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

From: Martyn Thomas < >
Date: Mon, 17 Jun 2013 12:13:27 +0100


I suggest that before any software is permitted as part of a safety-related system, certain mandatory processes should be put in place:
  1. The processes employed by the company to record failures in operation and changes to the software should be independently audited and certified to have enough integrity to justify the claims that are being made that the software has been "proven in use".
  2. These processes, once audited and agreed to be adequate, must remain in place and be subject to annual independent audit.
  3. Every failure, change to the software, (or change to the operating environment that makes it differ from the operating environment forecast in the safety case) must be reported to an independent safety assessor who must certify whether or not the safety case remains valid in the light of the failure or change.
  4. If the safety case is deemed no longer valid, the safety of the system must be assured by other means or the system mut be withdrawn from service until it can be shown once again to meet the safety criteria.

These steps are proposed on the basis that they (a) provide assurance that the evidence for "proven in use" is robust. (b) provide assurance that if in-service use or subsequent changes invalidate the safety case then it will not be concealed, and (c) put the risk on the system owner that the "proven in use" claim turns out to be false.

Martyn

On 17/06/2013 11:32, Peter Bernard Ladkin wrote:
> 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.
>
> http://www.rvs.uni-bielefeld.de/publications/WhitePapers/LadkinPiUessay20130614.pdf
>
>
> PBL
>



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systemsafety_at_xxxxxx Received on Mon Jun 17 2013 - 13:13:40 CEST

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