[SystemSafety] Units for Functional Safety variables ? - A Friday afternoon question on a Wednesday.

[Andrew Rae]

Jon,
Not everything has a dimension. (Although I suppose even dimensionless
constants could be considered dimensionless ratios).
Hardware Fault Tolerance (assuming you mean in the 61508 sense or
equivalent) is of the same "type" as a Safety Integrity Level - they are
assigned labels.
This is indicated in the fact that assignment and manipulation is according
to arbitrary rules, not normal arithmetic.

The other way of looking at it, if you reject the idea that HFT is a
"label", is that it is a count. The dimension of a count is whatever you
are counting, in this case
"levels of redundancy". This lets you perform dimension maths - you can
weigh your system, and come up with "Levels of redundancy per kilogram", or
cost your system and determine
"$$ per level of redundancy".



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On 2 October 2013 15:49, Jon Hind <jon at jonhind.co.uk> wrote:

> I posted this (only partially) frivolous question on out internal
> system, without any reply.
>
> So trying here for some erudition...
>
> What dimension and or units does Hardware Fault Tolerance (HFT) have ?
> - A Friday Poser on Functional Safety Terminology (FST).
>
> I remember being taught at university in the last century that
> everything has a dimension in terms of Length (L) , Mass (M) and or
> Time (T), or is a ratio (factor). A good check of the validity of an
> equation is that it must balance dimensionally.
>
> Availability Common Cause Factor (β, CCF) and Safe Failure Fractions
> (SFF) are dimensionless ratios, failures rates (λ) are T^-1, Proof
> Test intervals are T etc.
>
> And HFT ? HFT is an integer - In our (oil & gas process) world, 0 1 or
> 2 ; is it part of a case statement, or heaven forbid a pointer like in
> the C language ?
>
> I don't want to call it a factor as we already have that in CCF and SFF.
>
> Answers on a postcard please....
>
> Jon Hind
> jon at jonhind.co.uk
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> The System Safety Mailing List
> systemsafety at TechFak.Uni-Bielefeld.DE
>
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