communication modules critical, however, you need a GUI to display the status of the elements an operator has to control correctly. Some operations performed by the operator may not have the potential to generate a hazard with a high severity level, because there are mitigations in place. However, that doesn’t necessarily mean that the software responsible for displaying the information should not be programmed according to a safety critical standard. I am aware that these questions don’t have an “easy” answer; any opinions would be appreciated.

Kind Regards,


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systemsafety_at_xxxxxx Received on Thu Jul 25 2013 - 22:23:11 CEST

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