Re: [SystemSafety] NYTimes: The Next Accident Awaits

From: Peter Bernard Ladkin < >
Date: Tue, 04 Feb 2014 10:12:44 +0100

On 2/4/14 1:42 AM, Nancy Leveson wrote:
> were simply the first 15 references that came up when I googled "safety case" and "safety case
> regime." .....It would be best to
> create a new term rather than use a term that has been defined for decades and is widely used to
> have a specific meaning..

Oh, my!

A random Google search for the term "safety case" is as least as likely to turn up this discussion as anything else. That's not necessarily how you go about finding out what the term might mean. (My search turns up Nancy's paper in the first fifteen. Given the current discussion, I am tempted to regard that as *proof* that such a search isn't going to tell you what the term might mean :-) ).

A *goal-oriented* (rather than random Google) search in the WWW for the notion of "safety case" could profitably start with the UK HSE, since Lord Cullen is the source both for the original recommendation concerning the notion of "Safety Case", in those words, and the origin of the HSE.

One finds, on searching the HSE WWW site for the term "safety case", references to the legislation for offshore oil/gas installations, namely the England&Wales law known as Offshore Installations (Safety Case) Regulations 2005 (there are earlier versions). This law is *very prescriptive* as to what has to be included in an offshore installation Safety Case - you can read it at

One also finds references to the legislation for railways, namely the England&Wales law known as Railways (Safety Case) Regulations 2003 (there are earlier versions). This law is *prescriptive* as to what has to be included in a railway Safety Case - you can read it at

It should be evident that a Safety Case for an offshore installation and a Safety Case for a railway are *two different things*, because these two enterprises use different kit: pipes and pumps and drills versus rails and locomotives and carriages. So rather than
> a term that has been defined for decades and is widely used to have a specific meaning
there are at least two specific meanings in English law alone.

It is also absurd to suggest that either of these Safety Cases is just some argument put together to fulfil a goal-oriented safety standard. First, they are not there to fulfil any standard, they are the law of England and Wales (I say again: standards aren't laws). Second, they are *prescriptive* - what shall be in a Safety Case designated as such in these industries is specified precisely in the law.

So, sticking with Safety Cases with upper-case initial letters (or at least with mid-size initials), here is the definition of a Safety Case for the nuclear industry in Britain (notice this is not a law - the ONR surveys installations in Britain, and Britain has three separate legal regimes: England and Wales, Scotland, and Northern Ireland)

[begin quote from: Purpose and Scope of Safety Cases NS-TAST-GD-051 Revision 3]

5.1.	A nuclear safety case is a set of documents that describe the radiological hazards in terms of
a facility or site and modes of operation (including potential undesired modes) and the measures that prevent or mitigate against harm being incurred. The safety case should provide a coherent demonstration that relevant standards have been met and that risks to persons have been reduced to as low as reasonably practicable (ALARP).

[end quote]

So there is, already, the third meaning. (To spell it out: neither of the previous two regulations "describe the radiological hazards" of a railway or an offshore installation.) It is obviously not as prescriptive as the first two, but it is still prescriptive.

So let's move to a generic definition, as embodied in a standard which is, by the agreement forming the European Union, the standard governing safety related electronic systems for railway signalling in 28 European countries (EN 50129:2003, the document referenced by Les Chambers, in case readers haven't pursued the link):

[begin quote EN 50129:2003 Section 3 Terms and Definitions]

3.1.49 safety case
the documented demonstration that the product complies with the specified safety requirements

[end quote]

This is obviously a fourth definition, since it makes no reference to pipes&pumps and stuff, or to rails&locomotives and stuff, or to radiological hazards of a facility and stuff. Unlike the first three definitions, it is not prescriptive at all.

Then there is my definition, which might as well be this: a rigorous argument, given in detail in a specified set of documents which include all the evidence used in the argument, that a system is acceptably safe, both when it is operating and when it is not operating.

Notice that there is no requirement that all these documents have to reside in one place. It covers FAA and EASA certification practices and it is intended to cover the capital-letter Safety Case regimes above with a suitable interpretation of "acceptably safe".

I'll leave it as an exercise to point out the differences between my definition and the previous four.

Here's another, from

[begin quote UK MoD SMP 12 Version 22, citing Def Stan 00-56 Version 4]

A structured argument, supported by a body of evidence that provides a compelling, comprehensible and valid case that a system is safe for a given application in a given environment.

[end quote]

So, unlike offshore installations or UK railways, a given MoD system might have a plethora of safety cases, if there are a plethora of applications in a plethora of environments. As well as depending on the possibly-varying meaning of the system property "safe". (I'd guess this is intended to mean something similar to my term "acceptably safe".)

So much for "widely used to have a specific meaning"! More accurate would be to use the plural. It should also be evident that whether a safety case is narrowly prescriptive or more free in form depends on which industry, regulation and country you might be talking about.

It's perfectly possible for there to be a good argument that the regime in Offshore Installations (Safety Case) Regulations 2003 in England & Wales "would not work" in the US oil/gas industry, as Nancy has indicated she believes. Such an argument would not obviously generalise to, say, FAA civilian aerospace certification documentation, which also forms a safety case - indeed people like me would say that that civilian aerospace safety cases are pretty good in general, while pointing out that they are by no means perfect, indeed it is possible to think that overall they are pretty good but specifically concerning avionics SW they are pretty poor.

It is also possible to consider FAA safety cases de re free-form (a manufacturer can provide what evidence it wants to establish a certification goal set by negotiation between regulator and manufacturer, for example on certification of lithium-ion batteries for the Boeing 787) and de facto prescriptive (if manufacturers have provided such-and-such documentation for successful certification of kit X in the past, then it is pretty much de rigeur that a manufacturer will have to provide at least the equivalent of that for future kit-X certification proceedings. The certification requirements, and therefore the recertification documentation, after the Boeing 787 battery fires were *modified* from what they had been, not thrown away and rewritten).

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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