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

From: Jon Hind < >
Date: Thu, 10 Oct 2013 16:35:32 +0100


Thank you all for your comments, The answers are far more interesting than the question.

I think that Hardware fault tolerance has units of Hardware Fault tolerabilities ?
Not sure that there is enough room in an Excel cell to write that...

Tomorrow (Friday) another kind of, educational fault tolerance...

Jon Hind
jon_at_xxxxxx


> In measurement theory
> - a nominal scale are just labels
> - an ordinal scale has a rank order (1st, 2nd, 3rd, etc., or A,B,C,D ) - but
> you cannot do maths on the rank values
> - a ratio scale is used for expressing physical quantities relative to some
> base unit (where you can do maths)
>
> HFT could be represented on an ordinal scale, as there is implied ordering,
> i.e. 2 better than 1 {as more component failures can be tolerated) - but we
> do not know how much better.
>
>
>
> Andrew Rae wrote:
>>
>> 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".
>>
>>
>> My system safety podcast: http://disastercast.co.uk
>> My phone number: +44 (0) 7783 446 814
>> University of York disclaimer:
>> http://www.york.ac.uk/docs/disclaimer/email.htm
>>
>>
>> On 2 October 2013 15:49, Jon Hind <jon_at_xxxxxx >> <mailto:jon_at_xxxxxx >>
>> 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 (, CCF) and Safe Failure Fractions
>> (SFF) are dimensionless ratios, failures rates () 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
>>
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systemsafety_at_xxxxxx Received on Thu Oct 10 2013 - 17:36:02 CEST

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