Re: [SystemSafety] RE : Qualifying SW as "proven in use"

From: Matthew Squair < >
Date: Thu, 27 Jun 2013 17:18:36 +1000

I've been thinking about Peter's example a good deal, the developer seems to me to have made an implicit assumption that one can use a statistical argument based on sucessful hours run to justify the safety of the software.

I don't think that's true, in fact I'd go further and say that whether you operate for a thousand hours or a million hours has no bearing on demonstrating software safety, because what we're interested in are systematic failures rather than random ones.

Example, I have a piece of software and (despite my best efforts) there's a latent fatal fault within it, however testing hasn't discovered it and I'm also in luck in that the operating environment is sufficiently close to the test environment that the fault is not triggered in the operating environment. Now I could run the system for one, one hundred or a thousand years in that operating environment and I wouldn't see a problem. So according to the statistical treatment the software is safe, even with a fatal flaw isn't it?

So logically if the number of hours you run in service in a particular environment has nothing to do with proving the safety of software, why couldn't I say that after one hundred hours the software was 'proven in use', for that specific environment. Why not one hour?

In Peter's example the number of hours run on the original software version could have been one, or ten million and there still would have been the same end result, e.g a failure when put into a new operational context. In other words one hour of operations has as much weight as one thousand (in the same environment).

We use hours as a measure of exposure when we test for random failures because they're well, random, but systematic failures aren't random so I don't think that using hours actually works. So what's the 'unit of exposure' that is valid for systematic failures?

Another question, say I have developed a piece of software, it's now running in three quite different operating environments, in terms of evidence of 'safety' would I weight 300 hours of operation in a single environment the same as 100 hours from each of these different environments? If so why?   

