Re: [SystemSafety] words you cannot use at GM

From: Peter Bernard Ladkin < >
Date: Thu, 22 May 2014 12:33:15 +0200

On 2014-05-21 10:14 , Maier, Thomas wrote:
> A correction regarding IEC 615011:
> That minimum failure rate per IEC 61511 is specified in Part 1 clause 8.2.2: “The dangerous failure
> rate of a BPCS (which does not conform to IEC 61511) that places a demand on a protection layer
> shall not be assumed to be better than *10^-5 per hour*.”

I grant you that my point was badly expressed, a disadvantage of responding quickly while multitasking on the train. But there is no correction to be made. Bertrand's response to you is abstract but correct.

Let me be more concrete. Suppose you have a safety function SF with SIL 1, which functionality is also provided by the BPCS. The function the BPCS provides, call it BCPS-SF, is by definition not a safety function.

Suppose you implement code in your SIS which does the following.

Now, how reliable does this safety-related code SIS-Supplementary-SF have to be?

Here is the reasoning. The required safety function is SF. The executing code implementing SF is

        SF: IF <conditions> THEN BPCS-SF ELSE SIS-Supplementary-SF

The safety-related code here consists of SIS-Supplementary-SF (BPCS-SF is not safety-related by definition). The function SF gets SIL 1. <conditions> is determined by code part 1 above; the test for ELSE by code part 2. Let's assume they are perfect. You may assume that the rate at which the THEN fails is at most 10^(-5), and you need 10^(-6) overall. So....

.... all you need to demonstrate concerning SIS-Supplementary-SF is 10^(-1) reliability. QED.

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 Thu May 22 2014 - 12:33:30 CEST

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