[SystemSafety] SIL ratings to be scrapped?

Thu Aug 22 16:53:07 CEST 2013

Bertrand,

I wasn't discussing it from a tolerable hazard rate point of view.

I was simply (although I agree, maybe too simply), pointing out that a 
high demand system is a "demand mode" system where the hazard rate based 
on a PFD crosses the hazard rate defined by PFH, therefore PFH gets used 
provided the system meets the high demand requirements.

The maximum demand rate is defined by the slowest diagnostic test interval 
that has been used in the analysis. On a low & high demand system the 
process safety time is often independent from the diagnostic test 
interval.

In a continuous system the minimum process safety time is defined by the 
maximum diagnostic time plus the time to achieve the safe state.

They both have their place, the demand system can make use and take credit 
for diagnostics that would be to slow for continuous mode applications. 
Continuous mode systems can be used for application where failure of the 
SIS will cause a hazard, which a demand system can't.

My personal belief is that it is important to chose the correct system to 
fit the application. 
If we loose the distinction between the different types of operation I 
believe that we are likely to complicate the requirements to make sure all 
aspects have been covered.

(ok, lets not get into the discussion on what happens once in the shutdown 
state just at the moment :-).

Best regards,

Gerry Creech

From:   "RICQUE Bertrand (SAGEM DEFENSE SECURITE)" 
<bertrand.ricque at sagem.com>
To:     System Safety List <systemsafety at techfak.uni-bielefeld.de>
Date:   22/08/2013 15:26
Subject:        Re: [SystemSafety] SIL ratings to be scrapped?
Sent by:        systemsafety-bounces at lists.techfak.uni-bielefeld.de

Well said Jens, and things have not that much evoluated. This is normal as 
there are still industries that fiercely reject any approach to MTTH as 
the consequences might be disturbing in terms of necessary engineering 
effort and of low results...

Bertrand RICQUE
Program Manager, Optronics and Defense Division

T +33 (0)1 58 11 96 82
M +33 (0)6 87 47 84 64
23 avenue Carnot 
91300 MASSY - FRANCE 
http://www.sagem-ds.com

From: systemsafety-bounces at lists.techfak.uni-bielefeld.de [
mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of 
Braband, Jens
Sent: Thursday, August 22, 2013 3:55 PM
To: M Mencke; Gerry R Creech
Cc: System Safety List; 
systemsafety-bounces at lists.techfak.uni-bielefeld.de
Subject: Re: [SystemSafety] SIL ratings to be scrapped?

This discussion on operation modes  has been extensively lead in the 90s 
and early 00s. I was chairing the CENELEC WG at that time that rewrote 
annex A of EN 50129. I think the problem is oversimplified here. 

I would advise anyone really interested to go into the modeling details, 
e. g. as published by Prof. Sato in

Yoshimura, I., Sato, Y., Suyanma, K.: Safety Integrity Level Model for 
Safety-related Systems in Dynamic Demand State, Proceedings of the 2004 
Asian Inter-national Workshop on Advanced Reliability Modeling (AIWARM 
2004), Hiroshima, 577?584

We had a lot of discussions in this time, basically the same as in the 
90s, and I also wrote a paper for Safecomp with colleagues from TÜV 

Braband, J., vom Hövel, R. and Schäbe, H.: Probability of Failure on 
Demand ? the Why and the How , in: Proc. SAFECOMP2009, Hamburg, 2009, 
46-54

This sums up many discussions we had in the 90s when writing the EN 50129. 
These papers show that from a risk based perspective PFH and PFD are 
indeed two sides of the same coin and not so much different as suggested 
by IEC 61508. It explains also the reason why EN 50129 uses THR and NOT 
PFH. THR is an umbrella concept for both PFH and PFD and that is the main 
message.

Best regards

Jens Braband

PS I stated my personal technical opinion here, not necessarily that of my 
employer or any other organization.

