[SystemSafety] Software Safety Requirements according to IEC 61508

[Peter Bernard Ladkin]

Being as I'm on the Maintenance Team, I thought it was about time that I figured out what software
safety requirements are, according to IEC 61508. It took me a few hours. I thought I'd share the
derivation to save others the effort.

The answer is that they are a given reliability with which safety functions are executed, expressed
to a given confidence.

For those not familiar with the way safety is construed in IEC 61508, there is stuff being done in
an engineered system, called the Equipment Under Control (or EUC). The EUC does stuff, which might
result in harm (either because the actions undertaken can be dangerous, or because failure behaviour
might be dangerous, or both). So a robot arm mucking around doing its thing might take your head
off. Or a pipe containing superheated liquids might burst and boil everything in its vicinity.

So there are risks (defined in the usual way). Somewhere magically from outside the system comes a
specification as to what are acceptable risks. If all of the risks posed by the EUC are acceptable,
there is nothing extra to be done. If some of the risks posed by the EUC are inacceptable, then
so-called safety functions must be implemented, which reduce those risks (per risk, through
mitigation or avoidance) to a level deemed acceptable. So a sensor is placed in the vicinity of the
robot which will shut the robot off if a person is detected in the vicinity (a "virtual cage").
Cladding (with an expansion space) is added around the pipe which may burst.

It's a very industrial-process-oriented conception. But it's been tweaked enough to be deemed to be
generally applicable. But some things which you think should be up-front and center aren't. Like
what a software safety requirements specification is.

Here is the answer as an attached paper. The meat is in the first section of about 2pp, the
"Analytical Table of Contents", which consists of my observations from textual investigation. The
rest consists of those observations taken individually, along with the citations from IEC 61508
which constitute the proof. So if you just want to know the answer to the question "what is a
software safety requirements specification?" then the first section will give that.

I am distributing it this way because I can't get at our RVS WWW server at the moment. I'll be able
to at some point over this weekend. We are in the process of rehosting our WWW pages with a
commercial provider (they are already up at rvs-bi.de).

PBL

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




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