On Tue, Jun 18, 2013 at 6:41 PM, RICQUE Bertrand (SAGEM DEFENSE SECURITE) <bertrand.ricque_at_xxxxxx
> Hi Peter,
> Let's start some miles behind.
> There are 2 ways to claim that anything is compliant to 61508:
> 1 - Compliance. Yes, I know, it looks stupid as the way to claim compliance is nor ruled by the standard neither by the market. I can claim it on the ground of my reputation without having to bring any proof. Let's assume: on the basis of analysis made by firms having a strong reputation on the market.
> 2 - Proven use.
> Proven in use: what a nice idea.
> The standard is about safety. A manufacturer cand esign and put on the market a new equipment and get it "certified " for "compliance" for a given use (restricted operational and functional environment) for a "safety" purpose. Let's say safety critical pupose.
> Now let's go to your paper. First of all, you choose the example of a manufacturer, but we must rememeber that the standard adresses anybody. So a user could do it. Let's assume the user has more or less the same problems than the manufacturer.
> There are two possibilities for the previous use of the C equipment:
> 1 - previously used in a safety application with all the associated usual safety requirements (in particular dysfunctional requirements). Then it means obviously that the plant/application was NOT already compliant to 61508. Let's assume this is possible, although I doubt that plenty of users will shout it to the public...
> 2 - previously used in a non safety application. We can imagine that there are non-safety applications presenting very similar characteristics. I have however some doubts but lets assume it.
> >From a manufacturer point of view, it can be interesting to re-use C, most probably parts of C as C-SW, in new products. Honestly, I don't see any manufacturer going to the market saying: hey guys, here is my very old equipment that I have re-packaged and will sell you twice the price for safety applications with a nice SIL 3 stamp. The need is more like: hello, my dear certification company, here is my new product, it made of well known pieces, so please don't charge us too much for the certificate. You can consider C-SW as "proven in use", here are the data.
> >From an end-user point of view, (and this is what was in mind of the writers - "was" because it is becoming too demanding, and they are loosing the rules in 61511, pushing BTW 61508 for manufacturers only ...), if one can get rid of case 1, the issue is not to change anything in an existing plant and to, would an inspector ask strange questions, proove that the plant is the safest in the world and complies to any possible regulation. For competent and responsible end-users, the need it to have a framework to properly select equipments on sound basis and not on manufacturers data sheets.
> That's the context at business level. It is clear that points 1 and 2 of Martyn, in a non regulated context, will remain a dream for ever...
> When it goes to technical level:
> * Concerning manufacturers, in the industries at the origin of the standard, I don't know a single one having a solid process to collect data from the users. Some adjust the calculated MTBF with the return rate for repair in the factories. Concerning electronic equipment, if an expensive piece of hardware is not attached to the electronic board, it is usually thrown to the garbage... Most of them don't knwo at alla where and how are installed the equipement. It is actually the opposite The users are the best organised to gather data (OREDA, EIREDA, etc...). So I think that the issue is very theoretical for manufacturers.
> * Concerning end-users, before even entering in your considerations (I fully support BTW), and remaining within the concerns of the writers of the standard (shaping a much narrower picture actully):
> - Most systems operate on demand and, due to the functional characteristics of the applications, experience very few demands (otherwise, it means that the plants are permanently in trouble. This means that the chances that there could be a reasonable quantitative basis to extrapolate anything seem to me very low.
> - What is happening now in IEC 61511 discussions (with more or less the same persons as the ones at the origin of IEC 61508) is that three parties are building themselves:
> 1 - the ones (mostly end users) thinking that the concept dosn't apply to "re-use" but only to "already installed". They argue that if it is working properly, it will continue so. They keep a low profile on "re-use" but I suspect them to focus the discussion on "already installed" and use the concept for "re-use" later as it would not be clearly excluded in the standard ... They are also the ones that, rightly, don't want to buy expensive "certified" equipment with a lower performance than equipment they already have in their plant. The issue is real and interesting. These ones are mainly eurpoeans.
> 2 - the ones (mostly end-users) who want to re-use equipment because they want to reduce the number of different equipment in a plant (they are also strong opponents to diversity because of costs). These ones are mainly North American.
> 3 - the manufacturers are mainly against because they want to sell new equipment and they don't see how to actually "prove in use" as they lack data. The consultants are against because they don't where is the rationale (exactly what your paper is about).
> I am rather pessimistic about the outcomes in 61511. As far as 61508 is concerned, my opinion, converging more and more with several opinions expressed here is that:
> * Concerning software, the requirements must obviously be very stringent
> * This will implicitely so much limit the applicability of the concept that it will become useless for end-users and PLCs,
> * May-be it will be applicable for very tiny parts of firmware for manufacturers
> * It will de-facto create an insconsistency with 61511 if 61511 doesn't align on 61508 requirements
> Bertrand Ricque
> ________________________________________
> Date d'envoi : lundi 17 juin 2013 12:32
> À : systemsafety_at_xxxxxx > Objet : [SystemSafety] Qualifying SW as "proven in use"
> Folks,
> there is a significant question how SW can be qualified as "proven in use" according to IEC
> 61508:2010. There is a judgement in some quarters (notably the German national committee) that the
> criteria in IEC 61508:2010 are inappropriate. I think it wouldn't be out of place to say that many
> in the IEC 61508 Maintenance Teams find the current criteria unsatisfactory in one way or another.
> We in Germany have been discussing the issue and possible solutions for a couple of years, and
> recently the discussion has gone international. There seems to be a general feeling that qualifying
> SW statistically via the approach given by the exponential failure model is not practical, because
> the data requirements are overwhelming - it is regarded by most as implausible that companies will
> have the requisite data to the requisite quality even for SIL 2. But even if you qualify your SW for
> SIL 2 or higher without such data, then at some point some data will exist and people use such data
> as evidence that the original assessment was accurate. But what sort of evidence does it offer? The
> answer is probably a lot less than you might be convinced it does.
> There seems to me to be a lack of examples where things can go wrong - at least a lack of examples
> specifically adapted to assessments according to IEC 61508:2010. So I wrote one up - fictitious but
> I hope still persuasive - to illustrate what (some of) the assurance issues are. I hope it can aid
> the debate.
> --
> Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany
> Tel+msg +49 (0)521 880 7319 (tel:%2B49%20%280%29521%20880%207319) (
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Matthew Squair

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systemsafety_at_xxxxxx Received on Thu Jun 27 2013 - 09:18:39 CEST

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