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

From: Palin, Stuart (UK) < >
Date: Thu, 3 Oct 2013 08:46:11 +0000

Date: Wed, 2 Oct 2013 15:49:23 +0100
From: Jon Hind <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.
> 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
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