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

[Palin, Stuart (UK)]

Date: Wed, 2 Oct 2013 15:49:23 +0100
From: Jon Hind <jon at jonhind.co.uk>

> 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.
...
> Answers on a postcard please....

OK - probably aiming more for the frivolity than the erudite here, but ...

How does W/K (Watts/Kelvin) stand up?

Reasoning:  In considering a system we could consider a fully operational system to be more organized than a system with faults.  Indeed when a fault occurs we are likely to expend energy on it to fix it.  This suggests that hardware faults are an expression of entropy.  The SI units of entropy are J/K.  Now faults are likely to accrue over time, and at some time the accrued faults will cause the system to fail.  So the ability to tolerate faults is J/K/S - in other units W/K.  QED.

A Friday afternoon answer on a Thursday morning.

Stuart Palin
[I am sure my employer would like to disown this - so usual disclaimers, however the following is as required by UK law:

BAE Systems (Operations) Limited
Registered Office: Warwick House, PO Box 87, Farnborough Aerospace Centre, Farnborough, Hants, GU14 6YU, UK
Registered in England & Wales No: 1996687]
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