Von: systemsafety-bounces at lists.techfak.uni-bielefeld.de [
mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] Im Auftrag von 
M Mencke
Gesendet: Donnerstag, 22. August 2013 15:22
An: Gerry R Creech
Cc: System Safety List; 
systemsafety-bounces at lists.techfak.uni-bielefeld.de
Betreff: Re: [SystemSafety] SIL ratings to be scrapped?

Yes. They are different. The objective of my previous email was to point 
out that the classification of operation modes for Safety functions into 
different categories (one category containing ?High demand? and 
?Continuous? and the other ?Low demand?) in some standards and only 
referring to ?Continuous? mode in others could lead to confusion. In IEC 
61508-1, the PFH assigned for each SI level is the same for both ?High 
demand? and ?Continuous? mode, therefore grouping ?High demand? and 
?Continuous? in the same category, at least as far as PFH is concerned.
As you just mentioned, ?Continuous? mode and ?High demand? mode are not 
the same. However, if you consider the text I extracted from the EN 50129 
standard (it is a direct quote from the standard), the second point 
states:
?All demand mode systems can be modelled as continuous mode systems?.
This gives the (perhaps incorrect) impression that ?High demand? and 
?Continuous? could be considered to be equivalent, that is, according to 
what is suggested by the standard. 
Why? Because a logical interpretation of the above sentence is that 
?Continuous? mode and ?High demand? mode are in a single category, named 
by the standard as ?Continuous?. If you consider the definition of ?High 
demand? mode and ?Continuous? mode, for ?High demand?, the frequency of 
demands is greater than one per year, and for Continuous, the safety 
function retains the EUC in a safe state as part of normal operation. This 
indicates to me that a ?High demand? mode is a frequency of demand 
anywhere between greater than one per year and less than normal operation, 
a ?Continuous? frequency being the limit of this interval. The frequency 
of ?Low demand? can never be placed in this frequency range, as it is less 
than or equal to one per year.

prEN 50126-2:2012 (page 39, section 10.2) makes reference to ?continuous 
mode models?. However, in drafts 1 ? 5 of this standard I cannot find any 
definition of ?continuous?, ?high demand?, or other. It seems that these 
drafts are now in ?In hands of WG 14?. It may be a suggestion to include a 
definition of what ?continuous mode? includes, or specify that the 
category ?continuous? groups modes only in terms of the same PFH.
Regards.

2013/8/22 Gerry R Creech <grcreech at ra.rockwell.com>
Myriam, 

Isn't 'high demand' also a demand mode, that happens to use PFH and 
different from continuous mode? 

In continuous mode, we can only take credit for diagnostics that can 
detect a failure and carry out the specified action within the process 
safety time. 
In high demand mode, we can take credit for diagnostics where the ratio of 
the test rate to the demand rate equals or exceeds 100. 

Best regards, 

Gerry Creech 

From:        M Mencke <menckem at gmail.com> 
To:        Peter Bernard Ladkin <ladkin at rvs.uni-bielefeld.de> 
Cc:        System Safety List <systemsafety at techfak.uni-bielefeld.de> 
Date:        22/08/2013 11:10 
Subject:        Re: [SystemSafety] SIL ratings to be scrapped? 
Sent by:        systemsafety-bounces at lists.techfak.uni-bielefeld.de 

Regarding the high demand and low demand mode, it makes sense to apply 
these modes for some elements. However, in the railway standards, the 
concept of low demand is already not being considered. In EN 50129, the 
following is stated: 
?NOTE: In contrast to other standards the SIL table in this standard has 
only one column for 
frequencies (formerly called high demand or continuous mode) and does not 
have a column for 
failure probabilities on demand (formerly called demand mode). The reasons 
to restrict to one 
mode are 

· Less ambiguity in determination of SIL. 

· All demand mode systems can be modelled as continuous mode systems. 

· Continuous control and command signalling systems are clearly the 
majority in modern railway signalling applications. 

