On 28 Jul 2005, at 10:56, Michael.Ainsworth@xxxxxx wrote:


> I agree that there is a need to show that any system is fit for 
> purpose,
> and
> I'd expect to see, for example, a justification of radio system
> availability
> in such a system, but I'm a bit concerned about:
>
>> I believe it is unprofessional to treat safety-related systems any
>> differently just because their allowable failure probability is higher
> than
>> the SIL 1 threshold.
>
> Without getting into debates about the accuracy and value of quantified
> limits in the various standards - surely the level of assessment 
> applied
> has
> to be proportionate to the risk involved?
>
> ALARP is a general principle which applies to everything from nuclear
> reactors
> to office furniture, and while the general principle still applies to
> systems which
> are not usually categorised as safety-related, I'd expect the nature 
> of the
> assessment
> to be somewhat less onerous.
>
> Mike

I think that Martyn is correct. The way in which the system is treated 
should not differ. However, the amount that would need to be spent in 
order to claim that risk reduction is impracticable does differ - and, 
apparently, the increasing width of the carrot as the risk increases is 
intended to represent this.

Felix Redmill.