The SIL table has been constructed taking into account other relevant 
international standards.? 
In my opinion, the existence of two different approaches to the 
application of the SIL concept, where one only considers high demand mode 
and the other considers both, contributes to the reasons why there are 
misunderstandings regarding the use of SIL. This is particularly true for 
engineers new to the industry or potential customers who consult the 
standard relevant to their sector in order to try to gain an understanding 
of the SIL concept. 
Imagine a situation where a ?newcomer? to the railway industry consults 
the railway standards for an overview of SILs, and their understanding of 
the SIL concept is gained based on the assumption that only one mode of 
operation is considered, the high demand mode. This engineer (or 
technician, manager, etc.) then decides that he would like to extend his 
knowledge and reads, for example, the IEC 61508 where the ?high demand? 
and ?low demand? modes are introduced. This does not appear to aid the 
reader in providing a clear explanation of the application of the concept. 
Your response may be ?well, in that case the reader should read the 
available literature?, to gain an in-depth understanding. However, this 
may not always be possible, due to time constraints, etc., particularly in 
the case of a customer or a manager. 
Additionally, even though the standard argues that continuous demand are 
the majority in modern railway signalling applications, as Peter just 
mentioned, passenger emergency braking systems on trains are meant to be 
used only occasionally. Given that only high demand mode is considered in 
the railway standards, should the railway standard definition of ?high 
demand? then be applied for this type of system, or is it required to 
refer ?back? to IEC 61508?... 
Note: I write in Hiberno English. For example, words ending in the suffix 
?ing? preceded by ?l? are spelled with a double ?l? rather than a single 
"l", as in ?signalling?, ?modelling?. 
Regards, 
Myriam. 

2013/8/22 Peter Bernard Ladkin <ladkin at rvs.uni-bielefeld.de> 
To back up Martin's caveat with other reasons:

I would not argue for scrapping "low-demand" on the sole basis it is 
inappropriately applied - I think there need to be significantly more 
reasons than that.

Reactor SCRAM systems are only meant to be used occasionally. Similarly, 
passenger-emergency-braking systems on trains.

System functions which are invoked occasionally tend to not work when 
invoked. Emergency slides on commercial transport aircraft exits work as a 
rule-of-thumb about half the time, which is why the emergency-evacuation 
certification test is performed with only half the available exits.

So for such systems and functions there need to be defined proof tests and 
a defined interval for proof tests. And those intervals are dependent upon 
how often you think the demand for the function is likely to arise.

You don't have such things as proof tests or associated intervals for 
continuously-operating safety-relevant functions, such as fly-by-wire 
control systems or ETCS.

Now, I agree that such things as proof tests are not relevant for pure SW 
"elements" (to use the 61508 preferred terminology), but that SW mostly 
sits inside something which executes the function and for which proof 
tests are relevant. How are you going to deal with these differences 
appropriately if the standard scraps the distinction?

PBL 

On 8/22/13 9:30 AM, Jensen, Martin Faurschou Jensen wrote: 
I agree with the arguments below when it comes to systems, but we have to 
keep in mind that 61508 is also used for the development of single 
elements. For a sensor, designed and developed for use in a SIS, the 
demand mode makes sense, as this only needs to detect and report a 
situation, and does not need to contribute in maintaining the safe state 
afterwards. 

-----Original Message-----
......On Behalf Of ECHARTE MELLADO JAVIER 

Sent: 22. august 2013 09:20
To: Peter Bernard Ladkin; systemsafety at lists.techfak.uni-bielefeld.de
Subject: Re: [SystemSafety] SIL ratings to be scrapped?

I have discussed this mater several times. I think that low demand 
criteria should disappear because it is usually a fallacious argument. 

PBL 

Prof. Peter Bernard Ladkin, Faculty of Technology, University of 
Bielefeld, 33594 Bielefeld, Germany
Tel+msg +49 (0)521 880 7319  www.rvs.uni-bielefeld.de

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